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Class 10, , PHYSICS, Electricity, , TEXT BOOK QUESTIONS PAGE NO. 94, 1. What does an electric circuit mean?, Ans.An electric circuit consists of electric devices, switching devices,, source ofelectricity, etc. that are connected by conducting wires., 2. Define the unit of current., Ans.The unit of electric current is ampere (A). 1 A is defined as the flow of 1 C of, charge through a wire in 1 s., 3. Calculate the number of electrons constituting one coulomb of charge., Ans.One electron possesses a charge of 1.6 × 10−19C, i.e., 1.6 × 10−19C of charge is, contained in 1 electron., 1, 1.6 10-19, ∴ 1 C of charge is contained in =, = 6.25 ×1018 electrons, Therefore, 6.25 ×1018 electrons constitute one coulomb of charge., TEXT BOOK QUESTIONS PAGE NO. 96, 4. Name a device that helps to maintain a potential difference across a, conductor., Ans.A source of electricity such as cell, battery, power supply, etc., helps tomaintain a potential difference across a conductor., 5. What is meant by saying that the potential difference between two points is 1 V?, Ans.If 1 J of work is required to move a charge of amount 1 C from one point, toanother, then it is said that the potential difference between the two points is, 1 V., 6. How much energy is given to each coulomb of charge passing through a 6, Vbattery?, Ans.The energy given to each coulomb of charge is equal to the amount of, workrequired to move it. The amount of work is given by the expression,, Work done = Potential difference, Charge, Where, Charge = 1 C and Potential difference = 6 V, Work done = 6 1= 6 J, , Therefore, 6 J of energy is given to each coulomb of charge passing, through a battery of 6 V, TEXT BOOK QUESTIONS PAGE NO. 103, 1. On what factors does the resistance of a conductor depend?, Ans.The resistance of a conductor depends upon the following factors:, Length of the conductor, Cross-sectional area of the conductor, Material of the conductor, Temperature of the conductor

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2. Will current flow more easily through a thick wire or a thin wire of the same, material, when connected to the same source? Why?, Ans. Resistance of a wire,, Where, RA, =Resistivity of the material of the wire, l = Length of the wire, = Area of cross-section of the wire, Resistance is inversely proportional to the area of cross-section of the, wire. Thicker the wire, lower is the resistance of the wire and vice-versa., Therefore, current can flow more easily through a thick wire than a thin, wire., 3. Let the resistance of an electrical component remains constant while the potential, difference across the two ends of the component decreases to half of its former value., What change will occur in the current through it?, Ans.The change in the current flowing through the component is given by, Ohm’slaw as,, V = IR, 𝐼=, , 𝑉, 𝑅, , Where,, Resistance of the electrical component = R, Potential difference = V, Current = I, The potential difference is reduced to half, keeping resistance constant., Let the new resistance be R' and the new amount of current be I '., Therefore, from Ohm’s law, we obtain the amount of new current., , Therefore, the amount of current flowing through the electrical, component is reduced by half., Why are coils of electric toasters and electric irons made of an alloy rather, than a pure metal?, Ans.The resistivity of an alloy is higher than the pure metal. Moreover, at, hightemperatures, the alloys do not melt readily. Hence, the coils of heating, appliances such as electric toasters and electric irons are made of an alloy, rather than a pure metal.

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(a) Which among iron and mercury is a better conductor?, Ans. (a) Resistivity of iron =10.0 ×10-8 m, Resistivity of mercury = 94.0 ×10 -8, m, Resistivity of mercury is more than that of iron. This implies that iron is a better, conductor than mercury., (b) Which material is the best conductor?, It can be observed from Table 12.2 that the resistivity of silver is the lowest, among the listed materials. Hence, it is the best conductor., , TEXT BOOK QUESTIONS PAGE NO. 107, 1. Draw a schematic diagram of a circuit consisting of a battery of three cells of, 2 V each, a 5 Ω resistor, an 8 Ω resistor, and a 12 Ω resistor, and a plug key,, all connected in series., Ans.Three cells of potential 2 V, each connected in series, is equivalent to, abattery of potential 2 V + 2 V + 2 V = 6V. The following circuit diagram shows, three resistors of resistances 5 Ω, 8 Ω and 12 Ω respectively connected in series, and a battery of potential 6 V, , 2. Redraw the circuit of question 1, putting in an ammeter to measure the current, through the resistors and a voltmeter to measure potential difference across the 12 Ω, resistor. What would be the readings in the ammeter and the voltmeter?, Ans.To measure the current flowing through the resistors, an ammeter should, beconnected in the circuit in series with the resistors. To measure the potential, difference across the 12 Ω resistor, a voltmeter should be connected parallel, to this resistor, as shown in the following figure.

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TEXT BOOK QUESTIONS PAGE NO. 110, 1. Judge the equivalent resistance when the following are connected in parallel, (a) 1 Ω and 106Ω, (b) 1 Ω and 103Ω and 106Ω., Ans. (a) When 1 Ω and 106Ω are connected in parallel:, Let R be the equivalent resistance., , 2. An electric lamp of 100 Ω, a toaster of resistance 50 Ω, and a water filter, of resistance 500 Ω are connected in parallel to a 220 V source. What is the, resistance of an electric iron connected to the same source that takes as, much current as all three appliances, and what is the current through it?, Ans. Resistance of electric lamp, R1=, 100 Ω, Resistance of toaster, R2 = 50 Ω, Resistance of water filter, R3 = 500 Ω, Voltage of the source,, V = 20 V

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These are connected in parallel, as shown in the, following figure. Let R be the equivalent resistance of, the circuit., , 7.04 A of current is drawn by all the three given appliances., , Therefore, current drawn by an electric iron connected to the, same source of potential 220 V = 7.04 A, Let R’ be the resistance of the electric iron. According to Ohm’s law,, , Therefore, the resistance of the electric iron is 31.25 Ω and the, current flowing through it is 7.04 A., What are the advantages of connecting electrical devices in parallel, with the battery instead of connecting them in series?, Ans.There is no division of voltage among the appliances when, connected inparallel. The potential difference across each, appliance is equal to the supplied voltage. The total effective, resistance of the circuit can be reduced by connecting electrical, appliances in parallel., How can three resistors of resistances 2 Ω, 3 Ω and 6 Ω be connected, to give a total resistance of (a) 4 Ω, (b) 1 Ω?, Ans.There are three resistors of resistances 2 Ω, 3 Ω, and 6 Ω respectively., (a) The following circuit diagram shows the connection of the, three resistors. Here, 6 Ω and 3 Ω resistors are connected in, parallel.

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Therefore, their equivalent resistance will be given by, 1, , R=, , 1, , =, , 6x3, 6+ 3, , =2, 1, 1, +, +, R1, R2, 6, 3, This equivalent resistor of resistance 2 Ω is connected to a 2 Ω resistor in series., 1, , =, , 1, , Therefore, equivalent resistance of the circuit = 2 Ω + 2 Ω = 4 Ω, Hence, the total resistance of the circuit is 4 Ω, The following circuit diagram shows the connection of the three resistors., , All the resistors are connected in series. Therefore, their equivalent, resistance will be given as, 1, , R, 1, , 1, , R1, , R2, , +, , 1, , =, +, , 1, , 1, , R3, , 2, , =, , 1, , 1, , 3, , 6, , + +, , 1 = 6 =1, 3+2+, 1, 6, 6, , Therefore, the total resistance of the circuit is 1 Ω, , What is (a) the highest, (b) the lowest total resistance that can be secured by, combinations of four coils of resistance 4 Ω, 8 Ω, 12 Ω, 24 Ω?, Ans.There are four coils of resistances 4 Ω, 8 Ω, 12 Ω and 24 Ω respectively, If these coils are connected in series, then the equivalent resistance will be, the highest, given by the sum 4 + 8 + 12 + 24 = 48 Ω, If these coils are connected in parallel, then the equivalent resistance will, be the lowest, given by, 1, , R=, 1+, , 1+, , =, 1, , +, , 1, , 1, 1 + 1 + 1, , 1, , =, 1, , R1, R2, R3, R4, 4, 8 12, 24, Therefore, 2 Ω is the lowest total resistance., , 6 + 3+ 2+ 1, 24, , =, , 24 =2, 12

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INTEXT QUESTIONS PAGE NO.112, Why does the cord of an electric heater not glow while the heating element, does?, Ans.The heating element of an electric heater is a resistor. The amount of, heatproduced by it is proportional to its resistance. The resistance of the, element of an electric heater is very high. As current flows through the, heating element, it becomes too hot and glows red. On the other hand, the, resistance of the cord is low. It does not become red when current flows, through it., Compute the heat generated while transferring 96000 coulomb of charge in, one hour through a potential difference of 50 V., Ans. The amount of heat (H) produced is given by the Joule’s law of, heating as H=VIt, Where, Voltage, V = 50 V, Time, t = 1 h = 1 × 60 × 60 s, Amount of current, I=, , Amount of Charge =, 96000 = 80 A, Time of flow of, charge, 1x 60 x 60, 3, , 50 x 80/3 x 3600 = 4.8 x106J, Therefore, the heat generated is 4.8 x106J, , An electric iron of resistance 20 Ω takes a current of 5 A. Calculate the heat, developed in 30 s., Ans.The amount of heat (H) produced is given by the Joule’s law of, heating asH=VIt, Where, Current, I = 5 A, Time, t = 30 s, Voltage, V = Current × Resistance = 5 × 20 = 100 V, H = 100 x 5 x 30 = 1.5 x 104 J, Therefore, the amount of heat developed in the electric iron is 1.5 x 104 J, TEXT BOOK QUESTIONS PAGE NO. 114, What determines the rate at which energy is delivered by a current?, Ans.The rate of consumption of electric energy in an electric appliance is, calledelectric power. Hence, the rate at which energy is delivered by a, current is the power of the appliance., An electric motor takes 5 A from a 220 V line. Determine the power of the, motor and the energy consumed in 2 h., Ans. Power (P) is given by the expression, P = VI

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Where,, Voltage, V = 220 V, Current, I = 5 A, P = 220 x 5 = 1100 W, Energy consumed by the motor = Pt, Where,, Time, t = 2 h = 2 × 60 × 60 = 7200 s, P = 1100 × 7200 = 7.92 × 106JTherefore, power, of the motor = 1100 W, Energy consumed by the motor = 7.92 × 106 J, , EXERCISE QUESTIONS PAGE NO. 115

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Energy consumed by a TV set of power 250 W in 1 h = 250 × 3600 = 9 × 105 J, Energy consumed by a toaster of power 1200 W in 10 minutes = 1200 ×, 600 = 7.2× 105 J, Therefore, the energy consumed by a 250 W TV set in 1 h is more than the, energy consumed by a toaster of power 1200 W in 10 minutes., An electric heater of resistance 8 Ω draws 15 A from the service mains 2 hours., Calculate the rate at which heat is developed in the heater., Ans. Rate of heat produced by a device is given by the expression for power as, P = I2R, where, Resistance of the electric heater, R = 8 Ω, Current drawn, I = 15 A, P = 152 x 8 = 225 x 8 = 1800 J/s, Therefore, heat is produced by the heater at the rate of 1800 J/s., Explain the following., Why is the tungsten used almost exclusively for filament of electric lamps?, Why are the conductors of electric heating devices, such as bread-toasters and, electric irons, made of an alloy rather than a pure metal?, Why is the series arrangement not used for domestic circuits?, How does the resistance of a wire vary with its area of cross-section?, Why are copper and aluminium wires usually employed for electricity, transmission?, Ans. (a) The melting point and resistivity of tungsten are very high. It does not, burn readily at a high temperature. The electric lamps glow at very high, temperatures. Hence, tungsten is mainly used as heating element of electric bulbs., The conductors of electric heating devices such as bread toasters and electric, irons are made of alloy because resistivity of an alloy is more than that of metals., It produces large amount of heat., There is voltage division in series circuits. Each component of a series circuit, receives a small voltage for a large supply voltage. As a result, the amount of, current decreases and the device becomes hot. Hence, series arrangement is not, used in domestic circuits., Resistance (R) of a wire is inversely proportional to its area of cross-section, 1, (A), i.e., R ∝, 𝐴, , Copper and aluminium wires have low resistivity. They are good conductors of, electricity. Hence, they are usually employed for electricity transmission.

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Magnetic Effect of Electric current, TEXT BOOK QUESTIONS PAGE NO. 118, 1. Why does a compass needle get deflected when brought near a bar, magnet?, Ans.A compass gets deflected due to the forces acting on its poles due, to themagnetic field of the bar magnet., INTEXT QUESTIONS PAGE NO. 122, 2. Draw magnetic field lines around a bar magnet., a. Ans., , 3. List the properties of magnetic lines of force., A magnetic field lines originate from north pole and end at its south pole., A magnetic field line is a closed and continuous curve., The magnetic field lines are closer near the poles of a magnet where the, magnetic field is strong and farther apart where the magnetic field is weak., The magnetic field lines never intersect each other., A uniform magnetic field is represented by parallel and equidistant field lines., 4. Why don’t two magnetic lines of force intersect each other?, a. Ans. If two magnetic lines of force intersect then there would be two, directions ofmagnetic field at that point, which is absurd. That is why, they never intersect., TEXT BOOK QUESTIONS PAGE NO. 123 and 124, 5. Consider a circular loop of wire lying in the plane of the table. Let the, current pass through the loop clockwise. Apply the right-hand rule to find out, the direction of the magnetic field inside and outside the loop., a. For downward direction of current flowing in the circular loop, the, direction of magnetic field lines will be as if they are emerging from the, table outside the loop and merging in the table inside the loop. Similarly,, for upward direction of current flowing in the circular loop, the direction, of magnetic field lines will be as if they are emerging from the table, outside the loop and merging in the table inside the loop, as shown in, the given figure.

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.The magnetic field in a given region is uniform. Draw a diagram to represent it., , 6., Choose the correct option: The magnetic field inside a long straight, solenoid-carrying current (a) is zero. (b) decreases as we move towards its, end. (c) increases as we move towards its end. (d) is the same at all points., (a)The magnetic field for a point inside a long straight solenoid carrying current is, double than for a point situated at one of its ends. Thus, the correct option is (b)., TEXT BOOK.QUESTIONS PAGE NO. 125 AND 126, 7. Which of the following property of a proton can change while it moves freely in a, magnetic field? (There may be more than one correct answer.), (a) mass (b) speed(c) velocity (d) momentum, , Whenever a charged proton moves in a magnetic field, its velocity changes and, as a result of this its momentum change. Thus (c) and (d) are the properties which, change when a proton moves freely in a magnetic field., 8. In Activity 13.7, how do we think the displacement of rod AB will be affected if, current in rod AB is increased; (ii) a stronger horse-shoe magnet is used; and, length of the rod AB is increased?, If the current in the rod AB is increased, force also increases., When a stronger horse-shoe magnet is used, magnetic field increases as a, result force also increases., (iii)If the length of the rod AB is increased, force also increased., 9. A positively-charged particle (alpha-particle) projected towards west is, deflected towards north by a magnetic field. The direction of magnetic field is (a), towards south (b) towards east (c) downward (d) upward, The direction of the motion of proton is the direction of current. The direction of force of, the proton is towards north. Applying Fleming’s left hand rule, the direction of magnetic, field is upward. The correct option is (d)., 10. State Fleming’s left-hand rule., Ans. Fleming’s left hand rule states that if we arrange the thumb, the centre finger, and the, forefinger of the left hand at right angles to each other, then the thumb points towards the, direction of the magnetic force, the centre finger gives the direction of current, and the, forefinger points in the direction of magnetic field., 11. What is the principle of an electric motor?, Ans.The working principle of an electric motor is based on the magnetic effect, ofcurrent. A current-carrying loop experiences a force and rotates when placed in, a magnetic field. The direction of rotation of the loop is given by the Fleming’s lefthand rule., 12. What is the role of the split ring in an electric motor?, Ans.The split ring in the electric motor acts as a commutator. The commutator reverses, the direction of current flowing through the coil after each half rotation of the coil., Due to this reversal of the current, the coil continues to rotate in the same direction.

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TEXT BOOK QUESTIONS PAGE NO. 236, 13. Explain different ways to induce current in a coil., The different ways to induce current in a coil are as follows:, If a coil is moved rapidly between the two poles of a horse-shoe magnet, then an, electric current is induced in the coil., If a magnet is moved relative to a coil, then an electric current is induced in the coil, TEXT BOOK QUESTIONS PAGE NO. 237, 14. State the principle of an electric generator., An electric generator works on the principle of electromagnetic induction. It, generates electricity by rotating a coil in a magnetic field., 15. Name some sources of direct current., Some sources of direct current are cell, DC generator, etc, 16. Which sources produce alternating current?, AC generators, power plants, etc., produce alternating current, 17. Choose the correct option: A rectangular coil of copper wires is rotated in a, magnetic field. The direction of the induced current changes once in each, (a) two revolutions (b) one revolution (c) half revolution (d) one-fourth revolution, (c) When a rectangular coil of copper is rotated in a magnetic field, the direction, of the induced current in the coil changes once in each half revolution. As a, result, the direction of current in the coil remains the same, TEXT BOOK QUESTIONS PAGE NO.132, 18. Name two safety measures commonly used in electric circuits and appliances., Two safety measures commonly used in electric circuits and appliances are, as follows:, (i) Each circuit must be connected with an electric fuse. This prevents the flow of, excessive current through the circuit. When the current passing through the wire, exceeds the maximum limit of the fuse element, the fuse melts to stop the flow of, current through that circuit, hence protecting the appliances connected to the, circuit., (ii) Earthing is a must to prevent electric shocks. Any leakage of current in an electric, appliance is transferred to the ground and people using the appliance do not get, the shock., 19. An electric oven of 2 kW power rating is operated in a domestic electric circuit (220 V), that has a current rating of 5 A. What result do you expect? Explain. Current drawn by the, electric oven can be obtained by the expression,, , P=VI, Where, current = I. Power of the oven, P = 2 kW = 2000W, Voltage supplied, V = 220V, , 2000, 220, , =9.09A, , Hence, the current drawn by the electric oven is 9.09 A, which exceeds the safe, limit of the circuit. Fuse element of the electric fuse will melt and break the, circuit.

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20. What precaution should be taken to avoid the overloading of domestic, electric circuits?, The precautions that should be taken to avoid the overloading of domestic circuits, are as follows:, Too many appliances should not be connected to a single socket., Too many appliances should not be used at the same time., Faulty appliances should not be connected in the circuit, Fuse should be connected in the circuit., EXERCISE QUESTIONS PAGE NO.134, 1. Which of the following correctly describes the magnetic field near a long, straight wire?, The field consists of straight lines perpendicular to the wire, The field consists of straight lines parallel to the wire, The field consists of radial lines originating from the wire, The field consists of concentric circles centred on the wire, Ans. (d) The magnetic field lines, produced around a straight current-carrying, conductor, are concentric circles. Their centres lie on the wire., 2.The phenomenon of electromagnetic induction is, (a) the process of charging a body the process of generating magnetic field, due to a current passing through a coil, producing induced current in a coil due to relative motion between a, magnet and the coil, the process of rotating a coil of an electric motor, Ans. (c) When a straight coil and a magnet are moved relative to each other,, a current is induced in the coil. This phenomenon is known as electromagnetic, induction., 3.The device used for producing electric current is called a, (a) generator. (b) galvanometer. (c) ammeter. (d) motor., Ans. (a) An electric generator produces electric current. It converts, mechanical energy into electricity., 4.The essential difference between an AC generator and a DC generator is that, AC generator has an electromagnet while a DC generator has, permanent magnet., DC generator will generate a higher voltage., AC generator will generate a higher voltage., AC generator has slip rings while the DC generator has a commutator., Ans. (d) An AC generator has two rings called slip rings. A DC generator has, two half rings called commutator. This is the main difference between both, the types of generators., 5. At the time of short circuit, the current in the circuit, (a) reduces substantially. (b) does not change., (c) increases heavily., (d) vary continuously., Ans. (c) When two naked wires of an electric circuit touch each other, the amountof, current that is flowing in the circuit increases abruptly. This causes short-circuit.

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6.State whether the following statements are true or false., Ans. (a) False, An electric motor converts electrical energy into mechanical energy., (b)True, A generator is an electric device that generates electricity by rotating a coil in a magnetic, field. It works on the principle of electromagnetic induction., (c)True, A long circular coil is a long solenoid. The magnetic field lines inside the solenoid are, parallel lines., (d) False, Live wire has red insulation cover, whereas earth wire has green insulation colour in the, domestic circuits., 7. List three sources of magnetic fields., Ans.Three sources of magnetic fields are as follows:, Current-carrying conductors, Permanent magnets, Electromagnets, 8. How does a solenoid behave like a magnet? Can you determine the north and, south poles of a current–carrying solenoid with the help of a bar magnet? Explain., Ans.A solenoid is a long coil of circular loops of insulated copper wire., Magneticfield lines are produced around the solenoid when a current is allowed to, flow through it. The magnetic field produced by it is similar to the magnetic field of, a bar magnet. The field lines produced in a current-carrying solenoid is shown in the, following figure., , In the above figure, when the north pole of a bar magnet is brought near the end, connected to the negative terminal of the battery, the solenoid repels the bar, magnet. Since like poles repel each other, the end connected to the negative, terminal of the battery behaves as the north pole of the solenoid and the other end

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behaves as a south pole. Hence, one end of the solenoid behaves as a north pole, and the other end behaves as a south pole., 9. When is the force experienced by a current–carrying conductor placed in a, magnetic field largest?, Ans.The force experienced by a current-currying conductor is the maximumwhen, the direction of current is perpendicular to the direction of the magnetic field., 10. Imagine that you are sitting in a chamber with your back to one wall. An electron, beam, moving horizontally from back wall towards the front wall, is deflected by a, strong magnetic field to your right side. What is the direction of magnetic field?, Ans.The direction of magnetic field is given by Fleming’s left hand rule.Magnetic field, inside the chamber will be perpendicular to the direction of current (opposite to the, direction of electron) and direction of deflection/force i.e., either upward or, downward. The direction of current is from the front wall to the back wall because, negatively charged electrons are moving from back wall to the front wall. The, direction of magnetic force is rightward. Hence, using Fleming’s left hand rule, it can, be concluded that the direction of magnetic field inside the chamber is downward., 11. Draw a labelled diagram of an electric motor. Explain its principle and, working. What is the function of a split ring in an electric motor?, Ans.An electric motor is a rotating device that converts electrical energy, tomechanical energy. Electric motor is used as an important component in electric, fans, refrigerators, mixers, washing machines, computers, MP3 players etc., Principle: When a coil carrying current is placed in a magnetic field, itexperiences a, torque. As a result of this torque, the coil begins to rotate., Construction:, It consists of a rectangular coil ABCD of insulated copper wire. The coil is, placedbetween the two poles of a magnetic field such that the arm AB and CD are, perpendicular to the direction of the magnetic field. The ends of the coil are, connected to the two halves P and Q of a split ring. The inner sides of these halves are, insulated and attached to an axle. The external conducting edges of P and Q touch, two conducting stationary brushes X and Y, respectively, as shown in the below figure, , Working:, Current in the coil ABCD enters from the source battery through conducting brush X, and flows back to the battery through brush Y.Notice that the current in arm AB of the

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coil flows from A to B. In arm CD it flows from C to D, that is, opposite to the direction of, current through arm AB.On applying Fleming’s left hand rule for the direction of force, on a current-carrying conductor in a magnetic field.. We find that the force acting, on arm AB pushes it downwards while the force acting on arm CD pushes it upwards., Thus the coil and the axle O, mounted free to turn about an axis, rotate anticlockwise., At half rotation, Q makes contact with the brush X and P with brush Y. Therefore the, current in the coil gets reversed and flows along the path DCBA. A device that, reverses the direction of flow of current through a circuit is called a commutator. In, electric motors, the split ring acts as a commutator., The reversal of current also reverses the direction of force acting on the two arms AB, and CD. Thus the arm AB of the coil that was earlier pushed down is now pushed up, and the arm CD previously pushed up is now pushed down. Therefore the coil and, the axle rotate half a turn more in the same direction. The reversing of the current is, repeated at each half rotation, giving rise to a continuous rotation of the coil and to, the axle., , 12. Name some devices in which electric motors are used., Ans.Some devices in which electric motors are used are as follows:, (a)Water pumps (b) Electric fans (c) Electric mixers (d) Washing machines, 13. A coil of insulated copper wire is connected to a galvanometer. What will happen, if a bar magnet is (i) pushed into the coil, (ii) withdrawn from inside the coil, (iii) held, stationary inside the coil?, , i., , ii., , iii., , A current induces in a solenoid if a bar magnet is moved relative to it. This, isthe principle of electromagnetic induction, When a bar magnet is pushed into a coil of insulated copper wire, a current is, induced momentarily in the coil. As a result, the needle of the galvanometer, deflects momentarily in a particular direction., When the bar magnet is withdrawn from inside the coil of the insulated copper, wire, a current is again induced momentarily in the coil in the opposite, direction. As a result, the needle of the galvanometer deflects momentarily in, the opposite direction., When a bar magnet is held stationary inside the coil, no current will be, induced in the coil. Hence, galvanometer will show no deflection., , 14. Two circular coils A and B are placed closed to each other. If the current in the, coil A is changed, will some current be induced in the coil B? Give reason., Ans.Two circular coilsA and B are placed closed to each other.When the currentin, coil A is changed, the magnetic field associated with it also changes. As a result, the, magnetic field around coil B also changes. This change in magnetic field lines around, coil B induces an electric current in it. This is called electromagnetic induction., 15. State the rule to determine the direction of a (i) magnetic field produced around, a straight conductor-carrying current, (ii) force experienced by a current-carrying, straight conductor placed in a magnetic field which is perpendicular to it, and (iii), current induced in a coil due to its rotation in a magnetic field.

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Ans. (i) Maxwell’s right hand thumb rule, (ii)Fleming’s left hand rule (iii)Fleming’s right hand rule, 16. Explain the underlying principle and working of an electric generator by, drawing a labelled diagram. What is the function of brushes?, Ans.In an electric generator, mechanical energy is used to rotate a conductor in a, magnetic field to produce electricity., Principle: Whenever in a closed circuit, the magnetic field lines change, an, induced current is produced., Construction:, An electric generator, as shown in the below figure, consists of a rotating, rectangular coil ABCD placed between the two poles of a permanent magnet., The two ends of this coil are connected to the two rings R1 andR2.The inner side of, these rings are made insulated. The two conducting stationary brushes B1 and B2, are kept pressed separately on the rings R1 and R2, respectively. The two rings R1, and R2 are internally attached to an axle. The axle may be mechanically rotated, from outside to rotate the coil inside the magnetic field.Outer ends of the two, brushes are connected to the galvanometer to show the flow of current in the given, external circuit., Working:, When the axle attached to the two rings is rotated such that the arm AB moves up, (and the arm CD moves down) in the magnetic field produced by the permanent, magnet. Let us say the coil ABCD is rotated clockwise in the arrangement shown in, the above figure., By applying Fleming’s right-hand rule, the induced currents are set up in these arms, along the directions AB and CD. Thus an induced current flows in the direction ABCD., If there are larger numbers of turns in the coil, the currentgenerated in each turn adds, up to give a large current through the coil. This means that the current in the external, circuit flows from B2 to B1., After half a rotation, arm CD starts moving up and AB moving down. As a result, the, directions of the induced currents in both the arms change, giving rise to the net, induced current in the direction DCBA. The current in the external circuit now flows, from B1 to B2. Thus after every half rotation the polarity of the current in the respective, arms changes. Such a current, which changes direction after equal intervals of time,, is called an alternating current (abbreviated as AC). This device is called an AC, generator., To get a direct current (DC, which does not change its direction with time), a splitring type commutator must be used. With this arrangement, one brush is at all times, in contact with the arm moving up in the field, while the other is in contact with the, arm moving down. We have seen the working of a split ring commutator in the case, of an electric motor Thus a unidirectional current is produced. The generator is thus, called a DC generator. The difference between the direct and alternating currents, is that the direct current always flows in one direction, whereas the alternating, current reverses its direction periodically.

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17. When does an electric short circuit occur?, Ans.If the resistance of an electric circuit becomes very low, then the currentflowing, through the circuit becomes very high. This is caused by connecting too many, appliances to a single socket or connecting high power rating appliances to the light, circuits. This results in a short circuit., When the insulation of live and neutral wires undergoes wear and tear and then, touches each other, the current flowing in the circuit increases abruptly. Hence, a, short circuit occurs., 18.What is the function of an earth wire? Why is it necessary to earth metallic, appliances?, Ans. The metallic body of electric appliances is connected to the earth by meansof, earth wire so that any leakage of electric current is transferred to the ground. This, prevents any electric shock to the user. That is why earthing of the electrical, appliances is necessary., , Light Reflection and Refraction, INTEXT QUESTIONS PAGE NO. 78, 1. Define the principal focus of a concave mirror., Ans. Light rays that are parallel to the principal axis of a concave mirror converge at a, specific point on its principal axis after reflecting from the mirror. This point is known, as the principal focus of the concave mirror., The radius of curvature of a spherical mirror is 20 cm. What is its focal length?, Ans. Here R = 20 cm, We know that f=, , R, f= 20 =10cm, 2, 2, 3. Name a mirror that can give an erect and enlarged image of an object., Ans. When an object is placed between the pole and the principal focus of a concave mirror, the, image formed is virtual, erect, and enlarged.

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4. Why do we prefer a convex mirror as a rear-view mirror in vehicles?, Ans. Convex mirrors give a virtual, erect, and diminished image of the objects placed in front of, them. They are preferred as a rear-view mirror in vehicles because they give a wider field of view,, which allows the driver to see most of the traffic behind him., INTEXT QUESTIONS PAGE NO. 81, Find the focal length of a convex mirror whose radius of curvature is 32 cm. Ans., Ans. Here R = 32 cm, We know that f=, , R, , f= 32 =16cm, , 2, , 2, , Hence, the focal length of the given convex mirror is 16 cm., , A concave mirror produces three times magnified (enlarged) real image of an object, placed at 10 cm in front of it. Where is the image located?, Ans. Here, magnification, m = –3, object distance, u = –10 cm and image distance, v = ?, Putting these values in the magnification formula for a mirror, we get, 𝒗, , 𝒎=𝒖, , 𝒗, , =, , , −𝟑 = −𝟏𝟎, 𝒗 = −𝟑 × −𝟏𝟎 = 𝟑𝟎 𝒄𝒎, , INTEXT QUESTIONS PAGE NO. 86

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EXERCISE QUESTIONS PAGE No. 95 & 96, , 1. Which one of the following materials cannot be used to make a lens?, (a) Water (b) Glass (c) Plastic (d) Clay, (b) Ans:, (c) A lens allows light to pass through it. Since clay does not show such, property, it cannot be used to make a lens., 2. The image formed by a concave mirror is observed to be virtual, erect and, larger than the object. Where should be the position of the object?, (a) Between the principal focus and the centre of curvature, (b) At the centre of curvature, (c) Beyond the centre of curvature, (d) Between the pole of the mirror and its principal focus., (e) Ans:, (f) When an object is placed between the pole and principal focus of a, concave mirror, the image formed is virtual, erect, and larger than the, object., 3. Where should an object be placed in front of a convex lens to get a real image, of the size of the object?, (a) At the principal focus of the lens (b) At twice the focal length (c) At infinity, (b) Between the optical centre of the lens and its principal focus., (c) Ans:, (d) When an object is placed at the centre of curvature in front of a convex, lens, its image is formed at the centre of curvature on the other side of the, lens. The image formed is real, inverted, and of the same size as the object., 4. A spherical mirror and a thin spherical lens have each a focal length of −15 cm., The mirror and the lens are likely to be, (a) both concave (b) both convex (c) the mirror is concave and the lens is, convex, (b) the mirror is convex, but the lens is concave, (c) Ans:, (d) By convention, the focal length of a concave mirror and a concave lens are, taken as negative. Hence, both the spherical mirror and the thin spherical, lens are concave in nature., 5. No matter how far you stand from a mirror, your image appears erect. The, mirror, is likely to be (a) plane (b) concave (c) convex (d) either plane or convex, (a) Ans:, A convex mirror always gives a virtual and erect image of smaller size of the, object placed in front of it. Similarly, a plane mirror will always give a virtual, and erect image of same size as that of the object placed in front of it. Therefore,, the given mirror could be either plane or convex.

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6. Which of the following lenses would you prefer to use while reading small, letters found in a dictionary?, (a) A convex lens of focal length 50 cm, (b) A concave lens of focal length 50 cm, (c) A convex lens of focal length 5 cm, (d) A concave lens of focal length 5 cm, (e) Ans.:, A convex lens gives a magnified image of an object when it is placed between, the radius of curvature and focal length. Also, magnification is more for convex, lenses having shorter focal length. Therefore, for reading small letters, a convex lens of focal, length 5 cm should be used., 7., , We wish to obtain an erect image of an object, using a concave mirror of focal length 15, cm.What should be the range of distance of the object from the mirror? What is the, nature of the image? Is the image larger or smaller than the object? Draw a ray diagram, to show the image formation in this case., Ans:, a. Range of object distance = 0 cm to15 cm, b. A concave mirror gives an erect image when an object is placed between its pole, c. (P) and the principal focus (F)., d. Hence, to obtain an erect image of an object from a concave mirror of focal length 15, cm, the object must be placed anywhere between the pole and the focus. The image, formed will be virtual, erect, and magnified in nature, as shown in the given figure., , 8., , Name the type of mirror used in the following situations., e. Headlights of a car., f. Side/rear-view mirror of a vehicle., g. Solar furnace., h. Support your answer with reason., i. Ans:, j. Concave (b) Convex (c) Concave, k. Explanation:, l. Concave mirror is used in the headlights of a car. This is because concave mirrors can, produce powerful parallel beam of light when the light source is placed at their principal, focus., m. Convex mirror is used in side/rear view mirror of a vehicle. Convex mirrors give a, virtual, erect, and diminished image of the objects placed in front of it. Because of this,, they have a wide field of view. It enables the driver to see most of the traffic behind, him/her., n. Concave mirrors are convergent mirrors. That is why they are used to construct solar, furnaces. Concave mirrors converge the light incident on them at a single point known, as principal focus. Hence, they can be used to produce a large amount of heat at that, point.

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9. One-half of a convex lens is covered with a black paper. Will this lens produce a complete, image of the object? Verify your answer experimentally. Explain your observations., a. Ans:, b. The convex lens will form complete image of an object, even if its one half is, covered with black paper. It can be understood by the following two cases.

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THE HUMAN EYE AND THE COLOURFUL WORLD, INTEXT QUESTIONS PAGE No. 100, 1. What is meant by power of accommodation of the eye? Ans:, a. When the ciliary muscles are relaxed, the eye lens becomes thin, the focal length, increases, and the distant objects are clearly visible to the eyes. To see the nearby, objects clearly, the ciliary muscles contract making the eye lens thicker. Thus, the focal, length of the eye lens decreases and the nearby objects become visible to the eyes., Hence, the human eye lens is able to adjust its focal length to view both distant and, nearby objects on the retina. This ability is called the power of accommodation of the, eyes., 2. A person with a myopic eye cannot see objects beyond 1.2 m distinctly. What should be, the type of the corrective lens used to restore proper vision?, a. Ans:, b. The person is able to see nearby objects clearly, but he is unable to see objects beyond, 1.2 m. This happens because the image of an object beyond 1.2 m is formed in front of, the retina and not at the retina, as shown in the given figure.

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c. To correct this defect of vision, he must use a concave lens. The concave lens will, bring the image back to the retina as shown in the given figure., , 3. What is the far point and near point of the human eye with normal vision? Ans:, The near point of the eye is the minimum distance of the object from the eye, which can be seen, distinctly without strain. For a normal human eye, this distance is 25 cm., The far point of the eye is the maximum distance to which the eye can see the objects, clearly. The far point of the normal human eye is infinity., 4. A student has difficulty reading the blackboard while sitting in the last row. What, could be the defect the child is suffering from? How can it be corrected?, Ans:A student has difficulty in reading the blackboard while sitting in the last row. It shows that, he is unable to see distant objects clearly. He is suffering from myopia. This defect can be, corrected by using a concave lens., , EXERCISE QUESTIONS PAGE No. 107 & 108, 1. The human eye can focus objects at different distances by adjusting the focal length of, the eye lens. This is due to (a) Presbyopia (b) accommodation (c) near-sightedness (d), far-sightedness., Ans:Human eye can change the focal length of the eye lens to see the objects situated at various, distances from the eye. This is possible due to the power of accommodation of the eye lens., 2. The human eye forms the image of an object at its (a) cornea (b) iris, pupil (d) retina, The human eye forms the image of an object at its retina., 3. The least distance of distinct vision for a young adult with normal vision is about (a), 25 m (b) 2.5 cm (c) 25 cm (d) 2.5 m, Ans:The least distance of distinct vision is the minimum distance of an object to see clear and, distinct image. It is 25 cm for a young adult with normal visions., 4. The change in focal length of an eye lens is caused by the action of the, pupil (b) retina (c) ciliary muscles (d) iris, The relaxation or contraction of ciliary muscles changes the curvature of the eye lens. The, change in curvature of the eye lens changes the focal length of the eyes. Hence, the change, in focal length of an eye lens is caused by the action of ciliary muscles.

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different levels of the atmosphere. When the star light refracted by the atmosphere comes, more towards us, it appears brighter than when it comes less towards us. Therefore, it appears, as if the stars are twinkling at night., 11. Explain why the planets do not twinkle? Ans:, Planets do not twinkle because they appear larger in size than the stars as they are relatively, closer to earth. Planets can be considered as a collection of a large number of point-size, sources of light. The different parts of these planets produce either brighter or dimmer effect, in such a way that the average of brighter and dimmer effect is zero. Hence, the twinkling, effects of the planets are nullified and they do not twinkle., 12. Why does the Sun appear reddish early in the morning? Ans:, During sunrise, the light rays coming from the Sun have to travel a greater distance in the, earth’s atmosphere before reaching our eyes. In this journey, the shorter wavelengths of lights, are scattered out and only longer wavelengths are able to reach our eyes. Since blue colour, has a shorter wavelength and red colour has a longer wavelength, the red colour is able to, reach our eyes after the atmospheric scattering of light. Therefore, the Sun appears reddish, early in the morning., 13. Why does the sky appear dark instead of blue to an astronaut? Ans:, The sky appears dark instead of blue to an astronaut because there is no atmosphere in the, outer space that can scatter the sunlight. As the sunlight is not scattered, no scattered light, reach the eyes of the astronauts and the sky appears black to them., , Sources Of Energy, INTEXT QUESTIONS PAGE NO. 110, 1. What is a good source of energy?, Ans. A good source of energy fulfils the following criteria: It, produces a lot of heat per unit mass., It does a huge amount of work per unit mass. It is easily, accessible., It is easy to store and transport. It is economical., It produces less amount of smoke., 2. What is a good fuel?, Ans. A good fuel produces a huge amount of heat on burning, does not produce a lot of, smoke, and is easily available., 3. If you could use any source of energy for heating your food, which one would you use and, why?, Ans. Natural gas can be used for heating and cooking food because it is a clean source of energy., It does not produce huge amount of smoke on burning. Although it is highly inflammable, it is, easy to use, transport, and it produces a huge amount of heat on burning., INTEXT QUESTIONS PAGE NO. 115, , 1. What are the disadvantages of fossil fuels?, Ans. The disadvantages of fossil fuels are as follows:

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Burning of coal and petroleum produces a lot of pollutants causing air pollution., Fossil fuels release oxides of carbon, nitrogen, sulphur, etc. that cause acid rain, which affects, the soil fertility and potable water., Burning of fossil fuels produce gases such as carbon dioxide that causes global warming., 2. Why are we looking at alternate sources of energy?, Ans. Fossil fuels, which have been traditionally used by human beings as an energy sources,, are non-renewable sources of energy. These sources of energy are limited and cannot replenish, on their own. They are being consumed at a large rate. If this rate of consumption continues,, then the fossil fuels would be exhausted from the Earth. Therefore, we have to conserve the, energy sources. Hence, we should look for alternate sources of energy., 3. How has the traditional use of wind and water energy been modified for our, convenience?, Traditionally, waterfalls were used as a source of potential energy which was converted to, electricity with the help of turbines. Since waterfalls are few in number, water dams have been, constructed in large numbers. Nowadays, hydro-dams are used in order to harness potential, energy of stored water. In water dams, water falls from a height on the turbine, which produces, electricity., Earlier, the windmills were used to harness wind energy to do mechanical work such as, lifting/drawing water from a well. Today, windmills are used to generate electricity. In, windmills, the kinetic energy of wind is harnessed and converted into electricity. The rotatory, motion of the blades turns the turbine of the electric generator to generate electricity., , INTEXT QUESTIONS PAGE NO. 120, 1. What kind of mirror − concave, convex or plain − would be best suited for use in a solar, cooker? Why?, Ans. A solar cooker uses heat of the sunlight to cook and heat food. A mirror is used in order, to reflect and focus sunlight at a point. A concave mirror is used in a solar cooker for this, purpose. The mirror focuses all the incident sunlight at a point. The temperature at that point, increases, thereby cooking and heating the food placed at that point., 2. What are the limitations of the energy that can be obtained from the oceans?, Ans. The forms of energy that can be obtained from the ocean are tidal energy, wave energy,, and ocean thermal energy. There are several limitations in order to harness these energies., Tidal energy depends on the relative positioning of the Earth, moon, and the Sun., High dams are required to be built to convert tidal energy into electricity., Very strong waves are required to obtain electricity from wave energy., To harness ocean thermal energy efficiently, the difference in the temperature of surface, water (hot) and the water at depth (cold) must be 20ºC or more., 3. What is geothermal energy?, Ans. Geothermal power plants use heat of the Earth to generate electricity. This heat energy, of the Earth is known as geothermal energy., When there are geological changes, the molten rocks present in the core of the earth are pushed, to the earth’s crust. This forms regions of hot spot. Steam is generated when the underground, water comes in contact with these hot spots forming hot springs. This trapped steam is used to, generate electricity in the geothermal power plants.

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4.What are the advantages of nuclear energy?, Ans. The advantages of nuclear energy are as follows:, (a) Large amount of energy is produced per unit mass., (b) It does not produce smoke. It is a clean energy., Fission of one atom of uranium produces 10 million times the energy released by burning of, one atom of carbon., Fusion of four hydrogen atoms produces huge amount of energy approximately equal to 27, MeV., 5. Can any source of energy be pollution-free? Why or why not?, Ans. No source of energy can be pollution-free. It is considered that solar cells are pollutionfree. However, even their making causes environmental damage indirectly., Also, in the case of nuclear energy, there is no waste produced after the fusion reactions., However, it is not totally pollution-free. To start the fusion reactions, approximately 107 K, temperature is required, which is provided by fission reactions. The wastes released from, fission reactions are very hazardous. Hence, no source of energy is pollution-free., 6. Hydrogen has been used as a rocket fuel. Would you consider it a cleaner fuel than, CNG? Why or why not?, Ans. Hydrogengas is cleaner than CNG. CNG contains hydrocarbons. Therefore, it has carbon, contents. Carbon is a form of pollutant present in CNG. On the other hand, hydrogen is wastefree. The fusion of hydrogen does not produce any waste. Hence, hydrogen is cleaner than, CNG., INTEXT QUESTIONS PAGE NO. 121, 1. Name two energy sources that you would consider to be renewable. Give reasons for your, choices., Ans. Two renewable sources of energy are as follows:, Sun: The energy derived from the Sun is known as solar energy. Solar energy is produced by, the fusion of hydrogen into helium, fusion of helium into other heavy elements, and so on. A, large amount of hydrogen and helium is present in the Sun. Therefore, solar energy can, replenish on its own. The Sun has 5 billion years more to burn. Hence, solar energy is a, renewable source of energy., Wind: Wind energy is derived from air blowing with high speed. Wind energy is harnessed by, windmills in order to generate electricity. Air blows because of uneven heating of the Earth., Since the heating of the Earth will continue forever, wind energy will also be available forever., 2. Give the names of two energy sources that you would consider to be exhaustible. Give, reasons for your choices., Ans. Two exhaustible energy sources are as follows:, Coal: It is produced from dead remains of plants and animals that remain buried under the, earth’s crust for millions of years. It takes millions of years to produce coal. Industrialization, has increased the demand of coal. However, coal cannot replenish within a short period of time., Hence, it is a non-renewable or exhaustible source of energy., Wood: It is obtained from forests. Deforestation at a faster rate has caused a reduction in the, number of forests on the Earth. It takes hundreds of years to grow a forest. If deforestation is, continued at this rate, then there would be no wood left on the Earth. Hence, wood is an, exhaustible source of energy.

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EXERCISE QUESTIONS PAGE NO.121, 1. A solar water heater cannot be used to get hot water on, (a) a sunny day, , (b) a cloudy day, , (c) a hot day (d) a windy day, , Ans. (b) A solar water heater uses solar energy to heat water. It requires bright and intense, sunlight to function properly. On a cloudy day, the sunlight reflects back in the sky from the, clouds and is unable to reach the ground. Therefore, solar energy is not available for the solar, heater to work properly. Hence, solar water heater does not function on a cloudy day., 2.Which of the following is not an example of a bio-mass energy source?, (a) wood (b) gobar gas, , (c) nuclear energy, , (d) coal, , Ans. (c) Bio-mass is a source of energy that is obtained from plant materials and animal, wastes. Nuclear energy is released during nuclear fission and fusion. In nuclear fission,, uranium atom is bombarded with low-energy neutrons. Hence, uranium atom splits into two, relatively lighter nuclei. This reaction produces huge amount of energy. In nuclear fusion, reaction, lighter nuclei are fused together to form a relatively heavier nuclei. This reaction, produces tremendous amount of energy. Hence, nuclear energy is not an example of bio-mass, energy source.Wood is a plant material, gobar gas is formed from animal dung, and coal is a, fossil fuel obtained from the buried remains of plants and animals. Hence, these are bio-mass, products., , 3. Most of the sources of energy we use represent stored solar energy. Which of the, following is not ultimately derived from the Sun’s energy?, Geothermal energy, Wind energy, Nuclear energy, Bio-mass, Ans. (c) Nuclear energy is released during nuclear fission and fusion. In nuclear fission,, uranium atom is bombarded with low-energy neutrons. Hence, uranium atom splits into two, relatively lighter nuclei. This reaction produces huge amount of energy. In nuclear fusion, reaction, lighter nuclei are fused together to form a relatively heavier nuclei. The energy, required to fuse the lighter nuclei is provided by fission reactions. This reaction produces, tremendous amount of energy. These reactions can be carried out in the absence or presence of, sunlight. There is no effect of sunlight on these reactions. Hence, nuclear energy is not, ultimately derived from Sun’s energy., Geothermal energy, wind energy, and bio-mass are all ultimately derived from solar energy., Geothermal energy is stored deep inside the earth’s crust in the form of heat energy. The, heating is caused by the absorption of atmospheric and oceanic heat. It is the sunlight that heats, the atmosphere and oceans., Wind energy is harnessed from the blowing of winds. The uneven heating of the earth’s surface, by the Sun causes wind., Bio-mass is derived from dead plants and animal wastes. Chemical changes occur in these dead, plants and animal wastes in the presence of water and sunlight. Hence, bio-mass is indirectly, related to sunlight.

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4. Compare and contrast fossil fuels and the Sun as direct sources of energy., Ans.Fossil fuels are energy sources, such as coal and petroleum, obtained from underneath the, Earth’s crust. They are directly available to human beings for use. Hence, fossil fuels are the, direct source of energy. These are limited in amount. These are non-renewable sources of energy, because these cannot be replenished in nature. Fossil fuels take millions of years for their, formation. If the present fossil fuel of the Earth gets exhausted, its formation will take several, years. Fossil fuels are also very costly., On the other hand, solar energy is a renewable and direct source of energy. The Sun has been, shining for several years and will do so for the next five billion years. Solar energy is available, free of cost to all in unlimited amount. It replenishes in the Sun itself., 5.Compare and contrast bio-mass and hydro electricity as sources of energy., Ans.Bio-mass and hydro-electricity both are renewable sources of energy. Bio-mass is derived, from dead plants and animal wastes. Hence, it is naturally replenished. It is the result of natural, processes. Wood, gobar gas, etc. are some of the examples of bio-mass., Hydro-electricity, on the other hand, is obtained from the potential energy stored in water at a, height. Energy from it can be produced again and again. It is harnessed from water and obtained, from mechanical processes., , 6. What are the limitations of extracting energy from −, (a) the wind? (b) waves? (c) tides?, Ans.Wind energy is harnessed by windmills. One of the limitations of extracting energy from, wind is that a windmill requires wind of speed more than 15 km/h to generate electricity. Also, a, large number of windmills are required, which covers a huge area., Very strong ocean waves are required in order to extract energy from waves., Very high tides are required in order to extract energy from tides. Also, occurrence of tides, depends on the relative positions of the Sun, moon, and the Earth., 7. On what basis would you classify energy sources as, renewable and non-renewable?, exhaustible and inexhaustible?, Are the options given in (a) and (b) the same?, Ans.The source of energy that replenishes in nature is known as renewable source of energy., Sun, wind, moving water, bio-mass, etc. are some of the examples of renewable sources of, energy., The source of energy that does not replenish in nature is known as non-renewable source of, energy. Coal, petroleum, natural gas, etc. are some of the examples of non-renewable sources, of energy., Exhaustible sources are those sources of energy, which will deplete and exhaust after a few, hundred years. Coal, petroleum, etc. are the exhaustible sources of energy., Inexhaustible resources of energy are those sources, which will not exhaust in future. These, are unlimited. Bio-mass is one of the inexhaustible sources of energy., Yes. The options given in (a) and (b) are the same.

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8.What are the qualities of an ideal source of energy?, Ans.An ideal source of energy must be:, a) Economical, b) Easily accessible, c) Smoke/pollution free, d) Easy to store and transport, e) Able to produce huge amount of heat and energy on burning, 9. What are the advantages and disadvantages of using a solar cooker? Are there places, where solar cookers would have limited utility?, Ans.Solar cooker uses Sun’s energy to heat and cook food. It is inexhaustible and clean, renewable source of energy. It is free for all and available in unlimited amount. Hence, operating, a solar cooker is not expensive., Disadvantage of a solar cooker is that it is very expensive. It does not work without sunlight., Hence, on cloudy day, it becomes useless., The places where the days are too short or places with cloud covers round the year, have limited, utility for solar cooker., 10. What are the environmental consequences of the increasing demand for energy? What, steps would you suggest to reduce energy consumption?, Ans.Industrialization increases the demand for energy. Fossil fuels are easily accessible sources, of energy that fulfil this demand. The increased use of fossil fuels has a harsh effect on the, environment. Too much exploitation of fossil fuels increases the level of green -house gas, content in the atmosphere, resulting in global warming and a rise in the sea level., It is not possible to completely reduce the consumption of fossil fuels. However, some, measures can be taken such as using electrical appliances wisely and not wasting electricity., Unnecessary usage of water should be avoided. Public transport system with mass transit must, be adopted on a large scale. These small steps may help in reducing the consumption of natural, resources and conserving them.