Chapter 11: “Heat”

Q1 How do you distinguish between temperature and heat? Give example.

Ans: Heat is the energy flowing between a body and the surrounding due to difference of temperature. But temperature is the measure of the average translational kinetic energy of the molecules of body. If we dip a red hot iron ball in the sea, heat will flow from the ball to the water (from high temperature to low temperature), although the amount of internal energy possessed by the ball is very small as compared to the immense amount contained in the sea.

Q2 Why is the earth not in thermal equilibrium with the sun?

Ans: The earth is not in thermal equilibrium with the sun because while the earth is being warmed by the absorbed radiant energy, it is also in various ways, e.g. re-radiation, conduction, convection and evaporation, compensating the heat transfer. Moreover, the earth and the sun are also not in physical contact with each other. The average temperature of the earth is about 300 K.

Q3 It is observed that when mercury thermometer is put in a flame, the column of mercury first descends then rises. Explain.

Ans: When a mercury thermometer is put in a flame, the glass bulb of thermometer expands; thus the column of mercury descends. But no sooner the heat reaches mercury in the bulb, which expands, and this expansion is greater than that of the glass bulb so the mercury column rises.

Q4 It is correct that unit for Specific Heat Capacity is m2.s2.C−1.

Ans: Yes. It is correct. The formula for Specific Heat Capacity is given by:

The S.I. unit of

Q5 When a block with a hole in it is heated, why does not the material around the hole expand into the hole and make it small?

Ans: The thermal expansion of homogenous substance causes increase in all directions with the same linear thermal expansion coefficient. This increase in all directions causes an effective magnification of an object. So a hole in the block, on heating, expands outward (becomes big).

Q6 A thermometer is placed in direct sunlight. Will it read the temperature of air, or of sun, or of something else?

Ans: The thermometer will record the temperature of the surrounding.

Q7 One kilogram of hydrogen contains more atoms than one kilogram of lead. Explain.

Ans: One kilogram of hydrogen contains more atoms than one kilogram of lead because hydrogen atoms are much lighter than lead atoms (the atomic mass of hydrogen is 207 times less than that of lead).

Q8 The pressure in gas cylinder containing hydrogen will leak more quickly than if it contained oxygen. Why?

Ans: This is because the hydrogen molecules are lighter than oxygen molecules (the molecular mass of hydrogen is 16 times less than the molecular mass of oxygen). Molecular speed, and hence rate of diffusion, is inversely proportional to the molecular mass. Hence hydrogen will leak more quickly than oxygen.

Q9 What are some factors that affect the efficiency of automobile engine?

Ans: The efficiency of an automobile engine depends upon (i) temperature of hot reservoir, (ii) temperature of cold reservoir and (iii) friction and heat losses (dissipative effect).

Q10 What happens to the temperature of a room if an air-conditioner is left running on a table in the middle of the room?

Ans: When an air-conditioner is left running on a table in the middle of the room, heat is removed from the room by the air-conditioner. But heat is radiated on the other side to the room by the coils (condenser), at the back of the air-conditioner. The heat pumped out of the back of the air-conditioner and into the room is greater than the heat pulled into the front of

the unit, as the work done to remove the heat puts into the room an additional amount of heat .

Consequently, the air-conditioner warms the room.

Q11 When a sealed thermos of hot coffee is shaken, what are the changes, if any in: -a) The temperature of the coffeeb) The internal energy of the coffee

Ans: (a) The temperature of the coffee increases due to shaking. (b) The internal energy of the coffee increases. In fact the work done in shaking the coffee appears as increase in internal energy. Hence the temperature of the coffee increases (due to friction of walls of flask).

Q12 When an object is heated, not all the energy it absorbs goes into increasing the velocity of molecules. Explain where the remaining energy goes?

Ans: When an object is heated, not all the energy it absorbs goes into increasing the velocity of molecules. Some goes into the rotational motion of the molecules and some into the internal vibration motion.

Q13 If a pendulum clock is to keep the correct time at different temperatures, will it be better to use aluminium or steel?

Ans: It is better to use steel in a pendulum clock that has to keep the correct time at different temperatures because the coefficient of thermal expansion of steel is almost half that of aluminium.

Q14 Whys is the average velocity of the molecules of a gas zero but the squares of the velocities is not zero?

Ans: Due to the random motion of the molecules in a gas, the number of molecules on the average moving with certain velocities along x, y and z-axis is equal to the number of molecules moving along negative x, y and z-axis with the same velocities. Hence the average velocity of the molecules of a gas is zero. However, the square of a negative component of the velocity is also positive. Hence the square of the molecular velocities is not zero.

Q15 Why does the pressure of a gas in an automobile tyre increase if the automobile is driven for a while?

Ans: When an automobile is driven on road, it does work to overcome the friction between the tyres and the road; so heat is produced. This in turn, raises the temperature of the gas. Since pressure is directly proportional to temperature, hence pressure of the gas in the tyre increases.

Q16 Under what conditions can heat be added to a system without changing its temperature?

Ans: Heat can be added to a system without changing its temperature in the following ways:(i) For a gaseous system, it can be achieved in isothermal process.(ii) For a liquid, it can be achieved at the melting point of the solid.

Q17 Is it possible to cool a room by keeping the door of the refrigerator open?

Ans: A room cannot be cooled by keeping the refrigerator door open. Whatever heat is removed from the air directly in front of the refrigerator is pumped back into the room at the rear of the refrigerator. Also the work done to remove heat from the room put an additional amount of heat into the room.

Q18 Is it possible, according to second law of thermodynamics, to construct an engine that is free from thermal pollution?

Ans: It is not possible to construct an engine that is free from thermal pollution since heat rejected to sink is an essential requirement. The sink is the atmosphere to which the heat rejection results in thermal pollution. The small temperature changes have disruptive effect on the overall ecological balance. Thermal pollution is an inevitable consequence of the second law of thermodynamics.

Q19 When two systems are in thermal equilibrium, do they have same amount of kinetic energy?

Ans: Temperature is the measure of the average translational kinetic energy of the molecules of a system. However, the systems with the same average translational kinetic energy have the same temperature even if one has greater internal energy (due to greater rotational and vibration energy).

Q20 What do you mean by heat is ‘energy in transit’?

Ans: Heat is not the energy a body contains. It refers to the amount of energy transferred from hot body to cold body.

Q21 Why is the specific heat of polyatomic gases higher than the monatomic gases?

Ans: In a monatomic gas, the whole of the heat supplied is used up in increasing the translational kinetic energy, i.e. its temperature. But in a polyatomic gas, the heat energy supplied is wasted in increasing the rotational and vibration kinetic energy. Thus to obtain the same ranges of temperature, more heat is required for polyatomic gases.

Q22 Work can completely be converted into heat ( ). Can heat be completely converted into work?

Ans: A given amount of heat cannot be completely converted into work as some of the heat energy is used in increasing the internal energy of the system. If heat is completely converted into work, then efficiency of heat engine will be 100%. This violates the second law of thermodynamics.

Q23 Entropy has often been called as ‘time arrow’. Explain.

Ans: Entropy has often been called as ‘time arrow’ because it tells us in which direction the time is going. The normal sequence of events is that in which disorder increases with time.

Q24 Why is the molar specific heat of a diatomic gas usually greater than that of monatomic gas?

Ans: A monatomic gas contains only one atom in its molecule. When it is heated, the whole of the heat supplied is used in increasing the translational kinetic energy of the gas molecule, which determines the rise in temperature. In case of diatomic gas, each molecule contains two atoms. Thus its molecules may have vibratory as well as rotation motion in addition to translational kinetic motion, and partly increase vibration and rotational kinetic energies. Thus more heat is added or needed to increase the temperature of a diatomic gas by same amount.

Q25 Why must more work be done when gas is heated at constant pressure than at constant volume?

Ans: At constant volume, no work is done by the gas and heat supplied simply increases the internal energy or the temperature of the gas. At constant pressure, the gas expands and work is done. Hence heat supplied is partially used in increasing the internal energy of the gas and partially to do work. Hence more heat is required to raise the temperature of the gas through the same amount as in case of heating at constant volume. It means that more work must be done on the gas heated at constant pressure.

Q26 Why does the temperature drop in an adiabatic expansion process?

Ans: In an adiabatic process, heat neither enters into nor leaves the system. When a gas expands adiabatically, it does work against external pressure. This work is done at the cost of its internal energy. Hence its internal energy decreases which means that the temperature of the gas drops.

Q27 Why is it not possible to convert the given amount of heat completely into work? Is the reverse possible? Why?

Ans: A heat engine only works continuously when a part of heat is rejected to the cold body or the surrounding. Hence all the given amount of heat cannot be completely converted into work. The reverse is possible: the work done can be completely converted into heat. The second law of thermodynamics does not apply when work is being converted into heat.

Q28 Is it possible to convert internal energy into mechanical energy? Explain it with example.

Ans: Yes, it is possible to convert internal energy into mechanical energy. When a gas in a cylinder expands adiabatically, it does work in pushing the piston up at the cost of its internal energy. In this way, internal energy is converted into mechanical energy.

Q29 Under what conditions is it possible for the entropy of a system to decrease? Give an example.

Ans: The entropy of a system decreases when heat is taken out of it. When water is frozen, heat is taken away from it and thus its entropy decreases.

Q30 A blanket which keeps us warm in winter is also able to protect ice from melting. Explain.OR

A flannel keeps the ice cold but keeps people warm.

Ans: A blanket prevents the flow of heat due to the air trapped in its pores. In winter, it keeps us warm as heat from our body cannot flow to the surrounding. On the other hand, ice is saved from melting as heat cannot flow from the surrounding to the ice.

Q31 Stainless steel cooking pans are preferred with extra copper bottom. Why?

Ans: The coefficient of thermal conductivity of copper is quite large as compared to that of steel. When a cooking pan is fitted with extra copper bottom, it will allow more heat to flow into the pan when placed over flame. It will result in faster cooking of the food.

Q32 A liquid is generally heated from below. Why?

Ans: When a liquid is heated, it becomes rarer due to the decrease in density and rises up. The liquid in the upper part of the vessel comes down to take its place and thus convection currents will be set up and only the liquid in the upper part of the vessel will become hot. However, the temperature in the lower part of the vessel will rise slightly due to small amount of heat conducted by the hot liquid in the upper part of the vessel.

Q33 Animals curl into a ball when they feel very cold.

Ans: The total energy radiated by a body depends upon its surface area. When the animals feel cold, they curl their bodies into a ball in order to decrease the surface area of their bodies. Due to decrease in surface area, the loss of heat due to radiation also becomes small.

Q34 The earth constantly receives heat radiation from the sun and gets warm. Why does the earth not get as warm as the sun?

Ans: Due to the large distance of the earth from the sun, it receives only a small amount of heat radiation from the sun. Further, due to the loss of the heat from the surface of the earth, due to radiation and convection, the earth cannot maintain the temperature it attains due to the heat received from the sun.

Q35 The earth, without its atmosphere, will be inhospitably cold. Why?

Ans: The atmosphere does not allow the loss of heat from the surface of the earth in the form of radiation. In other words, the heat received by the earth from the sun during the day is kept trapped by the atmosphere. Therefore, the earth, without its atmosphere, will be inhospitably cold.

Q36 On a winter night, you feel warmer when clouds cover the sky than when the sky is clear. Why?

Ans: The surface of the earth absorbs sun rays during day-time and gets heated. At night, the earth radiates heat but due to the clouds, the radiations are reflected back towards the surface of the earth. However, when the sky is clear, the heat radiations escape the surface of the earth. For this reason, the cloudy nights are warmer than the nights when the sky is clear.

Q37 Give two examples each of an isothermal change and an adiabatic change.

Ans: Isothermal Change:1. A contained in a metallic cylinder is compressed or allowed to expand slowly.2. Boyle’s Law.

Adiabatic Change:1. A gas contained in a non-conducting cylinder is compressed or allowed to expand quickly.2. Air escaping from the cycle tube or removing the valve undergoes adiabatic expansion.

Q38 Is it possible to increase the temperature of a gas without adding heat to it? If yes, then explain how?

Ans: For an adiabatic change, first law of thermodynamics may be expressed as:

OR

The temperature of a gas will increase if is positive. Therefore, temperature of a gas can be increased without adding

heat to it, if it is compressed adiabatically.

Q39 If hot air rises, why is it cooler at the top of the mountain than near sea level?

Ans: Since atmospheric pressure decreases with height; pressure at the top of the mountain is lesser. When hot air rises up, it suffers adiabatic expansion at the top of the mountain. For an adiabatic change, the first law of thermodynamics can be written as:

OR

Therefore, work done by the air in rising up ( ) results in decrease in internal energy of the air ( ).

Hence temperature of air falls.

Q40 By applying the first law of thermodynamics to isobaric process, obtain relation between two specific heats of a gas.

Ans: In an isobaric process, pressure remains constant. If an amount of heat is supplied to one mole of a gas at constant

pressure and its temperature increases by , then:

Here, is the molar specific heat of gas at constant pressure. Therefore, for an isobaric process, the first law of

thermodynamics becomes:

------- (i)

From perfect gas equation, it follows that:

Substituting the value of and in equation (i), we have:

OR

Q41 On removing the valve, the air escaping from a cycle tube becomes cool. Why?

Ans: In the cycle tube, air is present at a pressure greater than the atmospheric pressure. As soon as the valve is removed, the gas expands suddenly, i.e. adiabatic expansion takes place. During adiabatic expansion, air performs work at the expense of its own internal energy. Hence the temperature decreases and the air escaping from the cycle tube becomes cool.

Q42 Why a gas has two principle specific heat capacities?

Ans: A solid or liquid when heated does not undergo any change in the volume or the pressure. But in case of gas, both the volume and the pressure change on heating. Hence specific heat of a gas is defined at either a constant volume or a constant pressure. Thus a gas has two specific heats.

Q43 An electric refrigerator transfers heat from the cold cooling coils to the warm surrounding. Is it against the second law of thermodynamics? Justify your answer.

Ans: According to the second law of thermodynamics, the heat cannot flow from a cold body to a hot body by itself. However, it can flow if an external agent performs work. In a refrigerator, heat transfers from the cold cooling coils to the warm surrounding due to the external work done through electric energy. Therefore, it is not against the second law of thermodynamics.

Q44 Why is it impossible for a ship to use the internal energy of sea water to operate?

Ans: To use the internal energy of sea water to operate a ship, the internal energy of the sea water must be converted into mechanical energy. Since whole of the internal energy cannot be converted into mechanical energy; a part has to be rejected to a cooler body (sink). Since no such body is available, the internal energy of the sea water cannot be used to operate the engine of a ship.

Q45 Why is the conversion of heat into work not possible without the sink at lower temperature?

Ans: For converting heat energy into work continuously, a part of the heat energy absorbed from the source has to be rejected. The heat energy can be rejected only if there is a body whose temperature is less than that of the source. This body at low temperature is called the sink.

Q46 Absolute temperature of a gas is increased four times its original value. What will be the change in r.m.s. velocity of its molecules?

Ans: We know that .

Therefore, when absolute temperature of a gas is increased four times its original value, its r.m.s. velocity will become double its initial value, i.e. change in r.m.s. velocity will be equal to its initial value.

Q47 What is the kinetic interpretation of temperature?

Ans: We know that:

Therefore, higher the temperature of a gas, more will be the average kinetic energy possessed by the gas, gives the measure of the average kinetic energy of the gas molecule.

If , then

Therefore, absolute zero is that temperature at which the molecular motion stops.

Q48 When a gas expands at constant temperature, why does the pressure decrease? Explain it on the basis of kinetic molecular theory of gas.

Ans: When a gas expands at constant temperature, the average translational kinetic energy of the gas molecules remains same. However, the gap between the molecules of the gas increases and they have to travel long distance before they collide with the wall of the container. As a result, the number of molecules colliding with the walls of the container per second decreases. Consequently, less momentum is transferred to the walls of the container per second. Hence the pressure exerted by the gas decreases.

Q49 Why a temperature less than absolute zero not possible?

Ans: According to kinetic interpretation of temperature, absolute temperature is directly proportional to mean kinetic energy of molecules. As the temperature of a gas is decreased, the average speed of gas molecules also decreases. At absolute zero, the average speed of gas molecules becomes zero, i.e. kinetic energy becomes zero. So no more decrease in kinetic energy is possible. Hence the temperature of the gas cannot be decreased to a temperature below absolute zero.

Q50 Cooling is caused by evaporation. Explain.

Ans: Evaporation is the escape of fast moving molecules from the surface of a liquid. Therefore, on evaporation, the average

kinetic energy of the molecules left behind will be lesser. Since , therefore, the temperature of the liquid left

behind will also be lesser. Thus cooling is caused by evaporation.

Chapter 13: “Current Electricity”

Q1 A potential difference is applied across the ends of a copper wire. What is the effect on the drift velocity of free electron if:

1. Potential Difference is increased.2. Length and temperature or wire is decreased.

Ans: A small uniform velocity that an electron gains in the presence of an electric field of the battery is called the drift velocity

in the direction of

a. The drift velocity of electron increases with the increase in potential difference. This is because by increasing the value of applied potential difference the value of electric field increases which cause an increase in the value of drift velocity of electron.

b. When the length of the wire is decreased then the value of resistance of wire will also be decreased. Thus the value of flowing current will increase.Similarly the resistance of wire will be decreased with the decrease in temperature. Thus the value of current will increase due to which the drift velocity will also increase.

Q2 What are the difficulties in testing whether the filament of a lighted bulb obeys Ohm's law?

Ans: The Ohm's law states that the potential difference is proportional to the current provided there is no change in the physical state of the conductor. The physical state of a conductor means its density, elasticity, temperature etc. In case of a lighted bulb, as the temperature increases, resistance of the filament goes on increasing. According to the relation:

Where α is the temperature coefficient and is the rise in temperature. It means that temperature of a lighted bulb does

not remain constant, so its filament doesn't obeys ohm's law. Thus the main difficulty in testing whether the filament of lighted bulb obeys ohm's law is the change in its temperature with the flow of current and electrical resistance.

Q3 Is the filament resistance lower or higher in a 500W, 220V bulb than in a 100W, 220V bulb?

Ans: For the first Light Bulb:

Applying the formula for power:

Putting the values, we get:

For the second Light Bulb:

Applying the formula for power:

Putting the values, we get:

It is clear, by comparing equations (i) and (ii) that .

It means that the filament resistance of 100W, 220V light bulb is higher than that of 500W, 220V light bulb.

Q4 Explain why the terminal potential of a battery decreases when the current drawn from it is increased?

Ans: As we already know a relation for terminal potential difference of a battery, i.e.:

Here, e.m.f. of the battery.

Internal resistance of the battery.

Potential difference across internal resistance.

Equation (i) shows that when is increased, the factor becomes large and becomes small. Thus terminal potential of

the battery decreases when the current drawn from it is increased.

Q5 Both P.D. and e.m.f. are measured in Volts. What is the difference between these concepts?

Ans: Potential Difference is the work done per unit charge across a resistor in a closed circuit. But e.m.f. is the total potential difference across the external & internal resistance. It refers to a source of current and is greater than the potential drop in an external circuit.

Q6 Can you construct two wires of same length; one of copper and other of iron, which would have the same resistance at the same temperature?

Ans: Yes, since the resistivity is proportional to cross sectional area, the resistivity of iron is about seven times higher than that of copper. Hence the iron wire must be seven times thicker than a copper of the same length to have the same resistance at the same temperature.

Q7 Why does the resistance of a conductor rise with the rise in temperature?

Ans: As the temperature of a conductor rises, the amplitude of the atoms in the lattice increases. This in turn increases the probability of their collision with the free electrons. This impedes the drift velocity of the electron. Hence the resistance of the conductor increases.

Q8 Why is heat produced in a conductor due to the flow of electric current?

Ans: As electric charge flows due to certain potential difference through a conductor, it suffers loss of electrical potential energy; the energy is delivered to the lattice. This energy is utilized in increasing the vibration kinetic energy which appears as heat. Consequently, the temperature of the conductor rises.

Q9 When a metallic object is heated, both its dimension and its resistivity increases. Is the increase in resistivity likely to be the consequence of the increase in length?

Ans: The resistivity is given by . The increase in the resistivity of a conductor due to heat is a consequence of the

increase in resistance and not a consequence of the increase in length.

Q10 It is sometimes said that an electrical appliance "uses up" electricity. What does such an appliance actually use in its operation?

Ans: An electrical appliance in its operation uses the kinetic energy carried by the moving electrons and not their quantity of charge.

Q11 Do bends in a wire affect its resistance?

Ans: No, bends in a wire do not affect its resistance. However, it depends upon the length, cross-sectional area, temperature and nature of the material.

Q12 Ten resistances of different values are connected in parallel. If the potential difference across one of them is 5V then what is the potential difference across the remaining nine resistors?

Ans: When resistors are connected in parallel then the same potential difference exists across each of them (as they all are connected to two common points). Hence the potential difference in this case will be 5V across all the nine resistors.

Q13 Is there any electrical field inside a conductor carrying an electric current? Explain motion of charges here.

Ans: When a potential difference is applied across a conductor by connecting it to a battery, an electric field is established

inside a conductor (parallel to the conductor, directed from the positive terminal to the negative terminal). This field exists here because the battery keeps the charge moving and prevents them in coming to equilibrium on the outer surface of the conductor (in contrast to the situation in electrostatics) where they would cause the net electric field on the interior to be zero.

Q14 How does the current flowing in a conductor depend upon the number of stationary charge carries per unit length on their average velocity and on the charge per carrier?

Ans: The current ‘I’ flowing through a conductor in unit time is given by:

If the number of mobile charges per unit length is given by:

The current flowing through the conductor is:

a. Directly proportional to the number of mobile charges per unit length .

b. Directly proportional to the average drift velocity .

c. Directly proportional to the charge per carrier .

Q15 What is the equivalent resistance of three resistors connected in series and parallel respectively? If potential

difference of 60V is applied across series connection, what is the current in each resistor?

Ans: In series:

In parallel:

In series connection, the current is same through all the resistances. Therefore the current ‘I’ is:

Q16 A wire carries a current of 1A. How many electrons pass through a point in the wire in each second?

Ans: Charge

Now number of electrons

Q17 Can the terminal voltage of a battery become zero?

Ans: When a battery is short-circuited, the existence of the negligibly small resistance in the circuit makes terminal voltage

zero, but current to a maximum value.

Q 18 Why is the internal resistance of a cell not constant?

Ans: The internal resistance of a cell depends upon the resistance of electrolyte, terminals and electrodes (and also on their area and separation) of the cell. Due to chemical changes (e.g. absorption of hydrogen and sulphate ions) in the electrolyte during the process of discharging the resistance of the electrolyte increases. Thus the internal resistance of a cell does not remain constant.

Q19 What is resistance? What is its mechanical analogue?

Ans: Resistance is a property of conductor which limits the flow of current. It is due to the collision of the drifted electrons with the crystal lattice which causes frequent scattering of the electrons under an electric field.

Q20 Often you might have noticed crows sitting safely on high tension wires. Why are not they electrocuted, even when sitting on a part of the wire where the insulation has worn off?

Ans: For electrocution, the current should pass through the bodies of crows. When a crow sits on a single wire, a potential difference is not developed for the flow of current, because both of its claws are at the same potential. Hence they are not electrocuted.

Q21 Why is it dangerous to touch a live wire while standing barefooted on the earth?

Ans: We may be electrocuted if we touch a live wire while standing barefooted on earth because we provide lower potential of

the earth through our bare feet (conductor). [The effective resistance of the body is which reduces to

when it is wet].

Q22 What does short circuit and open circuit means?

Ans: A short circuit is a closed circuit when no load is present and the external resistance is zero. But an open circuit implies an infinite resistance (or gap) along its conducting path (i.e. wires).

Q23 Is it possible to have a situation in which the terminal voltage will be greater than the e.m.f. of the battery?

Ans: In general: . V is greater than E in the case when a battery is being charged .

Q24 Why is the resistance of a conductor inversely proportional to its area of cross-section?

Ans: The larger the area of cross-section of a conductor, the wider path is provided by it for the flow of charges through it

. Hence, the resistance is decreased.

Q25 Electrons leave a dry cell and flow through a lamp back to the cell. Which terminal, positive or negative, is the one from which electrons leave the cell? In which direction is the conventional current?

Ans: Electrons leave through the negative terminal of the cell and move towards positive terminal. However, the conventional current is assumed to be positive electric charges moving from positive terminal to negative terminal of a coil. Current

through the cross-sectional area A is the net charge flowing through the area per unit time. .

Q26 Resistances of 10Ω, 30 Ω, 40 Ω are connected in series. If the current in 10Ω resistor is 0.1A, what is the current through the other resistances?

Ans: When resistors are connected in series then the same current flows through each of them because there is only one path. Hence the current in this case will be 0.1A through all the three resistors in series.

Q27 It is extremely dangerous to use a plug in radio near a bath tub when you are taking a bath. Why? Does the same reasoning apply to a battery operated radio?

Ans: The plug near the bath tub becomes wet and current can leak into the thin layer of water over it. This may cause a severe shock to a person plugging in a radio. Moreover, the resistance of wet body is greatly reduced and hence a large current can flow through the body. Hence it is extremely dangerous to do so.This reasoning does not apply to a battery operated radio as battery circuit is only completed when current has a path to flow from its one terminal to the other terminal.

Q28 If the positive and the negative connections to the terminals of an automobile battery were reversed, would you expect the automobile light to function?

Ans: Yes, the light will function because the light is produced due to heating effect of current and heating effect does not depend upon the direction of flow of current.

Q29 The current in an electric bulb rises to a maximum almost at the instant the bulb is turned on. The current then decreases significantly. Why does the current decreases?

Ans: Before the bulb is turned on, its filament is at room temperature, its resistance is less and large current passes through it. Due to the passage of current, heat is dissipated and temperature of filament rises, thus increasing its resistance. By ohm's

law , as V remains constant when resistance increases, the current decreases.

Q30 Why a thin region of light bulb filament has more possibility to burn that the thicker one?

Ans: The thin region of the light bulb has more resistance than the thicker one. Since the heat produced per second is . As

current I is constant through the whole filament, heat produced is greater where the resistance R is larger which is in the thin region. Therefore the temperature of thin region rises more and it has more possibility to burn than the thicker region.

Q31 In the normal operation of a resistor, what limits the temperature rise due to joule heating?

Ans: With the increase in temperature, the resistance of resistor increases and it results in the decrease of current which

subsequently limits the temperature rise since heat produced per second is . Moreover the radiation losses from the

hot resistor to its colder surrounding results in controlling the unlimited rise of temperature due to joule heating.

Q32 Describe the energy transformations for charges when a source of e.m.f. is connected to a light bulb.

Ans: When a source of e.m.f. is connected to a light bulb, the electrical energy is transformed into heat and light.