Ask a Teacher

I NEED CLASS NOTES OF THERMODYNAMICS

1. What is a cyclic process? What is the change in internal energy of a system after it completes one cycle of such a process?

Any process in which the system returns to its initial state after undergoing a series of changes is known as a cyclic process. The change in internal energy after complete cycle is zero because the system returns to its initial state.

2. State the first law of thermodynamics.

According to the first law of thermo dynamics, the amount of heat ∆Q absorbed by a system capable of doing mechanical work is equal to the sum of the increase in internal energy ∆U of the system and the external work ∆W done by the system. Mathematically,

∆Q=∆U+∆W= ∆U+P ∆V

3. Is the internal energy of a gas a function of pressure ? Comment.

The internal energy of an ideal gas depends only on the temperature of the gas, while that of a real gas depends on the temperature and volume, which in turn is dependent on pressure.

4. A gas does work during isothermal expansion. What is the source of mechanical energy so produced?

By first law of thermodynamics, ∆Q=∆U+∆W. But for an isothermal process, ∆U=0, so ∆W = ∆Q. Thus the energy required for doing mechanical work during an isothermal process is obtained as heat by the gas from the surroundings.

5. When a gas is suddenly compressed, its temperature rises. Why?

Sudden compression of a gas is an adiabatic process. The work done in compressing the gas increases the internal energy of the gas. Hence the temperature of the gas rises.

6. If an inflated tyre bursts, the air escaping out is cooled, why?

When the tyre bursts, there is an adiabatic expansion of air because the pressure of the air inside is sufficiently greater than the atmospheric pressure. During the expansion, the air does some work against the surroundings, therefore, its internal energy decreases, and as such temperature falls.

7. Does the mass of a body change when it is heated or cooled?

When a body is heated or cooled, it absorbs or releases energy and so its mass increases or decreases in accordance with the Einstein's relation : E= mc². As the speed of light c is very large, so the change in mass is extremely small.

8. First law of thermodynamics does not forbid flow of heat from lower temperature to higher temperature. Comment.

First law of thermodynamics simply tells about the conversion of mechanical energy into heat energy and vice- versa. It does not put any condition as to why heat cannot flow from lower temperature to higher temperature.

9. Can the work done during a cyclic process be zero?

Yes. The work done in a cyclic process is zero, if the process reverses exactly under similar conditions such that it retraces the same P-V diagram. In such a case, the area of the curve on a P-V diagram is zero and hence the work done is zero.

10. How many specific heats does a gas possess?

A gas can possess infinite number of specific heats, depending upon the conditions of temperature and pressure. However, generally we consider only two specific heats, one at constant pressure and the other at constant volume.

11. Why a gas has two principal specific heat capacities ?

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Gases have two specific heats, but the solids and liquids possess only one specific heat. Why?

When the gases are heated, there occurs an appreciable change in their volume. So energy is required for expansion and we have two specific heats one at constant pressure and the other at constant volume. In case of solids and liquids, expansion is negligible, so they have only one specific heat i.e, at constant volume.

12. What is difference between heat and work?

Heat and work are two different modes of energy transfer to a system. Heat is the energy transfer that occurs due to temperature difference between the system and the surroundings. Work in the energy transfer that is brought about by other means, such as moving the piston of a cylinder containing the gas, by raising or lowering some weight connected to it.

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

For converging 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 sink.

14. Why can a ship not use the internal energy of sea water to operate its engine?

For the operation of a heat engine we require a sink at temperature lower than the source and of sufficiently high thermal capacity, which is not possible in the sea.

15. What is meant by reversible engine? Explain why the efficiency of a reversible engine is maximum?

The engine in which the process can be retraced at any stage of its operation by reversing the boundary conditions is called reversible engine. Its efficiency is maximum because in such a device no dissipation of energy takes place against friction, etc.

16. The temperature of the surface of the sun is approximately 6000K. If we take a big lens and focus the sun rays, can we produce a temperature of 8000K?

No. According to second law of thermo dynamics, heat by itself cannot flow from a body at lower temperature to a body at higher temperature. This can be accomplished with the help of an external agency.

17. How a refrigerator can be used as a heat pump to heat a house in winter?

When the outside environment is colder than the inside of a room, we leave a refrigerator open with its radiator (backside) facing the room. The refrigerator pumps in heat from the environment to the room. This heats up the room.