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11th Class Punjab
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Heat And Thermodynamics Short Questions Preparation

Chapter 11 Heat And Thermodynamics Physics 11th Class Punjab Short Questions

Question No. 1 - Write kelvin statement of the second law of thermodynamics.

Answer No. 1 - It is impossible to devise a process which may convert heat, extracted from a single reservoir, entirely into work without leaving any change in the working system.

Question No. 2 - Is it possible to construct a heat engine that will not expel heat into the atmosphere?

Answer No. 2 - No, it is not possible to construct a heat engine that will not expel heat into the atmosphere. It is against 2nd law of thermodynamics. A heat engine works only when some of the total heat absorbed from the source is expelled to a sink or atmosphere.

Question No. 3 - What is an Adiabatic process? Also give its two examples.

Answer No. 3 - Adiabatic Process:An adiabatic process is the process in which no heat is transferred to or from the system but the temperature of the system changes.
Examples: Passage of sound through the air, rapid escape of air from a burst tyre and cloud formation etc.

Question No. 4 - Specific heat of a gas at constant pressure is greater than specific heat at constant volume why?

Answer No. 4 - At constant volume, no work is done and the entire heat is utilized in raising the internal energy of the system.But under constant pressure, heat is not only required to raise the internal energy but also to do work against constant pressure.
Hence specific heat of a gas at constant pressure is greater than specific heat at constant volume.

Question No. 5 - What is Diesel Engine?

Answer No. 5 - No spark plug is needed in diesel engine. Diesel is sprayed into cylinder at maximum compression. Because air is at high temperature after compression,fuel mixture ignites on contact with air is at high temperature after compression,fuel mixture ignites on contact with air in cylinder and pushes the piston outward.
Its efficiency is about 35% to 40%.

Question No. 6 - Define absolute zero using the Carnot cycle.

Answer No. 6 - The Carnot cycle provides us the basis to define a temperature scale that is independent of material properties, Absolute zero is the lower limit of the thermodynamic temperature scale, a state at which the enthalpy and entropy of a cooled ideal gas reaches its minimum value, taken as zero(0). By international agreement, absolute zero is taken as - 273.15°c.

Question No. 7 - Why absolute value of internal energy cannot be measured?

Answer No. 7 - By experiment it has been seen that the change in internal energy is independent of paths and it only depends upon change from initial to final state of the system. It is a function of state.
Internal energy is similar to the gravitational P.E.So like the potential energy, it is the change in internal energy and not its absolute value, which is important.

Question No. 8 - Define pressure of a gas.

Answer No. 8 - The pressure exerted by a gas is the momentum transferred to the walls of the container per sec unit area due to continuous collision of molecules of gas.

Question No. 9 - What happens to the temperature of the room, when an air conditioner in left running on a table in the middle of a room?

Answer No. 9 - No change will be observed because the heat is absorbed and expelled in the same room.Hence there will be no effect on the room's temperature.

Question No. 10 - Why we reduce pressure in the tyre while moving on motorway?

Answer No. 10 - We reduce the pressure in the tyre while moving on motorway because when a car is driven through some distance,its tyres become hot due to work done against friction between the tyres and the road which appears as heat. This heat is absorbed by the gas molecules contained in the tyre which results in an increase in pressure of the gas. If the pressure in the tyre is not reduced then the tyre may burst.

Question No. 11 - Give an example of a process in which no heat is transferred to or from the system but the temperature of the system changes.

Answer No. 11 - An adiabatic process is the process in which no heat is transferred to or from the system but the temperature of the system and velocities of molecules changes.
In an adiabatic expansion of a gas, the temperature decreases as the work is done in expanding the gas at the cost of its internal energy.

Question No. 12 - Under what circumstances the efficiency of a Carnot engine will be 100%? Is it possible?

Answer No. 12 - No, it is impossible.
The efficiency of a Carnot engines will be 100% only when cold reservoir is at absolute zero temperature (0k). Such reservoirs are not available and hence the maximum efficiency is always less than one or 100%.

Question No. 13 - Define Heat Engine.

Answer No. 13 - A device which converts heat energy into mechanical work is called heat engine.

Question No. 14 - Write down three example of adiabatic process.

Answer No. 14 - Three example of adiabatic process are:
1. Cloud formation in the atmosphere
2. The rapid escape or air from a burst tyre.
3. The rapid expansion or compression of a gas through
which sound wave is passing.

Question No. 15 - Does entropy of a system increase or decrease due to friction?

Answer No. 15 - The entropy of a system increase due to the friction as work done against friction is changed into heat and heat added to the system increase its entropy.

Question No. 16 - What is the similarity and difference between internal energy and gravitational P.E?

Answer No. 16 - Internal energy is similar to the gravitational P.E.So like the potential energy, it is the change in internal energy and not its absolute value, which is important.
Internal energy depends upon temperature of the system while gravitational P.E depends on position of the particle.

Question No. 17 - Define the term internal energy . Discuss in what form it is in an ideal gas.

Answer No. 17 - The sum of all forms of molecular energies such as kinetic and potential energy of a substance is called its internal energy.
Internal energy of an ideal gas system is generally the transnational kinetic energy of gas molecules.

Question No. 18 - A real heat engine is less efficient than cannot engine. Why?

Answer No. 18 - Carnot engine operates in an ideal reversible cycle and ideal gas is used as the working substance.
No practical heat engine can be perfectly reversible. All real heat engines are less efficient than Carnot engine due to friction and other heat losses.

Question No. 19 - State second law of thermodynamics in terms of entropy.

Answer No. 19 - It states that if a system undergoes a natural process, it will go in the direction that causes the entropy of the system plus the environment to increase.

Question No. 20 - No spark plug is used in Diesel Engine.How it gets ignition?

Answer No. 20 - Diesel is sprayed into the cylinder at maximum compression. Because air is at very high temperature immediately after compression, the fuel mixture ignites on contact with the air on the cylinder.

Question No. 21 - What is adiabatic process? Under what conditions this process occurs?

Answer No. 21 - Adiabatic Process: The process in which no heat enters or leaves the system but the temperature of the system changes.
Adiabatic change occurs when the gas expands or is compressed rapidly,particularly when the gas is contained in an insulated cylinder.

Question No. 22 - Write the four postulates of kinetic theory of gases.

Answer No. 22 - 1. A finite volume of gas consists of consists of very
large number of molecules.
2. The size of molecules is much smaller than the
separation between them.
3. The molecules do not exert force on each other except
during a collision.
4. The collision between gas molecules themselves and with
the walls of container is assumed to be perfectly
elastic.

Question No. 23 - Why does the pressure of gas in a car tyre increase when it is driven through some distance?

Answer No. 23 - When a car is driven through some distance on the road, its tyres has to overcome frictional force of the road. The work done against the friction heats up the gas in tyre. It increase the velocity and hence the pressure of the air in the tyre.

Question No. 24 - Define reversible and irreversible process.Give one example of each.

Answer No. 24 - Reversible Process: A Process which can be retracted in exactly reverse order without producing any change in the surroundings is called reversible process.For example, melting of ice into water and freezing of water into ice.
Irreversible Processes: A process which cannot be retracted in the backward direction by reversing the controlling factors is called an irreversible process.
For example, work done against friction.

Question No. 25 - Write two limitations of first law of thermodynamics.

Answer No. 25 - 1. 1st law of thermodynamics does not specify the
conditions under which conversion of heat into work is
possible.
2. It does not specify the direction in which heat transfer
takes place (high to low or low to high).
3. The fact that heat cannot be completely converted into
work, is a fact that the first law cannot explain.

Question No. 26 - What do you mean by triple point of water?

Answer No. 26 - The triple point of water is a state in which ice, water and vapour coexists in equilibrium and it occurs uniquely at one particular pressure and temperature.Its value is 273.16 k.

Question No. 27 - Name the four strokes of the petrol engine.

Answer No. 27 - Four strokes of the petrol engine are:
1. Intake stroke
2. Compression stroke
3. Power stroke
4. Exhaust stroke

Question No. 28 - State Carnot's theorem.

Answer No. 28 - It states that no heat engine can be more efficient than a Carnot engine operating between the same two temperatures.

Question No. 29 - What is thermodynamic scale of temperature? Give its unit.

Answer No. 29 - The thermodynamic scale of temperature is defined by choosing 273.16k as the absolute temperature of the triple point of water as one fixed point and absolute zero as the other.
The unit of thermodynamic scale is kelvin.

Question No. 30 - What is difference between isotherm and adiabatic process?

Answer No. 30 - Isothermal process: The process in which temperature of the system remains constant is called isothermal process.
T= constant
Adiabatic System: The process in which no heat enters or leaves the system is called adiabatic system.

Question No. 31 - We talk about molar specific heat of gases but not talk about molar specific heat of solids and liquids. Why?

Answer No. 31 - In case of solids and liquids the change in volume and hence work done against external pressure during a change of temperature is negligibly small.But same can not be said about gases which suffer variation in pressure as well as in volume with the rise in temperature.

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