11th Class Physics Chapter eleven Short Questions with Answers Solved

Question: 1

Why we reduce pressure in the tyre while moving on motorway?

Answer: 1

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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: 2

Why absolute value of internal energy cannot be measured?

Answer: 2

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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: 3

We talk about molar specific heat of gases but not talk about molar specific heat of solids and liquids. Why?

Answer: 3

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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.

Question: 4

Specific heat of a gas at constant pressure is greater than specific heat at constant volume why?

Answer: 4

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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: 5

Define Heat Engine.

Answer: 5

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A device which converts heat energy into mechanical work is called heat engine.

Question: 6

Define the term internal energy . Discuss in what form it is in an ideal gas.

Answer: 6

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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 translational kinetic energy of gas molecules.

Question: 7

Why does the pressure of gas in a car tyre increase when it is driven through some distance?

Answer: 7

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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: 8

Define reversible and irreversible process.Give one example of each.

Answer: 8

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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: 9

Define pressure of a gas.

Answer: 9

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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: 10

Write the four postulates of kinetic theory of gases.

Answer: 10

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A finite volume of gas consists of consists of very large number of molecules.
The size of molecules is much smaller than the separation between them.
The molecules do not exert force on each other except durings a collision
The collision between gas molecules themselves and with the walls of container is assumed to be perfectly elastic.

Question: 11

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: 11

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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: 12

Write two limitations of first law of thermodynamics.

Answer: 12

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1st law of thermodynamics does not specify the conditions under which conversion of heat into work is possible.
It does not specify the direction in which heat transfer takes place (high to low or low to high).
The fact that heat cannot be completely converted into work, is a fact that the first law cannot explain.

Question: 13

Is it possible to construct a heat engine that will not expel heat into the atmosphere?

Answer: 13

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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: 14

Write kelvin statement of the second law of thermodynamics.

Answer: 14

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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: 15

Write down three example of adiabatic process.

Answer: 15

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Three example of adiabatic process are

Cloud formation in the atmosphere
The rapid escape or air from a burst tyre.
The rapid expansion or compression of a gas through which sound wave is passing.

Question: 16

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: 16

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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: 17

Define absolute zero using the Carnot cycle.

Answer: 17

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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: 18

State second law of thermodynamics in terms of entropy.

Answer: 18

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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: 19

A real heat engine is less efficient than cannot engine. Why?

Answer: 19

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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: 20

What is an Adiabatic process? Also give its two examples.

Answer: 20

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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: 21

What is Diesel Engine?

Answer: 21

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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: 22

What is difference between isothermal and adiabatic process?

Answer: 22

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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: 23

What is the similarity and difference between internal energy and gravitational P.E?

Answer: 23

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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: 24

What do you mean by triple point of water?

Answer: 24

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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: 25

Does entropy of a system increase or decrease due to friction?

Answer: 25

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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: 26

Define entropy,how it changes with temperature.

Answer: 26

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Question: 27

State Carnot's theorem.

Answer: 27

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It states that no heat engine can be more efficient than a carnot engine operating between the same two temperatures.

Question: 28

What is negative entropy?Give example and its unit.

Answer: 28

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Question: 29

Name the four strokes of the petrol engine.

Answer: 29

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Four strokes of the petrol engine are

Intake stroke
Compression stroke
Power stroke
Exhaust stroke

Question: 30

Under what circumstances the efficiency of a Carnot engine will be 100%? Is it possible?

Answer: 30

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No, it is impossible.

The efficiency of a Carnot engines will be 100% only when cold reservoir is at absolute zero temperature (ok).Such reservoirs are not available and hence the maximum efficiency is always less than one or 100%

Question: 31

What would be the heat lost if internal energy decreased by 10j and 20j of work is done on the system simultaneously

Answer: 31

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Question: 32

What is adiabatic process?Under what conditions this process occurs?

Answer: 32

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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: 33

What is thermodynamic scale of temperature?Give its unit.

Answer: 33

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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: 34

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

Answer: 34

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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: 35

Give an example of a natural process that involves an increase in entropy .

Answer: 35

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11th Class Chapter eleven Physics Short Questions

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