# NCERT Class 11 Physics Solutions: Chapter 12 – Thermodynamics-Part 3

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**Question 12.7:**

A steam engine delivers of work per minute and services of heat per minute from its boiler. What is the efficiency of the engine? How much heat is wasted per minute?

**Answer:**

Work done by the steam engine per minute,

Heat supplied from the boiler,

Efficiency of the engine

Hence, the percentage efficiency of the engine is

Amount of heat wasted

Therefore, the amount of heat wasted per minute is

**Question 12.8:**

An electric heater supplies heat to a system at a rate of . If system performs work at a rate of . At what rate is the internal energy increasing?

**Answer:**

Heat is supplied to the system at a rate of

The system performs at rate of

From the first law of thermodynamics, we have:

Where,

Internal energy

Therefore, the internal energy of the given electric heater increases at a rate of

**Question 12.9:**

A thermodynamic system is taken from an original state to an intermediate state by the linear process shown in Figure.

Its volume is then reduced to the original value from to F by an isobaric process. Calculate the total work done by the gas from

**Answer:**

Total work done by the gas from

Area of

Where,

Change in pressure

Change in volume

Therefore, the total work done by the gas from

**Question 12.10:**

A refrigerator is to maintain eatables kept inside at . If room temperature is calculate the coefficient of performance.

**Answer:**

Temperature inside the refrigerator,

Room temperature,

Coefficient of performance

Therefore, the coefficient of performance of the given refrigerator is