Question

18 An engine absorbs 2000J of heat from a hot reservoir and expels 750J to a cold reservoir during each operating cycle. How much work is done during the cycle?
1250J
750J
2000J
350J

19 An engine absorbs 2000J of heat from a hot reservoir and expels 750J to a cold reservoir during each operating cycle. What is the power output of the engine if each cycle lasts for 0.5 seconds?
750W
1750W
3000W
2500W

20 The first law of thermodynamics is a restatement of which of these laws?
conservation of charge
conservation of momentum
conservation of entropy
onservation of energy

Answer 1

Answer on Question 58120, Physics, Molecular Physics | Thermodynamics

Question:

18. An engine absorbs $2000\,J$ of heat from a hot reservoir and expels $750\,J$ to a cold reservoir during each operating cycle. How much work is done during the cycle?

a) $1250\,J$

b) $750\,J$

c) $2000\,J$

d) $350\,J$

Solution:

We can find the work done by the engine during the cycle from the law of conservation of energy:

$$Q_H = W + Q_C,$$

here, $Q_H$ is the heat energy taken from the hot reservoir, $W$ is the work done by the engine during the cycle, $Q_C$ is the heat energy expelled to a cold reservoir.

Therefore, we can get:

$$W = Q_H - Q_C = 2000\,J - 750\,J = 1250\,J.$$

Answer:

a) $1250\,J$

19. An engine absorbs $2000\,J$ of heat from a hot reservoir and expels $750\,J$ to a cold reservoir during each operating cycle. What is the power output of the engine if each cycle lasts for $0.5\,s$?

a) $750\,W$

b) $1750\,W$

c) $3000\,W$

d) 2500 W

**Solution:**

By the definition of the power we have:

$$P = \frac {W}{t} = \frac {Q _ {H} - Q _ {C}}{t} = \frac {2 0 0 0 J - 7 5 0 J}{0 . 5 s} = \frac {1 2 5 0 J}{0 . 5 s} = 2 5 0 0 W.$$

**Answer:**

d) 2500 W

20. The first law of thermodynamics is a restatement of which of these laws?

a) conservation of charge

b) conservation of momentum

c) conservation of entropy

d) conservation of energy

**Solution:**

The first law of thermodynamics is the application of the law of conservation of energy to heat and thermodynamic process. The first law of thermodynamics states that the change in internal energy of a system is equal to the heat added to the system minus the work done by the system:

$$\Delta U = Q - W,$$

here, $\Delta U$ is the change in internal energy, $Q$ is the heat added to the system, $W$ is the work done by the system.

Thus, the correct answer is d) conservation of energy.

**Answer:**

d) conservation of energy

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