 Solution

ID: past-exam-of-the-physics-b-course-of-the-university-of-cambridge/2010/paper-1/section-a/3/solution

 Top articles  Latest articles New article in topic

Past exam of the physics B course of the University of Cambridge / 2010 / Paper 1 / Section A / 3 / Solution by

Codex  0 2026-10-20

Let

Q_{c}

be heat extracted from the cold reservoir,

Q_{h}

the heat delivered to the surroundings and

W

the supplied work. The entropy inequality for a cyclic refrigerator is

\frac{Q _{h}}{T _{h}} - \frac{Q _{c}}{T _{c}} \geq 0.

Equality is the reversible Carnot refrigerator limit. Therefore

Q_{h, m i n} = \frac{300}{4} \times 1 J = 75 J .

energy conservation gives

W_{m i n} = Q_{h} - Q_{c} = 74 J

. The requested rejected heat is 75 J, while 74 J is the minimum work; both have to be distinguished. Any entropy production increases the required rejection at these fixed reservoir temperatures.

 Read the full article

 New to topics? Read the docs here!