Thermodynamics

The first law of thermodynamics states that the change in internal energy of a system equals the heat added minus the work done, which is a direct statement of the conservation of energy. It does not deal with the conservation of momentum or mass, which govern mechanical collisions and chemical reactions respectively.

Efficiency is calculated using absolute temperatures in Kelvin, giving 1 - (300/500) = 0.4 or 40%. Option B is incorrect because 60% represents the fraction of heat rejected to the cold reservoir rather than the work converted. Using Celsius temperatures directly would yield an incorrect, much higher efficiency.

In physics, work done by an expanding gas is positive because the volume increases (dV > 0) against an external pressure. A diagram for this topic isn't available yet — the worked example below walks the same reasoning step by step.

• !Option B is incorrect because negative work corresponds to compression, where an external force does work on the system to decrease its volume.
• !Option B is incorrect because 60% represents the fraction of heat rejected to the cold reservoir rather than the work converted.
• !Option A is incorrect because a constant temperature describes an isothermal process, whereas temperature typically changes during an adiabatic expansion or compression.

Answer each from memory — you'll see instantly whether you're right and why.

Run a focused 10-question mini-mock on Thermodynamics and see it stick.