Catch the false line on heat engines

Exercise and reference answers

Questions

1. False statement

   A cyclical heat engine can turn all of its heat input into work.

   Statements

   1. A cyclical heat engine can turn all of its heat input into work.
   2. Heat moves on its own from a hotter body to a cooler one.
   3. Every real heat engine dumps some heat into its surroundings.

2. False statement

   A real engine can beat the Carnot efficiency between the same two temperatures.

   Statements

   1. A Carnot engine uses only reversible steps.
   2. A real engine can beat the Carnot efficiency between the same two temperatures.
   3. A Carnot cycle is built from two isothermal and two adiabatic paths.

3. False statement

   Heat transferred and work done are both independent of the path taken.

   Statements

   1. Internal energy depends only on the state a system is in.
   2. Change in internal energy equals heat in minus work done by the system.
   3. Heat transferred and work done are both independent of the path taken.

4. False statement

   The first law of thermodynamics is what forbids heat flowing cold to hot.

   Statements

   1. The first law of thermodynamics is what forbids heat flowing cold to hot.
   2. Reaching full efficiency would need a cold reservoir at absolute zero.
   3. Waste heat is the heat an engine gives up to its cold surroundings.

Instructions

In each set of three claims about heat engines, tap the one that is not true.

Explanation