The best engine allowed

Science · Thermal & Statistical Physics · ages 19-20

Name ______________________ Date ____________

In a Carnot cycle, heat flows only on the two isotherms.
Circle one: True False
Which order gives the Carnot cycle?
- Hot isotherm, adiabat, cold isotherm, adiabat
- Adiabat, adiabat, isotherm, isotherm
- Hot isotherm, cold isotherm, adiabat, adiabat
How is the Carnot ceiling computed?
- One minus hot over cold
- Cold over hot
- One minus cold over hot in kelvin
A student plugs 27 and 127 degrees Celsius into cold over hot. What is wrong?
- Celsius is fine for ratios
- Temperatures must be in kelvin
- Efficiency needs Celsius exactly
A Carnot engine runs between 600 K and 300 K. Give its efficiency as a decimal.
Answer: ______________
Why can no engine between the same stores beat Carnot?
- Carnot burns better fuel
- Carnot legs are fully reversible
- Carnot skips the cold store
A Carnot engine between 400 K and 300 K takes 800 J. Give the maximum work in J.
Answer: ______________
A real power station beats the Carnot ceiling between its steam and river temperatures.
Circle one: True False

Answer key

For grown-ups. Fold this page away before handing over the rest.

True · Adiabats by definition exchange no heat.
Hot isotherm, adiabat, cold isotherm, adiabat · Expansion legs alternate with the return legs in that order.
One minus cold over hot in kelvin · Only the two store temperatures in kelvin set the ceiling.
Temperatures must be in kelvin · The ratio only works on the absolute scale.
0.5 · One minus 300 over 600 is 0.5.
Carnot legs are fully reversible · Reversible legs waste nothing, leaving no room above.
200 · Ceiling is 0.25, and 0.25 times 800 is 200.
False · Real engines add friction, so they land below the ceiling.