Thermodynamic State Variables and PV Diagrams · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

State variables and the paths on a PV diagram

Science · Thermal & Statistical Physics · ages 18-19
Name ______________________   Date ____________

  1. How is work read from a PV path?

    • The area under the path
    • The slope at the start
    • The length of the curve

  2. Which list holds only state variables?

    • Work, heat, and temperature
    • Pressure, volume, and temperature
    • Work, heat, and path length

  3. Work and heat are state variables.

    Circle one:   True   False

  4. An expansion leg does 60 J of work while a compression leg takes 25 J. What is the net work out of the cycle, in joules?

    Answer: ______________

  5. Two paths join the same endpoints but report different work. Why?

    • The endpoints must differ
    • Volume is not a state variable
    • Work depends on the route taken

  6. Round a closed loop, what is the change in internal energy?

    • Zero, every state variable returns
    • Equal to the heat only
    • Equal to the work only

  7. A clockwise cycle expands at high pressure and compresses at low pressure. What is the net result?

    • No work either way
    • Net work out, paid by net heat in
    • Work in with no heat flow

  8. A student draws two isotherms that cross. What is wrong?

    • Crossings mark absolute zero
    • Isotherms must be vertical
    • Isotherms never cross, one temperature per point
LightMySky · lightmysky.comW1-mt_G3EYx84GZK-s1

Answer key

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

State variables and the paths on a PV diagram W1-mt_G3EYx84GZK-s1

  1. The area under the path · Adding pressure over volume along the path gives the area beneath it.
  2. Pressure, volume, and temperature · State variables ignore history, while work and heat describe a route.
  3. False · They describe the path taken, not the current state.
  4. 35 · Net work is work out minus work put back in.
  5. Work depends on the route taken · Same endpoints can pair with different areas, hence different work.
  6. Zero, every state variable returns · Internal energy is a state variable, so the loop brings it home.
  7. Net work out, paid by net heat in · The big expansion area beats the small compression area, and heat funds the gap.
  8. Isotherms never cross, one temperature per point · Each point holds one temperature, so two isotherms cannot share it.
Worksheet · LightMySky