The First Law of Thermodynamics for Any Process · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Energy in, energy out: the first law

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

  1. Which quantity is a state function?

    • Heat
    • Internal energy
    • Work

  2. Which equation states the first law with work by the system positive?

    • Change in U equals Q minus W
    • Change in U equals W minus Q
    • Change in U equals Q plus W plus pressure

  3. Round a complete cycle, the internal energy change is zero.

    Circle one:   True   False

  4. A gas expands with no heat flow. What happens to its temperature?

    • It stays exactly fixed
    • It falls, work drains internal energy
    • It rises without any cause

  5. A gas is squeezed while losing heat. Which balance rules?

    • Work in fights heat out, change is the balance
    • Both effects always raise internal energy
    • Temperature must stay fixed

  6. Why does heating at fixed pressure need more heat than at fixed volume?

    • Part of the heat leaves as expansion work
    • Pressure destroys energy
    • Volume change cools nothing

  7. A gas takes in 100 J of heat and does 40 J of work on its surroundings. What is its change in internal energy, in joules?

    Answer: ______________

  8. A student uses work done ON the gas as positive W in change equals Q minus W. What is the error?

    • Heat must always be negative
    • Internal energy depends on path
    • The rule counts work BY the system positive, so flip the sign
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Answer key

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

Energy in, energy out: the first law W1-mt_LIWgeWSXHz-s1

  1. Internal energy · Internal energy cares only about the current state.
  2. Change in U equals Q minus W · Heat added raises the inside while work done spends it.
  3. True · The loop ends at its start, so every state variable is restored.
  4. It falls, work drains internal energy · With no heat entering, the work spent must cool the gas.
  5. Work in fights heat out, change is the balance · Compression adds energy while leaking heat removes it.
  6. Part of the heat leaves as expansion work · At fixed pressure the gas spends some heat pushing outward.
  7. 60 · Apply heat in minus work out: 100 minus 40 is 60.
  8. The rule counts work BY the system positive, so flip the sign · One sign rule must hold every term, and this rule pays work outward.
Worksheet · LightMySky