The Photoelectric Effect and the Work Function · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

When light knocks electrons loose

Science · Waves, Light & Sound · ages 17-18
Name ______________________   Date ____________

  1. Dan says turning up the brightness makes each freed electron faster. Is he right?

    Circle one:   True   False

  2. A photon meets an electron at the metal surface. Where does the photon energy go?

    • Exit fee plus leftover kinetic energy
    • All into heat in the metal
    • All into making new photons

  3. Bright red light frees no electrons from a metal, but dim blue light does. What does this tell you?

    • Electron escape depends on brightness
    • Frequency decides whether electrons escape
    • Blue lamps always shine brighter

  4. The same colour of light is made brighter. More electrons leave each second, at the same top speed.

    Circle one:   True   False

  5. An electron leaves with 3.2e-19 J of kinetic energy from metal of work function 1.6e-19 J. What was the photon energy in joules?

    Answer: ______________

  6. The light is made twice as bright with no colour change. What happens to the top kinetic energy?

    • It doubles
    • It stays the same
    • It halves

  7. A 6.0e-19 J photon strikes metal with work function 4.0e-19 J. Give the freed electron's maximum kinetic energy in joules.

    Answer: ______________

  8. A photon of energy 6.0e-19 J strikes metal with work function 4.0e-19 J. What is the freed electron's maximum kinetic energy in joules?

    Answer: ______________

  9. Priya sees no electrons leaving and turns up her red lamp. Still nothing leaves. What should she change?

    • Use an even brighter red lamp
    • Warm the metal first
    • Use a higher frequency lamp

  10. The same light shines on two metals. Metal X has the larger work function. Which metal gives the faster electrons?

    • Metal X
    • Both give equal speeds
    • The metal with the smaller work function
LightMySky · lightmysky.comW1-mt_ADQK7WB2JN-s1

Answer key

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

When light knocks electrons loose W1-mt_ADQK7WB2JN-s1

  1. False · Brighter light frees more electrons, but each electron's top speed stays the same at fixed frequency.
  2. Exit fee plus leftover kinetic energy · Each photon pays the work function first, and the rest becomes the electron's top kinetic energy.
  3. Frequency decides whether electrons escape · Only photons above the threshold frequency can pay the exit fee, however many arrive.
  4. True · Brightness adds photons, so the rate rises while each photon keeps the same energy.
  5. 4.8e-19 · Add kinetic energy to work function: 3.2e-19 + 1.6e-19 = 4.8e-19 J.
  6. It stays the same · Top speed is fixed by frequency, so extra photons only raise the number leaving.
  7. 2e-19 · Subtract the exit fee: 6.0e-19 minus 4.0e-19 leaves 2.0e-19 J.
  8. 2e-19 · Subtract the work function: 6.0e-19 - 4.0e-19 = 2.0e-19 J of kinetic energy.
  9. Use a higher frequency lamp · Her photons sit below the threshold, so only a higher frequency can pay the exit fee.
  10. The metal with the smaller work function · The same photon leaves more leftover energy where the exit fee is smaller.
Worksheet · LightMySky