The Photoelectric Effect and the Work Function · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.
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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 kinetic energy
    • All into heat in the metal
    • Shared among several electrons

  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 they 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. Light of frequency 8.0e14 Hz falls on a metal with work function 3.6e-19 J. Take h = 6.6e-34 J s. Find the maximum kinetic energy of the freed electrons in units of 10^-19 J, to 2 significant figures.

    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 5.0e-19 J photon strikes metal with work function 3.4e-19 J. Give the freed electron's maximum kinetic energy in units of 10^-19 J.

    Answer: ______________

  8. A metal's work function is 4.4e-19 J. Take h = 6.6e-34 J s. Find its threshold frequency in units of 10^14 Hz, to 2 significant figures.

    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
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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 kinetic energy · Each photon pays the work function first, and the rest becomes the electron's top kinetic energy.
  3. Frequency decides whether they 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. 1.7 · Each photon carries h f = 6.6e-34 x 8.0e14 = 5.28e-19 J. Paying the 3.6e-19 J work function leaves 1.68e-19 J, which is 1.7 in units of 10^-19 J.
  6. It stays the same · Top speed is fixed by frequency, so extra photons only raise the number leaving.
  7. 1.6 · Subtract the exit fee: 5.0e-19 minus 3.4e-19 leaves 1.6e-19 J, which is 1.6 in units of 10^-19 J.
  8. 6.7 · At threshold, h f equals the work function: f = 4.4e-19 / 6.6e-34 = 6.7e14 Hz, which is 6.7 in units of 10^14 Hz.
  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