به انگلیسی

When light knocks electrons loose

Science · Waves, Light & Sound · ages 17-18

Name ______________________ Date ____________

Dan says turning up the brightness makes each freed electron faster. Is he right?
Circle one: True False
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
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
The same colour of light is made brighter. More electrons leave each second, at the same top speed.
Circle one: True False
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: ______________
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
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: ______________
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: ______________
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
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

Answer key

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

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False · Brighter light frees more electrons, but each electron's top speed stays the same at fixed frequency.
Exit fee plus kinetic energy · Each photon pays the work function first, and the rest becomes the electron's top kinetic energy.
Frequency decides whether they escape · Only photons above the threshold frequency can pay the exit fee, however many arrive.
True · Brightness adds photons, so the rate rises while each photon keeps the same energy.
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.
It stays the same · Top speed is fixed by frequency, so extra photons only raise the number leaving.
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.
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.
Use a higher frequency lamp · Her photons sit below the threshold, so only a higher frequency can pay the exit fee.
The metal with the smaller work function · The same photon leaves more leftover energy where the exit fee is smaller.