E-M radiation and Quantum Phenomena P08-12

Question 1

Describe the photoelectric effect.

Answer 1

The photoelectric effect is when light/photons are shined at a metal and the metal emits electrons.

Question 2

Define ‘threshold frequency’

Answer 2

The threshold frequency is the minimum amount of energy needed to release an electron from the surface of the metal.

Question 3

What do we mean by ‘one-to-one interaction’?

Answer 3

One photon is absorbed by one electron, which is why each photon’s energy is calculated by hf.

Question 4

Define ‘work function’.

Answer 4

The work function is the energy needed to liberate an electron.

Question 5

Explain why an electron might be emitted with less than Ekmax.

Answer 5

If the work function is not surpassed then the electron will not move off away from the surface. Therefore the incident photons will need to reach the electrons and have sufficient energy to give it KEmax at all.

Question 6

Sketch and label a graph of Ekmax vs frequency.

Answer 6

A graph of Ekmax = hf - work function, crosses the frequency axis at the threshold frequency and the EKmax axis at the work function (the minimum amount of energy needed to liberate an electron).

Question 7

What is meant by ‘stopping potential’?

Answer 7

The stopping potential is the amount of voltage going through a metal that stops the photoelectric effect from happening, stops any electrons getting to the anode. Normally equals the max KE in eV.

Question 8

What do we mean when we say an electron is excited?

Answer 8

When an electron gains enough energy to go to a higher energy level it has become excited.

Question 9

Explain the two ways by which we can excite an electron.

Answer 9

Absorbing enough energy from a photon or colliding with another atom or particle.

Question 10

Describe what happens when an electron de-excites.

Answer 10

The electron goes down back to ground state. Falls back down to its original energy level.

Question 11

What is an emission spectrum?

Answer 11

The spectrum of the electromagnetic radiation emitted by a source.

Question 12

What is an absorption spectrum?

Answer 12

The spectrum of the frequencies of light transmitted with dark bands when the electrons absorb energy in the ground state to reach higher energy states.

Question 13

Explain how a fluorescent tube converts electrical energy into visible light energy.

Answer 13

The electrical energy causes mercury vapor inside the tube to give off UV energy which is absorbed by the phosphor particles which coat the light bulb, which then glow giving off visible light.

Question 14

What do we mean by wave-particle duality?

Answer 14

Every particle can be described as a wave and every wave can be described as a particle.

Question 15

How can we show evidence for the wave behaviour of light?

Answer 15

Diffraction.

Question 16

How can we show evidence for the particle behaviour of light?

Answer 16

The photoelectric effect
Light photons are absorbed by electrons
one to one interaction

Question 17

How can we show evidence for the wave behaviour of electrons?

Answer 17

Wave-particle duality. When electrons pass through a double slit and strike the screen behind the lists, an interference pattern of bright and dark bands are formed on the screen.
Electrons produce dark rings in diffraction experiments.

Question 18

How can we show evidence for the particle behaviour of electrons?

Answer 18

The photoelectric effect occurs when a high energy photon strikes a metal surface and an electron is ejected while the photon disappears. Shows it can be a wave and a particle.

Question 19

What does the photoelectric effect prove?

Answer 19

Higher intensity of photons did not increase KEmax, therefore one photon absorbed by 1 electron. Light exists as quauta. But more electrons emitted.

Question 20

How do you measure the KE of an electron?

Answer 20

To measure the kinetic energy of electrons, turn up V until no electrons are reaching the other plate. This is the stopping potential. V = E/Q, Vs = KE/e, KEmax = eVs, where KEmax is the electrons liberated from the surface.

Question 21

What is the energy of one wave-packet in J?

Answer 21

hf or hc/(wavelength)

Question 22

What would happen if you used light with frequency lower than the threshold frequency of the metal?

Answer 22

Nothing, the electron wouldn’t have enough energy to escape the metal.

Question 23

What would you see if the intensity of the light was increased? (Fluorescence tubes)

Answer 23

Current increases in thr vacuum photocell. More photons per second would mean more electrons are freed per second and are crossing the gap.

Question 24

What happens if we connect the positive side of the cell to the metal surface?

Answer 24

Electrons would be attracted back to the metal. Only electrons with higher KE would get across the gap.

Question 25

If you started again with photons of even higher frequency, what effect would that have?

Answer 25

It would need a higher voltage, “stopping potential” to stop the most energetic electrons.

Question 26

What is the eqution relating photon energy coming in and the charge on the electron and the stopping potential?

Answer 26

hf = work function + eVs.

Question 27

What happens in de-excitation?

Answer 27

Electron returns to the energy level and a photon is emitted.

Question 28

In energy levels with n=1, n=2, n=etc, what is the jump that would give you the shortest wavelength?

Answer 28

The biggest jump.

Question 29

How do you calculate the ionisation energy from an energy level diagram?

Answer 29

Ground state energy (eV) x 1.6 x 10^-19.