IB DP Physics: HL复习笔记12.1.3 Observing the Photoelectric Effect

The photoelectric effect can be observed on a gold leaf electroscope

A plate of metal, usually zinc, is attached to a gold leaf, which initially has a negative charge, causing it to be repelled by a central negatively charged rod

This causes negative charge, or electrons, to build up on the zinc plate

UV light is shone onto the metal plate, leading to the emission of photoelectrons

This causes the extra electrons on the central rod and gold leaf to be removed, so, the gold leaf begins to fall back towards the central rod

This is because they become less negatively charged, and hence repel less

Some notable observations:

Placing the UV light source closer to the metal plate causes the gold leaf to fall more quickly

Using a higher frequency light source does not change the how quickly the gold leaf falls

Using a filament light source causes no change in the gold leaf’s position

Using a positively charged plate also causes no change in the gold leaf’s position

Typical set-up of the gold leaf electroscope experiment

Laws of Photoelectric Emission

The photoelectric effect is evidence that light does not purely behave as a wave

This is demonstrated by the following observations

Observation 1:

Explanation 1:

Placing the UV source closer to the plate increases the intensity incident on the surface of the metal

Increasing the intensity, or brightness, of the incident radiation increases the number of photoelectrons emitted per second

Therefore, the gold leaf loses negative charge more rapidly

Observation 2:

Explanation 2:

The maximum kinetic energy of the emitted electrons increases with the frequency of the incident radiation

In the case of the photoelectric effect, energy and frequency are independent of the intensity of the radiation

So, the intensity of the incident radiation affects how quickly the gold leaf falls, not the frequency

Observation 3:

Explanation 3:

If the incident frequency is below a certain threshold frequency, no electrons are emitted, no matter the intensity of the radiation

A filament light source has a frequency below the threshold frequency of the metal, so, no photoelectrons are released

Observation 4:

Explanation 4:

If the plate is positively charged, that means there is an excess of positive charge on the surface of the metal plate

Electrons are negatively charged, so they will not be emitted unless they are on the surface of the metal

Any electrons emitted will be attracted back by positive charges on the surface of the metal

Observation 5:

Explanation 5:

A single photon interacts with a single electron

If the energy of the photon is equal to the work function of the metal, photoelectrons will be released instantaneously

In the photoelectric effect, a single photon may cause a surface electron to be released if it has sufficient energy

Stopping Voltage

Stopping voltage, Vs, is defined as:

The voltage required to stop photoelectron emission from occurring

The photons arriving at the metal plate cause photoelectrons to be emitted

This is called the emitter plate

The electrons that cross the gap are collected at the other metal plate

This is called the collector plate

This set-up can be used to determine the maximum kinetic energy of the emitted photoelectrons

The flow of electrons across the gap results in an e.m.f. between the plates that causes a current to flow around the rest of the circuit

Effectively, it becomes a photoelectric cell producing a photoelectric current

If the e.m.f. of the variable power supply is initially zero, the circuit operates only on the photoelectric current

As the supply is turned up, the emitter plate becomes more positive (because it is connected to the positive terminal of the supply)

As a result, electrons leaving the emitter plate are attracted back towards it

This is because the p.d. across the tube opposes the motion of the electrons between the plates

If any electrons escape with enough kinetic energy, they can overcome this attraction and cross to the collector plate

And if they don't have enough energy, they can't cross the gap

By increasing the e.m.f. of the supply, eventually, a p.d. will be reached at which no electrons are able to cross the gap – this is the stopping voltage, Vs

At this point, the energy needed to cross the gap is equal to the maximum kinetic energy KEmax of the electrons

KEmax = eVS