Up Learn – A Level physics (AQA) – Quantum
Photoelectrons’ Maximum Kinetic Energy
Photoelectrons are emitted with a range of kinetic energies, but we’re normally interested in the maximum kinetic energy.
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Previously we saw that Newton thought that light was made up of particles…
Whereas, Huygens believed that it was a way…
So who was right?
To find out, we need to look at something called the photoelectric effect
And to do that, we’ll start with the gold leaf electroscope.
So… what is a gold leaf electroscope?
Well this is a gold leaf electroscope
It’s a device that’s very sensitive to changes in electric charge
Now, here’s how it works.
At the top, there’s a metal plate… which is usually either zinc or brass.
This plate is connected to two gold leaves.
Why gold? Not because scientists are fancy…
Instead, it’s because we can make tiny strips of thin gold that are very low mass and can give us a clear indication of changes in electric charge.
Now, we can charge this plate so that it’s negatively charged
And since the plate is connected to the leaves, that’ll mean that the leaves are also negatively charged .
And so they’ll…
The negatively charged gold leaves will repel each other
Next up, let’s look at the metal plate.
Now, within metals, electrons move freely… they’re not fixed in place
And at the surface of metals, the electrons are weakly attracted to the metal.
Which means that they can be removed from the metal.
Next, when electrons are removed from the metal plate….the plate will become less negatively charged.
When the gold leaves become less negatively charged … the leaves will stop repelling as much … and will move closer together
So, how can we change the amount of negative charge in these gold leaves… such that they go from repelling each other to coming together?
Well… we need to look at the surface of the metal plate.
And if we shine certain types of light… then this light can remove electrons from the surface of the plate!
The light will hit the plate… and then the electrons will be ejected from the surface…
And when this happens, the charge of the leaves will change… and they’ll come together.
Finally, when electrons are removed from metals because of certain types of light… we call this the photoelectric effect…
To sum up… a gold leaf electroscope is a very sensitive detector of electric charge that includes…
A gold leaf electroscope includes a metal plate which is attached to two gold leaves.
When the leaves are negatively charged, they will repel.
Electrons can be removed from the surface of the metal by certain types of light.
We call this the photoelectric effect .
When this happens, the gold leaves will move together.