the photoelectric effect

What is the photoelectric effect?

Electrons are emitted from the surface of a metal when electromagnetic radiation above a certain frequency was directed at the metal

What is the threshold frequency?

It is the minimum frequency of the incident electromagnetic radiation required to eject electrons from a metal surface. Below this frequency, no electrons are emitted

How does the threshold frequency relate to wavelength?

The wavelength of the incident light ray must be less than a maximum value, since λ = c/f

How does light intensity affect the number of emitted electrons?

If the light’s frequency is above the threshold frequency, the number of electrons emitted per second is proportional to the intensity of light

3 observations of the photoelectric effect

1. Existence of the threshold frequency

2. The number of electrons emitted per second is proportional to incident light ray intensity

3. Photoelectric emission happens without delay

Why is there a major problem with the 3 observations of the photoelectric effect?

They cannot be explained by using the theory of light as a wave

What happens when light is incident on a metal surface?

An electron at the surface absorbs a single photon from the incident light, and therefore gains an energy equal to hf

How many photons does each electron absorb during photoelectric emission?

Each electron absorbs exactly one photon, so it gains the energy (hf) of the photon

What is the work function?

When the electron absorbs a photon and gains its energy, hf, the electron can leave the metal surface if the energy gained from the single photon exceeds the work function.

So, the work function is the minimum energy needed by the electron to escape the metal (when the metal is at zero potential)

How can you calculate the kinetic energy of an emitted electron?

Eₖ = hf - work function

So it is the excess energy outside the work function

State the photoelectric equation

hf = Eₖ + work function

What is the stopping potential in the photoelectric effect?

The minimum potential needed to stop the photoelectric emission

What is the symbol for stopping potential?

Vs

what happens when the stopping potential is met?

Electrons that escape the metal plate can be attracted back to it by giving the plate a sufficiently positive charge

At this potential, the maximum kinetic energy of the emitted electron is reduced to zero because each electron must do extra work to escape the opposing electric field of the plate to equal the charge of the electron, e, multiplied by the stopping potential to leave the surface