You are Here: Home >< Physics

# Photoelectric effect questions watch

1. Two questions:

In the equation of hf = phi + 1/2 mv^2 max, why is the stressed importance on the 1/2mv^2 max? I don't fully understand it.

Also, for the IV graph, why does I become horizontal after V> roughly 0?

Attached Images

2. (Original post by Kolasinac138)
Two questions:

In the equation of hf = phi + 1/2 mv^2 max, why is the stressed importance on the 1/2mv^2 max? I don't fully understand it.

Also, for the IV graph, why does I become horizontal after V> roughly 0?

The consideration 1/2mv^2 max. means that the electrons have the highest energy by the maximum kinetic energy. This highest energy is spent whenever a the maximum voltage exists. At this level of voltage, a photo current does not exist, even if it is at the maximum kinetic energy. That is why the maximum (kinetic) energy is equal to the maximum voltage. So both the maximum voltage and maximum kinetic energy exists at the zero of the voltage axis (stopping potential).

The horizontal line (saturation current) means that there is a constant photo current at a positive voltage. So the current itself does not change at a certain voltage level, it cannot increase anymore, not even at higher positive voltages.
3. (Original post by Kallisto)
The consideration 1/2mv^2 max. means that the electrons have the highest energy by the maximum kinetic energy. This highest energy is spent whenever a the maximum voltage exists. At this level of voltage, a photo current does not exist, even if it is at the maximum kinetic energy. That is why the maximum (kinetic) energy is equal to the maximum voltage. So both the maximum voltage and maximum kinetic energy exists at the zero of the voltage axis (stopping potential).

The horizontal line (saturation current) means that there is a constant photo current at a positive voltage. So the current itself does not change at a certain voltage level, it cannot increase anymore, not even at higher positive voltages.
Sorry, I don't understand the max part fully. Are you saying that the energy of an electron MUST be the maximum kinetic energy?
4. (Original post by Kolasinac138)
Sorry, I don't understand the max part fully. Are you saying that the energy of an electron MUST be the maximum kinetic energy?
When an electron is removed from the surface of the metal then the maximum energy it can have is the energy of the light - energy taken to remove it from the surface (work function)
5. (Original post by Kolasinac138)
Sorry, I don't understand the max part fully. Are you saying that the energy of an electron MUST be the maximum kinetic energy?
No, it is not a 'must have' for electrons. Look: whenever electrons are released from a cathode, they are in moevement, and that is why they have a kinetic energy. This kinetic energy must be spent to go to the anode in the one hand, but to overcome the voltage in the other hand. If the voltage is at the maximum, it stands to reason that the kinetic energy of electrons must be at the maximum too, otherwise they have no chances to overcome the voltage.

But experiments like above showed that whenever the voltage is at the maximum, the electrons are not able to overcome this voltage and so have no chances to come to the anode, although the kinetic energy would be at the maximum in cases like that.

So the maximum of kinetic energy exists only, if the voltage is at the maximum. Otherwise the kinetic energy of electrons - and the energy itself - is below the maximum. That is why the kinetic energy has not to be at the maximum.
6. (Original post by Kallisto)
No, it is not a 'must have' for electrons. Look: whenever electrons are released from a cathode, they are in moevement, and that is why they have a kinetic energy. This kinetic energy must be spent to go to the anode in the one hand, but to overcome the voltage in the other hand. If the voltage is at the maximum, it stands to reason that the kinetic energy of electrons must be at the maximum too, otherwise they have no chances to overcome the voltage.

But experiments like above showed that whenever the voltage is at the maximum, the electrons are not able to overcome this voltage and so have no chances to come to the anode, although the kinetic energy would be at the maximum in cases like that.

So the maximum of kinetic energy exists only, if the voltage is at the maximum. Otherwise the kinetic energy of electrons - and the energy itself - is below the maximum. That is why the kinetic energy has not to be at the maximum.
I
think we're both talking about completely opposite things.

I'm referring to this equation:

7. Aha, perfect, thank you Langlitz.

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: May 13, 2015
Today on TSR

### Cambridge interviews

Find out which colleges are sending invitations

### University open days

• University of East Anglia
Fri, 23 Nov '18
• Norwich University of the Arts
Fri, 23 Nov '18
• Edge Hill University