# Photoelectric effect : retarding potential with back current

• crick
In summary, the method of calculating $h$ using V0 as determined by Eq. 1 is valid only if V0 is a value that "brakes" all the electrons always in the same way, which is not always the case in practice.
crick
In the experiment of the determination of ##h## using the photoelectric effect produced by light emitted by led's there is the systematic problem of the "dark current" or "back current", i.e. the current caused by photoelectric effect on the anode of the system which is used in the expreriment.

This current is opposite to the "normal" photocurrent and its effect is to make the I-V curves not going but keep decreasing (slightly).

Suppose that one wants to determine $h$ using two different led's of frequencies ##\nu_1## and ##\nu_2## using the following method (I know about other methods but I would like to know about this one).

Let's call ##V_{0_1}## and ##V_{0_2}## the values of ##V## where the (total) photocurrent is ##0## (for led ##1## and led ##2##). And suppose that the relation used is $$h=\frac{e(V_{0_1}-V_{0_2})}{(\nu_2-\nu_1)} \tag{1}$$

(work function is not involved at all)

My question is: under what theoretical assymptions and explanations is ##(1)## suitable to calculate ##h## (i.e. when to determine ##V_0##, even if current does not go asymptitically to zero but becomes negative, is still a good method to determine $h$ using ##(1)##)?

My guess is that this is a "good" method only if ##V_0##Vis such that it "brakes" all the electrons always in the same way, i.e. it makes them reach the same kinetic energy, for any frequency ##\nu## that is being used.

But how is this confermed/denied theoretically?

crick said:
In the experiment of the determination of ##h## using the photoelectric effect produced by light emitted by led's there is the systematic problem of the "dark current" or "back current", i.e. the current caused by photoelectric effect on the anode of the system which is used in the expreriment.

This current is opposite to the "normal" photocurrent and its effect is to make the I-V curves not going but keep decreasing (slightly).
View attachment 200294

Suppose that one wants to determine $h$ using two different led's of frequencies ##\nu_1## and ##\nu_2## using the following method (I know about other methods but I would like to know about this one).

Let's call ##V_{0_1}## and ##V_{0_2}## the values of ##V## where the (total) photocurrent is ##0## (for led ##1## and led ##2##). And suppose that the relation used is $$h=\frac{e(V_{0_1}-V_{0_2})}{(\nu_2-\nu_1)} \tag{1}$$

(work function is not involved at all)

My question is: under what theoretical assymptions and explanations is ##(1)## suitable to calculate ##h## (i.e. when to determine ##V_0##, even if current does not go asymptitically to zero but becomes negative, is still a good method to determine $h$ using ##(1)##)?

My guess is that this is a "good" method only if ##V_0##Vis such that it "brakes" all the electrons always in the same way, i.e. it makes them reach the same kinetic energy, for any frequency ##\nu## that is being used.

But how is this confermed/denied theoretically?

Please note that for Eq. 1 to be valid, you must have a linear relationship between the stopping potential and frequency. That equation is nothing more than calculating the slope of V vs frequency.

Unfortunately, since you only have two data points, there is no way for you to verify if this relationship is valid. It is valid "on paper" per the Einstein's photoelectric effect equation, but it is not known if it is valid in your experiment due to the inadvertent photoemission off the anode. And as far as experimental methodology goes, it is a BAD experiment to only collect two data points.

I do not know to what extent the negative value of the stopping potential will affect the result. If you can show, with more data, that what you have can be represented by a linear fit, then the negative potential will not matter since all you care about is the slope of this line.

Zz.

## What is the photoelectric effect?

The photoelectric effect is the phenomenon where electrons are emitted from a material when light of a certain frequency is shone on it. This was first observed by Heinrich Hertz in 1887 and was later explained by Albert Einstein in 1905.

## What is the retarding potential in the photoelectric effect?

The retarding potential is the voltage applied to a metal surface to prevent electrons from being emitted due to the photoelectric effect. If the voltage is high enough, it can stop all electrons from being emitted, and the current will drop to zero.

## What is back current in the photoelectric effect?

Back current is the flow of electrons in the opposite direction of the current caused by the photoelectric effect. This can occur if the retarding potential is not high enough to completely stop all electrons from being emitted.

## How does the intensity of light affect the photoelectric effect?

The intensity of light does not affect the photoelectric effect. The number of electrons emitted depends on the frequency of the light, not the intensity. However, increasing the intensity of light will result in a higher current of emitted electrons due to the increased number of photons hitting the material.

## What is the work function in the photoelectric effect?

The work function is the minimum amount of energy required to remove an electron from the surface of a material. It varies for different materials and is a crucial factor in the photoelectric effect, as the energy of a photon must be greater than the work function for electrons to be emitted.

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