Effects of Potential Difference on Photoelectric Current


All of you know about the sun and its importance. Well, it even helps in exploring some of the cool phenomena of physics and one of them is the photoelectric effect. I was researching the photoelectric effect and suddenly one thing strikes is what will be the effect of potential difference on photoelectric current? Then finally after some R&D, I got my answer.

As positive potential increases, current produced through photoelectric effect also increases but after reaching a certain point this current stops increasing even if we increase positive potential. This potential is known as saturated potential

Same as before if we increase a negative potential photoelectric current decreases and after reaching one point photoelectric current stops flowing. This potential is known as stopping potential.

What is Photoelectric current?

Before understanding the effect of potential difference on photoelectric current, we must understand how this photoelectric current is actually produced by the photoelectric effect experiment.

What is the photoelectric effect ?

photoelectric effect
Photoelectric effect

When the light of suitable frequency hits the metal surface, electrons from metal surface gets emitted and this process of electron emission from a metal surface due to light is called a photoelectric effect. The emitted electron is called photoelectrons.

Photoelectric effect experiment:

Setup for Photoelectric effect experiment
Setup for photoelectric effect experiment

In the above image, you can see the setup of the experiment. Light from source 'S' travels through quartz crystal to photosensitive plate 'C' and when light hits the plate 'C' surface, the electron gets emitted due to the photoelectric effect, due to potential difference created by battery electrons get attracted towards collector 'A' and this electron gets collected to collector 'A'.

P.S:- As you can see in the image above, electron emission and collection is carried in evacuated (vacuum) glass tube so that there is no involvement of the third party in the process.

Due to this flow of emitted electrons, current flows in the circuit. The amount of current is measured with the help of microammeter 'μA' placed in the circuit and potential difference is measured with voltmeter'V'.

Potential difference is kept variable in the circuit so that we can execute and take observation at different values of potential difference.

The polarity of plate C and A is maintained with the help of commutator. Plate A positive and negative potential with respect to emitter C can be maintained and changed accordingly with the help of commutator. Current flown in the above circuit due to photoelectric effect is called a photoelectric current.

So know you know about the photoelectric effect, photoelectric current and photoelectric effect experiment. So know it's time to find out what will be the effect of potential difference on photoelectric current in the above experiment.

Effect of potential difference on photoelectric current:

So now it's time to reveal the truth and understand what information and interesting facts we came to know from the taken observation.

1. When plate A is kept at accelerating positive potential with respect to emitter plate C, the photoelectric current is produced. The emitted electron gets attracted toward plate A due to positive potential on plate A with respect to plate C.

When accelerating positive potential on plate A is positively increased gradually, the amount of photoelectric current produced also gets increased. With the number of increase in positive potential at plate A, there was an increase in photoelectric current as well.

An interesting thing was that, after attending its maxed potential of photoelectric current, amount of photoelectric was now constant even if we increase the positive potential on plate A.

So after some time, this increase in photoelectric current is constant and after that, there is no increase in current even if we increase positive potential.

This maximum value of photoelectric current can be achieved in a photoelectric effect experiment is called Saturated Current

2. After applying negative retarding potential on plate A and increasing negative potential gradually with respect to plate C we see a downfall in the amount of photoelectric current produced.

This downfall is because when the polarity of plates is reversed and a negative potential is applied on plate A, electrons are repelled by plate A (negative potential on plate A and negative charge of electron repels each other) and due to these most energetic electrons are only able to reach collector plate A and current is produced.

When the negative potential on plate A is increased, now it's hard for even the most energetic electron to reach collector plate A and current stops flowing.

The point at which negative potential on the collector plate is so high that most energetic ion is also not able to reach plate A and currently does not flow in the circuit is called stopping potential.

Conclusion:

So when an accelerating potential is increased on collecting plate A with respect to an emitter plate C, photoelectric current increases, but after some time it becomes saturated current and amount of current do not increase even after increasing positive potential.

When the polarity is reversed on collector plate A and emitter plate C, plate A now has negative potential and when this potential is increased gradually in negative direction after sometime current stops flowing and this point of potential at which current stops is called stopping potential.

P.S:- Above observation is taken when intensity and frequency of light were kept fixed and the potential difference was changed. 

Interesting fact drove from observation when potential is changed for different intensity of light : 

Amount of saturated current depends upon the intensity of light. High intensity of light means the value of saturated current is high. But stopping potential was the same for all the intensity of light. 

So saturated current ∝ intensity of light but stopping potential does not depend upon intensity of light. Stopping potential is the same for all of them.

F.A.Q:

Q.1 What is Saturated Current ? 

This maximum value of photoelectric current can be achieved in a photoelectric effect experiment is called saturated current.

Q.2 What does observation graph show us ? 

Graph shows that stopping potential ∝ intensity of light but stopping potential does not depend upon intensity of light. It's the same for all of them.

Q.3 Why electrons get emitted due to light in photoelectric effect ? 

Electrons not always get emitted when light falls. It depends upon the frequency of light. If a suitable frequency of light is used which gives energy equivalent to work function or more then that only then an electron will leave the surface of the metal.

Q.4 What is work function ? 

The work function is the minimum energy required by an electron to leave the surface of the metal.

Q.5 Why we are using evacuated glass tube in the experiment ?

Evacuated glass tube is used so that no third party object or phenomena can interfere with our experiment.

Q.6 What is the use of commutator in photoelectric effect experiment ? 

Commutator is used to change the polarity of collector plate A and emitter plate C with respect to each other. 

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