explain the term stopping potential?

An electron in a metal is bound with an energy phi commonly called the work function. if light of a high frequency (ie hv>phi) is shined on the metal, electrons will be emitted. if the electrons are emitted into an electric field they will be accelerated towards the cathode.
Now imagine this. electron is emitted with energy hv = phi + KE
the electron is emitted in the direction of the ANODE
if the electron HITS the anode, a signal is registered
since the electron is emitted towards a negatively charged plate, it will slow down in the constant electric field
as the strength of the electric field is increased, it will slow down more and more until it never actually makes it to the anode
as soon as this occurs, ie if the strength of the electric field where the signal stops is known, the energy lost due to the field can be found, and the work function can be obtained. this field value (or potential) is the stopping potential. remember that a constant electric field is related to the potential by E = V/d where d is the distance between the plates, and V is the potential difference.

Thanks and Regards

Apoorva Arora

IIT Roorkee

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