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Intensity vs Photocurrent

A stream of p...

Question

A stream of photons having energy $5 e V$ each impinges on a potassium surface. The work function of potassium is $3.0 e V$ . The emerging photo - electrons are slowed down by a copper plate placed $5 m m$ away. If the potential difference between the two metal plates is $1 V$ , the maximum distance (in mm) the electrons can move away from the potassium surface before being turned back is

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Solution

Relation between maximum kinetic energy and work function is $K E_{m a x} = T E - ϕ = h v - ϕ = 5 - 3 = 2.0 e V .$
Using WE theorem: $F s = K E_{m a x}$
Here Electrostatic force $F = e E = \frac{e V}{d}$
$\Rightarrow s = \frac{K E}{F} = \frac{K E}{e (\frac{V}{d})} = \frac{(K E) d}{e V} = \frac{2 \times 5}{1} = 10 m m$

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