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A photoelectric current emerging in the ...

A photoelectric current emerging in the circuit of a vacuum photocell when its zinc electrode is subjected to electromagnetic radiation of wavelength `262 nm` is cancelled if an external decelerating voltage `1.5V` is applied. Find the magnitude and polarity of the outer contact potential difference of the given phtocell.

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The maximum `K.E.` of the photoelectrons emitted by the Zn cathode is
`E_(max) = (2picancelh c)/(lambda)-A_(zn)`
On calculating this comes out to be `0.993 eV = 1.0 eV`
Since an external decelerating voltage of `1.5V` is required to cancel this current, we infer that a contact potential difference of `1.5-1.0 = 0.5eV` exsists in the circuit whose polarity is oppsoite the decelearating voltage.
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