5) Electron magnetic radiation having 2 = 310Aºis subjected to a
metal sheet having work function=12.8ev what will be velocity of
photo electrons having maximum kinetic energy.
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a) 0, no emission will occur
c) 3.09 x 106 m/s
b)4.352 x 106 m/s
d) 8.72 x 10 m/s
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ANSWER
Energy of photon of wavelength, λ=310×10
−10
is
λ
hc
.
Therefore ,E(photon)=
(31×10
−9
)
(6.62×10
−34
)(3×10
8
)
=6.406×10
−18
J.
Work function, W=12.8eV=12.6×1.6x10
−19
J=2.016×10
−18
J.
Now, maximum kinetic energy of photo electron,
(0.5)mv
(max)
2
=E(photon)−W=6.406×10
−18
−2.016×10
−18
J=4.39×10
−18
J.
Therefore,v
(max)
2
=
(9.1×10
−31
)
(4.39×10
−18
)(2)
=9.648×10
12
m
2
s
−2
.
Therefore, v
(max)
=3.106×10
6
ms
−1
Hence,option C is correct answer
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