If an electron enters into a space between the plates of a parallel plate capacitor at an angle α with the plates and leaves at an angle β to the plates. The ratio of its kinetic energy while entering the capacitor to that while leaving will be
A
(cosαcosβ)2
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B
(cosβcosα)2
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C
(sinαsinβ)2
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D
(sinβsinα)2
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Solution
The correct option is B(cosβcosα)2 Let u be the velocity of electron while entering the field and v the velocity when it leaves the plates. Component of velocity parallel to plates will remain unchanged. Hence, ucosα=vcosβ or uv=cosβcosα The desired ratio is 12mu212mv2=(uv)2=(cosβcosα)2