Cbt4 Ps #42

[xlr8]

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#42 from the Physical Sciences from AAMC CBT 4

(Passage on photodiode)
An electron is ejected from the cathode by a photon with an energy slightly greater than the work function of the cathode. How will the final kinetic energy of the electron upon reaching the anode compare to its initial potential energy immediately after it has been ejected?

I figured that since it will accelerate through the plates its velocity is going to increase, therefore the KE by the time it reaches the other side will be larger, so I chose the only answer that said it increased (increase 2x).

Answer is "It will be approximately equal"

How is this possible? The work function is 2 eV (2 x 10^(-19) J), it will gain 50 eV as KE by the time it reaches the cathode, which will definitely increase the KE substantially as compared to the initial KE.

Thanks

 

[jheredia]

since it just barely has enough energy to eject the electron, the only potential energy it gets is from the potential difference between the plates

that potential difference is converted into kinetic energy by the time it reaches the cathode

not sure if that makes sense...

 

[shaqkillz451]

you have to remember the concept of conservation of energy for this one. As soon as the electron is ejected from the plate, it has potential energy only. This is the electrical potential energy given by EPE=qV (which u know is 50eV). Just as a ball in the air with potential energy drops towards the ground, the electron with EPE will travel towards the positive plate. How does the ball gain kinetic energy? acceleration due to gravity, right? As it accelerates, the ball looses potential energy and gains an equal amount of kinetic energy. Similarly, as the electron travels from the negative to teh positive plate, its EPE is reduced as its KE increases. When it is at top speed (final KE), its EPE is zero, since it'll be right next to the positive plate. However, the final KE can't excede the magnitude of the initial PE, and it'll be about 50eV also.

Hope that helps!

 

[xlr8]

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