Could someone please verify this (1 Viewer)

[sasquatch]

W = ∆KE = KE_f - KE_i

So if a certain voltage V_o is applied and the motion of the photoelectron is completely opposed, KE_f = 0.

Meaning, W = - KE_i. Is it safe to say that in this case, W is negative as the photoelectron is doing work travelling against the electric field set up. Such that W = KE_max, where W = qV?

I ask this because of this question:


One electron ejected from a clean zinc plate by UV light has kinetic energy of 4x10¹⁹ J.

What would be the kinetic energy of the electron when it reacher the anode if a retarding voltage of 0.9V was applied between the anode and cathode?

So as W = ∆KE = KE_f - KE_i, and the work done by the photoelectrons is W = -qV = -1.44x10^-19 as the photoelectrons are doing work by opposing the field. which results in:

-1.44x10^-19 = KE_f - 4x10¹⁹
KE_f = (4 - 1.44)x10^-19
= 2.56 x10^-19 J

which is the correct answer.

If you were to take W as positive, then you would end up with a final kinetic energy greater than it's initial which is obviously incorrect.

So my overall question is, is the work done by the photoelectron in this case negative as work is being done "by the body" meaning its kinetic energy is lost by doing this work against moving against the electric field?

Thanks in advance.

 

[drewgcn]

Its easier to think of it as the electric field (qV) is working against the electron.

If you take the kinetic energy of the electron in its original path as positive, the work done is obviously in the opposite direction (retarding voltage, not accelerating). Therefore you'd call it negative as it is working against the electron.

But you could even define the kinetic energy of the electron moving as negative in the first place (though you wouldn't, just for the sake of this example) and it would still work. You would get a negative answer, which you would know means it is still travelling in the original direction. If you got a positive answer (in this case) it would mean it was travelling in the other direction.

 

[sasquatch]

What you said is pretty much what i said:

If you take the kinetic energy of the electron in its original path as positive, the work done is obviously in the opposite direction (retarding voltage, not accelerating). Therefore you'd call it negative as it is working against the electron.

So as W = ∆KE = KEf - KEi, and the work done by the photoelectrons is W = -qV = -1.44x10-19 as the photoelectrons are doing work by opposing the field.

Its easier to think of it as the electric field (qV) is working against the electron.

Also the electric field working against the electron is the same as saying that work is being done by the electrons by moving against the electric field, except the perspective is reversed. So does that mean everything that i said in my original post was correct?

 

[Riviet]

Pretty much yes, and if you want to clarify your answer to the marker, you can state the direction of work done by the photoelectrons, e.g 2.56 x10-19 J against the electric field.

 

[sasquatch]

Hehe cool. I like your sig and your little quote thing "Nucking Futs". That movie was cool... its got me saying "Shizit" now around people whom i cant/shouldnt normally swear infront of. Started out as a joke though.. now its a habbit..

 

[Riviet]

Haha I never saw the movie, but got it off a friend who probably watched the movie that you're referring to. Only physics people like us would find that quote by Einstein funny.