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# In the experiment to confirm Planck's constant involving finding the stopping potenti watch

1. ...potenital and there are two sources of emf and when they cancle the current is zero. Why exactly is the stopping potenital equal to the (1/2mv^2)max ??????
2. (Original post by applestrudle)
...potenital and there are two sources of emf and when they cancle the current is zero. Why exactly is the stopping potenital equal to the (1/2mv^2)max ??????
http://www.thestudentroom.co.uk/show....php?t=1598648 Post #2

If it doesn't help, let me know.
3. (Original post by applestrudle)
...potenital and there are two sources of emf and when they cancle the current is zero. Why exactly is the stopping potenital equal to the (1/2mv^2)max ??????
The photoelectric effect:
Once the quickest of all the electrons slows down at a given stopping voltage between the circuit,it loses all the kinetic energy through which we get max k.e : 1/2mv^2.
The polarity is reversed in this case to appreciate the attraction of an electron to the cesium metal surface(technically) =)
=)
4. (Original post by jaroc)
http://www.thestudentroom.co.uk/show....php?t=1598648 Post #2

If it doesn't help, let me know.
Didn't help, sorry. Your gonna have to really dumb it down for me...
5. (Original post by applestrudle)
...potenital and there are two sources of emf and when they cancle the current is zero. Why exactly is the stopping potenital equal to the (1/2mv^2)max ??????
It should be Vs(Stopping potential) x e = K.E.(max) The work done on the electron by the (reversed) electric field causes the electron to lose all of its kinetic energy. (Kinda like pulling the electron back to the metal surface)
6. (Original post by applestrudle)
Didn't help, sorry. Your gonna have to really dumb it down for me...
(Original post by applestrudle)
...potenital and there are two sources of emf and when they cancle the current is zero. Why exactly is the stopping potenital equal to the (1/2mv^2)max ??????
First of all, what is current? It's a flow of charge (in this case, electrons). So if we want it to be 0, it means that we want no electrons to move. When they not move, they have zero kinetic energy.

Now let's say that each electron at first had some kinetic energy . If we want to ensure that no electrons pass, we have to slow down to 0 all of them, even the fastest ones. This can be done by putting them into an electric field of potential difference . The work done on each electron by this electric field (potential difference) is . Therefore if we want to bring to a halt all electrons, we have to let them through potential difference given by .

Is it clearer now?
7. (Original post by jaroc)
First of all, what is current? It's a flow of charge (in this case, electrons). So if we want it to be 0, it means that we want no electrons to move. When they not move, they have zero kinetic energy.

Now let's say that each electron at first had some kinetic energy . If we want to ensure that no electrons pass, we have to slow down to 0 all of them, even the fastest ones. This can be done by putting them into an electric field of potential difference . The work done on each electron by this electric field (potential difference) is . Therefore if we want to bring to a halt all electrons, we have to let them through potential difference given by .

Is it clearer now?
yeah much clearer now, thank you

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