physics doubt

[Sandy]

I don't get how to do this question. Can someone explain?

[kw0573]

1) Ionized hydrogen atoms is a fancy way of saying "proton"
2) Kinetic energy comes from an equal loss of potential energy.
3) Potential, by definition, is the such energy lost per unit charge.
4) change in E(nergy, potential) = -1.9 * 10^-18 J (potential energy is lost), q = 1.602 * 10^-19 C, E/q change is = -12 V. 
5) Now where there may be ambiguity. If the charge started exactly half way between, then it only gained half of the possible energy and experienced only half the voltage (potential difference is proportional to distance in equation V = E(letric field) * d) . I would assume this is what the question means. 
6) Finally answer is 12 * 2 = 24 V is the FULL potential difference. Sign does not matter because potential difference is expressed as a positive number.

Edited April 28, 2016 by kw0573

[kw0573]

1 hour ago, bb8-m8 said:

Why exactly is this? Isn't i it just change in PE = q * change in V --> V is roughly 12V. Why is V doubled?

Question asks for potential difference measured plate to plate. The proton in question has only traveled half of this voltage difference. It's as if there are two sets of parallel plates connected with the negative plate of one right next to the positive plate of the other. and the proton has only traveled through one of such parallel plates. The potential difference between the two outermost plates is thus adding the two smaller potential differences together. Or you can think of constant force on a positive charge inside the actual plates; the work done on the charge is F*s; s is only half of the plate separation so the work done is half of the max work done. 

Edited April 28, 2016 by kw0573

[Sandy]

Thanks a lot w0573  The answer is actually 12V so I guess we don't have to assume that its half way between the plates.

[kw0573]

11 hours ago, Sandy said:

Thanks a lot w0573  The answer is actually 12V so I guess we don't have to assume that its half way between the plates.

cool, my bad! I wasn't too sure about the half way plates thing, but if it were instead from midpoint then you would follow what I did. But yeah if the question meant plate to plate acceleration then 12V is correct.