I hate signs, a simple problem with electric potential

[flyingpig]

Homework Statement

(a) Calculate the speed of a proton that is accelerated from rest through an electric potential difference of 120V

(b) Calculate the speed of an electron that is accelerated from rest through an electric potential difference of 120V

The Attempt at a Solution

Basically it is

$$\Delta PE = q\Delta V = -\Delta KE$$

$$q\Delta V = -\frac{1}{2}mv^2$$

So what is the problem? It works fine with an electron sign the signs cancel out, but not the electron. Why?

 

[Andrew Mason]

Homework Statement

(a) Calculate the speed of a proton that is accelerated from rest through an electric potential difference of 120V

(b) Calculate the speed of an electron that is accelerated from rest through an electric potential difference of 120V

The Attempt at a Solution

Basically it is

$$\Delta PE = q\Delta V = -\Delta KE$$

$$q\Delta V = -\frac{1}{2}mv^2$$

So what is the problem? It works fine with an electron sign the signs cancel out, but not the electron. Why?

You have to reverse the field to accelerate the electron. So the $\Delta V$ has to change direction.

The convention is to assign to the electric field a direction which is the direction in which a positive charge will move (ie. the direction of the force on a positive charge). So the proton will accelerate from higher to lower potential (eg. from +V to 0). The electron, however, is accelerated from lower potential to higher potential (eg. from 0 to + V).

AM

 

[SammyS]

...

So what is the problem? It works fine with an electron sign the signs cancel out, but not the electron. Why?

So what is the problem? It works fine with an electron sign the signs cancel out, but not the electron. Why?

Read what you wrote!

Are you saying it works for an electron or are you saying it doesn't work for an electron?

 

[flyingpig]

*but not the proton

 

[SammyS]

Read what you wrote!

Are you saying it works for an electron or are you saying it doesn't work for an electron?

*but not the proton

Huh ??

To be fair, I know what you mean, but

... it appears that you didn't even take the time to recognize that my question required MORE than an implicit yes or no.​

I suppose you meant that:

"I (the flyingpig) can get this to work for an electron, but not for a proton."
​

At any rate, Andrew Mason gave you enough to resolve the problem.