Electric Potential/Electric Potential Energy question

[benji]

We just started this stuff in my AP Physics class and I'm not catching on to it too quickly... At least problem solving-wise. I thought I had all the concepts down, but maybe not.

Here's a question I'm having problems with:

Just as you touch a metal door knob, a spark of electricity (electrons) jumps from you hand to the knob. The electric potential of the knob is 2.0 X 10^4 V greater than that of your hand. The work done by the electric force on the electrons is 1.5 X 10^-7 J. How many electrons jump from your hand to the knob?

How do I go about this one? I'm sure it's a pretty simple problem because it's at the beginning of the chapter, but I guess I'm missing a connection or something.

 

[StatusX]

well, voltage is potential energy per charge, and the charge on a single electron is -1.6*10^-19 couloumbs.

 

[benji]

I see, so what I did was:

1.5 X 10^-7 / -1.6 X 10^-19 = -9.375 X 10^11 electrons
-9.375 X 10^11 / 2 = 4.68 X 10^11 elections

So 4.68 X 10^11 electrons was very close to the correct answer. Did I do this right?

 

[StatusX]

Your work is a little sketchy. First of all, Joules/charge does not have units of "electrons", and I don't know what a negative number of electrons means. I get the idea of what you're doing, but you should be a lot more careful with units. Then, you seem to have forgotten the power of ten on the voltage.