# Electric Magnetism

1. Mar 16, 2014

### brains

I need some help figuring #2 problem out.

The problem statement, all variables and given/known data

1. two charges of 6 microcoulombs and 10 microcoulombs are 10cm apart. Calculate their force of repulsion. Calculate the electric field midway between them.

2. For the charges in question 1 calculate the potential midway between them. Calculate the work to bring an electron from far away to midway between them.

So for the first question I used F= k |q1||q2|/r^2 for the second part of the first question i did E = k |q|/r^2

i calculated that the force would be 53.94N
As for the electric midway I calculate it to 3.6 x 10^6 N/c Although im not sure if i was supposed to add or subtract E1 and E2. So what i did is that i subtracted. But im not entirely sure if that is my correct answer

My work:
E1 = 8.99x 10^9 N*m^2/c^2 x |6 x10^-6 C|/(0.1m)^2

E1 = 5.39 x 10^6 N/c is my answer for E1

E2 = 8.99 x 10^9 N*m^2/c^2 x |10 x 10^-6C|/(0.1m)^2

E2 = 8.99 x 10 ^6 N/C

Then i did E = 8.99 x 10^6 N/C - 5.39 X 10^6 N/C
And I got 3.6 x 10^6 N/C however like i said before im not exactly sure if this was the correct approach to get the electric field midway

For the second question
I used this equation (U= k (q0)(q)/r ---> k being 8.99 x10^9n.m^2/c^2 for part of the 1st question in question 2 that asks for potential midway and got an answer of U= 10.78 N M

however when they asked me for the work in question two i assumed I should have used -W=q0Ed
but i don't know what numbers I would use for this equation. Would the correct way of going about it be
doing -W=(1.60 x 10^-19C)(8.99 x 10^6 N/C - 5.39 X 10^6 N/C)(0.1m) or is this wrong?

IF someone could please point me in the right direction and check if I've done these steps correctly it would be helpful. Thank you in advance.

Last edited: Mar 16, 2014
2. Mar 16, 2014

### majormaaz

Hey there!

Yep. That's right.

You're almost correct here - I'd say to take a look at your value for r again. As in, what are you measuring r to be again?

As for both parts of the second question:

The formula for U is for electrical potential energy, not the potential. But - there's a relationship between the two quantities, as well as one between W and U. Once you find those out, you're good to go.

3. Mar 16, 2014

### brains

You're almost correct here - I'd say to take a look at your value for r again. As in, what are you measuring r to be again?

Well I take the idea it's 10cm i converted that into 0.1m. Um Im guessing since they are asking me to calculate the electron from far away to midway. I would only take into account half of the original distance right?

4. Mar 16, 2014

### brains

Well I take the idea it's 10cm i converted that into 0.1m. Um Im guessing since they are asking me to calculate the electron from far away to midway. I would only take into account half of the original distance right?

5. Mar 16, 2014

### majormaaz

In regards to the Electric field problem, the question asks for the electric field at the midway point between the two charges. A radius of 10 cm here would imply that the point in question is at either charge.

With respect to the Potential Energy, yes. To be more technical, it's ΔU = Uf - U0, and if you use an r = ∞ for Uf, that expression becomes just -U0.

6. Mar 17, 2014

### brains

Sorry for the late reply and thank you very much i finally understood it :D.