# Metal sphere

1. Jul 4, 2008

### chukie

A metal sphere of radius R = 10 cm carries a total charge Q = 0.4 μC.

A. What is the magnitude of the electric field just outside the sphere, and in which direction is it pointing?

B. What is the magnitude and direction of the electric field just inside the sphere?

I have already found the magnitude of the electric field for each, but I am not sure about the direction.

2. Jul 4, 2008

### Hootenanny

Staff Emeritus

For case B, the fact that you are attempting to determine the direction of the electric field tells me you have calculated it's magnitude incorrectly.

3. Jul 4, 2008

### chukie

I used the equation E=q/(4pi*e*r^2)

4. Jul 4, 2008

### Hootenanny

Staff Emeritus
That is the electric field for a point charge q, so in what direction does the electric field of a point charge point?

5. Jul 4, 2008

### chukie

Outwards, since the charge is positive?

6. Jul 4, 2008

### chukie

Btw do you mean that the equation I used is wrong?

7. Jul 4, 2008

### Hootenanny

Staff Emeritus
Nope your equation is quite correct, I was simply giving you a hint.
Radially outwards from the centre of the sphere, correct .

8. Jul 4, 2008

### chukie

Okay thanks! For B then I know that the field is 0. Does this mean there is no direction?

9. Jul 4, 2008

### Hootenanny

Staff Emeritus
Correct again.

10. Jul 4, 2008

### chukie

11. Jul 4, 2008

### Hootenanny

Staff Emeritus
A pleasure

12. Jul 4, 2008

### K3nt70

im doing a question similar to this, except i need to find the charge on the ball. What does the "e" stand for in your equation?

13. Jul 4, 2008

### chukie

that's the electric constant.

14. Jul 4, 2008

### K3nt70

ohh ok. so, the question is:

The field just outside a 3.61 cm radius metal ball is 2.86×102N/C and points toward the ball. What charge resides on the ball?

my equation should look like:

286(4pi)(0.0361^2)(8.85E-12) = q

right?

15. Jul 4, 2008

### chukie

yah that should be right. check the sign of ur charge though.

16. Jul 4, 2008

### K3nt70

yup. the charge was neg since the feild pointed toward the center of the ball. Thanks!

17. Jul 4, 2008

No prob =)