# Calculate charge

1. Aug 23, 2009

### austin007

The e-field just outside the surface of a hollow conducting sphere of radius 20 cm is 500 N/C and is directed outward. Q is located at the center of the sphere. It is noted that the e-fields is still directed out ward with a magnitude of 100 N/C. What is the magnitude and sign of the unknown charge Q?

I can only find total charge which is 2.225nC. I can't get Q coz radius of the solide sphere is unknown. Pls help

2. Aug 23, 2009

### Staff: Mentor

Can you please state the problem completely, exactly as it was given. I don't understand what the field 100 N/C refers to.

3. Aug 23, 2009

### austin007

The electric field just outside the surface of a hollow conducting sphere of radius 20 cm is 500 N/C and is directed outward. An unknown charge Q is then located at the center of the sphere. It is noted that the electric field is still directed outward but its magnitude has decreased to 100 N/C. What is the magnitude and sign of the unknown charge Q?

This is the complete question as it is stated.

4. Aug 23, 2009

### Staff: Mentor

OK. That should tell you the charge on the conducting shell.
Ah... that additional word "then" makes all the difference!
Now that you've added charge Q, how does the net charge within the gaussian surface (taken just outside the sphere) change?

Apply Gauss's law twice.

Last edited: Aug 23, 2009
5. Aug 23, 2009

### austin007

thanks I understood finally....