Calculate Charge: Find Q at Center of Hollow Sphere of Radius 20 cm

[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

 

[Doc Al]

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

 

[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.

 

[Doc Al]

The electric field just outside the surface of a hollow conducting sphere of radius 20 cm is 500 N/C and is directed outward.

OK. That should tell you the charge on the conducting shell.

An unknown charge Q is then located at the center of the sphere.

Ah... that additional word "then" makes all the difference!

It is noted that the electric field is still directed outward with a magnitude of 100 N/C.

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.

 

[austin007]

thanks I understood finally...