# Electric Field and Magnetic Field

1. Apr 23, 2010

### nightseeress

I'm studying for an exam on Saturday and some of the old practice exams don't have explanations for the solutions so I came on here for help. Thanks in advance.

1. A conducting sphere of radium 2m carries a charge of 3uC. A point charge of -2uC is 3m away from the sphere center. Find the magnitude of electric field at point P halfway between the sphere center and the point charge.

The correct answer is 0. However, I'm not too sure I understand why it's 0. Is it because the point P is inside the conducting sphere?

2. A square loop is placed in the plane of the page next to a wire with current I going into the page. If the current I increases with time, what is the direction of the force on the loop?

The correct answer is "the force is zero". However, I thought it would be up because of the right hand rule for the magnetic field of wires carrying a current.

2. Apr 23, 2010

### Cilabitaon

If you draw yourself a circle and arbitrarily mark the charge on the spheres surface (+ve since the charge is 3uC).

Now you know the concept of an electric field line: it must follow the path a +ve charge would; with this you can see that there is no way to move a field line from the outside of a complete circle and find the end of that line inside the circle without breaking the shapes confines.

Now, if you can draw the field lines, what does that mean?