# Electrostatic charges

Donald Fisher

## Homework Statement

1. Select the answers which complete the statements below (e.g., if answer A completes the first statement, and answer B the others, enter ABBBB).

A) positive B) negative C) neutral D) cannot tell
1. A positive point charge is brought near the outside surface of a neutral conducting sphere and released. The initial position of the point charge is on the positive x axis and the sphere is centred on the origin. The point charge moves in the ____ x direction. (If the point charge does not move, select answer C.)
2. A positively charged glass rod attracts an object suspended by a non-conducting thread. The charge on the object is ____ .
3. Two neutral metal spheres are mounted on insulating supports. They are connected by a wire. A positively charged glass rod is brought near sphere 1. The wire connecting the spheres is removed and the charged rod is then taken away. Now the charge on sphere 2 is ____ .
4. A positively charged ball is brought close to a neutral isolated conductor. The conductor is then grounded, while the ball is kept close. If the ground connection is first removed and then the ball is taken away, the conductor has a ____ charge.
5. A positively charged glass rod repels an object suspended by a non-conducting thread. The charge on the object is ____

## The Attempt at a Solution

I thought I understood the intuition behind this but I can't seem to get it right. A classmate also could not figure it out, I've already tried CBACA & CBABA.

Mentor
Hello Donald, Welcome to Physics Forums.

We cannot tell where you and your classmate's reasoning is going wrong without you presenting it. Take one item at a time and tell us what your thinking was to arrive at the answer you chose.

Donald Fisher
Ok,

1. Since the conducting sphere is neutral, and even though other sphere is positive, it shouldn't attract it nor repel it, so I chose C because I don't think it would move.
2. Since the positively charged rod attracts the object, and opposites attract, it must be negative, so B.
3. Since the two spheres are connected by a wire, that means that electrons can move freely between them, so when a positively charged rod is brought near sphere 1, the electrons from sphere 2 come to sphere 1. Then because the wire is removed before the rod is, the electrons stay in sphere 1, therefore making sphere 2 positive because protons do not move, so A.
4. When the neutral conductor is brought near a positive sphere, the electrons in the conductor would be pulled towards the ball, so they wouldn't escape through the ground, then the ground is removed before the ball is taken away, so the conductor would stay neutral, so C.
5. Since the positive rod is repelling the object, the object must be positive as well. So A.

That is where I got CBACA, this seems to be making sense to me, but I must be missing something, I know there is an option for if you cannot tell, and I don't completely understand that, so maybe some of them are D.

Mentor
Ok,

1. Since the conducting sphere is neutral, and even though other sphere is positive, it shouldn't attract it nor repel it, so I chose C because I don't think it would move.
This situation trips up many students. Neutral objects are attracted by BOTH positive and negative charges. Your text probably says something about this and perhaps refers to the experiment where a charged rod can pick up neutral bits of paper from a surface.
2. Since the positively charged rod attracts the object, and opposites attract, it must be negative, so B.
Are your thoughts the same after reviewing (1)?
3. Since the two spheres are connected by a wire, that means that electrons can move freely between them, so when a positively charged rod is brought near sphere 1, the electrons from sphere 2 come to sphere 1. Then because the wire is removed before the rod is, the electrons stay in sphere 1, therefore making sphere 2 positive because protons do not move, so A.
Well reasoned. Correct.
4. When the neutral conductor is brought near a positive sphere, the electrons in the conductor would be pulled towards the ball, so they wouldn't escape through the ground, then the ground is removed before the ball is taken away, so the conductor would stay neutral, so C.
Not quite. While the conductor is connected to ground charges are free to move from ground to the conductor...
5. Since the positive rod is repelling the object, the object must be positive as well. So A.
Right. Unlike attraction, repulsion can only occur if there are opposite charges involved.

Donald Fisher
Ok that makes much more sense! Thank you for your help gneill!