Let's charge a conductor (we'll use an electroscope) by touching it with a charged insulating rod at one spot. Is there a more effective way to charge the electroscope?

1. Rubbing the rod over the electroscope is better
2. Touching and rubbing are equivalent
3. It depends on whether the insulating rod has + charge or - charge

If the charged object is an insulator, touching it to the conductor transfers charge only at the few points of contact. Rubbing the insulator over the conductor is much more effective at transferring charge. It makes no difference whether the rod has a positive charge or a negative charge.

The conductor acquires charge of the same sign as the charged object brought into contact with it.

Let's say we've given an electroscope a positive charge. We then bring a positively charged rod close to, but not touching, the electroscope. What does the indicator arm on the electroscope do?

1. Nothing
2. The arm deflects even more
3. The arm deflects less

The arm deflects more. Bringing a positively charged rod close to the top of the electroscope means electrons are attracted from the arms of the electroscope to the top. With even more positive charge on the arms than before the repulsive force is stronger.

Bringing a negatively charged rod close to the positively charged electroscope makes the arm deflect less. This means you can use a charged electroscope to distinguish between positively charge objects and negatively charged objects.

When charging a conductor by induction, a charged object is brought close to but does not touch the conductor. In the end the conductor has charge of the opposite sign as the charge on the object.

One way to carry out the four-step process:

1. Bring the charged object close to, but not touching, the conductor. Charge on the conductor shifts in response to the nearby charged object.

2. Connect the conductor to ground. Ground is basically a charge reservoir - anything that can give up or receive charge without noticing the change. Electrons flow from ground to the conductor if the charged object is positive, and the opposite way if the object is negative. The conductor now has a net charge with a sign opposite to the sign on the charged object.

3. Remove the ground connection. The transferred electrons can't get back to where they came from.

4. Remove the charged object. The net charge distributes itself over the surface of the conductor.