Note-A-Rific: Charge It!

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Charging by Friction

The only reason that we are able to use electricity in our modern world is that it is possible to separate positive and negative charges from each other.

• One way to do this is by rubbing two different materials together, known as charging by friction.
• Since the two objects are made of different materials, their atoms will hold onto their electrons with different strengths.
• As they pass over each other the electrons with weaker bonds to their nucleus will be “ripped” off of that material and collect on the other material.

Example 1: Rub a piece of ebonite (very hard, black rubber) across a piece of animal fur.

· The fur does not hold on to its electrons as strongly as the ebonite.

· At least some of the electrons will be ripped off of the fur and stay on the ebonite.

· Now the fur has a slightly positive charge (it lost some electrons) and the ebonite is slightly negative (it gained some electrons).

· The net charge is still zero between the two… remember the conservation of charge.

Example 2: Rub a glass rod with a piece of silk.

· Same sort of situation as the one above.

· In this case the silk holds onto the electrons more strongly than the glass.

· Electrons are ripped off of the glass and left on the silk.

· The glass is now positive and the silk is negative.

There is a list in the text on page 582 called an “Electrostatic Series” that lets you find out which material will be positive and which one will be negative.

• Find the two materials you are rubbing on the list.
• Whichever is higher on the list will be negative, whichever is lower will be positive.

Charging by Conduction

It is possible to transfer the charge that you have on one object onto another object by a couple of methods, one of which is conduction.

• Conduction just means that the two objects will come into actual physical contact with each other (this is why it is sometimes called “charging by contact”).
• A negatively charged object is brought near a metal object (usually on an insulated stand) and touches it
• Some of the negative charge will transfer over to the metal object.
• These charges will quickly spread evenly across the metal object since they are repulsive to each other.
• When the negative object is removed, it will not be as negative as it was.
• The metal object now has some of the negative charge… how much depends on the size of the objects and the materials they are made of.
• Overall the total negative charge remains constant.
• Notice that a negative object causes a negative charge in another object.

What would happen if you used a positive rod to touch the metal sphere?

• Follow the same model as above and see if you can explain what you think would happen.

Charging by Induction

It is possible to charge a conductor without coming into direct contact with it.

• You do have to follow a couple special procedures, though, the most important one being the use of a grounding wire.
• A grounding wire is simply a conductor that connects the metal sphere to the ground.
• Think of the earth as a huge reservoir of charge… it can both gain or donate electrons as needed.
• Depending on what the situation is, either electrons will travel up the wire to the metal conductor, or travel down to the ground.

Charging by induction is a more complex process than conduction, as the example below shows…

1. The metal sphere is on an insulating stand. It also has a ground wire attached to it.
2. We bring a negative object nearby. This will cause a separation of charge in the sphere as shown. If you think about it, those electrons piled up at the edge want to get as far away as possible, and since they are free to move they do just that. They will travel down the ground wire.
3. This step is VERY important. Keeping the negative object nearby we snip the ground wire. Now there is no way for the electrons to travel back up they wire to the sphere. If we had skipped this step and just moved the negative object away without snipping the ground wire, the negative charges would have just gone back up the wire onto the sphere and it would be neutral again.
4. We remove the negative object… now the sphere has a net positive charge.

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The same sort of thing happens if you bring a positively charged object near to a grounded sphere.

• In that case, electrons would come up the grounding wire to be closer to the object.

Notice that when you charge by induction you get the opposite charge on the metal sphere.

How can you tell if an object has a charge, especially if you’re doing research in the 1700’s?

• Very early on physicists started using electroscopes to measure very small charges on objects.
• An electroscope is made up of a couple of very thin metal leaves that hang down near to each other.
• They are connected to a metal rod that extends upwards.
• The whole apparatus is usually insulated from outside effects by being in a metal container with a glass window to look in at the leaves.
• A rubber stopper insulates the rod from touching the metal container.
• Imagine what happens to the metal leaves if a charged object is brought nearby…

1. In this situation a negative object is brought nearby the electroscope. This causes free moving electrons in the electroscope to move down into the leaves. Since the leaves both have negative charge they repel each other and move apart.

2. Bring a positive object nearby and the free electrons in the electroscope all start moving up towards the top. This means the bottom has a net positive charge. The leaves will spread apart again.

3. Touch the electroscope with any charged object and you’ll give it an overall charge by conduction. The leaves will stay spread apart even if you remove the object.