Electricity and Magnetism

Static electric charge

There are two types of electric charge. They can both be created by rubbing two materials together to create static electricity.

Ben Franklin named the two types of charge plus, +, and minus, -.

These names help us remember that when you add an equal amount of positive charge to negative charge the result is a sum which is zero. This is the same result as for adding positive and negative integers.

Plus and minus are just names. Ben assigned them arbitrarily. The charge on glass when it was rubbed with silk he chose to be positive. The charge on amber, fossil tree sap, when rubbed with wool he chose to be negative.

Positive charges attract negative charges.

Positive charges repel positive charges.

Negative charges repel negative charges.

Summarized in a table with R = repel and A = attract

 + - + R A - A R

This is what the text wants you to know.

The reality however is more complicated.

We can add uncharged objects, 0, to our table.

 + - 0 + R A A - A R A 0 A A 0

So we see that charged objects attract uncharged objects.

There is only one combnation that has neither attraction or repulsion, if you see neither you know that both objects are uncharged.

If you see repulsion then you know both objects have the same charge.

If you see attraction it could be that the objects have opposite charges, but it could be that one is charged and one is uncharged.

Charge separation by chemistry

Charge separation can also happen due to chemical reactions within a battery.

One end of the battery has positive charges the other end has negative charges.

For example think of what happens when a neutral sodium atom touchs a neutral chlorine atom. An electron is transferred from the sodium to the chlorine creating a sodium positive ion and a chlorine negative ion. Similar charge transfer happens at electrodes inside batteries.

Charge separation by paricle physics

Energy can be converted into matter, however when matter is created the total amount of electric charge in the universe remains the same. Equal quantities of positive and negative charge are always created at the same time.

 Scientific Explorations with Paul Doherty © 2005 10 October 2005