What is Charge?

Electric charge is a property which is possessed by certain objects. If you've ever rubbed a balloon on your head and stuck it to a wall, you've seen an object with electric charge (the balloon, in this case). Socks which stick together in the dryer are electrically charged, as is the piece of plastic wrap that sticks to your hand, the dust which collects on a TV screen, and your body when you shuffle your feet along a carpet on a cold, dry day. (The spark that you get when you touch something afterwards is electric charge leaving your body.)

Electric charge comes in two varieties, which are called positive and negative because they have the ability to cancel each other out: an object containing equal amounts of positive and negative charge behaves just like an object with no charge at all. On the macroscopic level, charge can be transferred from one object to another and can move through certain materials (called conductors) but not others (insulators).

Interactive 2.1.1

In most (but not all) cases, the flow of positive charge in one direction is identical in effect to the flow of negative charge in the opposite direction. For example, if I want to make a neutral sphere positive, I can add positive charge into it, or I can remove negative charge from it, and the two actions are nearly equivalent in their macroscopic effect.

We will see later that in most real-life situations, it is the negative charge that flows, while positive charge remains stationary. However, from a theoretical, calculational point of view, we can imagine positive charge moving, or both types moving at the same time, and the result will be exactly the same.

Like charges repel, opposite charges attract

Given any two electric charges in the universe, each charge exerts a force on the other one. If they are both positive or both negative, then that force is repulsive: the charges push each other away. If they are opposite charges, however, then the force is attractive: the charges pull each other closer together. (Hollywood stole the phrase "opposites attract" from physicists, you know.) Newton's Third Law does apply in such situations: both charges feel a force of the same magnitude, but in opposite directions. This force decreases as the charges get farther apart, but never goes to zero: a positive charge in your body is pushing on positive charges inside the Sun, in Alpha Centauri, in the galactic core, and so forth.

All macroscopic objects contain a large number of positive and negative charges. If an object contains equal numbers of both, then we say the object is neutral; if there is an imbalance, then the object is charged. All objects can become charged: whenever two objects come in contact, it's possible (and usually rather likely) that charge will flow from one to the other, depending on the types of materials.