Conservation Laws

Electric charge is the property that makes you experience a spark when you touch a metal door knob after shuffling your feet across a rug. It is also the property that produces lightning. Electric charge comes in two varieties, positive and negative. Like charges repel, that is, they tend to push one another apart, and unlike charges attract, that is, they tend to pull one another together. Therefore, two negative charges repel one another and, likewise, two positive charges repel one another. On the other hand, a positive charge will attract a negative charge. The net electric charge on an object is found by adding all the negative charge to all the positive charge residing on the object. Therefore, the net electric charge on an object with an equal amount of positive and negative charge is exactly zero. The more net electric charge an object has, the greater will be the force of attraction or repulsion for another object containing a net electric charge.

Electric charge is a property of the particles that make up an atom. The electrons that surround the nucleus of the atom have a negative electric charge. The protons which partly make up the nucleus have a positive electric charge. The neutrons which also make up the nucleus have no electric charge. The negative charge of the electron is exactly equal and opposite to the positive charge of the proton. For example, two electrons separated by a certain distance will repel one another with the same force as two protons separated by the same distance and, likewise, a proton and electron separated by this same distance will attract one another with the same force.

The amount of electric charge is only available in discrete units. These discrete units are exactly equal to the amount of electric charge that is found on the electron or the proton. It is impossible to find a naturally occurring amount of electric charge that is smaller than what is found on the proton or the electron. All objects contain an amount of electric charge which is made up of a combination of these discrete units. An analogy can be made to the winnings and losses in a penny ante game of poker. If you are ahead, you have a greater amount of winnings (positive charges) than losses (negative charges), and if you are in the hole you have a greater amount of losses than winnings. Note that the amount that you are ahead or in the hole can only be an exact amount of pennies or cents, as in 49 cents up or 78 cents down. You cannot be ahead by 32 and 1/4 cents. This is the analogy to electric charge. You can only be positive or negative by a discrete amount of charge.

If one were to add all the positive and negative electric units of charge in the universe together, one would arrive at a number that never changes. This would be analogous to remaining always with the same amount of money in poker. If you go down by five cents in a given hand, you have to simultaneously go up by five cents in the same hand. This is the statement of the law of conservation of electric charge. If a positive charge turns up in one place, a negative charge must turn up in the same place so that the net electric charge of the universe never changes. There are many other subatomic particles besides protons and electrons which have discrete units of electric charge. Even in interactions involving these particles, the law of conservation of electric charge is always obeyed.