Electric Charges

All matter is composed of atoms. Atoms have a nucleus composed of protons and neutrons. Protons have a positive electrical charge and neutrons are neutral. In orbitals surrounding the nucleus of an atom are found particles called electrons. Electrons have a negative electrical charge. Matter which has more protons than electrons is positively charged. Matter which has more electrons than protons is negatively charged. Matter which has an equal number of protons and electrons is electrically neutral.

Matter having opposite electrical charges will be attracted to each other. Matter having the same electrical charges will repel each other.

When you rub one object against another, only the electrons are free to move from one object to the other. Since an uneven distribution of electrons produces forces which attract or repel, electrons can produce energy. This energy is called electricity. Electricity is defined as the energy associated with electrons.

A build up of electrons on an object is called static electricity. There are three ways that objects can become charged with static electricity: friction, conduction and induction.

Friction involves rubbing electrons off of one object on to another. If you rub a balloon against your hair, electrons are removed from your hair on to the balloon. Your hair now has a positive electrical charge and the balloon now has a negative electrical charge. When you rub your shoes on carpet, electrons are removed from the carpet and collect on your shoes. Conduction is charging by direct contact with an object. In conduction the electrons flow through one object to another object. If after rubbing your shoes on a carpet you touch something, the electrons are conducted through your body to the object which you touched. Materials through which electrons can easily move are called conductors. Metals are usually good conductors. This is why metals such as gold, silver, cooper and aluminum are used as electrical conduction wires. Materials through which electrons cannot move are called insulators. Glass, wood, rubber and plastic are good insulators.

Induction is a method of charging an object with static electricity that involves the rearrangement of electric charges. However, unlike conduction, the objects do not touch each other. If one object is negatively charged and placed near another object, it will cause the electrons on that other object to be repelled. If an object which is positively charged is placed near another object, it will cause the electrons of that other object to be attracted to it. Remember, opposite charges attract and like charges repel. It is by this method that the electrical charges are rearranged. You will hear more about induction in our next unit on electric generation.

Lightning is static electricity which occurs because of induction. In a thunder cloud, the weight of moisture causes it to move downward. As it does so, it rubs the atmosphere removing electrons from the matter which composes the atmosphere. This causes the bottom of a cloud to become negatively charged and the top to become positively charged. Therefore, an electrical discharge can occur between the top and bottom of a cloud, or between the bottom of one cloud and the top of another cloud. Because the bottom of a thunder cloud is negatively charged, through induction, the electrons on the ground are rearranged. The electrons on earth are repelled by the negatively charged bottom of the thunder cloud. Therefore a discharge can occur between the earth and the bottom of the cloud. This discharge, called a lightning bolt, heats the air causing it to expand rapidly. It is this rapid expansion of air that produces the sound of thunder.

Benjamin Franklin experimented with electricity and hypothesized that a lightning bolt will tend to hit the highest object on earth. He also discovered that the positively charged earth and the negatively charged thunder cloud will connect better if you have a sharp pointed conductor placed in the earth. Using these hypotheses, Franklin invented the first lightning rod. Lightning rods are sharp pointed conductors placed above a building and grounded to the earth. The lightning hits the rod and not the building.

Most of the electrical uses that we are familiar with involve electric current. Electric current is electrons flowing along a conductor. Electric current is used to power electric appliances such as televisions, hair dryers, refrigerators and light bulbs. Electric current is measured according to how many electrons pass a given point in a second. The metric unit for measuring electric current is the ampere or amp.

The opposition to the flow of electrons is called resistance. The metric unit for resistance is called the ohm. The thicker the conductor, the less resistance. The longer the conductor, the greater the resistance. Resistance to the flow of electrons produces heat. This is why the tungsten filament of a light bulb is very thin and is curled up to make it longer. Being thin and long, the increased resistance results in the heat which makes the tungsten filament glow.

If electrons are to flow along a conductor, something must push them to make them flow. The measure of the energy available to push electrons along a conductor is called voltage.

Electric Charges Study Sheet