(EAM) Static Electricity Lesson

Static Electricity

All objects surrounding us (including people!) contain large amounts of electric charge. There are two types of electric charge: positive charge and negative charge. If the same amounts of negative and positive charge are found in an object, there is no net charge and the object is electrically neutral. If there is more of one type of charge than the other on the object then the object is said to be electrically charged.

Positive charge is carried by the protons in a material, and negative charge is carried by electrons. The overall charge of an object is usually due to changes in the number of electrons. To make an object:

  • Positively charged: electrons are removed making the object electron deficient.
  • Negatively charged: electrons are added giving the object an excess of electrons.

Objects may become charged in many ways, including by contact with or being rubbed by other objects. This means that they can gain or lose negative charge. For example, charging happens when you rub your feet against the carpet. When you then touch something metallic or another person, you feel a shock as the excess charge that you have collected is discharged.

Charge, like energy, cannot be created or destroyed. We say that charge is conserved.

When you rub your feet against the carpet, a negative charge is transferred to you from the carpet. The carpet will then become positively charged by the same amount.   Another example is to take two neutral objects such as a plastic ruler and a cotton cloth (handkerchief). To begin, the two objects are neutral (i.e. have the same amounts of positive and negative charge).

Note: We represent the positive charge with a + and the negative charge with a -. This is just to illustrate the balance and changes that occur, not the actual location of the positive and negative charges. The charges are spread throughout the material and the real change happens by increasing or decreasing electrons on the surface of the materials.

ruler with pluses and minuses: See description

Before Rubbing description Links to an external site.

Now, if the cotton cloth is used to rub the ruler, a negative charge is transferred from the cloth to the ruler. The ruler is now negatively charged (i.e. has an excess of electrons) and the cloth is positively charged (i.e. is electron deficient). If you count up all the positive and negative charges at the beginning and the end, there is still the same amount, i.e. total charge has been conserved!

the ruler has 9 positive charges and 12 negative charges. See description

After rubbing description Links to an external site.

Force Between Charges

The force exerted by non-moving (static) charges on each other is called the electrostatic force. The electrostatic force between:

  • like charges is repulsive
  • opposite (unlike) charges is attractive.

In other words, like charges repel each other while opposite charges attract each other.

Attractive force: negative & positive attract Repulsive force: 2 negative & 2 positive charges oppose

Some materials allow electrons to move relatively freely through them (e.g. most metals, the human body). These materials are called conductors. Other materials do not allow the charge carriers, the electrons, to move through them (e.g. plastic, glass). The electrons are bound to the atoms in the material. These materials are called non-conductors or insulators.

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