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## Physics - Electricity and Magnetism

Suppose a rod is made of copper and that it has more electrons than protons. That means that the copper rod is negatively charged. Copper is a transition metal, so its valence electrons are very mobile. So, if this rod touches something, it will make it negatively charged by passing its electrons to it.

That is called contact charge.

When a charged object touches another object and gives its charge to that other object, the other object is said to undergo contact charge. Any and every substance, element, compound or mixture of compounds will readily produce and undergo contact charge if it has a large number of relatively mobile electrons. On the other hand, if a substance does not have a large number of relatively mobile electrons, it will not readily produce or undergo contact charge.

Conductor

Remember the copper rod, it is called a good conductor that is because one, it readily gives up electrons to an object which relative to itself is positive and by the same token; two, it readily receives electrons from an object that relative to itself is negative.

Insulator

The opposite of conductor is insulator. To say that something is a poor conductor is the same as saying it is a good insulator. Contact charge is one way to transfer charge. Here are two other ways; one, rubbing; two, induction. Remember that rubbing is different from contact charge.

Now, that you know about transferring charge, you should know about the relationship between force and charge. This relationship is given by Coulomb’s Law. Coulomb’s Law allows us to figure out the force that two charged particles produce when they are separated by a certain distance.

Coulomb’s Law also tells us that the force of attraction or repulsion between two separated charges is proportional to Q1xQ2/R2. When there is a force between two particles, there is an area all around the particles which would exert a force on any charged particle that happened to be there. This area is called an electrostatic field.

The electrostatic field is stronger in some places that it is in others. The electrostatic field strength for any point in an electrostatic field is equal to the force that a charged of +1 would experience if it was placed there. Electrostatic field strength is measured in Newton per Coulomb.

Rule Number 27:
Force experienced by a charge in an electric field equals field strength times charge or F = EQ.