The electromagnetic force is one of four fundamental forces. It is responsible for frictional forces, elastic forces and contact forces to name a few. Electricity and magnetism were viewed as distinct forces until it was discovered by Oersted that they were connected. While gravity is important over very large distances and nuclear forces apply to the very small distances on the order of an atom's nucleus, electromagnetic forces are important over intermediate distances such as those occurring between molecules.

In order for us to understand electricity, it is necessary for us to start with electrostatics, the study of stationary electricity. People have been aware of electrostatic forces or static electricity for millennia. It was recognized by people in ancient Greece who coined the term elektron, which means amber. This term came about because it was noticed that when amber was rubbed with fur, small bits of paper and dust were attracted to the amber.

The Electrical Nature of Matter

Matter consists of positively charged protons and negatively charged electrons as well as neutrons, which have no charge.

Because matter cannot be created or destroyed, neither can electrical charge.

When objects acquire charge, for example an amber rod rubbed with fur:

It is the electrons that are transferred.

If the amber acquires 5 electrons, it has a charge of -5, while the fur, which loses the 5 electrons, now has a charge of +5. All charged objects eventually lose charge because of humidity in the atmosphere. A negatively charged object will which give up electrons to the water vapor eventually neutralizing the object., A positively charged object will eventually be neutralized by accepting electrons from water vapor . Unlike gravity, which is only attractive, electrostatic forces can be attractive or repulsive.

Like charges repel and opposite charges attract.

Like gravity, electrostatic forces act over a distance through a field.

Materials vary in their ability to allow electricity move through them.

Charging Objects

There are two ways to put a charge on an object.

If the charged object is negative, electrons can flow into the neutral object resulting the object acquiring negative charge. Although the originally negatively charged object loses some of its negative charge, it still is negative. The originally charged object and the newly charged object will repel one another because they have the same charge.

 f the charged object is positive, it is the neutral object that loses electrons to the positively charged object. Since the neutral object loses negative charge, it acquires a positive charge. The originally charged object ends up with slightly less positive charge than what it originally had. The originally charged object and the newly charged object will repel one another because they have the same charge.

Charging by induction is different than charging by contact because no contact is made between the charged object and the neutral object.

To charge an object by induction we must first induce a charge separation with a charged object. If we consider a negatively charged object brought in close proximity to a neutral conductor, the negatively charged object will repel the electrons on the neutral object. This results in a charge separation or polarization with the electrons pushed as far away from the negative charge as possible. This leaves an excess of positive charge next to the negatively charged object. This explains the attraction that occurs between the charged object and the neutral object.

At this point, the object is still neutral. In order to charge the object, we need to provide a passage out for the negative charges. We accomplish this by grounding the object. Since the electrons repel one another, they will freely flow into the earth where they can spread out. As the electrons flow out of the neutral object, the object becomes positively charged. It will now attract (and be attracted to) the negative charge that induced the original charge separation.

If we consider a positively charged object brought in close proximity to a neutral conductor, the electrons on the neutral object will be attracted to the positively charged object. This results in a charge separation or polarization with the electrons pulled as close to the positive charge as possible. This leaves an excess of negative charge next to the positively charged object and an excess of positive charge away from the positively charged object. This explains the attraction that occurs between the charged object and the neutral object.

Electric Discharge

Objects that become charged can be thought of as somewhat unstable. A negatively charged object has an excess of electrons that repel one another. One can imagine that given the opportunity, the electrons would spread out away from each other. A similar problem exists for positively charged objects. However the positive charges will remain stationary and electrons will be attracted to the object to alleviate the excess positive charge.

Similarly, an object with a high concentration of positive charge will induce a charge separation in the neutral object causing its electrons to pile up on the side facing the positive object. As in the previous example the electrons will be repelled by each other and attracted to the positive object. The electrons will arc from the neutral object to the positive charge.

Lightning is just arcing on a large scale where a cloud induces a charge separation on the ground below it and electrons flow from one to the other. Lightning is a fairly complicated process and is not treated with much detail here.

Measurement of Electrical Charge

Robert Millikan devised his oil drop experiment that determined the charge on an electron, which is termed the elementary charge. The smallest charge that Millikan was able to determine was - 1.6 x 10^-19 Coulombs. He determined this by measuring numerous quantities of charge and figuring out that they all were multiples of this number. The charge on a proton is the same number but positive.

A Coulomb of charge is defined as the charge on 6.24 x 10²⁴ electrons (or protons).

The total charge on an object can be calculated by the following formula:

Q = N*e

Electrostatic Force - Coulomb's Law

The magnitude of the force between two charged objects (points or spheres) can be calculated by the formula derived by Charles Augustin-Coulomb.

F_e = k*Q₁*Q₂
            d²

where F_e = the electrostatic force of attraction (-) or repulsion (+)
k = a constant 9.0 x 10⁹ N*m²/C²
Q₁ = charge on object 1
Q₂ = charge on object 2
d = distance between the two charges (m)

Electric Fields

The electric field (E) surrounding a negative charge looks like:

The electric field can become quite interesting when more than one charge is in an area. The charges act to distort the field of their neighbors. Electric fields are vector quantities and add in a way you would expect vectors to add.


The electric field surrounding two different charges looks like:

The electric field surrounding two like charges looks like:

Electric Field Intensity (E)

When we discussed gravity, we became acquainted with the gravitational field intensity, g. On the surface of the earth the value of g is 9.8 Newtons / kg. The strength of the gravitational field is the gravitational field intensity. It expresses the amount of force acting per unit mass (kg). Electric charges are surrounded by an electric field just as a gravitational field surrounds any mass. This means that we can express the strength of the field as the electric field intensity in Newtons / Coulomb. The equation for electric field intensity around a point or spherical charge is

E₁ = F_e / Q₂ =  kQ₁
                      d²

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