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# Electric Charge and Matter

"A new scientific truth does not triumph by convincing its opponents and making them see the light, but rather because its opponents eventually die, and a new generation grows up that is familiar with it"
Max Planck

- In order to feel an electric force an object must possess ELECTRIC CHARGE.
- Electric charge can be either positive (+) or negative (-).
- Electric charge is a fundamental property of matter (similar to mass).
  The unit of electric charge is the Coulomb (C)
  One Coulomb is the amount of charge which flows through a wire carrying a current of 1 Ampere in one second.
  Formally, as we will see later in this course, the unit of electric current - the Ampere - is the defined unit, one of the seven basic units (meters, kilograms, seconds, amperes, temperature, candela, mole). The Coulomb is then defined as Ampere.Seconds. However, in developing the basic theory of electricity it is more convenient to "pretend" that the Coulomb is the basic unit; the Ampere is then Coulomb/sec.

Electric charge is "quantized" in units of the charge on the electron, e = 1.6 x 10-19 C. That is charge always appears in integer multiples of "e". Unless we are dealing with individual atoms or sub-atomic particles, since the value of "e" is so small, this quantization is not apparent and need to be considered.

- Conservation of charge

"The net electric charge of an isolated system is constant"

However, Note that in calculating the net charge the sign of the charges must be taken into account, e.g. the net charge of a system containing objects with +2 C and -1 C is +1 C.

The conservation of charge is a basic physical "law" in the same manner as mass, energy and momentum conservation.

- The force between two charges can be attractive or repulsive, depending on the relative signs of the charges. This fact can be expressed simply,

"Like charges repel - unlike charges attract"

- The magnitude of the force between charged objects is given by Coulomb 's Law (see later section for detailed description).

- Most useful/interesting electrical phenomena occur as electric charge is transferred from one place to another. Where possible electric charge will "flow" to minimize the total energy of a system. However, initially, we will discuss static situations, where the charges are held in place by some external mechanism.

"If I were as rich as Rockerfeller, I'd be richer than Rockerfeller"
"How's that?"
"I'd do a bit of window cleaning on the side"

Ronnie Barker

Dr. C. L. Davis
Physics Department
University of Louisville
email: c.l.davis@louisville.edu