Physics Study Guide/Electricity

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F = \frac{k\cdot q_1\cdot q_2}{r^2}

The force resulting from two nearby charges is equal to k times charge one times charge two divided by the square of the distance between the charges.

E = \frac{F}{q}

The electric field created by a charge is equal to the force generated divided by the charge.

E = \frac{k\cdot q}{r^2}

Electric field is equal to a constant, “k”, times the charge divided by the square of the distance between the charge and the point in question.

U = \frac{k\cdot q_1\cdot q_2}{r}

Electric potential energy is equal to a constant, “k” multiplied by the two charges and divided by the distance between the charges.


F: Force (N)
k: a constant, 8.988×109 (N·m2/C2)
q1: charge one (C)
q2: charge two (C)
r: distance between the two charges, (m)

Electricity acts as if all matter were divided into four categories:

  1. Superconductors, which allow current to flow with no resistance. (However these have only been produced in relatively extreme laboratory conditions, such as at temperatures approaching absolute zero)
  2. Conductors, which allow electric current to flow with little resistance.
  3. Semiconductors, which allow some electric current to flow but with significant resistance.
  4. Insulators, which do not allow electric current to flow.

Charges are positive (+) or negative (-). Any two like charges repel each other, and opposite charges attract each other.

Electric fields[edit]

A charge in an electrical field feels a force. The charge is not a vector, but force is a vector, and so is the electric field. If a charge is positive, then force and the electric field point in the same direction. If the charge is negative, then the electric field and force vectors point in opposite directions.

A point charge in space causes an electric field. The field is stronger closer to the point and weaker farther away.

Electricity is made of subatomic particles called Electrons and so are Electric Fields and Magnetic Fields.

  • For a good introduction to Gauss' Law and Ampere's Law, check out this website