Apply Coulomb's law to the force between point charges and describe the electric field around a charge.

What this dot point is asking

You need to apply Coulomb's law to find the force between point charges and describe the electric field surrounding a charge.

Coulomb's law

The structure is identical to Newton's law of gravitation - an inverse-square law proportional to the product of the "source quantities" (here charge instead of mass). Key differences:

• Charge comes in two signs, so the electric force can attract or repel: like charges repel, unlike charges attract. Gravity is always attractive.
• The electric force is vastly stronger than gravity for everyday particles.

The elementary charge is $e = 1.60\times10^{-19} \text{ C}$ (the charge of a proton; an electron carries $-e$).

The electric field

Rather than describing the force between every pair of charges, we say each charge creates an electric field in the space around it. A second charge then feels a force from that field.

Once you know the field $E$ at a point, the force on any charge $q$ placed there is simply $F = qE$.

Field lines

We draw electric fields with field lines:

• Lines point away from positive charges and toward negative charges.
• The field is stronger where lines are closer together.
• Lines never cross (the field has one direction at each point).

A single positive charge has radial lines pointing outward; a pair of opposite charges (a dipole) has lines curving from positive to negative.