The forces between two charged particles $q_1$ and $q_2$ separated by distance $r$ are

The forces are repulsive for two like charges, attractive for two opposite charges.

To solve electrostatic force problems:

There are two kinds of charge, positive and negative.

• Fundamental charges are protons and electrons, with charge $\pm e$ where $e=1.60\times {10}^{-19}\text{C}$.

• Objects are charged by adding or removing electrons.

• The amount of charge is $q=(N_{\text{p}}-N_{\text{e}})e$.

• An object with an equal number of protons and electrons is neutral, meaning no net charge.

Charged objects exert electric forces on each other.

• Like charges repel, opposite charges attract.

• The force increases as the charge increases.

• The force decreases as the distance increases.

There are two types of material, insulators and conductors.

• Charge remains fixed in or on an insulator.

• Charge moves easily through or along conductors.

• Charge is transferred by contact between objects.

Charged objects attract neutral objects.

• Charge polarizes metal by shifting the electron sea.

• Charge polarizes atoms, creating electric dipoles.

• The polarization force is always an attractive force.

Charges interact with each other via the electric field $\vec{E}$.

• Charge A alters the space around it by creating an electric field.

  

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• The field is the agent that exerts a force. The force on charge $q_{\text{B}}$ is $\vec{F}{}_{\text{on B}}=q_{\text{B}}\vec{E}$.

An electric field is identified and measured in terms of the force on a probe charge $q$:

• The electric field exists at all points in space.

• An electric field vector shows the field only at one point, the point at the tail of the vector.

The electric field of a point charge is

Unit vector $\widehat{r}$ indicates “away from $q$.”