Electric fields
The comb-and-paper trick
- Rub a comb on your hair and it lifts tiny bits of paper — no contact needed.
- The charged comb sets up an electric field around it.
- A field is any region where a charge feels a force.
Field strength
- Electric field strength is the force per unit positive charge: $E = \dfrac{F}{q}$.
- Unit: $\dfrac{\text{N}}{\text{C}}$. It is a vector, pointing the way the force pushes a positive charge.
Practice
Electric field strength is the force per unit:
$E = \dfrac{F}{q}$ — the force on each coulomb of a small positive test charge.
Practice
A charge of $2.0\ \text{C}$ feels a force of $6.0\ \text{N}$. What is the field strength?
$E = \dfrac{F}{q} = \dfrac{6.0}{2.0} = 3.0\ \dfrac{\text{N}}{\text{C}}$.
Force on a charge
- The force on a charge is $F = qE$.
- A positive charge is pushed along the field; a negative charge against it.
Practice
The force on a negative charge is opposite to the field direction.
Field direction is defined for a positive charge; a negative charge feels the opposite force.
Field lines
- Lines start on positive charges and end on negative ones; they never cross.
- Closer lines mean a stronger field.
Practice
Electric field lines:
They run from + to −, never cross, and are closer where the field is stronger.
Practice
Field lines drawn closer together mean a ____ field.
Line spacing shows the strength — closer lines, stronger field.
You've got it
Key idea
- electric field strength $E = \dfrac{F}{q}$ (force per unit positive charge)
- force on a charge $F = qE$ (positive with the field, negative against)
- field lines go + to −, never cross; closer = stronger