Electric fields
| English | Chinese | Pinyin |
|---|---|---|
| electric field | 电场 | diàn chǎng |
| electric field strength | 电场强度 | diàn chǎng qiáng dù |
| positive charge | 正电荷 | zhèng diàn hè |
| negative charge | 负电荷 | fù diàn hè |
| field line | 场线 | chǎng xiàn |
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 电场 around it.
- A field is any region where a charge feels a force.
Field strength
- Electric field strength 电场强度 is the force per unit positive charge 正电荷: $E = \dfrac{F}{q}$.
- Unit: $\dfrac{\text{N}}{\text{C}}$. It is a vector, pointing the way the force pushes a positive charge.
Lightning is a giant spark driven by a huge electric field
Electric fields
E ∝ Q / r²
A charge sets up a radial field — out for +, in for −, obeying the inverse-square law.
Electric field strength is the force per unit:
$E = \dfrac{F}{q}$ — the force on each coulomb of a small positive test charge.
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 负电荷 against it.
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 场线
- Lines start on positive charges and end on negative ones; they never cross.
- Closer lines mean a stronger field.
Electric field lines:
They run from + to −, never cross, and are closer where the field is stronger.
Field lines drawn closer together mean a ____ field.
Line spacing shows the strength — closer lines, stronger field.
You've got it
- 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