Electric Fields

Lesson goal: describe electric fields using field lines, field strength, potential difference, and field-force relationships.

Electric field diagrams are representations of force per unit positive test charge. They are not paths taken by electrons.

Electric field strength is force per unit charge:

$$E = \frac{F}{q}$$

For a point charge:

$$E = k\frac{|Q|}{r^2}$$

Potential difference is energy transferred per unit charge:

$$V = \frac{W}{q}$$

Representation	Meaning
field-line direction	force direction on a positive test charge
closer field lines	stronger field
uniform field	constant magnitude and direction, often between parallel plates
equipotential	locations with equal electric potential

1. Electric field strength is:

   • A. force per unit positive charge
   • B. charge per unit time
   • C. energy per kilogram
   • D. resistance per metre

   Answer: A.

2. Electric field lines point in the direction of force on:

   • A. a neutron
   • B. a positive test charge
   • C. an electron only
   • D. a magnetic pole

   Answer: B.

3. Closer field lines usually indicate:

   • A. weaker field
   • B. stronger field
   • C. zero potential
   • D. no force

   Answer: B.

A charge experiences a force of $0.18\ \text{N}$ in an electric field. The charge is $3.0\times10^{-6}\ \text{C}$. Calculate the electric field strength.

$$E = \frac{F}{q} = \frac{0.18}{3.0\times10^{-6}} = 6.0\times10^4\ \text{N C}^{-1}$$

Final answer: $6.0\times10^4\ \text{N C}^{-1}$.

Prompt: explain why field lines around a positive point charge point outward and why their spacing changes with distance.