Orientation
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.
Core Content
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 |
Concept Check
-
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.
-
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.
-
Closer field lines usually indicate:
- A. weaker field
- B. stronger field
- C. zero potential
- D. no force
Answer: B.
Applied Practice
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: $6.0\times10^4\ \text{N C}^{-1}$.
Deep Practice And Writing
Prompt: explain why field lines around a positive point charge point outward and why their spacing changes with distance.
Maintenance Loop
State field definition, field-line direction, and what field-line density represents.