For SL and HL: Sketch electric field lines so that their arrows show the force direction on a positive test charge. Lines leave positive charges, enter negative charges, never cross, and are closer together where the field is stronger.
The Reasoning and Method
An electric field line is an imaginary line whose tangent gives the direction of the electric field strength . Because field direction is defined using a positive test charge, arrows point away from positive source charges and toward negative source charges.
Use these conventions systematically:
- Identify the sign and arrangement of the source charges.
- Draw lines beginning on positive charges and ending on negative charges. If only one sign is present, the lines extend to or from infinity.
- Add arrowheads showing the direction of the field.
- Keep the pattern symmetrical when the charge arrangement is symmetrical.
- Use line density to represent field strength: closer spacing means a stronger field.
- Never allow field lines to cross, because the electric field cannot have two directions at one point.
| Charge arrangement | Required field-line pattern |
|---|---|
| Single positive charge | Straight radial lines directed outward |
| Single negative charge | Straight radial lines directed inward |
| Equal opposite charges | Curved lines connecting positive to negative, with symmetry about the axis between them |
| Equal like charges | Lines curve away from the region between the charges and do not connect |
| Oppositely charged parallel plates | Straight, parallel, equally spaced lines from the positive plate to the negative plate, except for edge effects |
A common misconception is that field lines show the path a charge must follow. They do not: they show the field direction at each point. A moving charge may have an initial velocity that produces a different trajectory.
Exam Technique
This is assessed in D.2 Electric and magnetic fields at both SL and HL. For a Draw question, include correct arrow directions, symmetry, spacing, and charge labels; use a ruler for straight lines between parallel plates.