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Electric Field & Field Lines

Key idea: O Level electric fields: what an electric field is and how to draw electric field line patterns.

  • SEC G3 Physics 2027
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Learning objectives

  • state that there are positive and negative charges and that charge is measured in coulombs
  • state that unlike charges attract and like charges repel
  • Describe an electric field as a region where charge experiences force
  • Recall that the field lines of an isolated point charge give the direction of the force on a positive test charge
  • Draw the directed electric field of an isolated point charge
  • Interpret the electric-field pattern between two isolated point charges
  • Draw the electric-field pattern between two isolated point charges
  • Explain charging by rubbing as electron transfer
  • Describe experiments that charge a conductor by induction
  • Describe potential electrostatic-charging hazards
  • Describe an electrostatic precipitator and transfer the principle

1. Definition

A. Electric field

An electric field is a region in which an electric charge experiences an electric force.

B. Electric field lines

Electric field lines show the direction of the force on a positive test charge placed at that point.

What you need for this course

You should be able to:

  • define an electric field
  • draw the electric field of an isolated point charge
  • draw the field patterns between two isolated point charges

2. Key Ideas

A. Rules for drawing field lines

  • Field lines start on positive charges and end on negative charges (or extend to infinity).
  • Arrows show the direction of force on a positive test charge:
    • away from +
    • towards −
  • Field lines never cross.
  • Closer lines mean a stronger field (qualitative idea).

B. Linking field lines to forces

  • If a positive charge is placed on a field line, it is pushed along the arrow direction.
  • If a negative charge is placed there, the force is opposite the arrow direction.

3. Detailed Explanations

A. Field pattern around a single positive charge

Field lines radiate outwards symmetrically from a positive charge.

B. Field pattern between unlike charges (+ and −)

Field lines go from the positive charge to the negative charge.

C. Field pattern between like charges (+ and + or − and −)

Field lines do not join between like charges. They bend away from the region between the charges because like charges repel.

Electric-field patterns for one and two point chargesThree panels show radial outward arrows around one positive point charge, field lines joining a positive and negative pair, and field lines curving apart between two positive charges.A: isolated positive+Arrows point awayB: unlike charges+−Lines run + to −C: like charges++Lines curve apart
Scroll diagram horizontally to read all labels.
Field arrows show the force direction on a positive test charge. Lines leave positive charges, enter negative charges and never cross.

For two equal like charges, the field is zero exactly at the midpoint because the two field contributions there are equal and opposite. Do not draw a field line through that point or join the charges.

4. Common Mistakes

  • Drawing arrows from − to + (correct direction is from + to −).
  • Drawing field lines that cross.
  • Forgetting to use symmetry for equal charges.
  • Using too few lines so the pattern is unclear.

5. Exam Tips

  • Always add arrowheads and label +/− clearly.
  • If asked for field direction, use the phrase: “direction of force on a positive test charge”.
  • Keep spacing reasonable and make the pattern smooth (no sharp kinks).
  • For like charges, don’t draw lines connecting the charges.

6. Worked Examples

Modelled example 1

Direction of force (positive test charge)

Core

Problem

At a point where the electric-field arrow points right, state the force direction on (a) a positive test charge and (b) a negative charge.
Study the worked solution

  1. Use the field definition

    Method

    Direct the positive test charge to the right.

    Reason

    Electric-field direction is defined as the force direction on a positive test charge.

    Working

    Positive charge: force → right.

  2. Reverse for a negative charge

    Method

    Direct the negative charge’s force to the left.

    Reason

    A negative charge experiences force opposite to the field direction.

    Working

    Negative charge: force → left.

Guided practice 2

Identifying the correct pattern

About 4 min

Problem

Which two-charge field pattern represents attraction: like charges or unlike charges? State the arrow direction.

Use force behaviour and arrow convention

Attracting pair

Hints

Hint 1: use attraction
Opposite charge signs attract.
Hint 2: add arrowheads
Field lines begin at positive and end at negative.
View solution step by step

  1. Choose the pair

    Method

    Select unlike charges.

    Reason

    Positive and negative charges attract each other.

    Working

    + paired with -.

  2. Describe the pattern

    Method

    Connect smooth field lines from positive to negative with arrows toward the negative charge.

    Reason

    The arrows show positive-test-charge force direction.

    Working

    Field direction: + → -.

Common misconception 3

Field strength from line spacing

Find and correct the mistake

Learner response

Field lines are closer together at A than at B. A student says B has the stronger field because each line has more space to act. Diagnose the interpretation.

Use the diagram convention qualitatively

Stronger field

View solution step by step

  1. Read the spacing convention

    Method

    Select point A as the stronger-field region.

    Reason

    A greater density of drawn field lines represents greater field magnitude.

    Working

    Closer spacing at A → stronger field at A.

  2. Keep the interpretation qualitative

    Method

    Do not treat individual lines as physical agents or calculate an exact value from spacing alone.

    Reason

    Field lines are a representation of direction and relative strength.

    Working

    Diagram convention, not literal strands.

Examiner practice 4

Field around a negative charge

3 marks

Examination question

Describe the electric-field-line pattern around an isolated negative point charge, including shape, arrow direction and spacing trend. [3 marks]

State all three drawing features

View solution step by step

  1. Describe geometry and direction

    2 marks

    Method

    Draw or describe straight radial lines with arrows pointing inward.

    Reason

    A positive test charge would be attracted toward the negative source charge.

    Working

    Symmetric radial pattern; arrows toward -.

  2. Describe relative strength

    1 mark

    Method

    Place lines closer together near the charge and farther apart with distance.

    Reason

    The field is qualitatively stronger nearer the point charge.

    Working

    Line density decreases outward.

Challenge 5

Midpoint check (equal like charges)

Minimal support

Symmetry transfer

Two equal positive charges are fixed a distance apart. Determine the net electric field at the exact midpoint on their joining line and explain what this means for the field-line drawing.

Combine the two field contributions as vectors

Hints

Hint 1: compare distances and source charges
The midpoint is equally far from equal charges.
Hint 2: compare directions
Each positive charge’s field points away from that charge.
View solution step by step

  1. Compare the contributions

    Method

    Assign equal field magnitudes in opposite directions along the joining line.

    Reason

    The source charges and distances are equal, while each positive-charge field points away from its source.

    Working

    vector E₁ = - vector E₂ at the midpoint.

  2. State the net field and drawing consequence

    Method

    Conclude that the net field is zero and do not draw a field line through the cancellation point.

    Reason

    Vector contributions cancel exactly there, leaving no unique field direction.

    Working

    vector Eₙₑₜ = 0

7. Mind Stretchers

Mind stretcher 1: Why field lines can’t crossExtension

Why can’t electric field lines cross at a point?

Show Answer

At any point, the electric field has one direction (force direction on a positive test charge). If field lines crossed, it would imply two different directions at the same point, which is impossible.

Mind stretcher 2: Symmetry checkExtension

Two equal positive charges are placed the same distance apart. What should be true about the field pattern?

Show Answer

The pattern should be symmetric about the midpoint between the charges (mirror symmetry).

8. Practice and next step

Draw the isolated positive, isolated negative, unlike-pair and like-pair patterns from memory, including arrowheads. Check them in the Static Electricity Explorer, then attempt the topic quiz.

Continue with the next resource in this course.

Course and syllabus information
Course
SEC G3 Physics
Edition
SEC G3 Physics 2027