AQA · A-Level · Physics
Course version: 7408 · Version from Sept 2016
Representation of electric fields by electric field lines.
Full specification wording
Electric field strength (A-level only): Representation of electric fields by electric field lines.
Electric field lines represent the electric field in a region: at any point the tangent to a field line gives the direction of the force on a positive test charge, and the spacing of lines…
Checked against the AQA 7408 specification (opens in a new tab) on .
- Point 1 of 2
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Electric field lines represent the electric field in a region: at any point the tangent to a field line gives the direction of the force on a positive test charge, and the spacing of lines indicates field strength. Lines are drawn with arrows pointing away from positive charges and towards negative charges. Near an isolated point charge the lines are radial and the field is non-uniform, becoming weaker with distance as the lines spread out. Between two oppositely charged parallel plates, the lines are straight, parallel and equally spaced, representing a uniform field. Field lines never cross, because the field has a single direction at each point, and their density is proportional to field strength.
What earns the marks
- Draw field lines with arrows showing the direction of force on a positive test charge.
- Show radial field lines around an isolated point charge, with spacing increasing with distance to indicate decreasing field strength.
- Draw straight, parallel, equally spaced lines between oppositely charged parallel plates to represent a uniform field.
- State that field lines never cross because the electric field has a unique direction at any point.
- Use line spacing or density to compare field strength in different regions of a diagram.
Common mistakes and how to put them right
Drawing arrows pointing towards a positive charge.
Put it right by directing arrows away from positive and towards negative charges.
Showing field lines crossing near a charge.
Put it right by ensuring each point has only one field direction.
Drawing curved lines between parallel plates.
Put it right by using straight, parallel, equally spaced lines for a uniform field.
Exam tips
- Always add arrowheads to field lines, as direction is part of the representation.
- Use a ruler for parallel-plate fields so lines are straight and evenly spaced.
- When comparing field strengths, refer explicitly to the spacing or density of the lines you have drawn.
Try the lesson
The start of the Electric field strength (A-level only): Representation of electric fields by electric field lines. lesson, made from checked teaching notes for this specification point.
Have a go first
Which property of electric field lines represents the direction of the electric field at a point?
Listen
Electric field lines represent the electric field in a region: at any point the tangent to a field line gives the direction of the force on a positive test charge, and the spacing of lines indicates field strength. Lines are drawn with arrows pointing away from positive charges and towards negative charges. Near an isolated point charge the lines are radial and the field is non-uniform, becoming weaker with distance as the lines spread out. Between two oppositely charged parallel plates, the lines are straight, parallel and equally spaced, representing a uniform field. Field lines never cross, because the field has a single direction at each point, and their density is proportional to field strength. A strong answer earns marks for these points. Draw field lines with arrows showing the direction of force on a positive test charge. Show radial field lines around an isolated point charge, with spacing increasing with distance to indicate decreasing field strength. Draw straight, parallel, equally spaced lines between oppositely charged parallel plates to represent a uniform field. State that field lines never cross because the electric field has a unique direction at any point. A common slip: Drawing arrows pointing towards a positive charge. Instead, directing arrows away from positive and towards negative charges. Exam tip: Always add arrowheads to field lines, as direction is part of the representation.
Key words
Say what each word means before you turn the card.
Field representation
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Direction and magnitude
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Uniform and radial fields
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Vector fields
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See every definition
- Field representation
- Electric field lines represent the electric field in a region, with the tangent at any point giving the direction of the force on a positive test charge.
- Direction and magnitude
- Arrows on field lines show the direction of force on a positive test charge, and the spacing of lines indicates field strength.
- Uniform and radial fields
- Between oppositely charged parallel plates the lines are straight, parallel and equally spaced, representing a uniform field, while near an isolated point charge the lines are radial and the field is non-uniform.
- Vector fields
- Field lines never cross, because the field has a single direction at each point, and their density is proportional to field strength.
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