OCR · A-Level · Physics
Course version: H556 · Version from Sept 2015
Point and spherical charges: (c) electric field lines to map electric fields
Electric field lines are a visual way to represent an electric field.
Checked against the OCR H556 specification (opens in a new tab) on .
- Point 3 of 4
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Electric field lines are a visual way to represent an electric field. The direction of a field line at any point gives the direction of the force on a positive test charge there. The spacing of the lines indicates field strength: closely spaced lines mean a strong field, widely spaced lines mean a weak field. Field lines start on positive charges and end on negative charges, and they never cross. To map a field, draw lines with arrows showing direction and vary their spacing to show how strength changes with position. For a point charge the lines are radial and evenly spaced in angle, becoming further apart with distance.
What earns the marks
- Field lines show the direction of the electric field at each point.
- The arrow on a field line points in the direction of the force on a positive test charge.
- Closer spacing of field lines indicates a stronger field; wider spacing indicates a weaker field.
- Field lines begin on positive charges and end on negative charges.
- Field lines never cross each other.
Common mistakes and how to put them right
Drawing field lines that cross.
Put it right by ensuring lines never intersect because the field has only one direction at each point.
Using uniform spacing for a point charge.
Put it right by increasing spacing with distance to show weakening field.
Omitting arrows; correct this by adding arrows that show the direction of the field.
Drawing field lines that start or end in empty space.
Put it right by starting them on positive charge and ending them on negative charge.
Exam tips
- Always include arrows on field lines in your diagrams.
- Use spacing to convey relative field strength, not just the number of lines.
- Check that no two field lines cross before finalising your sketch.
Try the lesson
The start of the Point and spherical charges: (c) electric field lines to map electric fields lesson, made from checked teaching notes for this specification point.
Listen
Electric field lines are a visual way to represent an electric field. The direction of a field line at any point gives the direction of the force on a positive test charge there. The spacing of the lines indicates field strength: closely spaced lines mean a strong field, widely spaced lines mean a weak field. Field lines start on positive charges and end on negative charges, and they never cross. To map a field, draw lines with arrows showing direction and vary their spacing to show how strength changes with position. For a point charge the lines are radial and evenly spaced in angle, becoming further apart with distance. A strong answer earns marks for these points. Field lines show the direction of the electric field at each point. The arrow on a field line points in the direction of the force on a positive test charge. Closer spacing of field lines indicates a stronger field; wider spacing indicates a weaker field. Field lines begin on positive charges and end on negative charges. A common slip: Drawing field lines that cross. Instead, ensuring lines never intersect because the field has only one direction at each point. Exam tip: Always include arrows on field lines in your diagrams.
See it
Key words
Say what each word means before you turn the card.
Visual representation of fields
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Direction and strength from field lines
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Field lines and charge sign
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Rules for drawing field diagrams
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See every definition
- Visual representation of fields
- Electric field lines are a visual way to represent an electric field.
- Direction and strength from field lines
- The direction of a field line at any point gives the direction of the force on a positive test charge there, and the spacing of the lines indicates field strength.
- Field lines and charge sign
- Field lines start on positive charges and end on negative charges, and they never cross.
- Rules for drawing field diagrams
- To map a field, draw lines with arrows showing direction and vary their spacing to show how strength changes with position.
Label the diagram
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1Box 1
2Box 2
3Box 3
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