Draw the uniform field between oppositely charged parallel plates
Represent the central field with equally spaced parallel lines from the positive plate to the negative plate, excluding edge effects.
Extended only (Supplement); Physics 0625, 2026–2028. Follow the selected magnetism or charge section. Field lines are explanatory models, not material threads or measured force values. Written practice is self-checked.
Before you begin
- Use the positive-test-charge convention.
- Distinguish radial patterns from a uniform pattern.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which sign defines electric-field direction?
What you will learn
- Draw the central parallel-plate pattern and arrow direction.
- Determine charge-force directions while retaining the stated uniform-region approximation.
Construct the central pattern
Between two oppositely charged parallel conducting plates, the central electric field is represented by straight, parallel, equally spaced lines normal to the plates. Arrows point from the positive plate toward the negative plate, because a positive test charge is repelled by the positive plate and attracted by the negative one.
The central-region approximation is uniform: the direction and strength do not vary from one central position to another in this ideal model. Equal representative spacing carries this qualitative meaning; it is not a calibrated numerical field-strength value. Cambridge excludes end effects from the simple pattern required here.
Apply the pattern to a test charge
With a positive left plate and negative right plate, field arrows point right. A positive test charge has a rightward electric force and an electron a leftward one. Reversing both plate signs reverses the field and these electric-force directions. The negative test particle does not make the field arrows reverse in an unchanged plate arrangement.
Keep arrows within the stated central gap rather than importing the radial rays of a point source. Real plate edges can differ, so the straight uniform pattern should not be asserted everywhere around finite plates. Neither a motion trajectory nor a numerical force follows without the additional initial and quantitative information.
Pause and explain
The left plate is negative and the right plate positive. What is the central field direction?
Worked example
Draw the central field between a positive left plate and a negative right plate; give an electron's force direction.
Show the worked solution
- Use equally spaced horizontal lines normal to the vertical plates.
- Point field arrows right, from + to −.
- The negative electron's force points left.
Answer Uniform central field rightward; electron force leftward.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Describe the central field sketch for positive left and negative right parallel plates.
Give me a hint
Draw normal lines with equal representative spacing and a common arrow sense.
Compare my reasoning
- Use straight parallel horizontal lines between the vertical plates.
- Space them equally in the central region.
- Point every arrow rightward, from + to −.
Equally spaced straight parallel lines directed rightward from the positive to negative plate.
Look for these in your work
- Include geometry and direction.
- Restrict the sketch to the central-region approximation.
Reverse those plate signs. Give the new field direction and separate positive-charge and electron force directions.
Give me a hint
Reverse the source field, then apply the two test-charge signs.
Compare my reasoning
- The positive plate is now right and the negative plate left.
- The field and positive-charge force point left.
- The electron force points right.
Field left; positive force left; electron force right.
Look for these in your work
- Use the changed source signs.
- Reverse force only for the negative particle.
A learner extends equally spaced straight lines around every outer edge and calculates 5 N from five drawn lines. Assess both claims.
Give me a hint
Separate the central approximation from edge behaviour and representative ray count from calibration.
Compare my reasoning
- The uniform pattern was specified only for the central gap.
- Edge effects are omitted rather than claimed absent everywhere.
- Five schematic lines do not establish a numerical force of 5 N.
Neither claim follows: the pattern is central and uncalibrated; edge behaviour and force values need further information.
Look for these in your work
- State the approximation boundary.
- Reject force inference from line count.
Common mistakes
- Arrows go from positive to negative plate.
- Negative charge force opposes the field.
- The central approximation excludes edge effects.
What can you explain now?
How do the representative lines show a uniform central field?
Compare with the explanation
They are straight, parallel and equally spaced.
The common pattern represents unchanged local direction and qualitative strength in the ideal central region.
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Make it stick
Draw the central uniform plate field and infer signed-charge forces with explicit edge and calibration limits.
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