Determine electric field direction using a positive test charge
Use positive charge as the direction convention and reverse the force direction for a negative test charge.
Section P4.2.1 · Electric charge
Mapped to the named 0654 section using teaching from the corresponding single-science course. A 0654-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Define an electric field.
- Read named left/right directions and charge signs.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which sign does an electron carry?
What you will learn
- State the positive-test-charge convention.
- Determine electric-force directions for both charge signs in a stated local field.
Fix the convention
The direction of an electric field at a point is the direction of the force on a positive charge at that point. If a small positive test charge would be pushed right, the local field is rightward. Choose this positive reference even when the particle actually supplied in the question is an electron.
A negative test charge experiences an electric force opposite to that field direction. In a stated rightward field, a positive charge has a rightward electric force and a negative charge a leftward one. Moving the test charge between trials does not redefine the convention for the same undisturbed source arrangement.
Separate source, field and test charge
A positive source has an outward field because it repels a positive test charge; a negative source has an inward field because it attracts that test charge. These source patterns and the force on an actual negative particle are separate steps. An electron near a positive source is attracted inward even though that source's field points outward.
Draw local force arrows at the particle's position. A path of motion also depends on initial velocity and other forces, so a field arrow alone does not prescribe an entire trajectory. If the resultant field is zero at the stated point, no unique direction is defined there. This lesson determines direction without assigning unprovided numerical force values.
Sources for this lesson
Pause and explain
A negative test charge has an upward electric force. Which way is the local field?
Worked example
The electric field points right. Give the electric forces on separate positive and negative test charges.
Show the worked solution
- A positive test charge defines the field direction.
- Its force is rightward.
- A negative charge's force reverses to leftward.
Answer Positive force rightward; negative force leftward.
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Test this topic →From guided practice to a new situation
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.
A uniform local field points downward. State the electric-force directions for separate positive and negative test charges.
Give me a hint
Use the positive reference, then reverse for the negative sign.
Compare my reasoning
- The positive charge's force follows the field downward.
- The negative charge's force is opposite.
- Its force is upward.
Positive downward; negative upward.
Look for these in your work
- Include both signs.
- Describe electric forces rather than guaranteed motion paths.
A positive test charge is forced left in one trial and an electron right in another at the same point. Are these consistent with one unchanged field?
Give me a hint
Translate both observations to the positive-reference convention.
Compare my reasoning
- The positive trial gives a leftward field.
- The electron's rightward force implies a leftward field.
- Both are consistent with the same local direction.
Yes; both imply a leftward electric field.
Look for these in your work
- Reverse only the electron case.
- Compare the inferred fields.
An electron is attracted toward an isolated positive source. A learner draws all field arrows toward that source. Correct the drawing rule.
Give me a hint
Define the arrows using a positive test charge rather than the electron.
Compare my reasoning
- The positive source repels a positive test charge.
- Its field points outward.
- The electron's inward force is opposite to that outward field.
Field arrows point outward; the electron's force points inward.
Look for these in your work
- Separate field and actual particle force.
- State the positive-reference convention.
Common mistakes
- The reference charge is positive.
- A negative charge's force reverses the field direction.
- A force direction is not a complete trajectory.
What can you explain now?
An electron has a leftward electric force. What is the local field direction?
Compare with the explanation
Rightward.
The electron is negative, so its force is opposite to the positive-test-charge field convention.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Translate positive and negative force observations into one consistent electric-field direction.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.