Predict forces between charged objects
Apply like-sign repulsion and opposite-sign attraction while recognising why attraction to a neutral object needs more information.
Section 4.2.1 · Electric charge
Exam scope and assessment notes
Core and Extended · Physics 0972, 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. Human subject-teacher review, supervised practical competence and cumulative assessment remain pending. Cambridge states that IGCSE 0972 has the same subject content as IGCSE 0625, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Identify positive and negative net charge.
- Distinguish a force from resulting motion.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which object is negatively charged?
What you will learn
- Predict interactions for two specified charged objects.
- Draw a force on each object and interpret the limits of attraction evidence.
Use the two specified signs
Two positive charges repel, and two negative charges repel. Positive and negative charges attract. For separated charged objects in the stated simple electrostatic model, attraction means each force is toward the other object and repulsion means each is away from it.
Draw one force on each object. These do not cancel on a single object because they act on different objects. A clamp can hold a charged object still despite a force, so motion is not required for the force rule to apply.
Do not infer too much from attraction
A charged rod may also attract a neutral object after charge redistribution. In a neutral conductor, mobile electrons rearrange; in a neutral insulator, bound charge can shift slightly. Opposite charge closer to the rod can give an overall attraction despite zero net charge. This qualitative explanation does not assign a new permanent net sign without transfer.
When a question specifies two charged objects, use their signs. If an unknown object is merely attracted by a known positive rod, it could be negative or neutral under suitable conditions. Clear controlled repulsion supplies stronger evidence of matching charge signs; attraction alone does not uniquely determine the unknown sign.
Pause and explain
A known positive object clearly repels an unknown charged object in a controlled electrostatic test. What sign is supported?
Worked example
Two freely responsive objects are both negative. Predict their interaction and draw the directions.
Show the worked solution
- Their signs are alike.
- Like signs repel.
- The left object has a leftward force and the right object a rightward force.
Answer Repulsion; each force is away from the other object.
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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.
Predict the interactions of + with +, − with −, and + with −.
Give me a hint
Classify each pair as like or unlike before giving the force rule.
Compare my reasoning
- Two positives are alike and repel.
- Two negatives are alike and repel.
- Opposite signs attract.
Repulsion; repulsion; attraction.
Look for these in your work
- Cover all three pairs.
- Use signs rather than magnetic labels.
A positive object is left of a negative object. Draw each electrostatic force direction; then hold the left object in a clamp.
Give me a hint
A clamp changes the motion conditions, not the stated charge-sign interaction.
Compare my reasoning
- Unlike signs attract.
- The left object's electric force is rightward; the right object's is leftward.
- The clamp can oppose the left object's rightward electric force.
Electric forces point toward one another; a clamp can prevent translation without removing attraction.
Look for these in your work
- Place one force on each object.
- Distinguish the support force from the electric force.
A positive rod attracts an unknown object. Does this prove that the object's net charge is negative?
Give me a hint
Consider a neutral object whose charge can redistribute.
Compare my reasoning
- A negative object is one possible explanation.
- A neutral object can be attracted after redistribution.
- Attraction alone does not decide the net sign.
No; negative charge and suitable neutral-object redistribution are both possible.
Look for these in your work
- Give both possibilities.
- Avoid claiming attraction proves charge transfer.
Common mistakes
- Two negatives repel rather than attract.
- Put each force on its own object.
- Neutral attraction does not prove a transferred net charge.
What can you explain now?
What do a positive and a negative charge do?
Compare with the explanation
Attract.
Unlike electric charge signs attract in the stated two-charge model.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Predict signed-charge forces and explain the limited information supplied by attraction alone.
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.