Define an electric field by its force on charge
Describe a region where charge experiences an electric force and distinguish the local field from a drawn pattern or object motion.
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
- Recall charge signs and forces.
- Distinguish force from movement under supports.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which pair contains opposite electric signs?
What you will learn
- Define an electric field through force on an electric charge.
- Separate field presence, force direction and numerical information.
Define the region
An electric field is a region in which an electric charge experiences a force. A charged source can exert an electric force on a suitable test charge placed in the region without physical contact. Use 'electric charge' rather than 'magnetic pole' in this definition; they are different interactions.
A small test charge probes a local field without substantially disturbing the source arrangement in the ideal model. The field exists as a description of the source arrangement before the test charge is inserted. A sketch's lines represent the pattern; they are not strings that must physically touch a charge.
Describe the stated field, not unprovided values
A force observation can support a field explanation even if a clamp prevents motion. Conversely, seeing no motion does not prove zero electric force when other forces or friction may act. The simple definition concerns the electric contribution, not necessarily the object's resultant force or acceleration.
A drawn arrow gives direction under its stated convention. Without a supplied calibrated strength and charge, it does not provide a numerical force in newtons. At a point where the resultant electric field is zero, the model supplies no unique field direction there; that does not prove the whole surrounding region or every source is uncharged.
Pause and explain
Which is the appropriate electric-field definition?
Worked example
A charged object is clamped in a stated electric field and stays still. Does remaining still invalidate the field definition?
Show the worked solution
- The definition concerns an electric force on charge.
- A clamp can provide an opposing force.
- No translation need not mean no electric force.
Answer No; a support force can balance the electric force on the charged object.
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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.
State what is meant by an electric field.
Give me a hint
Identify both the region and the object used in its force definition.
Compare my reasoning
- It is a region.
- An electric charge placed there experiences an electric force.
- This does not require contact with the source.
A region in which an electric charge experiences a force.
Look for these in your work
- Name electric charge.
- Describe the force rather than compulsory motion.
A support holds a charged bead still while an electric force acts. Explain why these statements are compatible.
Give me a hint
Separate the electric contribution from the combined forces.
Compare my reasoning
- The field exerts an electric force on the charge.
- The support can supply an opposing force.
- The resultant can be zero and prevent acceleration.
An opposing support force can balance the electric force; a field can still be present.
Look for these in your work
- Include both forces.
- Do not equate stillness with absence of field.
A field sketch gives arrow direction but no calibrated strength or charge magnitude. Can it establish a force of 2 N?
Give me a hint
Check whether numerical force information was supplied.
Compare my reasoning
- The arrows give a stated direction convention.
- No calibrated strength or charge magnitude is given.
- A numerical force of 2 N cannot be inferred.
No; direction alone does not establish a numerical force.
Look for these in your work
- State the missing numerical information.
- Keep schematic lines distinct from calibrated measurements.
Common mistakes
- Define the force on charge, not a magnetic pole.
- Field lines are a representation.
- No movement is not proof of zero electric force.
What can you explain now?
What experiences force in the electric-field definition?
Compare with the explanation
An electric charge.
The definition identifies the electric interaction with a test charge in the region.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Define an electric field and identify the limits of force, motion and schematic-arrow evidence.
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.