OCR · A-Level · Physics
Course version: H556 · Version from Sept 2015
Point and spherical charges: (d) electric field strength; E = F/q.
Electric field strength E at a point is defined as the force F per unit positive charge q placed at that point, giving E = F/q.
Checked against the OCR H556 specification (opens in a new tab) on .
- Point 4 of 4
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Electric field strength E at a point is defined as the force F per unit positive charge q placed at that point, giving E = F/q. The unit is newtons per coulomb, N C⁻¹, which is equivalent to volts per metre, V m⁻¹. Rearranging gives F = Eq, so a charge q in a field E experiences a force Eq. Field strength is a vector, so direction matters: the force on a positive charge is in the direction of E, while the force on a negative charge is opposite to E. A reliable method is to identify the charge sign first, then use E = F/q to find magnitude and decide direction from the sign.
What earns the marks
- Electric field strength is defined as force per unit positive charge: E = F/q.
- The unit of electric field strength is N C⁻¹, equivalent to V m⁻¹.
- Rearranging gives F = Eq, the force on a charge q in a field E.
- Field strength is a vector; its direction is the direction of the force on a positive charge.
- For a negative charge the force is opposite to the field direction.
Common mistakes and how to put them right
Using the charge magnitude without considering sign when determining force direction.
Put it right by noting the force on a negative charge is opposite to E.
Confusing E = F/q with E = kQ/r².
Put it right by recognising the first is the definition and the second applies to a point charge in a vacuum.
Forgetting that q in E = F/q is the test charge, not the source charge.
Put it right by identifying which charge experiences the force.
Treating field strength as a scalar.
Put it right by remembering it has direction as well as magnitude.
Exam tips
- Write down the definition of E before substituting values.
- Check the sign of the charge to determine the direction of the force.
- Remember N C⁻¹ and V m⁻¹ are equivalent units for field strength.
Try the lesson
The start of the Point and spherical charges: (d) electric field strength; E = F/q. lesson, made from checked teaching notes for this specification point.
Listen
Electric field strength E at a point is defined as the force F per unit positive charge q placed at that point, giving E = F/q. The unit is newtons per coulomb, N C⁻¹, which is equivalent to volts per metre, V m⁻¹. Rearranging gives F = Eq, so a charge q in a field E experiences a force Eq. Field strength is a vector, so direction matters: the force on a positive charge is in the direction of E, while the force on a negative charge is opposite to E. A reliable method is to identify the charge sign first, then use E = F/q to find magnitude and decide direction from the sign. A strong answer earns marks for these points. Electric field strength is defined as force per unit positive charge: E = F/q. The unit of electric field strength is N C⁻¹, equivalent to V m⁻¹. Rearranging gives F = Eq, the force on a charge q in a field E. Field strength is a vector; its direction is the direction of the force on a positive charge. A common slip: Using the charge magnitude without considering sign when determining force direction. Instead, noting the force on a negative charge is opposite to E. Exam tip: Write down the definition of E before substituting values.
See it
Key words
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Electric field strength
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Force on a charge in a field
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Vector nature of field strength
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Units of field strength
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See every definition
- Electric field strength
- The force F per unit positive charge q placed at that point, giving E = F/q.
- Force on a charge in a field
- Rearranging gives F = Eq, so a charge q in a field E experiences a force Eq.
- Vector nature of field strength
- Field strength is a vector, so direction matters: the force on a positive charge is in the direction of E, while the force on a negative charge is opposite to E.
- Units of field strength
- The unit is newtons per coulomb, N C⁻¹, which is equivalent to volts per metre, V m⁻¹.
Label the diagram
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