Topic 7: Electric and Magnetic Fields — Edexcel A-Level Physics
Test yourself on Topic 7: Electric and Magnetic Fields with PEARSON EDEXCEL A-Level practice questions.
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Topic 7: Electric and Magnetic Fields explained
Electric field strength E at a point is the force per unit positive charge experienced by a small test charge placed there, so E = F/Q.
Read the full explanation
Rearranged, F = EQ gives the force on a charge Q in a field of strength E. The direction of E is the direction of the force on a positive charge; a negative charge feels force opposite to E. Units follow from the definition: newtons per coulomb (N C⁻¹), equivalent to volts per metre (V m⁻¹). Work through a typical calculation: a charge of 4.0 × 10⁻⁶ C experiences a force of 2.0 × 10⁻³ N, so E = F/Q = (2.0 × 10⁻³)/(4.0 × 10⁻⁶) = 5.0 × 10² N C⁻¹. Always identify which charge is the test charge and which produces the field.
Your focus
- Define electric field strength as force per unit positive charge using E = F/Q.
- Calculate E, F or Q by rearranging E = F/Q with correct SI units.
- Determine the direction of force on positive and negative charges in a field.
Topic 7: Electric and Magnetic Fields exam tips
Marking Points
- States that electric field strength is force per unit positive charge, E = F/Q.
- Uses E = F/Q with consistent SI units, giving N C⁻¹ (equivalently V m⁻¹).
- Rearranges correctly to F = EQ when the force on a given charge is required.
- Recognises that E is a vector: its direction is the force direction on a positive charge, opposite for a negative charge.
- Selects the correct charge as Q (the one experiencing the force) rather than the source charge.
Examiner Tips
- 💡Write the equation, substitute values with units, then state the unit N C⁻¹ explicitly.
- 💡Check the sign of the charge to decide the direction of force relative to E.
- 💡Convert microcoulombs and millinewtons to SI before calculating.
Common Mistakes
- Treating E as a force rather than force per unit charge; correction: divide the force by the charge to obtain field strength.
- Substituting the source charge into Q when finding the force on a different test charge; correction: Q is the charge on which the force acts.
- Omitting the direction of E for a negative charge; correction: the force on a negative charge is opposite to E.