Retrieval Practice
Magnetic Fields & Forces — AQA A-Level Physics
Q1. Define a magnetic field.
A region of space in which a magnetic pole will experience a force.
Q2. State two sources of a magnetic field.
A moving electric charge, and a permanent magnet.
Q3. Define magnetic flux density B and state its unit.
- The number of magnetic flux lines passing through a region of space per unit area.
- Measured in teslas (T).
Q4. Define one tesla.
The flux density that causes a force of 1 N on a 1 m wire carrying a current of 1 A at right angles to the flux.
Q5. State the equation for the force on a current-carrying conductor in a magnetic field.
F = BIL sin θ, where F = force (N), B = flux density (T), I = current (A), L = length of conductor in the field (m), θ = angle between conductor and field.
Q6. When is the force on a current-carrying conductor (a) maximum and (b) zero?
(a) Maximum at θ = 90° (perpendicular to field): F = BIL.
(b) Zero at θ = 0° (parallel to field): sin 0° = 0.
Q7. State Fleming's left-hand rule.
- Thumb = Force/motion, First finger = magnetic Field (B), Second finger = conventional Current (I).
- All three are mutually perpendicular.
Q8. What do dots and crosses represent in a magnetic field diagram?
- Dots = field coming out of the page (arrow tip towards you).
- Crosses = field going into the page (arrow flights away from you).
Q9. State the equation for the force on a moving charged particle in a magnetic field.
F = BQv sin θ, where B = flux density (T), Q = charge (C), v = speed (m s⁻¹), θ = angle between velocity and field.
Q10. Explain why a charged particle follows a circular path in a uniform magnetic field.
The magnetic force is always perpendicular to velocity, so it acts as a centripetal force — changing direction without changing speed.
Q11. Derive the expression for the radius of a charged particle's circular path.
- Equate centripetal force to magnetic force: mv²/r = BQv.
- Cancel v: mv/r = BQ.
- Rearrange: r = mv/BQ.
Q12. State four factors affecting the radius and whether each increases or decreases it.
- Increase r: faster speed (r ∝ v), greater mass (r ∝ m).
- Decrease r: stronger field (r ∝ 1/B), greater charge (r ∝ 1/Q).
Q13. State the roles of the magnetic field and the electric field in a cyclotron.
- Magnetic field: provides centripetal force to bend particles into circular arcs within the dees.
- Electric field: accelerates particles in the gaps between dees, increasing speed and kinetic energy.
Q14. In the required practical for measuring B, what do you plot and how do you find B?
- Plot mass (m) on y-axis against current (I) on x-axis.
- Gradient = BL/g.
- Rearrange: B = (g × gradient) / L.