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A Level Electromagnetic Forces Hub

A Level Electromagnetic Forces hub covering fields due to currents, forces on wires and moving charges, current balances, particle deflection and velocity selection.

  • GCE A-Level H2 Physics 2027
Learning goals
  • Calculate and represent magnetic fields produced by currents.
  • Sketch magnetic field lines due to currents in a long straight wire, a flat circular coil and a long solenoid.
  • Analyse forces on current-carrying conductors, current balances and interactions between parallel currents.
  • Analyse forces and paths of moving charges in uniform fields.
  • Apply crossed electric and magnetic fields to velocity selection.

Electromagnetic Forces connects current, fields and motion. Start with how currents produce magnetic fields, then use field direction to analyse forces on conductors and charged particles.

Start here

Prerequisites: A Level Currents, Circular Motion, Electric Fields, and the O Level Magnetism Hub.

Route: field patterns and field equations → force on a wire → current balance → moving charges → beam deflection → velocity selection.

After this hub: take the Electromagnetic Forces Quiz, then the Electromagnetism Structured Set.

Lessons

Work through these lessons in order.

  1. Magnetic fields produced by currents
  2. Conductor force, flux density, current balance and parallel currents
  3. Force on a moving charge
  4. Charged-particle beams in uniform fields
  5. Velocity selection in crossed fields
  6. Magnetic Fields Due to Currents

    Use standard results for B due to a long straight wire, circular coil centre and long solenoid, and solve B and force-per-length questions (A Level Physics).

  7. Magnetic Flux Density

    Define magnetic flux density B in tesla, use F = BIl sinθ, and solve force and B calculations for wires in magnetic fields (A Level Physics).

  8. Current Balance (Measuring Magnetic Flux Density)

    Use a current balance to measure magnetic flux density B from mass changes, including reversed-current readings, correct directions and units (A Level Physics).

  9. Motion of a Moving Charge in a Uniform Magnetic Field

    Use F = Bqv sinθ and circular-motion ideas to analyse the motion of a charged particle in a uniform magnetic field, including helical paths (A Level Physics).

  10. Deflection of Charged Particles (Electric & Magnetic Fields)

    Analyse charged-particle beam deflection in uniform electric and magnetic fields using qE, Bqv and circular/projectile motion methods (A Level Physics).

  11. Velocity Selector

    Use crossed electric and magnetic fields to select particles of speed v = E/B, with clear force directions and exam-style calculations (A Level Physics).

Revision

Field-pattern checkpoint
Magnetic field patterns produced by currentsThree labelled diagrams show concentric magnetic field lines around a straight wire, the axial field through a circular coil, and the nearly uniform field inside a long solenoid.Long straight wirecurrent out of pageconcentric circles; B decreases with rFlat circular coilB at centrefield is strongest through the centreLong solenoidinside: parallel, equally spaced field lines
Scroll diagram horizontally to read all labels.
Use the right-hand grip rule for every current direction. Inside a long solenoid, the closely spaced parallel lines represent an approximately uniform field.

The current direction fixes the field direction. In calculations, identify the conductor geometry before choosing an equation.

Quick reference
SituationRelationshipDirection or check
Long straight wireB = μ₀I/(2π r)circles around the wire; B ∝ 1/r
Flat circular coil centreB = μ₀NI/(2r)field along the coil axis
Long solenoid interiorB = μ₀nInearly uniform inside
Wire in a magnetic fieldF = BIl sin θFleming’s left-hand rule
Moving chargeF = B| q| v sin θreverse the positive-charge result for an electron
Perpendicular particle motion$r=mv/(q
Velocity selectorv = E/Brequires equal, opposite electric and magnetic forces
Particle-motion checkpoint
Charged-particle paths in a uniform magnetic fieldOne panel shows a positive charge following a circle when velocity is perpendicular to the magnetic field. A second panel shows a helical path when velocity has both perpendicular and parallel components.velocity perpendicular to B+vF toward centrespeed constant; radius r = mv/(|q|B)perpendicular and parallel componentsB; parallel motion →+parallel speed sets pitch; perpendicular speed sets radius
Scroll diagram horizontally to read all labels.
The perpendicular velocity component produces circular motion. A parallel component is unchanged, so the combined path is helical.

Use magnitudes in the calculation and state the force or curvature direction separately. The sign of the charge changes direction, not force magnitude.

Exam traps
  1. Flux density is not magnetic flux. B is measured in tesla; flux Φ is measured in weber.
  2. The angle is with the field. In both force equations, θ is between the current or velocity and vector B.
  3. Negative charge reverses direction. Find the positive-charge force first, then reverse it for an electron.
  4. Magnetic force does no work. It can bend a path without changing kinetic energy.
  5. A selector needs opposing forces. Equal magnitudes alone are insufficient if the electric and magnetic forces point the same way.
  6. Field formula depends on geometry. Do not transfer the straight-wire 1/r dependence to a solenoid.

Practice

Quiz and structured practice
A Level Electromagnetic Forces QuizElectromagnetism Structured Set

Use the quiz to diagnose a weak outcome, then complete one fields-from-currents question and one charged-particle question.

Next hub: Electromagnetic Induction

Back to A Level Physics

Continue with the next resource in this course.

Course and syllabus information
Course
GCE A-Level H2 Physics
Edition
GCE A-Level H2 Physics 2027