Electromagnetism

Study H1 Physics electric and magnetic fields, predict forces on currents and moving charges, and explain velocity selection.

GCE A-Level H1 Physics 2027
3 lessons

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Before you begin

Electric and magnetic fields both push on charges, but in very different ways. An electric field pushes on a charge whether it moves or not, and can speed it up; a magnetic field pushes only on a moving charge, always at right angles to its motion. The lessons start with charged particles in a uniform electric field, then sketch the magnetic fields of currents and use the force on a current to define and measure flux density. They finish with the force on a moving charge, beam deflection and velocity selection.

Be comfortable with: fields and potential energy, because the electric field strength is force per unit charge, and current and drift velocity, because a current is moving charge.

Learning goals

Calculate field strength and force in uniform electric fields.
Analyse charged-particle motion in uniform electric fields.
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 in magnetic fields, including current balances.
Recall and use F = BIl sin θ, with Fleming's left-hand rule for direction
Explain how a current balance uses the force on a current-carrying conductor to measure magnetic flux density
Analyse forces and paths of moving charges in uniform fields.
Apply crossed electric and magnetic fields to velocity selection.

Syllabus statements covered

calculate the force on a charge in a uniform electric field
describe the effect of a uniform electric field on the motion of a charged particle
show an understanding that a magnetic field is an example of a field of force produced either by current-carrying conductors or by permanent magnets
sketch magnetic field lines due to currents in a long straight wire, a flat circular coil and a long solenoid
show an understanding that a current-carrying conductor placed in a magnetic field might experience a force
recall and solve problems using the equation F = BIlsinθ, with directions as interpreted by Fleming’s left-hand rule
define magnetic flux density as the force acting per unit current per unit length on a conductor placed perpendicular to the magnetic field
show an understanding of how the force on a current-carrying conductor can be used to measure the flux density of a magnetic field using a current balance
predict the direction of the force on a charge moving in a uniform magnetic field
recall and solve problems using the equation F = BQv sin θ
describe and analyse deflections of beams of charged particles by uniform electric fields and uniform magnetic fields
explain how perpendicular electric and magnetic fields can be used in velocity selection for charged particles

Lessons

Work through them in order.

Practise and check

Or choose

Topic reference

Quick reference

Idea	Relationship	Note
force in an electric field	F = qE	along E for a positive charge; opposite for a negative one
acceleration in a uniform field	a = qE/m	constant; path is a parabola if entering at right angles
force on a conductor	F = BIℓ sin θ	direction from Fleming’s left-hand rule
magnetic flux density	B = F/Iℓ	conductor at right angles to the field; 1 T = 1 N A⁻¹ m⁻¹
force on a moving charge	F = Bqv sin θ	at right angles to v, so no work is done
velocity selector	qE = Bqv, so v = E/B	independent of charge and mass

Core knowledge to remember

A magnetic field is a field of force produced either by current-carrying conductors or by permanent magnets.
Around a long straight wire the field lines are concentric circles; a flat circular coil has a field like a short bar magnet; inside a long solenoid the field is uniform.
A current-carrying conductor in a magnetic field may experience a force, with direction given by Fleming’s left-hand rule.
Magnetic flux density is the force acting per unit current per unit length on a conductor placed perpendicular to the magnetic field.
A current balance measures B from the force on a conductor of known length carrying a known current.
A magnetic force on a moving charge is perpendicular to its velocity, so it changes the direction of motion but not the speed.
In perpendicular electric and magnetic fields, particles with speed v = E/B pass through undeflected.

Common mistakes

Making a negative charge accelerate in the direction of the electric field.
Mixing up the right-hand grip rule (field around a current) and Fleming’s left-hand rule (force on a current).
Leaving out sin θ, or using the whole wire length when only part of it is in the field.
Saying a magnetic field changes a charged particle’s speed.
Forgetting to reverse the force for a negative charge in a magnetic field.