Theme D — Fields

D.3 Motion in Electromagnetic Fields — IB Physics SL Revision Notes & Topical Questions

5 min read 10 parts Lesson 15 of 21 1 quiz

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Lesson summary

D.3 Motion in Electromagnetic Fields — IB PHYSICS SL notes: Analyse the motion of charged particles in uniform electric and magnetic fields and calculate the forces on moving charges and current-carrying conductors.Force — F = qE is constant in size and directionAcceleration — a …

Zero force
Centripetal force
Arrangement
Undeflected particles
Velocity selector
What the symbols mean

Part 1 of 8Charge in a uniform electric field

Force

F = qE is constant in size and direction

Acceleration

Acceleration a = qE/m

Path

Path a charge entering at right angles keeps its original velocity component and accelerates along the field, following a parabola like a projectile under gravity

Part 2 of 8Magnetic force on a moving charge

Equation

F = qvB sin θ, where θ is the angle between v and B

Direction

Direction perpendicular to both v and B
The left-hand rule gives it for conventional current and is reversed for negative charges.

Zero force

Zero force none acts on a stationary charge or one moving parallel to the field

No work

No work the force does no work, so the speed is constant

Part 3 of 8Circular motion in a magnetic field

Centripetal force

Centripetal force the force is always perpendicular to the velocity, so qvB = mv²/r

Radius

Radius r = mv/(qB)

Period

T = 2πm/(qB), which does not depend on speed

Part 4 of 8Crossed fields

Crossed fields

Arrangement

E and B are perpendicular and the forces oppose each other

Undeflected particles

Undeflected particles qE = qvB, so v = E/B

Velocity selector

Velocity selector this arrangement passes only particles of speed E/B

Part 5 of 8Force on a conductor

Equation

F = BIL sin θ
What the symbols mean: F = force on the wire (N) · B = magnetic flux density (T) · I = current (A) · L = length of wire in the field (m) · θ = angle between wire and field

Flux density

B is defined from this, and 1 T = 1 N A⁻¹ m⁻¹

Part 6 of 8Parallel wires

Each wire lies in the magnetic field of the other

Force per unit length

F/L = μ₀I₁I₂/(2πr), with μ₀ = 4π × 10⁻⁷ T m A⁻¹

Parallel currents attract and antiparallel currents repel

Part 7 of 8Solving problems

Identify the sign of the charge, the velocity direction and the field direction.

Use F = qE along the field (reversed for a negative charge) or the left-hand rule for the magnetic force.

For an electric field use projectile-style motion with a = qE/m.

For a magnetic field set qvB = mv²/r for circular motion.

Part 8 of 8Electric and magnetic fields compared

Feature	Uniform electric field	Uniform magnetic field
Force	qE	qvB sin θ
Direction	Parallel to the field	Perpendicular to field and velocity
Acts on a stationary charge?	Yes	No
Work done	Yes, speed changes	None, speed constant
Path (entering at 90°)	Parabola	Circle

Key terms

Term	Meaning
Magnetic flux density	The force per unit current per unit length on a conductor at right angles to the field; unit tesla.
Tesla	1 T = 1 N A⁻¹ m⁻¹.
Velocity selector	An arrangement of perpendicular electric and magnetic fields that passes only particles of speed E/B.
Left-hand rule	A rule giving the direction of the force on a current or positive charge moving in a magnetic field.
Permeability of free space	The constant μ₀ = 4π × 10⁻⁷ T m A⁻¹.

Avoid these mistakes

Saying the magnetic force acts along the direction of motion.

Claiming a magnetic force changes the speed of a charge.

Using electron-flow direction in the hand rule instead of conventional current.

Last updated: 18 October 2026

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