Magnetism Basics — Fields, Poles and the Right-Hand Rule

High School⏱ 10 min readElectricity & Magnetism

Magnetism: poles, magnetic fields, fields from currents, the force on moving charges (F = qvB), and the right-hand rule.

Quick answer

Magnetism is the force between moving charges. Magnets have inseparable north/south poles; electric currents generate magnetic fields; magnetic fields exert forces on moving charges — always perpendicular to their motion.

$F = qvB\sin\theta$

Unit: tesla (T) for B

Example: An electron moving at 10⁶ m/s perpendicular to a 0.5 T field feels F = (1.6×10⁻¹⁹)(10⁶)(0.5) ≈ 8×10⁻¹⁴ N — tiny, but enough to bend its path into a circle.

What you will learn

Describe magnetic poles and fields
Explain how currents create magnetic fields
Use F = qvB for the force on a moving charge
Apply the right-hand rule correctly

Poles always come in pairs

Every magnet has a north and south pole — and they are inseparable. Slice a bar magnet in half and each half is a complete magnet with its own N and S. Slice again: same result, forever.

Like poles repel, opposites attract — the familiar rule. But unlike electric charge, there is no isolated 'magnetic charge': magnetism is not a separate substance but a consequence of moving electric charge, as Einstein's relativity reveals.

Magnetic fields from currents

Oersted's 1820 accident — a compass needle twitching near a current-carrying wire — united electricity and magnetism. Moving charges create magnetic fields:

Straight wire: circular field lines around it, $B = \mu_0 I / 2\pi r$ .
Loop/coil: field concentrated through the center — an electromagnet.
Direction given by the right-hand grip rule: thumb along current, fingers curl along B.

Every electromagnet, motor, and MRI magnet is this principle scaled up.

Force on a moving charge: F = qvB

A charge $q$ moving at velocity $v$ through field $B$ feels:

$F = qvB\sin\theta$

Maximum when motion is perpendicular to the field ( $\theta = 90°$ ).
Zero when moving parallel to the field ( $\theta = 0°$ ) — sliding along field lines feels nothing.
Direction from the right-hand rule: point fingers along $v$ , curl toward $B$ — thumb gives $F$ (for positive charge; flip for electrons).

F = qvB\sin\theta

θ = angle between v and B

The right-hand rule

The right-hand rule is worth practicing until it's reflex:

Force on charge: fingers = $v$ , curl toward $B$ , thumb = $F$ (positive charge).
Field around wire: thumb = current, curled fingers = $B$ direction.
Force on current wire: fingers = current, $B$ into palm... (variants exist — pick one and drill it).

Because $F \perp v$ always, magnetic forces bend paths into curves and circles — the basis of cyclotrons and the aurora's spiraling particles.

Key idea

Magnetic force steers but never speeds up: always perpendicular to velocity, so it changes direction only. Any problem asking about magnetic work has answer zero.

Earth's magnetic field: the compass

Earth itself is a giant (weak) magnet — field ~ $50\ \mu\mathrm{T}$ at the surface, generated by swirling liquid iron in the outer core (the geodynamo).

A compass needle is a tiny magnet: its north-seeking end aligns with Earth's field lines, pointing roughly toward geographic north. Note the subtlety: Earth's magnetic north pole is actually a magnetic south pole (opposites attract) — one of physics' favorite naming ironies.

Variables and units

Symbol	Name	Unit	Meaning
$B$	Magnetic field	T	Magnetic field strength
$q$	Charge	C	Charge of the moving particle
$v$	Velocity	m/s	Velocity of the charge
$θ$	Angle	degrees	Angle between velocity and field

Common mistakes

✗ You can isolate a north pole by cutting a magnet.

Every fragment becomes a complete dipole with its own north and south. Poles are inseparable — a consequence of magnetism arising from moving charge, not magnetic 'stuff'.

✗ Magnets attract all metals.

Only ferromagnetic materials (iron, nickel, cobalt, some alloys) feel strong attraction. Aluminum, copper, gold — no noticeable pull.

✗ Magnetic force does work on moving charges.

F = qvB is always perpendicular to velocity, so it bends trajectories without changing speed. No work, no kinetic energy change — just steering.

Real-world applications

Compasses align with Earth's magnetic field — a planetary-scale magnetometer in your pocket.
Electric motors spin by arranging magnetic forces on current-carrying wires; every EV and appliance depends on it.
MRI scanners use superconducting magnets ~60,000× Earth's field to align hydrogen nuclei in your body.

Frequently asked questions

What is the unit of magnetic field?

The tesla (T) — a strong unit; Earth's field is ~50 microteslas. MRI magnets run 1.5–3 T. The older gauss: 1 T = 10,000 G.

Do magnetic monopoles exist?

As far as anyone has found, no. Cut a magnet in half and you get two smaller dipole magnets, not isolated poles. Maxwell's equations are written assuming zero magnetic charge.

Can magnetic fields do work on charges?

No — magnetic force is always perpendicular to velocity, so it changes direction but never speed. It does zero work on the charge (though it can do work via induced electric fields).

How do I make an electromagnet?

Wrap the wire in a coil around an iron core and run current through it. More turns and more current mean a stronger field — and it switches off with the current.