Magnetism and Electromagnetism Explained
Magnetism and electricity are two aspects of the same fundamental force. Understanding their relationship — how an electric current creates a magnetic field and how a changing magnetic field drives an electric current — explains everything from electric motors and generators to the transformers that bring electricity to your home.
Magnetic Fields and Field Lines
A magnetic field is a region of space in which a magnetic material or moving charge experiences a force. We represent this invisible field with magnetic field lines: lines that show the direction a free north pole would move. Several rules govern field lines: they run from north to south outside the magnet; they never cross; they form closed loops (passing south to north inside the magnet); and the closer together the lines, the stronger the field.
Every magnet has two poles: north and south. Like poles repel; unlike poles attract. This behaviour is a result of magnetic domains — microscopic regions within a magnetic material where atoms are aligned. In an unmagnetised iron bar, domains point in random directions and cancel out. Stroking the iron repeatedly with a permanent magnet, or placing it in a strong magnetic field, aligns the domains to produce a net magnetisation.
Electromagnetism: Current Creates a Field
In 1820, Hans Christian Oersted discovered that a compass needle deflected when placed near a wire carrying an electric current — proof that electric currents generate magnetic fields. The field around a straight wire forms concentric circles in the plane perpendicular to the wire. The right-hand grip rule for a straight wire: point the thumb of your right hand in the direction of conventional current; your fingers curl in the direction of the magnetic field.
Coiling the wire into a solenoid (a helix of loops) concentrates and reinforces the individual field circles so that the combined field resembles that of a bar magnet — with a clear north pole at one end and a south pole at the other. For a solenoid, the right-hand grip rule becomes: curl the fingers of the right hand in the direction of current around the coil; the thumb points toward the north pole. Inserting a soft-iron core inside the solenoid concentrates the field dramatically, creating an electromagnet. The field strength can be increased by raising the current, increasing the number of turns, or using a more permeable core material.
The Motor Effect
A current-carrying conductor placed inside an external magnetic field experiences a force — the motor effect. This is because the two fields (the conductor's own field and the external field) add on one side and subtract on the other, creating an unequal field that pushes the conductor. The direction of the force is given by Fleming's left-hand rule: hold the thumb, index finger, and middle finger of the left hand mutually at right angles. The index finger points in the direction of the field (north to south), the middle finger in the direction of conventional current, and the thumb shows the direction of the resulting force (the thrust or motion).
The force is maximised when the current and field are perpendicular, and is zero when they are parallel. A rectangular current-carrying coil placed between the poles of a magnet experiences forces on its two sides in opposite directions, producing a turning effect — a torque. This is the principle of the DC electric motor: a split-ring commutator reverses the current direction every half-turn, keeping the coil spinning continuously in the same direction.
Faraday's Law (1831): The magnitude of the induced EMF in a circuit is directly proportional to the rate of change of magnetic flux linkage through that circuit. In short: move a magnet faster into a coil, or use more turns, and the induced voltage is larger.
Lenz's Law: The direction of the induced current is always such that it opposes the change that caused it. If you push a north pole into a coil, the induced current flows so as to create a north pole facing you — repelling the approaching magnet. This is an expression of the conservation of energy: you must do work against the opposing force, and that work appears as electrical energy.
Together, these laws are captured in Faraday's equation: EMF = −N ΔΦ/Δt, where N is the number of turns, Φ is the magnetic flux, and the negative sign encodes Lenz's law.
Generators and Transformers
An AC generator (alternator) works by rotating a coil inside a magnetic field. As the coil turns, the rate at which field lines are cut changes continuously, producing a sinusoidally alternating EMF. Slip rings maintain a continuous connection as the coil rotates, so the output alternates rather than being rectified by a commutator.
A transformer uses the principle of electromagnetic induction to change the voltage of an AC supply. An alternating current in the primary coil creates a constantly changing magnetic flux in a soft-iron core; this changing flux induces an EMF in the secondary coil. The ratio of voltages equals the ratio of turns: Vs/Vp = Ns/Np. A step-up transformer (more secondary turns) increases voltage and reduces current; a step-down transformer does the reverse. Power = V × I, so reducing current proportionally raises voltage, transmitting power over long distances with low heat losses in the cables. This is why the National Grid transmits electricity at hundreds of thousands of volts before stepping it down for safe household use.
Summary
Magnetic field lines map the invisible force around magnets and current-carrying conductors. The right-hand rule links current direction to field direction; Fleming's left-hand rule predicts the motor force. Electromagnetic induction — governed by Faraday's and Lenz's laws — converts mechanical energy into electrical energy in generators, and allows transformers to alter voltage levels for efficient power transmission. Together these principles underpin virtually all modern electrical technology.