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Electromagnetism

14 min read

Magnetic effect of an electric current

A wire carrying an electric current is surrounded by a magnetic field. Hans Oersted discovered this in 1820. The field lines are circles around a straight wire. The direction is given by the right-hand grip rule: grip the wire with the thumb pointing along the current; the fingers curl in the direction of the field. A coil of wire (a solenoid) has a field like a bar magnet. Putting a soft iron core inside makes a strong electromagnet.

Magnetic fields of currentsOn the left, a wire carrying current towards the viewer has circular magnetic field lines around it, in the direction given by the right-hand grip rule. On the right, a solenoid, a long coil of wire, has a field like a bar magnet with a north pole at one end and a south pole at the other.•Straight wire: circular fieldcurrent towards youSolenoid: field like a bar magnetN at one end, S at the other

The strength of an electromagnet can be increased by: a larger current, more turns of wire and a soft iron core. It can be switched on and off, so it is used in cranes, relays, electric bells and loudspeakers.

Force on a current-carrying conductor

A wire carrying a current in a magnetic field feels a force, because the field of the wire interacts with the magnetic field. Fleming’s left-hand rule: with the thumb, first and second fingers at right angles, the First finger points along the Field, the SeCond finger along the Current and the Thumb along the Thrust (force).

F = force on the wire (N), B = magnetic flux density (tesla, T), I = current (A), L = length of wire in the field (m). The force is greatest when the wire is at right angles to the field.
  • magnetic flux density (tesla, T)
Try itForce on a wire

Choose the field strength, the current and the length of wire in the field.

The d.c. motor

A motor turns electrical energy into movement. A coil in a magnetic field has a current going up one side and down the other, so the forces on the two sides are opposite and make the coil turn. A split-ring commutator reverses the current every half turn so the coil keeps turning in the same direction. A motor turns faster with a bigger current, stronger magnets or more turns.

A simple d.c. motorA rectangular coil lies between the north and south poles of a magnet and is connected through a split-ring commutator and carbon brushes to a battery. Current flows up one side and down the other, so the forces on the two sides are opposite and turn the coil. The commutator reverses the current every half turn so the coil keeps turning the same way.NSforce upforce downcoilbrushessplit-ring commutator

Electromagnetic induction

If a conductor cuts magnetic field lines, or the magnetic field through a coil changes, an e.m.f. is induced. Faraday’s law: the induced e.m.f. is proportional to the rate of change of magnetic flux. Lenz’s law: the induced current flows in a direction that opposes the change that causes it. A bigger induced e.m.f. is obtained by moving the magnet faster, using a stronger magnet or more turns.

The generator and the transformer

An a.c. generatorA coil turns between the poles of a magnet. As it cuts the magnetic field lines, a voltage is induced. Slip rings and brushes carry it out. The output changes direction twice every turn, so it is an alternating voltage drawn as a sine wave. A faster turn or a stronger magnet gives a bigger voltage.NSslip rings and brushesturnvoltage against time

A generator (dynamo) turns movement into electricity. A coil rotating in a magnetic field has an e.m.f. induced; slip rings carry the output, which is alternating current (a.c.). Power stations use the same idea with huge turbines driven by steam, water or wind.

A transformerTwo coils of wire wound on a soft iron core. An alternating current in the primary coil makes a changing magnetic field in the core, which induces an alternating voltage in the secondary coil. The ratio of the voltages equals the ratio of the numbers of turns. A step-up transformer has more turns on the secondary and a step-down has fewer.primary coilNₚ turns, Vₚsecondary coilNₛ turns, Vₛsoft iron coreVₛ ÷ Vₚ = Nₛ ÷ Nₚ
A transformer works only with alternating current. A step-up transformer has more turns on the secondary and a step-down fewer. If it is 100% efficient the power in equals the power out, so a higher voltage means a lower current.
Try itA transformer

Choose the primary voltage and the number of turns on each coil. Find the secondary voltage and the current if the transformer is 100% efficient and the secondary current is 2 A.

Power transmission: electricity is sent over long distances at very high voltage (for example 132 000 V). A high voltage means a small current, so little energy is wasted as heat (I²R) in the cables. Step-down transformers then reduce the voltage for homes and factories. Tanzania’s national grid uses this system.

Field lines leave the north pole and enter the south pole.
Check yourself The direction of the force on a current-carrying wire in a magnetic field is found by…
Check yourself A transformer has 500 primary turns and 50 secondary turns, with a primary voltage of 240 V. The secondary voltage is…
Check yourself Why is electricity transmitted at a very high voltage?

Key terms

Solenoid
A long coil of wire that produces a magnetic field when a current flows.
Electromagnetic induction
The production of an e.m.f. by a changing magnetic field.
Commutator
A split ring that reverses the current in a motor every half turn.
Transformer
A device that changes the size of an alternating voltage.

Summary

  • A current produces a magnetic field; a solenoid with an iron core is an electromagnet.
  • A current-carrying wire in a magnetic field feels a force F = BIL (Fleming’s left-hand rule); motors use this.
  • A changing magnetic field induces an e.m.f. (Faraday, Lenz); generators use this.
  • Transformers: Vs/Vp = Ns/Np; high-voltage transmission reduces energy loss.
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