Magnetism and electromagnetism

Revision notes

Magnets have north and south poles. Like poles repel and unlike poles attract. A magnetic field is a region where a magnetic material, magnet or current-carrying conductor experiences a force. Field lines show the direction of the force on a north pole; around a bar magnet they run from north to south outside the magnet. Iron, nickel and cobalt are common magnetic materials. An induced magnet becomes magnetic in an external field and usually loses most or all of its magnetism when the field is removed.

A current in a wire produces a magnetic field. A solenoid produces a field similar to a bar magnet, and an iron core can strengthen it. A current-carrying conductor placed in a magnetic field can experience a force: this is the motor effect. For a conductor perpendicular to a uniform field, F = BIL, where B is magnetic flux density, I current and L the length in the field. Reversing the current or field reverses the force.

Electromagnetic induction occurs when the magnetic flux linking a conductor changes, producing a potential difference. A larger rate of change produces a larger induced potential difference. Transformers use alternating current and changing magnetic fields; for an ideal transformer \(\dfrac{Vp}{Vs}\) = \(\dfrac{Np}{Ns}\), where N is the number of turns.

Magnetic fields and forces

A magnetic field is a region where a magnetic material, magnet or current-carrying conductor can experience a force. Outside a bar magnet, field lines are conventionally drawn from north to south. The closer the field lines, the stronger the field. Permanent magnets produce their own field; induced magnets become magnetic because of an external field and may lose much of that magnetism when the field is removed.

A current in a wire produces a magnetic field. Around a straight wire the field forms concentric circles, while a solenoid produces a field similar to a bar magnet. Increasing current or the number of turns per unit length strengthens a solenoid's field; adding a suitable iron core can make an electromagnet much stronger.

Motor effect

A current-carrying conductor in a magnetic field can experience a force because the field around the current interacts with the external field. The force direction is perpendicular to both current and field and is found with Fleming's left-hand rule: first finger = field (north to south), second finger = current (conventional, positive to negative), thumb = motion (force). For a conductor at right angles to the field, F = BIL, so increasing current, magnetic flux density or the length of conductor in the field increases the force.

In an electric motor, forces on opposite sides of a coil form a turning effect. A split-ring commutator in a simple d.c. motor reverses current every half-turn so the turning effect continues in the same rotational sense.

Electromagnetic induction

A potential difference is induced when the magnetic field linking a conductor changes. This can happen by moving a magnet relative to a coil, moving a conductor through a field, or changing the current producing the field. Faster change generally produces a larger induced potential difference. If the circuit is complete, an induced current flows.

The induced effect opposes the change that produced it. This is why mechanical work is required to keep a generator turning under load: electrical energy output is not created for free.

The generator effect is used in alternators (a rotating coil with slip rings gives a.c.) and dynamos (a split-ring commutator gives d.c.). A moving-coil microphone uses induction to turn sound vibrations into a varying p.d.; a loudspeaker uses the motor effect to turn a varying current into vibrations.

Transformers and generation

A transformer has two coils on an iron core. An alternating current in the primary coil produces a changing magnetic field in the core, which induces an alternating p.d. in the secondary coil; a direct current would not work because the field would not change. For an ideal transformer \(\dfrac{Vp}{Vs}\) = \(\dfrac{Np}{Ns}\), and if it is 100% efficient the power in equals the power out: VpIp = VsIs. A step-up transformer has more turns on the secondary; a step-down transformer has fewer. National-grid systems use high transmission voltages so a given power can be carried with lower current, reducing I²R heating losses in cables.

Exam technique

Keep three ideas separate: current produces magnetic fields; a current in an external field can experience a force; a changing magnetic linkage can induce a potential difference. Many weak answers mix these mechanisms together.