Magnetism: Electromagnetism

Magnetism: Electromagnetism


  • An electromagnet is a type of magnet that is created by an electric current.
  • It consists of a coil of wire, often wound around a soft iron core. The iron core becomes magnetised when current flows through the coil.
  • The strength of the magnetic field produced by an electromagnet can be increased by using more turns of wire in the coil, increasing the electric current through the coil, or using a larger and more permeable core material.
  • Electromagnets have the advantage of being able to be turned on and off by controlling the electrical current flowing through the coil.
  • Unlike permanent magnets, electromagnets can have their polarity reversed by changing the direction of the electric current.
  • Common uses of electromagnets include in electric motors, generators, loudspeakers, memory devices, and magnetic resonance imaging (MRI) machines.

The Motor Effect

  • The motor effect is a phenomenon where a force is exerted on a wire which is carrying an electric current and is placed in a magnetic field.
  • The force is at a maximum when the wire is perpendicular to the magnetic field lines, and is zero when the wire is parallel to the field lines.
  • The direction of the force can be determined using Fleming’s left-hand rule.
  • The magnitude of the force is directly proportional to the strength of the magnetic field, the amount of current, and the length of wire in the magnetic field.
  • This effect is employed in electric motors to produce rotational motion.

Electromagnetic Induction

  • Electromagnetic induction is the phenomenon where a voltage or electromotive force (EMF) is induced in a conductor when it experiences a changing magnetic field.
  • The rate of change of the magnetic field, the area of the loop, and the number of turns in the loop all affect the magnitude of the induced EMF.
  • This principle is used in generators to convert mechanical energy into electrical energy and in transformers to increase or decrease the voltage of alternating current (AC).
  • Lenz’s law states that the direction of the induced current is such that it will oppose the change that created it.

The Generator Effect

  • The generator effect takes advantage of electromagnetic induction to transform mechanical energy into electrical energy, typically by rotating a coil in a magnetic field.
  • The output is an alternating current. The frequency of the AC output is directly proportional to the speed of rotation.
  • If a piece of metal wire moves relative to a magnetic field or if the magnetic field through a loop of wire changes, a potential difference is induced. This is called the generator effect.
  • The generator effect underpins the operation of generators, alternators, and dynamos.