Magnetism and magnetic fields Study Guide

When an electric current flows through a conductor, it generates a magnetic field around that conductor. This phenomenon is fundamental to electromagnetism and has numerous applications, from electric motors to loudspeakers. The strength of this magnetic field is not uniform; it varies depending on two primary factors: the magnitude of the current flowing through the conductor and the distance from the conductor itself.

Firstly, the strength of the magnetic field is directly proportional to the current. This means that if you increase the current flowing through the wire, the magnetic field produced will become stronger. Conversely, if you decrease the current, the field will weaken. This relationship is intuitive: more moving charges (higher current) create a more pronounced magnetic effect.

Secondly, the strength of the magnetic field is inversely proportional to the distance from the conductor. This means that as you move further away from the wire carrying the current, the magnetic field becomes weaker. The field lines, which represent the direction and strength of the magnetic field, become more spread out as you move away from the source. Therefore, a point close to the wire will experience a much stronger magnetic field than a point further away, assuming the current remains constant. For a long, straight wire, the field strength decreases proportionally to 1/r, where 'r' is the distance from the wire.

This understanding is crucial for designing and understanding electromagnetic devices, as it allows engineers to predict and control the magnetic effects produced by electric currents.

The generation of electrical current in both alternators and dynamos relies on the principle of electromagnetic induction. This occurs when a conductor moves through a magnetic field, or when the magnetic field around a conductor changes, inducing an electromotive force (e.m.f.) across the conductor which, if part of a complete circuit, drives a current. The direction of the induced current is given by Fleming's Right-Hand Rule. The magnitude of the induced e.m.f. depends on the rate at which the magnetic field lines are cut; this means that a faster movement, a stronger magnetic field, or more turns in the coil will all increase the induced e.m.f.

An alternator uses slip rings and carbon brushes to connect the rotating coil to an external circuit. As the coil rotates continuously within a magnetic field, its sides cut the magnetic field lines in opposite directions, causing the induced current to reverse direction every half turn. This produces an alternating current (a.c.), which changes direction periodically. The slip rings maintain continuous contact, allowing the a.c. to be delivered to the external circuit.

A dynamo, in contrast, uses a commutator (a split-ring commutator) instead of slip rings. The commutator is a device that reverses the connections of the coil to the external circuit every half rotation precisely when the direction of the induced current in the coil reverses. This means that although the current within the coil still alternates, the current delivered to the external circuit always flows in the same direction, producing a direct current (d.c.). The output of a simple dynamo is not perfectly constant d.c.; it is a pulsating d.c. because the magnitude of the induced e.m.f. varies during each rotation, peaking when the coil sides are moving perpendicular to the magnetic field lines and dropping to zero when they are moving parallel to them. This pulsating effect can be reduced by using multiple coils and commutator segments.