CURRENT ELECTRICITY AND MAGNETISM (4)

1. The motor effect:

When a current-carrying conductor is placed in a magnetic field, there is an interaction between the magnetic field produced by the current and the permanent field, which leads to a FORCE being experienced by the conductor:

This is demonstrated by the following experiment: if a conductor is placed between the poles of two bar magnets that are aligned as shown in the diagram on the right, and a current allowed to flow as shown, the conductor will suddenly jump to the right. If either the current flow or the polarities of the magnets are reversed, the conductor will move in the opposite direction.

The magnitude of the force on the conductor depends on the magnitude of the current which it carries. The force is a maximum when the current flows PERPENDICULAR to the field (as shown in diagram A on the left below), and it is zero when it flows parallel to the field (as in diagram B, on the right):


The greater the length of the conductor perpendicular to the magnetic field, the greater the force on it.

The direction of the force is defined by the diagram on the left, where the axes are at right angles to each other. (F = force, , B = magnetic field, and I = current).


Most people find it easy to remember this diagram by applying Fleming's left hand motor rule: If you point your left forefinger in the direction of the magnetic field, and your second finger in the direction of the current flow, then your thumb will point naturally in the direction of the resulting force!


The motor effect provides a definition for the unit of MAGNETIC FLUX DENSITY, the TESLA, T, named in honour of the Serbian-American inventor Nicola Tesla:

The tesla is the density of a magnetic field such that a conductor carrying a current of 1 ampere at right angles to that field has a force of 1 newton acting upon it.

2. Galvanometers

The force that is exerted on a current-carrying conductor in a constant magnetic field is dependent on the strength of the current - the greater the current, the greater the force on the conductor. This principle is used in instruments used to determine the current strength. These instruments are called GALVANOMETERS. The internal circuits built into these instruments will determine whether they will be used to measure current, in which case they are called AMMETERS, or whether they will be used to measure potential difference, in which case, they are called VOLTMETERS.

A wire carrying the current to be measured is wound as shown on the right around a soft iron core. A needle is attached to the outgoing wire. The current flow causes a force which tends to twist the wire, and this is seen as a needle deflection. The deflection is measured on a scale (not shown) which is calibrated in units of current or potential difference.

3. The electric motor

The principle that a current flowing through a conductor which is placed in a magnetic field results in a mechanical effect, that is the conductor experiences a force, is exploited in ELECTRIC MOTORS, essential components of our day-to-day life. Note that heat is also produced, so there is never a 100% conversion of electrical energy into mechanical energy.

In its most basic form, an electric motor consists of a rectangular coil of insulated wire, which makes up the armature, or moving part of the motor. The commutator acts as a current- reversing switch after every half-revolution of the coil.

The brushes serve to make contact between the battery and the rotating commutator, which is mounted on an insulated shaft, not shown in the picture.

When the current is switched on, it flows in opposite directions along the two segments of the coil, generating equal but opposite thrusts, which form a couple tending to rotate the coil. The momentum of the coil carries it past the point where the current is short-circuited, and beyond that point, the current is reversed in the coil, but the thrusts remain in the same direction, ensuring the continuous rotation of the coil while the current is flowing.

If the direction of the current, or if the poles of the magnet, are reversed, rotation will proceed in the opposite direction. The combination of field, current and thrust (F) are shown in the diagram on the right.

4. Additional questions