Current and magnetism

The direction of the magnetic fields set up by the current flowing in the conductor can be illustrated as in figure 6.5 (a) and (b).

Figure 7.5

Figure 7.5 (a) indicates a magnetic field in a clockwise direction around a conductor for current which flows towards the observer. The spot in the centre indicates this. Figure 7.5 (b) indicates a magnetic field in an anti-clockwise direction around a conductor for current which flows away from the observer. The cross in the centre indicates this.

Conductors will follow the same rules as permanent magnets in that attraction and/or repulsion of these magnetic fields can take place. This is indicated by figure 7.6 (a) and (b). Figure 7.6 (a) shows that two conductors carrying current in the same direction will cause the magnetic fields to be attracted to one another and figure 6.6 (b) shows that two current in the opposite direction will cause the magnetic fields to be repelled from one another.

N S

+

Current toward

the observer Current away

from the observer

( a ) ( b )

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+

Current in opposite directions and magnetic fields will repel

( b ) Current in the same direction

and magnetic fields will attract ( a )

Figure 7.6

A force will be exerted onto the current carrying conductors and the direction of this force can be illustrated by Fleming’s left-hand rule as follows. Arrange your left-hand index finger, middle finger and thumb at right angles to one another. Point the index finger in the direction of the magnetic field and the middle finger in the direction of the current flowing through the conductor. Your thumb will then indicate the direction of the force exerted on the conductor. Irrespective of whether the magnetic fields attract or repel one another, a force will be exerted onto the conductors carrying the current.

This force being exerted forms the basis of the unit of current namely the ampere to be formulated.

This definition may be expressed mathematically as follows.

_-7

F = 2 × 10 × I1 × I2 × ℓ where I₁ = current in conductor in ampere ^d I₂ = current in conductor in ampere

ℓ = length of conductor in metres

d = distance apart in metres

F = force exerted in N/m

It should be noted that this force exerted between the two conductors will be the same irrespective of whether they are attracted or repelled from one another.

Definition 7.2 Ampere

The ampere is that current flowing in two infinitely long conductors with negligible cross-sectional area and placed one metre apart in a vacuum, that will exert a force of 2 × 10^-7 Newton/meter per meter length of the conductors.

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Example 7.1

Two conductors carrying a current of 180 ampere each and 1600 meters long are placed next to one another at the following distances:

(a) 1, 3 meters; and (b) 4, 3 meters.

Calculate in each instance the force exerted between these conductors.

Solution:

(a) F = 2 × 10^-7 × I₁ × I₂ × ℓ

d

= 2 × 10^-7 × 180 × 180 × 1600 1, 3

= 7,975 N/m (b) F = 2 × 10^-7 × I₁ × I₂ × ℓ

d

= 2 × 10^-7 × 180 × 180 × 1600 4, 3

= 2,411 N/m

The direction of the magnetic field around a current carrying conductor can also be determined by using the right-hand-rule. Hold the conductor in your right hand.

The thumb will indicate the direction of the current flow and your fingers around the conductor will indicate the direction of the magnetic field.

The screw-rule is another method that can be used in that the direction in which the screw travels indicates the direction of the current and the direction in which the screw is turned will indicate the direction of the magnetic flux.

When a conductor is shaped in the form of a coil the right-hand-rule can also be used to find the North-pole of the given coil. Should your fingers be wrapped around the coil in the direction of the current flow then the thumb will indicate the North-pole of the coil.

7.5 Electromagnets

The strength of the magnetism produced by a coil through which an electric current is flowing is dependant upon the following factors:

• The number of turns in the coil;

• The magnitude of the current flowing through the coil;

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• The type of core material; and

• The ratio of the coil diameter to its length.

The product of the number of turns and the current in amperes is known as the ampere-turns of the coil and is a measurement of the field strength of the coil. This field strength may be increased by inserting an iron core since it will provide a better path for the magnetic flux lines in comparison to a magnetic path for air. The result will be that the strength of the magnetism is increased considerably. This concept is then known as an electromagnetism and is extensively used in the electrical industry in the manufacturing of relays, buzzers, circuit breakers and door-bells. This principle is illustrated in figure 7.7 (a) and (b).

Figure 7.7

Figure 7.7 (a) illustrates a coil without an iron core and is more commonly referred to as an air core whereas figure 7.7 (b) illustrates a coil with an iron core. The type of iron core used will have an effect on the operation of an electromagnet. Should a very hard iron be used the iron will retain its magnetism which is not desired for the use of relays for instance. This magnetism remaining in the iron core is referred to as residual magnetism. To utilise a coil as a controlled electromagnet an iron core must be used that will not retain much or little of its magnetism. Relays make use of this concept and are illustrated in figure 7.8.

The circuit depicts a relay used to control some sort of device indicated as the load. The contacts of the relay are shown in the normally open (N/O) position. Should a current now be allowed to pass through the coil the core will be magnetised and the magnetic strength of the core will overcome the mechanical tension of the spring, to which the armature is attached, and will pull the contacts of the relay into the normally closed (N/C) position which will now connect the supply and the load will be energised.

Should the current to the relay coil now be interrupted the core will loose its magnetism and the contacts of the relay will return to its normally open (N/O) position.

Magnetic flux

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This should indicate to you why it is so important that the core should have no residual magnetism qualities. If this was not so then the relay would have stayed in the normally closed (N/C) position permanently even with the current removed and the load will also be energised permanently.

Figure 7.8