Electromagnetism and Its Applications

What is Magnetism?

Magnetism is the force exerted by magnets when they attract or repel each other. This magnetism force is caused by the motion of electric charges. Every element or substance is made up of tiny units called atoms. Each atom has electrons (particles that carry electric charges). The electron spins around the nucleus in a circle. Their movement generate an electric current and causes each electron to act like a microscopic magnet.

If you were to experiment with a simple magnet, and iron fillings placed on a paper, you notice lines of force traced on the paper. The quantity of lines of force that come out from the magnet is called the flux and is measured in webers (wb). Flux density is represented by symbol, B and unit, tesla (T).

Electromagnetism

The field around a conductor carrying a current

When a conductor carries a current, a magnetic field is produced around that conductor. This field is in the form of concentric circles along the whole length of the conductor. The direction of the field depends on the direction of the current – the field is clockwise for a current flowing away from the observer and anti-clockwise for a current flowing towards the observer.

You can also quickly determine the direction of the magnetic field around a current-carrying conductor using the screw rule.

Imagine a screw being twisted into or out of the end of a conductor in the same direction as the current, the direction of rotation of the screw will indicate the direction of the magnetic field.

Related: Basic electric circuits

The force between current-carrying conductors

If we were to place two current-carrying conductors side by side, there exists a force between them due to the flux. The direction of this force will depend on the direction of the current flow as shown in the diagrams below:

In figure 2(a) there is more flux, between the conductors than on the either side of them, hence they will be forced a part.

In figure 2(b), the flux between the conductors is in opposite directions and tends to cancel out leaving more flux on the outside of the conductors than in between them, so they will be forced together.

The direction of movement can be found using Fleming’s left-hand rule, which states that if the thumb, first and second fingers of the left hand are placed at right angles to one another, they will indicate:

First finger –Field

Second finger –Current

Thumb –Motion

The force on a conductor carrying a current in a magnetic field

When a current carrying conductor is placed in a magnetic field, it experiences a force. Experiment shows that the magnitude of the force depends directly on the current in the wire and the strength of the magnetic field, and that the force is greatest when the magnetic field is perpendicular to the conductor.

If we were to have a current-carrying conductor placed at right angles to a magnetic, a force will be exerted on that conductor. This force is measured in newtons.

In the first figure (x) above the flux above the conductor is greater the flux below and the conductor is therefore forced downwards. In figure (y), the current and hence the field around the conductor is opposite to that in figure (x) and the conductor is forced upwards.

The magnitude of this Force is dependent on 3 things:

1. The current flowing in the conductor (I)
2. The density of the magnetic field (B)
3. The length of the conductor in the magnetic field (L)

Therefore, force F (newtons) = B (tesla) x l (metres) x I (amperes)

F = BLI

The e.m.f induced in a moving conductor

As discussed above, a current through a conductor in a magnetic field produces a movement of that conductor. If we reverse the process and physically move the conductor through in a magnetic field such that it cut across the flux, then a current would flow in that conductor.

Note that, a pressure is required for a current to flow. Hence, if a current flows as indicated in the figure above, then an e.m.f. must be producing it. This e.m.f. is called an induced e.m.f and its direction is the same as that of the current flow. This direction can be determined using Fleming’s right-hand rule.

Fleming’s right-hand rule

If the thumb, first and second fingers of the right hand are arranged, at right angles to one another, they indicate:

First finger – Field (north to south)

Second finger –Current (and e.m.f.)

Thumb –Motion

The magnitude of the induced e.m.f (E) depends upon:

1. The flux density of the field (B)
2. The length of the conductor (L)
3. The velocity at which the conductor cuts across the flux (V)

Therefore E (Volts) = B (tesla) x L (metres) x V (metres per second)

Applications of Magnetic Effects of Electric Current

There are several applications that make use of the magnetic effects of electric current, they include:

• Electromagnetics used in Relays, circuit tripping mechanisms etc.
• Direct current generators
• Alternating current generators
• Transformers