202/3 – Know the principles of electro-magnetism

Preliminary understanding

Unit 202 Principles of electrical science

Magnetism is often found to be one of the hardest subjects to

come to grips with. It is essential however that you take your time and get a grasp of it. It is one of the most important subjects. Without a proper understanding of magnetism, electrical work will make no sense at all.

Without magnetism, we would have no motors or generators, no fluorescent or discharge lights. Just from these few examples you can see how important it is.

Magnetism and where it comes from is not fully understood, but at this level that doesn’t really matter, as all we have to do is use some of the concepts.

No doubt at school or elsewhere, you will have come across the patterns that permanent magnets make when iron filings are scattered over them. These iron filings seem to follow a series of lines across the paper and look something like waves around a rock in water.

Magnetism

Unlike poles attract each other

When two N poles or two S poles are together the magnetic flux lines are pushed outwards away from each other.

When a N and S pole are placed together the magnetic flux lines are straight and are attracted to each other.

N N

Like poles repel each other

SN

Magnetic flux

Magnetic flux

Draw the lines of flux you would expect to see?

Bar Magnet

The amount of lines of flux in any particular area is called the flux density.

The lines of flux appear to go through the North pole of the magnet round the outside and enter into the South pole.

If the lines of flux were close together you would have a strong magnetic field.

If they were further apart then the magnetic field is weak.

Magnetic flux

Magnetic flux

• The direction of magnetic flux is from the north seeking pole to the south seeking pole.

• Each line of magnetic flux forms a closed loop.

• The lines of magnetic flux never cross.

• The lines of magnetic flux always follow the shortest route.

• Lines of magnetic flux which are parallel and in the same direction repel each other.

• Line of magnetic flux which are parallel and in the opposite direction attract each other.

Properties of Magnetic flux

Magnetic flux vs density

To understand the meaning of magnetic flux (Φ) and magnetic flux density (B) think first about an ordinary bar magnet. Around the magnet there is a magnetic field and this gives a ‘flow of magnetic energy’ around the magnet. It is this flow of energy that we call magnetic flux (Φ). We think of magnetic flux as flowing from the north pole of a magnet round to its south pole as shown by the arrows on the lines in the diagram. Looking at the diagram you should see that there is as much flux flowing ‘from the north pole’ as there is ‘flowing into the south pole’.

Magnetic flux is given the symbol Φ and is measured in units called Webers (Wb).

However the amount of magnetic flux flowing through a given area will change from one point to another around the magnet and you can understand this by thinking about a loop of wire placed in the field at two different points (A and B). You can see that in position B there are a smaller number of magnetic field lines passing through the loop than there is when it is in position A.

We call the amount of flux passing through a unit of area at right angles to the magnetic field lines the flux density (B) at that point.

Flux (Φ) = Flux density (B) x area (A)

   = BA

Magnetic flux vs densityThere is no equation for flux but there is one for flux density that we can simply transpose to calculate flux.

To measure the flux in a particular area

Symbol = B

Unit is Tesla (T) or (Wb/m2)

The formula for flux density is B = A

Magnetic flux density

The two diagrams show the idea of flux density being based on area.

The more lines passing through the unit area, then the stronger the magnetic field is.

When a conductor carries a current, a magnetic field forms around it.

The magnetic field is circular and extends across the length of the conductor.

Current flow and Magnetism

Direction ofcurrent flow

Direction ofmagnetic field

Direction ofmagnetic field

There are arrows showing the direction of the magnetic field around the conductor.

The direction of the field (N to S) can be determined using the corkscrew or right-hand thread rule.

If I turn the corkscrew clockwise as into the bottle, then the current and the magnetic field is assumed to be flowing clockwise.

If I unscrew the cork anti-clockwise then the current and magnetic field flow is anti-clockwise.

Current flow and Magnetism

To determine the direction of the magnetic field assume that the fingers on the right hand are showing the direction of current in the coil or solenoid and that the thumb is pointing north.

Current flow and Magnetism

Right Hand Grip Rule

Magnetic field

ConductorFlight ofa dart

Magnetic field

ConductorPoint ofa dart

When drawing the direction of flow on paper, it should be drawn as shown in the diagrams. The dot indicates that the current is coming out of the paper and the cross indicates that it is flowing into the paper.

Think of it as a dart with a point at one end and a flight at the other.

Drawing Current flow

Magnetic field producedby conductor A

Magnetic field producedby conductor B

Movement of conductor A Movement of conductor B

A B

When currents flow in opposite direction, the conductors A and B will repel each other.

It is not obvious but the lines of magnetic flux are closer together therefore there are more of them. This reduces the overall magnetic field to zero.

Current flow and Magnetism

When currents flow in the same direction the conductors A and B are attracted to each other.

The lines of magnetic flux between the conductors deceases but the overall magnetic field increases.

Current flow and Magnetism

Resultant magnetic field

A B

Magnetic field producedby conductor B

Magnetic field producedby conductor A

Movement of conductor BMovement of conductor A

Current flow and MagnetismWhen currents flow in the same direction the conductors

A and B are attracted to each other.

Lets look at this more in depth in stages?

Eddy currents

As current passes through the conductors, a magnetic field is induced in the steelwork. This current circulates continually unless stopped.

When a number of current carrying conductors pass through a metal plate, this could be a consumer unit or the edge of some metal trunking etc, eddy currents occur.

There are two ways of stopping eddy currents.

Eddy currents

One is to make sure that conductors of opposing currentflow are placed in the same hole.

The second is to cut a slot in the metalwork so that the eddy current

cannot circulate.