This document discusses the key principles of electromagnetism. It explains that magnetism is difficult to understand but is essential for electrical work. Without magnetism, technologies like motors and generators would not function. The document then describes magnetic flux lines and how they behave depending on whether magnetic poles are like or unlike. It also discusses how magnetic fields are generated by electric currents based on the direction of current flow.

1. 
202/3 – Know the principles of electro-
magnetism
Preliminary understanding
Unit 202 Principles of electrical science

2. 
 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

3. 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

4. Magnetic flux
Draw the lines of flux you would
expect to see?

5. 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

6. Magnetic flux

7. • 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

8. 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.

9. Flux (Φ) = Flux density (B) x area (A)
 = BA
Magnetic flux vs density
There 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

10. 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.

11. 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

12. Direction of
current flow
Direction of
magnetic field
Direction of
magnetic 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

13. 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

14. Magnetic field
ConductorFlight of
a dart
Magnetic field
ConductorPoint of
a 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

15. Magnetic field produced
by conductor A
Magnetic field produced
by 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

16. 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

17. Resultant magnetic field
A B
Magnetic field produced
by conductor B
Magnetic field produced
by conductor A
Movement of conductor BMovement of conductor A
Current flow and Magnetism
When 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?

18. 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.

19. There are two ways of stopping eddy currents.
Eddy currents
One is to make sure that conductors of opposing current
flow are placed in the same hole.
The second is to cut a slot in the
metalwork so that the eddy current
cannot circulate.