1. Part of Physics Achievement Standard
2.6 – Demonstrate understanding of
electricity and magnetism.

2. V 1 2
E F  Eq E p  Eqd Ek  mv
d 2
q E E
I V V  IR P  IV P
t q t
1 1 1
Rt  R1  R2 ...   ...
Rt R1 R2
F  BIL(sin  ) F  Bqv V  BvL

3. • Where have you had experience with
magnetism in life so far?
• Permanent Magnets
• Electromagnetism
• Can a frog really levitate?

4. All permanent magnets have a north pole and a south
pole. What will happen if we cut a bar magnet in
half?
Opposite poles attract, like poles repel, just like
charges in electricity.
The earth has a magnetic field, which is very
important to protect the planet from dangerous
radiation.
The Auroa (northern lights) is actually a result of the
charged particles being channelled to the poles and
colliding with the particles in the atmosphere.

5. Magnetic field lines are drawn in a similar way to
electric field lines. They show the direction that
a small north pole would move.
Field lines go from NORTH to SOUTH.
The force is stronger where the field lines are
closer together. Field lines never cross or meet.

6. Magnetic field lines can be visualised in two main
ways:
 Compass needles always line up with the
magnetic field lines.
 Iron filings will become induced magnets and
temporarily line up with field lines.

7. The Earth has a magnetic field,
as though there is a large bar
magnet inside the Earth.
Note that the geographic and the
magnetic poles are different.
When we look at a compass, we
read the needle pointing
towards the North pole, so we
assume that is the north end
of the earth’s magnet.
But is the north end of the
compass attracted to the
north pole of a magnet?

8. Some metals are easily magnetised (e.g.
iron). This means that the dipoles
inside the metal are free to move
around. When they feel a magnetic
field around them the dipoles align
and this causes the object to become
a temporary magnet.
The method of induced magnetism is
used to store most of the worlds
information. Hard Disk Drives
(HDDs) use temporary(ish)
magnetism to store data. This is why
you should never put your computer
too close to a speaker.

10. Sometimes it is necessary for us to draw a current
going into or out of the page. These are the
symbols we use...
Magnetic Field Lines
Current Into Page Current Out Of Page
It turns out that there is a magnetic field that exists
around the wire, depending on the direction of the
current. The magnetic field circles the current.

11. So we can see that magnetism and electricity are
linked in a very important way. It turns out
that currents create circular magnetic fields
around themselves.
The magnetic field created
can be found using the
right hand rule:
1. Your right thumb
points in the direction
of the current.
2. Your fingers will wrap
in the direction of the
magnetic field lines.

12. The symbol for magnetic field strength is B, with
units NA-1m-1 , more commonly called Tesla, T.
A bit more about Tesla...

13. Solenoids are an example of an application of the
magnetic field created by a current. There are three
ways to increase the magnetic field produced by a
solenoid:
1. Increase the current.
2. Increase the number of turns.
3. Inserting an iron core.
There is a right hand rule for solenoids which can be
used to find the direction of the magnetic field:
1. Curl your fingers in the direction of the current
through the coil.
2. Your thumb points in the direction of north.

14. Below: The right hand solenoid rule.
Above: The magnetic field
produced by a solenoid.

15. Magnetic fields interact with each other when
there is more than one in the same space.
This interaction results in a force.
Therefore, a wire carrying a current will
experience a force when it is in a magnetic
field.
This is how we turn electrical energy into kinetic
energy – an electric motor!
It’s called the motor effect.

16. The direction of this force can be found using the
Right Hand Slap rule:
Your thumb should point in
the direction of the current.
Your fingers should be in
the direction of the magnetic
field.
The direction of your “slap”
will show how the force will
act.

17. We are able to calculate the size of the force that the
wire will feel by considering 3 factors that will
have an effect on that force:
The size of the force depends on:
1. The strength of the magnetic field, B
2. The size of the current in the wire, I
3. The length of the wire that is inside the magnetic
field, L
This gives the equation: F = BIL
Note that in some instances, the wire and the field are
not at right angles. These cases will introduce a
factor of angle, making the equation F = BILsin(θ),
where θ is the angle between the wire and the
magnetic field. Note that θ will always be between
0° and 90°.

18. The most important application of the motor
effect is the DC motor. Approximately 85% of
power generated is used to power DC Motors.
The DC motor
works by taking
advantage of the
motor effect to
make a loop of
wire turn.

20. Note that the split ring commutator is the main
component that make the motor possible.
There are a number of ways to make the motor
force more powerful:
1. Thicker wire to allow a bigger current
2. Insert an iron core to make the magnetic field
larger
3. Put more than one coil on the loop.

21. The actual reason that the current feels a force when
in a magnetic field is because a current is made up
of moving charges. It is these charges that are
feeling the force and making the wire move.
This means that a free charge moving through a
magnetic field will also experience a force. This
force will depend on:
1. The magnetic field strength, B.
2. The velocity of the charged particle, v.
3. The size of the charge, q.
F = Bvq is the equation. Note that the charge must be
moving in perpendicular direction.
What kind of path would a moving charge in a
magnetic field follow?

22. We know that a charge must follow a circular
path , unless we change the direction of the
magnetic field. This is a direct consequence of
the right hand “slap” rule.
(This is how old TV’s and Monitors work – they
use a changing magnetic field to tell electrons
exactly where to go.)
The circular path can be calculated with precision:
remember the equation for force in a circular
situation: F = mv2/r.
The forces are equivalent, so given the mass, we
can find the radius, or vice versa.

23. A positron is the anti –
matter equivalent of
One type of radiation is
an electron. A
known to have a charge
magnetic field is used
of +3.2x10-19C, and a
to keep the positron
mass of 6.64x10-27 kg.
contained. Calculate
the size of B if r must Calculate the radius of it’s
be at most 5cm, and path in a magnetic field
the positron is moving of 1 Tesla if it is
at 0.5c. travelling at 300ms-1

24. If (perchance) instead of sitting back and waiting
for a current and magnetic field to interfere and
cause a force on a wire, what if we gave the
wire some force? What would this do?
You shouldn’t be too surprised to hear that this
would cause a current to exist in the wire.
The free electrons in the wire feel a force on them,
and all start moving in the direction of that
force.
Electron movement (flow) = current!
We have just generated some electricity.

25. The movement of the wire in the magnetic field
causes the current to flow in the wire – this induces
a potential difference (voltage) in the wire.
The induced voltage is proportional to the velocity
the wire is moving, the length of the wire in the
magnetic field, and the magnetic field strength:
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Note that it could be the field moving instead of the
wire, or even the field oscillating in strength to
generate the voltage. As long as the field is
changing somehow relative to the wire.
We could increase the size of the voltage/current
generated by…

26. There is a right hand slap rule for the induced
current:
Your thumb is in the direction of
the motion, your fingers in the
direction of the magnetic field,
and your slap shows the direction of the current.
This is the conventional current – electron flow
would be the opposite way.
This is how electricity is generated around the
world. Note that energy is conserved in this
situation.

27. The diagram shows a
metal rod moving to
the right through a
magnetic field.
The induced current
in the rod flows as
shown.
If energy is to be conserved, then the mechanical work
done must equal the electrical energy generated.

28. Let the speed of the wire be v, and its effective length
L.
• Then the induced voltage is given by V = vBL.
• The electrical power is VI = vBLI.
• The force the magnetic field B makes on the current
I is given by F = BIL. This force opposes the motion.
• If the rod is to move at constant speed, then the
force sustaining the motion must be the same size,
BIL.
• Then mechanical power = F × v = BIL × v = vBLI
We see that the mechanical power put in equals the
electrical power produced and so energy is
conserved.

29. The alternating current generator is also known as an
alternator. In such a device, a coil rotates in a
magnetic field and a current is produced.
The current flows first in one direction, then the
other, then the first again, and so on.
Because it keeps reversing direction, the current is
known as alternating current (AC).
This is the most common type of electricity, because it
is much easier to use (in motors and almost all other
appliances) but it does have one major drawback.
Can you guess what the disadvantage is?
We learn a lot more about AC in year 13!