Magnets and Magnetic
Fields
A Magnet attracts certain
materials to itself.
A magnet will attract Iron, Steel, Nickel,
Cobalt and some alloys of these. A magnet
has no noticeable effect on other materials.
A Bar Magnet is strongest
at each end.
Dip a bar magnet into iron filings or a box of
pins. It attracts the filings or the pins to itself.
Most cling on at each end of the magnet.
The regions of greatest strength at each end
are called Magnetic Poles.
If a Bar Magnet is suspended freely it will line up
approximately North-South.
The pole of the magnet that always points North is called
the North-Seeking Pole or the North
Pole.
The pole that points south is called the South-Seeking
Pole or the South
Pole.
Magnetic Poles occur in Pairs.
You cannot have a single pole on its own. For every north pole there is
always a south pole. The strength of the north pole is the same as the
strength of the south pole.
Like poles repel and unlike poles attract
The North Pole of one magnet repels the North Pole of another.
The South Pole of one magnet repels the South Pole of another.
The North Pole of one magnet is attracted to the South Pole of another
magnet.
A Magnet causes some
materials brought near it
or touching it to become
magnetised.
This magnetism is called Induced
Magnetism. If the magnet is taken away
some materials (called permanent
magnets) hold on to their magnetism but
others (called temporary magnets) lose
most of it.
Hard steel holds onto magnetism very
well whereas soft iron does not.
Ordinary nails are usually made from soft
iron and do not retain their magnetism
very well.
What is a Magnetic Field?
A Magnetic Field is any region of space where
magnetic forces can be felt.
What is the Direction of a Magnetic Field ?
The Direction of the Magnetic Field at a point is the
direction of the force on a north pole if it was placed at that
point.
What is a Magnetic Field Line?
A line drawn in a magnetic field so that the tangent to it at
any point shows the direction of the magnetic field at that
point is called a Magnetic Field Line.
What are Magnetic Poles?
Magnetic poles are the regions at each end of a magnet
where the magnetic forces are greatest. Magnetic poles are
always found in pairs.
A Plotting Compass is a
small magnet that can
rotate about a vertical
axis. If no other magnets
are nearby it will line up
North-South.
If another magnetic field is present it will deflect the compass needle from its N-S
position.
If the other field is strong enough the compass needle will line up almost parallel to
the field rather than North-South.
The Magnetic Field around a Bar Magnet
The magnetic field lines
start at the north pole and
end at the south pole.
The magnetic field lines
never cross each other.
Near the poles - where the
magnetic field is strongest the lines are close together.
Further away, where the
field is weaker, the lines are
far apart.
Iron Filings showing the Magnetic Field around a Bar Magnet
The magnetic field
around a U-shaped
Magnet
Earth’s Magnetic Field,
pointing approximately
North
What is the Magnetic Effect of an Electric
Current?
Every Current-Carrying Conductor has a Magnetic
Field around it caused by the current.
As long as the current is flowing the Magnetic Field exists.
If the current stops flowing the Magnetic Field disappears.
Experiment to show the Magnetic Effect of an
Electric Current
Set up the equipment with the wire
lined up North-South. The plotting
compass also lines up N-S.
Close the switch, sending current
through the wire. The compass
needle will deflect.
Reverse the direction of the current
and the needle deflects in the
opposite direction.
Open the switch, no current flows,
the magnetic field disappears and
the compass again lines up N-S.
Conclusion: Every current
carrying conductor has a
magnetic field around it caused
by the current.
The Magnetic Field
around a Long
Straight Wire
The Right-Hand Grip Rule
If a Right Hand clasps a
conductor
with the Thumb pointing in the
direction of the Current
Then the Fingers give the
direction of the Magnetic Field
around the conductor.
The thumb points in the direction of
conventional current, i.e. from + to -
A Circular Loop of Wire carries
a current in the direction shown.
Using the Right-Hand Grip Rule at a
number of points on the wire shows
us the shape of the magnetic field
around the loop.
The side of the loop facing us
behaves like a South Pole (the
magnetic field lines are going in)
The other side is like a North Pole (the
magnetic field lines are coming out).
The Magnetic Field around a Current-Carrying
Coil (or a loop)
The Magnetic Field around a Current-Carrying Solenoid
What is an Electromagnet?
A Solenoid carrying a current and containing a soft
iron core is known as an Electromagnet.
Electromagnets are used in:
Scrap yard cranes
Electric motors
Electric bells
Moving coil loudspeakers
Induction coils
By turning the current on or off the magnet can be turned on or off.
By varying the size of the current the strength of the magnet can be varied.
A powerful
Electromagnet
lifting scrap Iron
A Magnetic Compass shows the direction of the Earth’s Magnetic Field
and it is used in navigation.
The Earth’s Magnetic Field forms a protective layer (from charged
particles) around the Earth.
The Magnetic Compass has been used for hundreds of years in marine navigation, since it
enables you to know direction. The angle between True North and Magnetic North is also
of importance. Charts and maps used in navigation have its value in the locality of the chart
noted on them since navigators must allow for it in their calculations.
To show that a Current-Carrying Conductor in a
Magnetic Field experiences a Force
Send a current
through the tinfoil.
The foil will move
forwards. Reverse the
current and the foil
will move backwards.
Conclusion: A
current-carrying
conductor in a
magnetic field
experiences a force.
The moving coil meter and the moving coil loudspeaker
are based on the principle that a current-carrying conductor
in a magnetic field experiences a force.
A simple d.c. Motor is based on the principle that a currentcarrying conductor in a magnetic field experiences a force.
Why would you expect a current-carrying
conductor placed in a magnetic field to
experience a force?
A current-carrying conductor has a magnetic field around it
due to the current.
When this conductor is placed in another magnetic field,
the two magnetic fields interact (push off each other!).
This causes the force on the current-carrying conductor.
What is the Direction of the Force on a
Current-Carrying Conductor in a Magnetic
Field?
The direction of the force is always:

Perpendicular to the current

Perpendicular to the magnetic field
NOTE: A Current-Carrying Conductor in a magnetic field experiences
no force if the conductor is parallel to the magnetic field.
Fleming’s Left-Hand Rule:
If the thumb, first finger and
second finger of the left hand are held at right angles,
with the first finger in the direction of the magnetic field and the
second finger in the direction of the current, then the thumb points in
the direction of the force.
What determines the Size of the Force on a
current-carrying conductor in a magnetic
field?
The size of the Current
The Length of the conductor
How strong the Magnetic Field is
The Angle between the conductor and the magnetic field
Magnetic Flux Density (B)
Is the magnitude of the magnetic field strength(B):
F  I
It follows that:

F  l
and
F  Il
F =
IlB
where B is a constant.
The value of B depends on how strong the magnetic field is.
In a strong magnetic field B is large and in a weak field B is small.
Thus B is a measure of how strong the magnetic field is.
B is called the Magnetic Flux Density.
Define Magnetic Flux Density
At a point in a magnetic field the Magnetic Flux Density ( B )
is a vector whose:
•direction is the direction of the force on a north pole placed at
that point
•magnitude is the value of B from the equation F = I l B
The SI unit of magnetic flux density is the tesla (T)
or put another way:
The magnetic Flux Density (B) at a point in a magnetic field is a vector whose:
magnitude is equal to the force that would be experienced by a conductor of length
1 m carrying a current of 1 A at right angles to the field at that point. Its direction is
the direction of the force on a north pole placed at that point.
If the conductor is not perpendicular to the field resolve the B into two
perpendicular components - one parallel to the conductor and the
other at right angles to the conductor. It is the component of B that
is perpendicular to the conductor that causes the force on it. The
parallel component has no effect on the wire.
F = B I l Sin 0o
The coil is free to rotate about the axis.
Convince yourself that the directions of the forces on
the sides of the coil are correct and that the coil will
begin to rotate.
To Show the Force on a current-carrying coil in a
magnetic field
Use the equipment above. The coil is free to rotate about the axis.
When the current is switched on the coil starts to rotate as shown.
A Beam of Electrons in a cathode ray tube is an
Electric Current
A beam of electrons in a
cathode ray tube moves in a
vacuum.
The beam passes close to a
fluorescent screen and shows
up as a beam of light.
The moving electrons have
negative charge and thus are
an electric current.
They, therefore, have a
magnetic field around them.
Force on a Moving Charge in a Magnetic
Field
This magnetic field, due to the beam of moving charges (the
electrons), will interact with any other magnetic field placed
near it.
The picture shows the beam of electrons deflecting due to
the presence of a bar magnet.
The Size of the Force on a Moving Charge in a
Magnetic Field
A charge of q coulombs moving with a speed of v metres
per second at right angles to a magnetic field of flux
density B teslas experiences a force of F newtons, given
by;
F = qvB
A charged particle moving at constant speed enters a
uniform magnetic field and moves at right angles to
the field. Explain why the particle moves in a circle?.
When the charged particle enters the
magnetic field there is a force on it.
The force is at right angles to its direction of
motion. Therefore its speed does not
change. Only its direction of motion changes.
The force on it has a constant magnitude
(F = q v B.).
As it turns the force always remains at
right angles to the direction of motion.
Thus the particle moves in a circular path.
Electric Current and Electric Charge
What is an Electric Current?
An Electric Current is a flow
of charge.
What is the SI Unit of
electric current?
The ampere (A).
What is the SI Unit of
electric charge?
The coulomb (C).
Define the coulomb.
1 coulomb is the amount of
charge that passes any point
in a circuit when a current of
1 ampere flows for 1 second.
Electric Current and Electric Charge
What is the relationship between Electric Current and
Electric Charge?
The current (I) is the amount of charge (Q) passing
per second
Q = It
Where:
Q is charge gone past
I is the steady current
t is the time taken.
Magnetic Forces between Currents
Two parallel conductors carry
current in opposite directions.
Each current creates a
magnetic field around itself.
The magnetic fields interact with
each other and cause a force
on each conductor, pushing the
conductors apart.
If the conductors carry current
in the same direction the force
between them is attractive.
State the principle on which the definition
of the ampere is based.
The definition of the ampere is based on the
principle that:
Two current carrying conductors exert a force on
each other due to their magnetic fields.
The Ampere
The ampere is that current which:
•if maintained in two infinitely long
parallel wires, is of negligible cross
section
•is placed 1 metre apart in a
vacuum
•would produce a force on each
wire of 2 × 10-7 newtons per metre
of length
What is Magnetic Flux?
Magnetic flux  is defined as:
 = BA
Where:
 = magnetic flux
B
A
= magnetic flux density
= area
Magnetic flux through Area A is equal to
Magnetic Flux Density × Area
What is the SI Unit of Magnetic Flux?
The SI Unit of Magnetic Flux is the weber (Wb).
Magnetic flux is a Scalar Quantity.
B = 3T
What is the magnetic flux
through a loop of area
0.5 m2 placed at right
angles to a magnetic field
of flux density 3 T?

=
BA
= (3)(0.5)
= 1.5 Wb
A = 0.5 m2
What if the magnetic field is not perpendicular to
the area?
Resolve the magnetic flux density B
into components parallel and
perpendicular to the area.
Flux through A =
Component of B perp to A × (area A)
In the diagram :
Component of B perp. to coil
= B Sin 30o = 2 Sin 30o = 1 T
Flux through coil = B × A
=
(1)(0.4) = 0.4 Wb
What is Electromagnetic Induction?
Whenever the magnetic field passing through a
coil changes an emf appears in the coil. This is
Electromagnetic Induction.
To Demonstrate Electromagnetic Induction
Move the magnet towards (or away from) the coil.
The galvanometer deflects, indicating that current flows and
that an emf appears.
When the magnet is not moving the meter reads zero.
State Faraday’s Law of Electromagnetic
Induction.
Faraday’s Law states that the induced emf is
directly proportional to the rate of change of
magnetic flux.
Average Induced emf E 
Final Flux - Initial Flux
Time Taken
Experiment to demonstrate Faraday’s Law of
Electromagnetic Induction
Move the magnet towards the coil
slowly.
The galvanometer gives a small
deflection, indicating a small
induced emf.
Move the magnet towards the coil
quickly.
The galvanometer gives a large
deflection, indicating a large
induced emf.
Conclusion: The induced emf is directly proportional to the rate of
change of magnetic flux through the coil.
As the man runs towards a coil the magnetic flux through the coil increases. An emf
is induced in the coil.
By Lenz's Law the current induced in the coil flows in a direction that opposes the north
pole of the magnet approaching.
The current flows in a direction that causes the end of the coil facing the approaching
magnet to behave like a north pole (north repels north).
Because of this opposition the man must do work to bring the magnet nearer the coil.
The work he does appears as electrical energy in the coil.
As the man runs away from the coil the direction of the induced current
changes so that his motion is still opposed.
The direction of the induced current changes so that a south pole
appears at the right hand side of the coil (south attracts north).
The work the man does in pulling a north pole away from a south pole
again appears as electrical energy in the coil.
State Lenz’s Law.
Lenz’s Law states that the induced current flows in such
a direction as to oppose the change causing it.
Lens’s law follows from the
Principle of Conservation of Energy.
Experiment to demonstrate Lenz’s Law
Drop a metal cylinder through a copper
pipe. Note how long it takes to fall
through the pipe.
Drop a strong cylindrical magnet (same
size and weight as the metal cylinder)
through the copper pipe. It takes much
longer to fall through.
As the magnet falls through the pipe, its
changing magnetic field induces currents in
the pipe. By Lenz’s Law these currents flow in
such a direction as to oppose the change
producing them, i.e the moving magnet. They
exert forces on the magnet slowing it down.
The non magnetic metal cylinder does not
experience these forces
Thus Lenz’s Law is demonstrated.
Experiment to demonstrate Lenz’s Law
Hang a light aluminium ring from a piece of thread.
Move the north pole of a bar magnet quickly
towards the ring.
The ring moves away from the magnet.
Move the north pole quickly away from the ring
and the ring follows the magnet.
Conclusion: As the north pole approaches
current is induced in the ring. A north pole
appears at the side of the ring nearest the magnet.
The magnet repels it and the ring moves away.
When the north pole is taken away the direction of
the induced current changes, a south pole
appears at the side of the ring facing the magnet.
The ring is attracted to the magnet and follows it.
This agrees with Lenz’s Law and thus the law
is demonstrated.
What is an Electric Generator?
An Electric Generator is a device that converts
mechanical energy into electrical energy.
The alternator in a car is an
electrical generator. This
one has a maximum power
output of 1 kW.
Electric Generators
Electromagnetic Induction is the principle on which the
Electric Generator is based.
In an electricity generating station some form of energy e.g.
chemical energy from coal or oil is used to produce steam
which causes a turbine to rotate - i.e. the turbine is given
kinetic energy.
The turbine rotates a coil in a magnetic field thereby causing
the magnetic flux through it to change and an emf is induced
in it.
Thus the kinetic energy is converted to electrical energy.
Everyday examples of Electric
Generators
Electricity Power Stations which generate huge
quantities of electricity.
The Alternator in a Car is turned by the engine and
generates electricity to supply power to the car’s electrical
system to continually keep the car battery charged.
The Dynamo on a bicycle generates electricity to operate
the bicycle’s lights.
What is alternating Current (a.c.)?
An electric current that periodically reverses the direction
in which it flows is called Alternating Current (a.c.).
The current that flows through
an ordinary domestic light bulb
when connected to the mains
electricity supply reverses
direction 100 times every
second.
Mains Electricity is
alternating current.
Alternating Current
A Graph of Current against Time for alternating current
Alternating Voltage
To produce alternating current an
alternating voltage is needed.
The diagram shows a graph of an
a.c. voltage against time on an
oscilloscope.
If alternating voltage is applied
across a pure resistor the current
flowing at any instant is found from
Ohm's Law.
Voltage at that instant
Current at any instant 
Resistance
v
i 
R
Alternating Voltage
Voltage / V
Graph of Voltage against Time for A.C.
A.C. and Heating
3 A d.c. flows in a 6  resistor. Heat is produced in the resistor at a rate
given by Joule's Law: P = I 2R = (3)2(6) = 54 J s-1
If Alternating Current flows in the resistor what is the maximum value of
the a.c. if it produces heat at the same rate as the 3 A d.c.?
Clearly if the a.c. only reaches 3 A in each direction it will not produce heat
at the rate of 54 J s-1, because the current is less than 3 A at all other
times in each cycle.
rms Value of an Alternating Current (or Voltage)
When we state that the value of an Alternating Current is
5 A, we mean that this alternating current has the same
heating effect as a 5 A direct current.
Since alternating current varies with time, to have the same
heating effect as a 5 A d.c. it must have a maximum value in
each direction which is greater than 5 A.
The stated value of an alternating current is called its rms
value ( symbol Irms ).
The Maximum or Peak Value of the current is symbolised: I0
It can be shown that:
I rms
I0

2
and
I 0  I rms  2
The same thing applies to alternating
voltages:
Vrms
V0

2
and
V0  Vrms  2
Peak voltage (V0)
and rms Voltage (Vrms)
The peak voltage of the mains electricity in Europe is 325 V.
Calculate the rms voltage of the mains.
Vrms
V0

2
325

2
 229.8 V
Mains voltage is supplied at an rms value of 110 V in the US.
Calculate the peak value of the voltage.
V0  Vrms  2  110  2  156 V
What is Mutual Induction?
If a changing electric current in one coil causes an
induced emf to appear in a nearby coil there is said to be
Mutual Inductance between the two circuits.
How can the amount of Mutual Induction
between two coils be increased?
Move the coils nearer each other.
Wind the coils on the same soft Iron core.
Increase the number of turns on either or both of the
coils.
Mutual inductance occurs in the Transformer and the
Induction Coil.
Experiment to show Mutual Induction
When the switch S is opened (or closed) the current in coil 1 changes
and thus the magnetic field around it changes. This changing magnetic
field passes through coil 2.
As this happens the galvanometer in coil 2 gives a deflection showing
that an emf is induced in coil 2.
There is thus mutual inductance between the two coils.
What is Self Induction?
Whenever the current passing through a coil changes the
magnetic field surrounding that coil changes.
This changing magnetic field induces an emf in the coil
that opposes the changing current. (This emf is called a
back emf ).
This phenomenon is called Self Induction.
Experiment to demonstrate Self Induction
When the switch is closed the bulb does not light
immediately. It takes a number of seconds for the bulb to
reach full brightness.
This is due to the self inductance of the coil.
Explanation of the Experiment to
demonstrate Self Induction
When the switch is closed the current starts to flow and
immediately produces a magnetic field around the coil. This
field is increasing.
Since the coil now has a changing magnetic field in it, by
Faraday's Law an emf will be induced in the coil.
By Lenz's Law the direction of the emf opposes the change
producing it, i.e. it opposes the increasing current.
The induced emf opposes but does not succeed in
preventing the current from increasing. Such an emf is called
a Back emf.
The Effect of an Inductor on a.c.
A 12 V battery sends a 2 A d.c current
through a 6 Ω resistor.
If a 12 volt a.c. source (Vrms = 12 V) is
connected to a coil of resistance 6  with a
soft Iron core in it, current still flows.
However the current that now flows, is much
less than 2 A. The reason is:
Alternating current continually changes and
so the magnetic field around the coil also
changes. By Faraday's and Lenz's Laws an
emf is induced in the coil that always
opposes the changing current.
It is this back emf that causes the coil to
offer more opposition to a.c. than to d.c.
A filament lamp lights when connected in series with
an a.c. power supply and a coil. Explain why the
lamp goes out when a soft Iron core is inserted into
the coil.
The core causes an increased magnetic flux in the coil and hence a greater
rate of change of magnetic flux.
A coil carrying a.c. has a back emf. The greater rate of change of magnetic
flux causes the back emf to increase.
This reduces the net voltage and the current decreases. The lamp goes out
because the current decreases.
A.C. and Inductors
A coil (an inductor) opposes the flow of direct
current (d.c.) with its Ohmic Resistance.
A coil (an inductor) opposes the flow of
alternating current (a.c.) with its Ohmic
Resistance and the Back emf Induced in it.
A Dimmer Switch
What is a Transformer?
A Transformer is a device used to change the
value of an Alternating Voltage.
Structure of a Transformer
A Transformer consists of two coils
of wire wound on a soft Iron core.
One coil, called the Primary Coil
has an alternating voltage, called the
Input Voltage, applied to it.
The transformer causes a different
voltage to appear across other coil,
called the Secondary Coil.
The voltage across the secondary
coil is the Output Voltage.
How a Transformer Operates
The Input Voltage V
​ i​ across the
primary coil causes alternating
current in the primary coil.
This current causes an
alternating magnetic flux in the
Iron core.
This alternating flux passes
through the secondary coil and
induces an emf ​Vo in it. V​o is the
Output Voltage.
The size of the Output Voltage, ​Vo, depends on the number of turns in
the secondary ​Ns​ and N
​ p​, the number of turns on the primary coil.
Transformer Formula
Vi
Vo
Vi

Np
Ns
= Input Voltage = Voltage across Primary Coil
Np = Number of Turns on Primary coil
Vo = Output Voltage = Voltage across Secondary Coil
Ns = Number of Turns on Secondary Coil
If Ns is greater than Np then Vo is greater than
Vi and it is called a Step Up Transformer.
If Ns is less than Np then Vo is less than Vi and
it is called a Step Down Transformer.
To Demonstrate the Action of a Transformer
With an a.c. voltmeter measure the voltage across the primary
coil and the emf across the secondary coil.
It will be seen that if Ns > Np then Vo > Vi and vice versa.
By noting the number of
turns on each coil the formula:
can be verified
Vi
Vo

Np
Ns