Download Electromagnetism

The closer the field
lines are to each
other the stronger the
magnetic field at
that point
 The field lines run
from north to south
outside the magnet
and from south to north
inside the magnet.

Magnetic field about straight conductor
There is a magnetic field about a straight currentcarrying conductor.
This field consists of concentric circles and can be
studied by using iron filings or small compasses as in
the sketches below.
Direction of magnetic field about
straight conductor
Clasp the conductor with the right
hand – thumb pointing in the
direction of conventional current
flow.
The fingers now curl in the direction
of the magnetic field.
Direction of magnetic field about
straight conductors
Direct current into page (x in
circle) produces a clockwise
field.
Current out of page (dot in
circle) produces an
anticlockwise field.
Check with right-hand rule.
Now see how the fields interact if there are currents in parallel
conductors.
Currents in same direction –
wires expierence a force
towards each other.
Currents in opposite directions
–wires expierence a force
away from each other.
Direction of magnetic field about
straight parallel conductors
Magnetic fields
about parallel
conductors
The field lines add up if they are in same direction
resulting in more field lines and so a stronger field. The
magnetic field lines subtract if in the opposite direction
resulting in less field lines and a weaker field. The wires
will experience a force towards a region of lower
magnetic field density
Direction of magnetic field about a
solenoid
A solenoid is just a coil of wire – as in sketch below.
Draw this circuit and then draw the magnetic field
about the coil.
Is there any resemblance to the field about a
magnet?
Direction of magnetic field about a
solenoid
Right hand solenoid rule – Amperes rule
Clasp the solenoid in the right hand – pointing the
curled fingers in the direction of the direct current
flow.
The thumb now points to the North pole of the field
about the solenoid.
To increase the strength of the magnetic field, you
could:
wrap more coils in the solenoid;
increase the current flowing through the solenoid; and
wrap the coils around a magnetic material, for
example, a length of iron. The iron core then
concentrates the magnetic filed, making it stronger.
Ampere’s Rule for a Solenoid:
Grasp the coil in the right hand with the fingers
circling the coil in the direction of the conventional
current. The extended thumb will point in the direction of
the North pole of the core.
Electromagnets
A simple electromagnet can be made by winding a
length of wire about an iron ( ferromagnetic material)
nail and connecting to a battery.
Apply the right hand wire rule to identify the N & S
poles.
N
S
Each of these smaller
nails in turn – also
becomes a magnet.
When we place an iron bar, called an iron
core, in the solenoid, it can be magnetised
temporarily.
If the current in the solenoid is switched off,
the magnetic field around the solenoid no
longer exists and in turn the iron core loses
its magnetism.
.
The core must be made of soft iron, so that
it can lose its magnetism immediately
the current is switched off in the solenoid.
If a hard iron, such as steel, is used it will
become a permanent magnet when
magnetised by the solenoid.
This means that when the
current in the solenoid
is switched off the core
keeps its magnetism
A current carrying conductor that is
placed in a magnetic field will
experience a force
 The magnetic field of the current
carrying wire is a set of concentric rings
and the magnetic field of the magnet is
a set of parallel lines that run from north
to south.

Flemmings Left hand motor rule
The concentric circular field about the conductor, interacts with
the permanent field between the magnets to form this
resulting magnetic field.
This now produces motion of the conductor in the direction of
F - as a result of the strengthening and weakening of the
resultant fields to the left & right of the conductor – as seen in
the second sketch. The force is in the direction of lower
magnetic field density Field between Strengthening of
resultant field
magnets.
Field about
conductor
Weakening
of field
Left hand motor rule
Thrust
Magnetic field
Current
This rule is used to establish the direction of movement of the
current carrying conductor in the magnetic field.
Note: The first finger points in the direction of the magnetic
field, the middle finger points in the direction of the
conventional current flow and the thumb in the
direction of the thrust of the conductor.
Now apply this rule to this sketch.
An experiment, to find the effect of a magnetic field on a
current-bearing conductor, is set up as in the
photograph.
.
When the switch is closed, a current flows
through the metal swing hanging in the
magnet.
a) In which direction will the metal swing
experience a force: into or out of the
magnet?
b) Name and describe the rule you used to
determine the direction of the force
experienced by the metal swing

The force that acts on the wire is actually
acting on the moving electrons in the
wire. Therefore, if a freely moving
charged particle moves in a magnetic
field it will experience a force
The direction of movement of
the charge, q, is indicated by the arrow
labelled v, and the force is indicated
with the arrow labelled, F.
The field is going
Into the page (X)


The movement of a charged particle in a
magnetic field has a useful application in
the screens of our television sets. The screen
works by using a (CRT). The cathode ray
tube is essentially made up of a stream of
electrons, which deflects in the presence of
a magnetic, or sometimes an electric, field.
Inside the tube electrons are directed onto
the screen, to produce an image. Modern
TVs do not use cathode ray tubes.
A charge q moving with
velocity v perpendicular
to a magnetic field feels
a maximum magnetic
force F=qvB
 The force is
perpendicular to v and
to B ( Flemming’s left
hand motor rule)

The magnitude of the force depends on:
the magnitude of the charge, Q – the
magnitude of the force and the charge are
directly proportional, which may be represented
as F α Q;
 the speed, v, at which the charge is moving – the
magnitude of the force is proportional to the
speed, which is written as F α v; and
 the direction of movement of the charge relative
to the direction of the field. The magnetic field is
given the symbol, B.

Where:
F is the force measured in Newtons (N)
Q is the charge measured in coulombs(C)
v is the speed of the charge measured in
metres per second (ms-1)
θ is the angle between the direction of
movement of the charge and the
magnetic field.
B is the magnetic field strength or the
magnetic field intensity and is measured in
tesla abbreviated T.
Calculate the force acting on a 5C charge
when it moves at 300 to a magnetic field
of strength 0,9 Ns-1C-1m-1. The charge
moves with a speed of 3ms-1.
The power generators at power plants use
electromagnetic induction
to make electricity and
then transformers, which also work on
electromagnetic induction help transfer
the electricity to our homes.
Teacher will demo with
a Galvanometer , coil
and magnet
DEFN: The emf induced in a circuit is
directly proportional to the time rate of
change of magnetic flux through the
circuit.
Where: B is the magnetic field intensity
measured in tesla (T)

A is the cross sectional area through
which the magnetic field passes
 θ is the angle that the field line makes
with the normal (an imaginary line at 900
to the surface)
 Φ is the magnetic flux and is measured in
Tm2

Calculate the magnetic flux if the
magnetic field intensity is 0,002 T, the
cross-section area over which the
magnetic field acts is 2 m2 and the
magnetic field enters the material at 400
to the normal.
From Faraday’s Law, the magnitude of the
induced emf depends on:
 the rate at which the field is changed;
 magnetic flux; and
 the number of coils or loops in the
solenoid.
http://www.tutorvista.c
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Where:
ε is the induced emf measured in Volts (V)
N is the number of cools
Δ Φ = change in magnetic flux
(Φfinal – Φinitial) measured in Tm2
Δt is the time of movement measured in
seconds (s)
The negative sign is added to show that the
emf that is induced in the coil opposes the
inducing motion. This is explained in the
discussion of Lenz’s Law below.
A square coil of side 5 cm contains 100 loops
and is positioned perpendicularly to an
uniform 0,6 T magnetic field. It is quickly and
uniformly pulled from the field (moving
perpendicularly to B) to a region to where B
drops to 0,2 T.
It takes 0,1 s for the coil to move between the
to points in the magnetic field. What is the
emf induced during this period?
First we calculate the area of the field:
A = length x breadth
= 0,05 x 0,05
= 0,0025 m2
Φf = Bf A cos θ
= (0,2)(0,0025)(cos 0)
= 0,0005 Tm2
Δ Φ = 0,0005 - 0,0015
= - 0,001 Tm2
Second we must calculate Δ Φ, by calculating the initial
and final Φ:
Φi = Bi A cos θ
= (0,6)(0,0025)(cos 0)
= 0,0015 Tm2
Δ Φ = 0,0005 - 0,0015
= - 0,001 Tm2
Finally calculate the emf:
ε== 1V
The magnetic field shown
in the figure
decreases from 1.0 T to 0.4
T in 1.2 s. A 6.0 cm
diameter loop with a
resistance of 0.010 Ω is
perpendicular to the field.
What is the size and
direction of the
current induced in the
loop?
E = 1,4mV and
 I = 141 mA
 Current is clockwise

In order to find the direction of the induced
current we use Lenz’s Law.
DEFN:
The induced current in a coil has a
direction such that its own magnetic
field opposes the change brought about
by the external magnetic field.
Electromagnets near a coil
An electromagnet can induce a magnetic field & thus a current in
a nearby coil.
Secondary coil
Primary coil
N
S S
N
ç
This only takes place while the current is increasing to
a maximum and also decreasing to a minimum – once
switched off.
Current is thus only induced in the right hand coil –
when the current is switched on and off in the left hand
circuit.
This phenomenon is used in transformers.
Mutual Induction
A potential difference, & thus a current will only be
induced in a secondary coil when:
 Current strength & thus magnetic field, in the
primary coil is increasing
And then
 Current strength & thus magnetic field in the primary
coil is decreasing
Achieved by
 The (D. C)current is switched on or off
Or using alternating current (A.C.)
The varying current that producers a varying magnetic
field results in magnetic flux linkage and so induces an
EMF according to Faradays Law.
This is equivalent to moving the magnet in the coil –
the magnetic field moves across the coils = magnetic
flux linkage occurs
If the current is steady, this will result in a steady
magnetic field and there will be on magnetic flux
linkage and no induced emf.
This is equivalent to holding the magnet stationary in
the coil- no magnetic flux linkage
Transformers
Transformers consist of a Primary coil and a Secondary coil –
connected by means of a ‘soft iron core’.
The primary coil is connected to A.C. – in order to achieve &
maintain a fluctuating current and changing magnetic field in the
primary coil. NOT D.C.- This in turn induces a fluctuating
potential difference & current in the secondary coil.
Transformers
There are 2 types of transformers – step up and step down
transformers.
Step up transformers step up the voltage from the primary to
the secondary coil – as a result of more turns in the secondary.
a.c.
source
220 V
Primary
coil
440 V
Secondary
coil
Double the number of turns in the secondary coil produces
double the voltage in the secondary coil.
Step down transformers
In a step down transformer there are fewer turns on the
secondary coil & this produces a lower voltage in the
secondary coil.
a.c.
source
220 V
Primary
coil
110 V
Secondary
coil
Half the number of turns in the secondary coil produces half
the potential difference in the secondary coil.
Calculations for transformers
There are at least 3 equations that can be used for
transformers:
Vp
Np
Vs = Ns
IP = Np
Is
Ns
VpIp = VsIs
Where V = potential difference
N = number of turns in coil
I = current in coil
s = secondary coil &
p = primary coil
A transformer has 20 windings on the primary
coil and 40 windings on the secondary coil.
An input emf of 120 V is used.
1.
Is this a step up or step down
transformer? Give a reason for your choice.
2. Calculate the output emf.
3. If the current in the secondary coil is 3A,
what is the current in the primary coil?
Uses of transformers
Eskom provides the 220 V a.c. current that is required
throughout S.A.
Dynamos produce the required current and voltage at
the respective power stations throughout S.A.
Transformers at the power stations step up potential
difference to 700 000 V and this is distributed to the
various towns across the country via the national grid.
Step down transformers are required in each town to
step the potential difference down to 240V – for
household use.
Less energy is lost in the national grid if very high
voltages are used.