PHYSICS
Electricity & Magnetism
Electricity &
Magnetism
O-LEVEL
1
● Field strength decreases with distance
from the magnet.
Plotting lines of force
4.1) Simple phenomena of magnetism:
 Magnetic forces are due to interaction
between magnetic fields
 Law of magnetic poles :
‘Like poles repel,opposite poles attract’
 When a magnet is freely suspended so
Induced magnetism:
that it can swing in a horizontal plane, it
comes to rest in N-S direction.
 N-pole points towards the north &
South pole points towards the south of
the earth.
● Magnetic materials : are attracted to
magnets, can be magnetized (e.g. iron,
steel, cobalt, nickel).
● Magnetic materials can be:
● Non-magnetic materials : are not
* Magnetized by stroking with a magnet,
attracted to magnets and cannot be
hammering them in a magnetic field, or
magnetised (e.g. glass, plastic)
putting them inside a coil with a D.C.
through it. A piece of steel becomes
permanently magnetized when placed
near a magnet, but its magnetism is
usually weak.
Magnetic fields :
* They can be demagnetized by
It is the space surrounding a magnet
hammering them, heating them or
where it produces a magnetic force.
putting them inside a coil with an A.C.
 Field lines around magnet point from
through it.
north to south.
 Magnetic materials that can be
● The direction of a magnetic field line
permanently magnetised are
shows the direction of the force on a north
magnetically hard (e.g. steel).
pole at that point.
 Magnetic materials that are only
temporarily magnetised are magnetically
soft (e.g. soft iron).
Dr. Islam Abbas
01003177143
PHYSICS
Electricity & Magnetism
Soft iron
VS
O-LEVEL
2
Steel
is measured in coulombs (C).
• Gets magnetized
• Slow to be
 There are positive and negative charges;
faster but loses its
magnetized but
Unlike charges attract and like charges
magnetism as
retains magnetism
repel.
soon as magnet
for a long time.
 The attraction is stronger than repulsion
is removed.
• Low susceptibility
because the attracting charges are closer
• High
but high retentively
than repelling ones.
susceptibility
• Use: making
● Conductors allow electrons to flow
but low retentively
permanet magnets
through them (e.g. iron, copper,
• Use:
aluminum..) whereas insulators impede
Transformer’core
the flow of electrons (e.g. plastic,
paper……).
Permanent Magnet
VS
Electromagnet
Charging an insulator :
• Design: hard
• Design: Uses a
(a)
Rubbing method : When two
magnetic
solenoid to
insulators are rubbed together,
material
create magnetic
electrons move from one to the other
• Use: for
field
and they become charged.
applications
• Use: For
where magnetism
applications
is needed over
where magnetic
long periods –
field
fridge doors
needs to be turned
on &off-scrap
metal moving
4.2) Electrical quantities
 Matter is neutral since it contains equal
numbers of positive charges and negative
(b) Induction method: When a charged
body
touches an uncharged object, the
two bodies will share the charge. Both
will carry a part of the same charge.
electrons.
 If a body gains electron it becomes
negatively charged, if it loses some
electrons it becomes positively charged.
 Electric charge is the physical property of
matter that causes it to experience a force
when placed in an electromagnetic field; it
Dr. Islam Abbas
01003177143
PHYSICS
Electricity & Magnetism
O-LEVEL
3
 The presence of an electrostatic charge
* Current
can be detected using a leaf electroscope
 It is the flow of charges in a conductor;
* If a charged object is placed near the
SI unit is the Ampere (A).
cap, charges are induced.
 Ammeter : is the device used to
* The metal cap gets one type of charge
measure electric current, it is
(positive or negative) and the metal
connected in series with circuit.
stem and gold leaf get the other type of
charge so they repel each other.
 Current is a rate of flow of charge.
𝑸
I =
𝒕
Q = ne
Electric field: It is the region in which
n = number of electrons
electric charge experiences a force.
e = charge of an electron (1.6 × 10-19 C)
• The direction of an electric field at a
 There are two directions of current
point is the
direction of the force on a positive charge
at that point.
 The field lines around a charged
conducting sphere are as if the charge
was concentrated at the centre of the
sphere.
○ The field lines between two charged
plates go in straight lines from the
positive plate to the negative plate and
are equally spaced apart.
* Potential difference (P.D)
 Potential difference, is also known as
voltage.
 Voltage is the amount of energy the cell
gives the electrons it pushes out.
 Voltage is measured in volts (V) and is
measured by a voltmeter (connected in
parallel).
 If a cell has 1 Volt, it delivers 1 Joule of
energy to each coulomb of charge (J/C).
𝑬
V=
𝑸
Dr. Islam Abbas
01003177143
PHYSICS
Electricity & Magnetism
O-LEVEL
4
* For semi-metal conductors:
Higher temperature = less resistance.
𝑳
𝑳
R= ρ = ρ 𝟐
𝑨
𝝅𝒓
Experiment to determine the resistance
* Electromotive force (EMF)
 The maximum voltage a cell can
produce is called the electromotive force
(EMF), measured in volts.
 When a current is being supplied, the
voltage is lower because of the energy
wastage inside the cell.
 A cell produces its maximum PD when
not in a circuit and not supplying
current.
* Resistance
 It is the opposition that faces electric
of unknown resistance: - (Ohm`s law)
 Vary
the
potential
difference
across
unknown resistor by changing resistance
of variable resistor.
 Measure the p.d across unknown resistor
by using voltmeter as well as measuring
current intensity by ammeter.
 Repeat
by
changing
the
length
of
resistor the measure p.d and electric
current.
 Plot
graph
between
p.d
on
x-axis
current
current when it moves in a wire.
 The greater the resistance, the harder it
is for current to flow Through.
𝑽
Resistance (R) =
𝑰
Its unit is ohm (Ω).
Factors affecting the resistance :1- Length of wire:
(Increasing length, increasing resistance).
2- Diameter of wire:
(Increasing diameter, decreasing resistance)
3- Material:
(Better conductor = less resistance)
4- Temperature:
* For metal conductors:
Higher temperature = more resistance
Dr. Islam Abbas
01003177143
PHYSICS
Electricity & Magnetism
O-LEVEL
5
* Electrical working
● Energy is transferred from chemical
energy in the battery to electrical energy
used by circuit components and then to
the surroundings.
 This electric energy is converted to heat
energy in the conductor which raises its
temperature, or can be converted to
motion or light or sound ……….
● The power of a component is given by
P=IV =V2/R =I2R
 The electric energy is given by:
E = IVt
4.3) Electric circuits
* Circuit diagram
Semiconductor diode
 A device that has an extremely high
resistance in one direction and a low
resistance in the other, therefore it
effectively only allows current to flow
in one direction.
 Forward bias is when the diode is
pointing in the direction of the
conventional current and reverse bias
is the opposite.
 It can be used in a rectifier; turns AC
current into DC current .
Dr. Islam Abbas
01003177143
PHYSICS
Electricity & Magnetism
O-LEVEL
6
●The potential difference is the same
VT = v1=v2=v3
●The total resistance of two resistors in
parallel is less than the resistance of
either resistor
𝟏
Series and Parallel Circuits
Series:
 Components are connected end to end
in one loop.
 The same current flows through every
component
𝑹𝑻
=
𝟏
𝑹𝟏
+
𝟏
𝑹𝟐
+
𝟏
𝑹𝟑
…..
If there are two resistor only then:
RT =
𝑹𝟏 𝑿 𝑹𝟐
 Connecting
lamps
in
parallel
advantageous
because
if
one
𝑹𝟏+𝑹𝟐
is
breaks,
current can still pass through the rest.
IT=I1=I2=I3
 The potential difference is shared across
each component
VT = v1+v2+v3+….
 The total resistance is the sum of the
resistances of each component
RT = R1 + R2+ …
Action and use of circuit components
A potential divider :
Divides the voltage into smaller parts.
To find the voltage (at VOUT) we use :
𝑹
Vout = VIN x (𝑹𝑻)
●Parallel
•A variable potential divider
●Components are connected to the
(potentiometer) is the same as the one
power supply in separate branches
above but using a variable resistor; it acts
 The current is shared between each
like a potential divider, but you can
branch.
change output voltage.
IT=I1+I2+I3
Dr. Islam Abbas
01003177143
PHYSICS
Electricity & Magnetism
O-LEVEL
7
• Input Transducers:
*Noise is amplified
* The noise is
Thermistor: input sensor and a
too when the
normally
transducer.
signal is amplified,
a lower amplitude
It is a temperature dependent resistor.
so the quality is
than the high/low
At higher temperature there is less
reduced.
states used,
resistance.
so it can be ignored.
Logic gates
Light dependent resistor (LDR):
Input sensor and a transducer. When
light intensity
increases, resistance
decreases.
Logic gates are processors that are
circuits containing transistors and other
components. Their function is shown by
the truth table below (3 columns from
the right)
Digital electronics
Analogue signals
Digital signals
*It varies
* They are a series
continuously in
of pulses with two
amplitude,
states, a high state
frequency or both.
and a low state.
*All signals get
* Carry more
weaker as they
information per
travel longer
second and
distances and
maintain their
need to be
quality better over
amplified so they
longer distances
can be returned to
the original.
Dr. Islam Abbas
01003177143
PHYSICS
Electricity & Magnetism
O-LEVEL
8
* Dangers of electricity
Fuses:
Hazards:
● A fuse is a thin piece of wire which
● Damaged insulation : contact with the
overheats and melts if the current is too
wire due to gaps in the insulation can
high, protecting the circuit.
cause an electric shock or pose a fire
hazard by creating a short circuit.
● Overheating of Cables : high currents
passing through thin wire conductors
cause the wires to heat up to very high
temperatures which could melt the
● Fuses have a current rating which
insulation and cause a fire.
should be slightly higher than the current
● Damp Conditions : water can conduct a
used by the device in the circuit. The
current so wet electrical equipment cause
most common are 3A, 5A and 13A.
an electric shock.
* Electromagnetic effects
Electromagnetic Induction:
- There are three main wires running
parallel in home in every electrical circuit.
The earth wires :
If a wire is passed across a changing
magnetic field, a small EMF is induced
and can be detected by a galvanometer.
• The direction of an induced EMF
opposes the change causing it (Lenz’s
● Connect to the metal body of appliance
rule).
so that if the body connected to live wire,
the current will flow through the earth
wire since it has very low resistance
which breaks the fuse and disconnects the
appliance and prevent electric shock.
Dr. Islam Abbas
01003177143
PHYSICS
Electricity & Magnetism
O-LEVEL
9
An experiment to show electromagnetic
* Applications
induction:
1- AC generator
● In a direct current, the current only
flows in one direction whereas in an
alternating current, the current
continuously changes direction.
• Connect a coil of a large number of
● An AC generator consists of a coil of
turns to a center- zero galvanometer.
insulated copper wire between two
)1( Move a bar magnet quickly into coil.
permanent magnets. The slip rings are
The pointer of the galvanometer will
fixed to the coil and rotate with it. The
momentarily deflect in on direction
brushes are 2 contacts which rub against
which means that a current has been
the slip rings and keep the coil connected
induced.
to the outside part of the circuit, usually
)2( Keep the magnet still inside the coil,
made of carbon.
the galvanometer will read zero as the
● As the coil rotates, the magnetic field
movement stops.
through the coil changes, which induces
(3) Now, move the magnet out of the
an e.m.f. in the coil.
coil, you will observe that the deflection
● The magnitude of the e.m.f. is
of the galvanometer is reversed.
maximum when the coil is horizontal as
the field lines are cut the fastest, and
zero when vertical (no field) lines are
being cut.
- Fleming’s right-hand rule Current
direction
• The direction of the current is given by
● The induced EMF can be increased by:
the right-hand grip rule:
* Moving the coil faster.
The fingers point in the conventional
* Using a stronger magnet.
current direction and the thumb gives
* Increasing length of wire in magnetic
the North Pole.
field (increasing number or turns of the
coil).
Dr. Islam Abbas
01003177143
PHYSICS
Electricity & Magnetism
O-LEVEL
10
• The current and EMF direction can be
● A step down transformer has fewer
reversed by:
turns on the secondary which means the
* Moving the coil in the opposite direction
voltage of the secondary is less than that
* Changing the poles.
of the primary.
- Each side of the coil travels upwards
then downwards then upwards etc. so
the current flows backwards then
forwards then backwards etc. so it is an
alternating current.
𝑽𝒑𝒓𝒊𝒎𝒂𝒓𝒚
𝑽𝒔𝒆𝒄𝒐𝒏𝒅𝒂𝒓𝒚
=
𝑵𝒑𝒓𝒊𝒎𝒂𝒓𝒚
𝑵𝒔𝒆𝒄𝒐𝒏𝒅𝒂𝒓𝒚
●For a 100% efficient transformer,
because the power used is constant,
IPVP= ISVS
●Transformers are used to step up the
voltage in power lines which reduces
power loss.
- Because a higher voltage means a
smaller current and the loss of power due
to P=I2R will be lower.
2- Transformer
● A transformer consists of 2- coils
wrapped around a iron core and used to
transform voltages.
● An alternating current in the primary
coil creates a changing magnetic field;
* The magnetic effect of a current
When an electric current in a wire
produces a magnetic field around it,
which causes the deflection of the
magnetic needle of a compass.
this changing magnetic field links with
the secondary coil and induces an
alternating e.m.f. in it.
● A step up transformer has more turns
on the secondary which means the
voltage of the secondary is greater than
that of the primary.
Dr. Islam Abbas
01003177143
PHYSICS
Electricity & Magnetism
O-LEVEL
11
* The right hand grip rule determines the
• By reversing current, direction of force
direction of the magnetic field produced
is reversed
by a current carrying wire.
• By reversing the field, direction of force
● The magnetic field created by a
is reversed
solenoid is like the field produced by a
• If both are reversed nothing happen.
bar magnet.
* The strength of this force can be
●Increasing the current through the wire
increased by:
increases the strength of the magnetic
a. increasing the length of conductor .
field.
b. Using strong magnet
●Reversing the direction of the current
c.
through the wire reverses the direction of
conductor.
Increasing
the
current
in
the
the magnetic field.
●The direction of a magnetic field line at
a point is the direction of the force on
the N pole of a magnet at that point
• Magnetic effect of current is used in a
relay and a circuit breaker.
* Force on a Current-Carrying Conductor
* The direction of the force, current or
magnetic field is given by Fleming’s lefthand rule.
* A force is also exerted on charged
particles moving in a magnetic field
• If a current carrying conductor is in a
magnetic field, the conductor experience
motion due to a force is called motor
force.
* This force is a result of interaction of
following two magnetic fields:
(because moving charged particles are
current).
* If a beam of charged particles moves
through a magnetic field, it will be
deflected, showing that
there is a force.
a. The magnetic field due to magnet .
b. Magnetic field of the current carrying
conductor
Dr. Islam Abbas
01003177143
PHYSICS
Electricity & Magnetism
* Applications
O-LEVEL
12
Reed switch
1- Electromagnet:
An electromagnet is made of a solenoid
wound around a core made of soft iron.
It is magnetized only during the flow of
current, when the current is switched off
3- Circuit breakers:
it loses its magnetization, its magnetism
is only temporary.
Uses: electric bells, magnetic relays,
telephone.
2- Relays
* Relay is an electrical device in which
large current circuit can be operated by
a low current DC circuit.
● Circuit breakers consist of an automatic
*It is a switch operated by an
electromagnet switch which breaks the
electromagnet
circuit if the current rises over a certain
value.
● This is better than a fuse as it can be
reset and used again, and they operate
faster.
Dr. Islam Abbas
01003177143
PHYSICS
Electricity & Magnetism
4- D.C. motor
O-LEVEL
13
Turning effect increased by:
• Increasing the current.
• Using a stronger magnet.
• Increasing the strength of the magnetic
field.
• Increasing the number of turns on the
coil.
Reversing rotation can be done:
When coil not
When coil
• Reversing the battery.
energized,
energized,
• Reversing the poles.
switch is closed,
switch is closed,
completing
completing
circuit
circuit
When a current-carrying coil is in a
magnetic field, it makes a turning effect.
•A DC motor runs on a direct current.
•The coil is made of insulated copper
wire and is free to rotate between the
poles of the magnet.
•The commutator (split-ring) is fixed to
the coil and rotates with it.
•When the coil overshoots the vertical,
the commutator changes direction of the
current through it, so the forces change
direction and keep the coil turning.
•The brushes are two contacts which rub
against the commutator and keep the
coil connected to battery, usually made of
carbon.
•Max. turning effect is when the coil is
horizontal.
•There is no force when the coil is
vertical but it always overshoots this
position.
Dr. Islam Abbas
01003177143