Download Unit 2 Section 3 - Belfast Royal Academy

In order to get the G.C.S.E. grade you are capable of, you must make
your own revision notes using your Physics notebook.
When summarising notes, use different colours and draw
diagrams/pictures. If you do, you will find them easier to remember.
Once you have made revision notes for a topic, re-visit these regularly
(on the day of your examination you will not remember something you
revised 4 weeks previously). Each time you re-visit a note tick the top
of the page/card. This will allow you to identify any notes you have
neglected.
WARNING: DO NOT RELY SOLELY ON THE REVISION
POWERPOINTS!
The electricity Board sells electricity in units known as kilowatt
hours.
One kilowatt hour (kWh) is the amount of electrical
energy converted by a (1kW) one kilowatt appliance
in one hour.
The kilowatt hour is a measure of the amount of ENERGY converted.
Energy = power x time
= 1000 x (1x60x60)
= 3600000J
=3.6 x 106 J
= 3.6MJ
i.e. Electrical energy into other forms:
1kWh = 3.6MJ
The number of units or kilowatt hours used
can be calculated using the formula:
Number of kilowatt hours = power (kW) x time (h)
The cost can be calculated using:
COST = power (kW) x time (h) x cost of 1 unit
or
COST = number of kWh x cost of 1 kWh
Always be careful to use the correct units!!!
Voltage or current
D.C. or direct current
flows around a circuit in
ONE DIRECTION ONLY.
Batteries produce D.C.
+
o
time
-
Voltage or current
A.C. or alternating current
continually CHANGES
DIRECTION, i.e. current flows
one way and then the other.
Mains voltage produces A.C.
+
o
-
+
-
+
+
-
time
Live Wire:
Essential for a complete circuit. It is at a high voltage and
is brown in colour. It is always connected to the fuse and
switch.
Neutral Wire: Essential for a complete circuit. It has zero voltage and is
blue in colour.
Earth wire:
Used as a safety measure (see safety section).
Fuse:
Used as a safety measure. It melts when the current
becomes too high.
Memorize the diagram of the plug so that you can
recognise any faults and/or label the diagram.
Common faults:
1.
Bare wire visible.
2. Wire connected
to the wrong pins.
3. Wires visible
below the cable
grip.
There are two main types of switch:
(A) ONE-WAY SWITCH
If the switch is open – no current
(B)
TWO-WAY SWITCH
A two-way switch can direct current
along one of two paths.
A switch is always connected to the Live Wire.
The diagram shows a
combination of 2 two-way
switches. This is used in
long hallways and
staircases. It enables the
light to be switched on
and off at two different
locations.
Staircase
The THREE ways to reduce the dangers associated with
‘Mains’ Electricity are:
(A) Fuses
(B) The Earth Wire
(C) Double Insulation
Note: these all have the sole purpose of preventing somebody
getting an electric shock when using an electrical device. They do
this either by isolating the device from the live connection (earth
wire, fuses and circuit breakers) or by constructing the device so
it isn’t possible for a user to touch any live parts (double
insulation)
(A) FUSES
CIRCUIT SYMBOL:
•
A fuse is a short piece of thin wire with a low melting point.
•
When the current gets too high the fuse heats up and melts,
breaking the circuit and preventing damage to electrical
appliances and fires.
•
The fuse is always connected to the Live wire..
There are two types of fuse:
1]
The “cartridge type” which cannot be rewired.
2] A single piece of fuse wire which is rewireable.
Choosing a fuse: Use the equation I = P/V (from P=IV).
The chosen fuse must be close to this value but
slightly greater so to allow the current to pass
through.
(B) THE EARTH WIRE
This is a low resistance wire connecting the case of an appliance to the
Earth.
If a fault develops in the appliance and the live wire touches the metal case
then a large current will flow from the live wire to the low resistance earth
wire and the fuse will burn out. This isolates the appliance from the live
terminal
If there is no Earth connection the case of the faulty appliance would be at
240V and anyone touching the ‘live’ case would receive an electric shock.
(C) DOUBLE INSULATION
Symbol:
Used when an appliance does not have an earth connection, e.g. some
electric drills and hair dryers.
Defining Double Insulation:
The external metal parts which could become live if a fault developed
are insulated from all the external parts which can be touched by the
user.
1. Bar Magnet
Put a magnet on a page and draw an
outline around it.
N
Dot 2
.
Dot 1
.
.
.
.
S
Place a compass at the north pole of
the magnet and mark the ends of
the needle.
N
S
Move the compass as shown and
mark the position of the needle
head. Repeat this until you reach
the south pole of the magnet.
3
2
1
N
S
When this is repeated
from several different
points the magnetic field
is shown to have the
appearance opposite.
N
S
Remember:
1. Field lines never
touch or cross
2. Field lines point away
from the north pole
and towards the
south pole.
2. Coil of wire
A. Place magnets around a coil
of wire. They should all point
in the same direction – north.
B. Supply a current to the coil
and note the direction in which
the compasses point.
Conclusion
The shape of the magnetic field surrounding a current carrying
solenoid (coil of wire) is similar to that which surrounds a bar
magnet.
S
N
+
-
To determine which end of the
solenoid is the north pole, grip the
solenoid with your right hand so your
fingers curl in the direction of the
current. Your thumb will point to the
north pole.
The strength of the magnetic field can be increased by:
1.
Increasing the current
2. Increasing the turns density (number of turns per cm)
3. Placing a soft iron core through the centre of the solenoid
A major advantage of an electromagnet over a bar magnet is that it
can be switched on and off.
Electromagnets are used in electric bells and electric relays.
If a conductor carries a current at right angles to a magnetic field
it will experience a force.
The direction of the force is reversed if:
1.
The current flows in the opposite direction (see animation above)
2. The magnetic field changes direction (see animation above)
Using your left hand, point you first finger in the direction of the
magnetic field and your second finger in the direction of the current.
Your thumb will then point in the direction of the force.
An electric motor changes electrical energy into kinetic energy.
1.
Current flows around the rectangular coil.
2.
Current flows in opposite directions along sides AB and DC.
3.
Sides AB and DC experience forces in opposite directions.
4.
The coil turns.
This is the inducing (generating) of electricity in a conductor by
changing the magnetic field through a conductor.
The following experiments can be used to demonstrate electromagnetic
(em) induction. Learn these!
A current is only induced when the magnetic field through the
conductor changes
Electromagnetic Induction is used in the following:
1) The A.C. generator.
To wire is wound in the shape of a coil and
rotated between the poles of a magnet.
When the coil is rotated (by hand, or using a
steam engine or windmill etc.) the magnetic
field through it changes. A voltage is
therefore induced in the coil causing an
alternating current (A.C.) to flow.
2) Transformers
A transformer consists of two circuits, each
circuit containing a coil. The primary circuit is
connected to an a.c. supply and therefore
causes a voltage to be induced in the
secondary circuit. (see next two slides for
more detail)
To make the magnetic field change continuously the current through the
primary coil must be alternating (a.c.).
This changing magnetic field induces a current (and voltage) in the secondary
coil.
The coils can be linked using iron ring core. The voltage induced will depend
on: (1) the number of turns in both coils and (2) the primary voltage.
The voltage produced can be
calculated using the following
equation:
Vs N s

Vp N p
Where: Vp = Primary voltage
Vs = Secondary voltage
Np= Number of turns in the secondary coil
Ns= Number of turns in the primary coil
Transmission of Electricity to your home.
In order to REDUCE the ENERGY LOST when transmitting current through overhead
cables, electricity is TRANSMITTED AT A VERY HIGH VOLTAGE from the power
station to our homes.
Using P = IV, the higher the voltage is the lower the current will be and therefore
the less energy that will be lost as heat.
An A.C. VOLTAGE can easily be increased and decreased easily using a transformer.
There are two types of Transformers:
1. STEP-UP TRANSFORMERS (Ns>Np) are used to increase the voltage at the POWER
STATION. This is done to reduce power loss through the generation of heat.
2. STEP-DOWN TRANSFORMERS (Ns<Np) are used to reduce the voltage again at our
HOMES.
Cost Questions
1. At 7p per unit, how much more would it cost to run a 3kW fire for 4 hours
than a 100W lamp for 100 hours? (click for solution)
Cost = power x time x cost of 1 unit = 3 x 4 x 7 = 84 p
Cost = power x time x cost of 1 unit = 0.1 x 100 x 7 = 70 p
Difference = 14 p
2. An electric shower is rated at 7kW. How much would it cost to run for a
whole year at 7 pence per unit if it was used on average for 3 hours per
week? (click for solution)
Cost = power x time x cost of 1 unit = 7 x (3 x 52) x 7 = 7644 p = £76.44
A.C./D.C. Questions
1) Draw the two graphs which represent alternating and direct
current. (Click for solutions)
Current
Current
Time
Time
A.C.
D.C.
2) Name a source of alternating current and a source of direct
current. (Click for solution)
a.c. – mains
d.c. - battery
Plug Question
Paper 1 June 2009
4a) The diagram below shows a fused 13A plug. Five parts are labelled A, B, C, D and E.
B
D
A
C
E
(i)
Complete the table to identify the wires connected to the three pins of the plug.
Wire connected to:
Letter (A, B or C)
(ii)
Live pin
C
Neutral pin Earth pin
A
B
What names are given to the parts labelled D and E?
Fuse
D _____________________
Cable grip
E _____________________
Click for solutions
A washing machine has a metal frame. A fault occurs so that the live wire
connected to the motor is detached and makes contact with the metal frame.
(iii) Explain fully how the fuse and the earth wire in the previous plug work together to
prevent a person touching the frame being electrocuted. Click for solution
•
A large current flows through the low resistance earth wire.
•
The fuse blows.
•
The appliance is isolated from the live terminal.
Switch Questions
1) Draw a circuit to show how a single light bulb can be switch on
and off at the top and bottom of a staircase. (click for solution)
x
2) Which wire is the switch always connected to in a mains
electricity circuit? (click for solution)
Live wire
Switch Questions
4e)
Paper 2 June 2009
Mr Henderson is an electrician. He wires an electrical heater up to the
mains supply. The incomplete circuit is shown below.
Symbols:
Live
Fuse
Mains supply
Heater
Neutral
Switch
(i)
Complete the diagram by inserting its symbols shown above in the correct
place. (click for solution)
(ii) Placing the switch in the wrong position in mains circuit is dangerous.
Explain why this is so. (click for solution)
As when the switch is open the appliance will not be isolated from the live
wire. Therefore, there is a risk of an electric shock.
Safety Questions
1)
Calculate the operating current and suggest a suitable
fuse for the following electrical appliances:
1)
2)
1000W kettle operating at 240 V (click for solution)
2000W washing machine operating at 240V (click for solution)
Possible fuses: 1 A, 2 A, 5A, 8A and 10 A.
2)
Describe the purpose of a fuse. (click for solution)
3)
What is the function of double insulation and when is it
not used in an appliance? (click for solution)
1) (a) I = P/V = 1000/240 = 4.17 A  5 A fuse
1) (b) I = P/V = 2000/240 = 8.33 A  10 A fuse
2) If the current exceeds the rating of the fuse the fuse melts. This
isolates the appliance from the mains
3) To insulate all internal parts that can become live from the user. It is
not used when there is an earth wire connected to the appliance.
Safety Questions
4dii)
Paper 2 June 2008
An electric drill operates at 240 V and has power of
950W. Calculate the correct fuse to fit to the plug of this
drill. You should select your fuse from the following: 1A,
3A, 5A, 10A, 13A. You are advised to show clearly how
you get your answer.
P = VI
I = P/V = 950/240 = 3.96 A
Therefore, a 5 A fuse should be used in the plug
Safety Contents
Circuit Breaker Questions
1) What are the advantages of using a circuit breaker instead of a
fuse? (click for solution)
They don’t have to be replaced
They respond quicker
2) Explain how the circuit breaker below works. (click for solution)
If the live wire touches the outer
casing of an appliance a large
current flows through the low
resistance earth wire. The
electromagnet in the live wire
becomes strong enough to
break the circuit
Circuit Breaker
Contents
EMI Question
1) A bar magnet is placed inside a coil
of copper wire as shown below. The
ends of the coil are joined to a sensitive
centre-zero ammeter.
What would be observed on the sensitive ammeter when the following actions are
carried out? Record the observations by ticking (√) in the appropriate box below.
(click for solutions)
Action
1
2
3
4
5
No
deflection
of the
pointer
The coil and magnet are
moved together to the left.
The coil and magnet are
moved together to the right.
The magnet is moved quickly
out of the coil to the left and
held stationary.
The magnet is pushed, just as
quickly as action 3, back into
the coil and left there.
The coil is moved quickly
away from the magnet and
held stationary beyond the
magnet.
Pointer
deflects
and quickly
returns to zero
√
√
√
√
(in the opposite
direction to 3)
√
(in the same
direction as 3)
Steady
deflection
of the
pointer
EMI Question
Paper 2 June 2007
4c) A strong permanent magnet is placed inside a coil of insulated copper wire
(Step 1).
Permanent magnet
(Step 1)
The ends of the coil are now attached to a sensitive ammeter (Step 2).
Permanent magnet
(Step 2)
(i)
Sensitive ammeter
Will an electric current be detected on the ammeter? Give a reason for
your answer.
No. The magnetic field through the coil does not change.
(ii) Using no extra equipment, describe how you could make a current flow in
the ammeter.
Remove the magnet from the coil.
(iii) With the same equipment, what must be done to make the current larger?
Move the magnet faster.
(iv) Explain why the current would be larger.
There would be a greater change in the magnetic field through the coil.
(v) Name the process used to produce electric currents in this way.
Electromagnetic induction
Transformers Questions
1.(a) Calculate the number of turns on the secondary coil of a step-down transformer
which would enable a 12V bulb to be used with a 240V a.c. mains power if there
are 480 turns on the primary. (click for solution)
Ns/Np = Vs/Vp  Ns / 480 = 240 / 12  Ns = 9600 turns
2.
Complete the following table:
(Click for solutions)
Primary
Primary
p.d.
p.d.
Secondary
p.d.
Primary
turns
Secondary
turns
Step-up
Step-up or
or
step-down
100V
100V
100V
100V
1000V
10
100
100
10
Step-up
Step-down
200
10
Step-down
1000
12000
Step-up
240V
240V
10V
12V
11000V
11000V
132000
Transformers Question
Paper 2 June 2007
4d) The diagram represents a transformer with a primary coil of 90 turns and a
secondary coil of 300 turns.
(i)
What type of operating voltage does a transformer use?
Alternating
(ii) What name is given to the type of transformer shown above?
Step up
(iii) The voltage connected to the primary coil is 12V. Calculate the output
voltage of this transformer.
Vs/Vp = Ns/Np  Vs/12 = 300/90  Vs/12 = 3.33  Vs = 40 V