Transcript Mains Electricity

Mains Electricity
Syllabus
Lesson 1 Safe Electricity
Lesson 2 Power
Lesson 3 Heating
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Mains electricity
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identify common materials which are electrical conductors or insulators,
including metals and plastics (P2.2)
recall the hazards of electricity including frayed cables, long cables, damaged
plugs, water around sockets, and pushing metal objects into sockets (P2.3)
describe the uses of insulation, double insulation, earthing, fuses and circuit
breakers in a range of domestic appliances (P2.4)
know some of the different ways in which electrical heating is used in a
variety of domestic contexts (P2.5)
understand that a current in a resistor results in the electrical transfer of
energy and an increase in temperature (P2.6)
recall and use the relationship
power = current × voltage (P = I × V)
and apply the relationship to the selection of appropriate fuses (P2.7)
use the relationship between energy transferred, current, voltage and time:
energy transferred = current × voltage × time (E = I × V × t) (P2.8)
recall that mains electricity is alternating current (a.c.) and understand the
difference between this and the direct current (d.c.) supplied by a cell or
battery (P2.9).
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Safe Electricity
12/04/2017
Aim
 Explain the safety features in electrical equipment
 Connect a plug
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How can electricity be used safely?
 Do not overload sockets.
 Wire plugs correctly, and check
they are not damaged.
 Never mix water and electricity.
 Regularly check cables for fraying.
 Do not stick anything other than a plug in a socket.
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What is inside an electrical cable?
copper
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The wires are made of
copper (a good conductor)
The wires are surrounded
by plastic (an insulator)
In each cable there are two
or three wires
insulating plastic
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What does each wire do?
The live wire carries a current
that alternates between a negative
and positive voltage.
The earth wire is a
safety wire that is needed
to earth appliances with
a metal case.
This makes it safer to
touch the appliance if it
develops a fault.
The neutral wire completes the
circuit. It is kept at a zero voltage
by the electricity company.
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How does earthing work?
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The fuse does two jobs.
• It protects the wiring if something goes
wrong.
• It can also protect us
• A fuse has a rating in amps (ie 13A) the
rating of the fuse must be greater than
the current used by a device
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You choose which fuse to use!
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How does a circuit breaker work?
Too much current makes the
magnetic field produced by the
electromagnet strong enough to
open the switch.
The circuit breaker is said to
‘trip’ and switches off the
current.
switch
electromagnet
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What does ‘double insulated’ mean?
Another way of improving the safety of electrical appliances is to
make them double insulated.
• Double insulated appliances have
plastic cases
• Thus the case cannot become live
• So even if a wire inside the case
breaks the appliance is still safe
symbol on double insulated
appliance
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Safety device
How it works / Notes
• Melts when too much current flows.
Fuse
• Value of fuse must be higher than current drawn
by the device
• Must be in live wire
• Breaks the circuit when too much current flows
Circuit Breaker
• Must be in live wire
• Can be reset
Earth
Double Insulated
Plastic
Insulator
• Protects from shock with metal cased appliances
• Any charge on outside case will be dissipated to
earth (also causing the fuse to melt)
• Plastic exterior means that even if live wire
touches case user will not receive a shock
• Grips outer cable and prevents strain on the inner
coloured wires.
• Is an insulator, stops current flowing from wire
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Earth Wire
Live Wire
Fuse
Neutral Wire
Outer
Insulation
Cable Grip
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Identifying wrongly-wired plugs
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Live wire
Brown
Neutral
wire
Blue
Earth
wire
Green and yellow
stripes
Practical: Connect your plug up safely
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Electrical Safety
An electric iron is fitted with an
earth and a plug containing a fuse
a)
Describe briefly how a fuse
works
b)
Give two reasons why a fuse in a
plug is important for the safe use
of mains-operated appliance
c)
Why should the fuse in the plug
be connected to the live wire?
d)
Explain how earthing the iron
protects the user from receiving
an electric shock
2) A fault in an electrical circuit can
cause too great a current to flow.
Some circuits are switched off by a
circuit breaker. One type of Circuit
breaker is shown. A normal current
is flowing. Explain, in detail, what
happens when a current which is
bigger than normal flows.
1)
Worksheet
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Materials and properties of plug parts
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What does each wire do?
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Can you wire a plug?
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Earthing and fuses – true or false?
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Safe Electricity
recap
Aim
 Connect a plug
 Explain the safety features in electrical equipment
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Power
12/04/2017
Aim
 explain alternating current
 compare ac and dc traces
 calculate Power
Starter:
a
b
c
d
e
f
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1½ V
6V
230V
12 V
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+
1 ½ V
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AC supply
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AC supply
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AC supply
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AC supply
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Comparing a.c and d.c.
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electricity is
Alternating Current.
 Small Circuits
(Batteries) use Direct
Current
 This means that the
current changes from
positive to negative.
 Frequency (Hz) is
number of waves per
second
voltage
 Mains
voltage
Mains Electricity
a.c.
time
d.c.
time
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Which type of current – a.c. or d.c.?
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Power
• Power is “the rate of doing work”
• Or how much energy does it use a second
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Calculating Power - Mechanical Method:
 Whose
the Most Powerful in the Class?
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Power
• Power is “the rate of doing work”
(or how much energy it uses in a second)
Energy = Power
( joule)
(Watt )
x
time
(seconds) E = Energy (J)
P = Power (W)
t = time (s)
E
P
t
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Some example questions
1) What is the power rating of a light bulb that transfers 120
joules of energy in 2 seconds?
2) What is the power of an electric fire that transfers
10,000J of energy in 5 seconds?
3) Farhun runs up the stairs in 5 seconds. If he transfers
1,000,000J of energy in this time what is his power rating?
4) How much energy does a 150W light bulb transfer in
a) one second, b) one minute?
5) Shaun’s brain needs energy supplied to it at a rate of 40W.
How much energy does it need during a physics lesson?
6) Damien’s brain, being more intelligent, only needs energy at
a rate of about 20W. How much energy would his brain use
in a normal day?
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Calculating Power – Electrical Method
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• What would happen if a greater
voltage was applied
• What would happen if the bulb
was replaced with a more
powerful one?
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Power and fuses
Power is “the rate of doing work”.
The amount of power being used in
an electrical circuit is given by:
P
Power = current x voltage
in W
Peter
in A
in V
Is a
Veggie
I
V
Using this equation we can work out the fuse rating for any
appliance. For example, a 3kW (3000W) fire plugged into a
240V supply would need a current of _______ A, so a
_______ amp fuse would be used (fuse values are usually 3,
5 or 13A).
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Power and fuses
P
Copy and complete the following table:
Appliance
Power rating
(W)
Voltage (V)
Toaster
720
240
Fire
2000
240
Hairdryer
300
240
Hoover
1000
240
Computer
100
240
Stereo
80
240
Current
needed (A)
I
V
Fuse needed
(3, 5 or 13A)
Mains Electricity
recap
Aim
 explain alternating current
 compare ac and dc traces
 calculate Power
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Heating
12/04/2017
Aim
 To understand the mechanism of electrical heating
 To name uses for resistors in homes
 To calculate the energy transferred by a heater
Resistance Simulation
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Electrical Heating
 As
the electrons flow through a wire they will
occasionally collide with an atom.
 This results in a transfer of energy to the
wire causing it to heat.
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Electrical Heating
 Resistors
are used in the home mostly for
heating:
 Kettle
 Electric Fire
 Electric Oven
 Heat Bulb
 Hair Dryer
 Electric Radiators / Heaters
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Power Equations
Power = current x voltage
Energy = Power x time
Combining the equations:
Energy = Power
time
Energy = voltage x current
time
Energy = voltage x current x time
E=VxIxt
(J) (V) (A) (s)
(or E = V I t)
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Example Question:
A
12V immersion heater draws 2 A of current for 100
seconds how much energy is transferred?
 What
do we know?
 V = 12V
 I = 2A
 t = 100s
E = V x I x t
E = 12 x 2 x 100
E = 2400 J
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Experiment: Measuring the Energy Transferred
Method:
1.
Measure 250 ml of water into a 250 ml beaker
2.
Connect up the circuit
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Use a clamp to suspend the immersion heater in
the beaker
4.
Turn on the immersion heater
5.
Measure the temperature, Voltage and Current
and start your stopwatch
6.
Take readings every minute for 10 minutes
Temperature (°C)
Time (s)
Voltage (V)
A
Current (A)
V
Energy (J)
0
60
Results:
Plot a graph of Energy (X) against Temperature (Y)
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Heating
recap
Aim
 To name uses for heaters in homes
 To understand the mechanism of electrical heating
 To calculate the energy transferred by a heater
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