Transcript General

Intermediate 1
Physics
Practical Electricity

Electrical Circuits

Resistance

Mains Electricity
Beath High School - Int 1 Physics
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Intermediate 1
Physics
Practical Electricity

Electrical Circuits
Beath High School - Int 1 Physics
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Electrical Circuits
A circuit will always have,

a source of
e _ _ electrical
_ _ _ _ _ _ _ energy

an electrical comp onent
_ _ _ _ t

wires forming a c _ complete
_ _ _ _ _ _ path from one
end of the source back to the other end.
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Electrical Circuits
If the circuit is ccomplete
_ _ _ _ _ _ _ , there will be
current.
If the circuit is incomplete, there will be
no current.
c______.
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Circuit Symbols
Each electrical component has a symbol - called a
"circuit symbol". When we draw circuit diagrams,
we draw the circuit symbol instead of trying to
draw the component itself.
The following table is a list of the circuit symbols for
all the electrical components needed for this
course as well as a brief description of the function
of the component.
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Circuit Symbols 1
Component
Battery
lamp
switch
Symbol
Description
supplies electrical energy
Converts electrical
energy into light energy
Open: breaks a circuit
Closed: completes a circuit
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Circuit Symbols 2
Component
heater
Symbol
Description
converts electrical energy
into _ heat
_ _ _ energy
resistor
Opposes current; it converts
electrical energy into heat
variable
resistor
A resistor whose
resistance can be changed
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Circuit Symbols 3
Component
Motor
Ammeter
Voltmeter
Symbol
Description
M
Converts electrical energy
into kinetic energy
A
Used to measure electric
current – always connected in
series
V
Used to measure voltage –
always connected in parallel
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Circuit Symbols 4
Component
Ohmmeter
Fuse
Symbol
W
Description
Measures resistance directly –
use when the component is not
connected.
A protection device. It melts
when the current gets too high.
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Series Circuits
When components are connected to allow only
one path for the current, we say that the
components are connected in series.
three l _lamps
_ _ _ in series
A
lamp, ammeter and fuse
connected in series.
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Series Circuits
If the components form a circuit, the circuit is
called a s _series
_ _ _ _ circuit.
A
A series circuit. The battery,
switch, lamp, variable resistor
and ammeter are all connected
in series.
There is only oone
_ _ path for
the c _current
_ _ _ _ _ from one end
of the battery, through each
component in turn to the other
end of the battery.
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Parallel Circuits
When components are connected to allow mmore
_ _ _ than
one path for the current, we say that the components
are connected in parallel.
two lamps in pparallel
_______
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Parallel Circuits
lamp, heater and voltmeter
connected in parallel.
V
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Parallel Circuits
A parallel circuit.
The b _battery
_ _ _ _ _ , lamp and
voltmeter are all connected in
pparallel.
_______.
There is more than one separate path for
the c _current
_ _ _ _ _ . You can trace a
V
separate path from the battery
b______
through either component back to the
oother
_ _ _ _ end of the battery.
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Measuring Current

Current is measured using an ammeter
a______

Current is measured in amperes (aamps
_ _ _ ).
(The shorthand for amperes is ‘A’)

To measure the current through a component, always
connect the ammeter in s _series
_ _ _ _ with the
component. This means b _breaking
_ _ _ _ _ _ the circuit to
insert the ammeter.
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Measuring Current
Connecting an ammeter
A
Before
After
The circuit is altered to include the
ammeter in series with the lamp.
The reading on the ammeter is the
current t _through
_ _ _ _ _ the lamp.
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Measuring Voltage

Voltage is measured using a voltmeter

Voltage is measured in vvolts
____.
(Shorthand for volts is ‘V’)

To measure the voltage across a component, always
connect the voltmeter in p _ _parallel
_ _ _ _ _ with the
component.
The voltmeter forms another parallel branch across
the component.
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Measuring Voltage
Connecting the voltmeter
Before
After
V
The voltmeter is added to make
another branch in parallel
p_______
with the lamp.
The reading on the voltmeter is the
voltage across the lamp.
l___.
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Current and Voltage in Series Circuits
The current through every component in a series circuit
is i _ _identical
_ _ _ _ _ _ and is the same as the supply
current.
A
A
To measure the current in each lamp, connect an ammeter
in s _series
_ _ _ _ with each lamp.
Each ammeter will have the ssame
_ _ _ reading.
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Current and Voltage in Series Circuits
The sum of the voltages across each component in a
series circuit adds up to the supply voltage.
The voltage across each lamp can be measured directly.
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Current and Voltage in Series Circuits
This voltmeter measures the voltage a _across
_ _ _ _ the
battery.
This is the supply voltage.
V
V
V
To measure the voltage across each lamp, connect a
voltmeter in parallel with each lamp.
The voltmeter readings across the lamps aadd
_ _ up to the
supply voltage.
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Examples
1.
In the circuit shown, the current reading on one of
the ammeters is 0.2 amperes. Find the current
reading on the other ammeter and the current
through each lamp.
6V
A
A
0.2A
In a series circuit, the current is the ssame
_ _ _ at all points.
The current reading on the other ammeter = 0.2 A
also the current through each lamp
= 0.2 A
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Examples
2. Find the supply voltage in the circuit shown below.
V
V
V
1V 2V
In a series circuit, the voltages across each component add
up to the supply voltage.
The supply voltage = 2 + 1
=3 V
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Current in parallel circuits
The sum of the currents through each component in a
parallel circuit adds up to the supply current.
The current through each lamp and the battery can be
measured directly.
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Current in parallel circuits
To measure the current in each lamp, connect an ammeter
in s _series
_ _ _ _ with each lamp.
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Current in parallel circuits
u _r _
r e_ _n _
t through the
This ammeter measures the c _
battery.
This is the supply current.
A
A
A
To measure the current in each lamp, connect an ammeter
in s _series
_ _ _ _ with each lamp.
To measure the current through the battery, connect an
a _ammeter
_ _ _ _ _ in series with the battery.
The ammeter readings aadd
_ _ up to the ammeter reading of
the current through the supply.
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Voltage in parallel circuits
The voltage across every component in a parallel circuit
is i _ _identical
_ _ _ _ _ _ and is the same as the supply
voltage.
The voltage across each lamp can be measured directly.
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Voltage in parallel circuits
This v _ voltmeter
_ _ _ _ _ _ _ measures the voltage across the
battery.
V
This is the supply voltage.
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Voltage in parallel circuits
To measure the voltage
across each lamp, connect a
r _a _
l _
l e
voltmeter in p a_ _
_ _l
with each lamp.
V
V
Each voltmeter will have the
s a_ m
__
e reading.
The supply voltage is the
s a_ m
__
e as the voltage across
each lamp.
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Intermediate 1
Physics
Practical Electricity

Resistance
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Resistance
Materials oppose current and some materials oppose it
more than others.
The opposition to current is called r _
e _s _
i _
s _t _
a _n _c _
e.
e _c _
re
s e_ in
An increase in resistance causes a d _
_ a_ _
current.
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Resistance
A resistor causes electrical energy to be converted
into h _
e_
a t_ energy in the material. Sometimes this is
a nuisance – when circuits heat up and get too h _
o t_ .
Sometimes it is useful – when heat is required, for
example in k e t _
t _l _
e_
s or cookers.
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Measuring Resistance

Resistance is measured in o _
h_
m _s .
(Shorthand for ohms is “W” – the Greek letter omega)

h_
m _m_e _t _
e_
r .
Resistance can be measured using an o _
To measure resistance, connect the ohmmeter directly
across the resistor or component whose resistance you
want to measure (nothing else connected).
W
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Calculating resistance from ammeter and voltmeter values
Put the resistor or component into a circuit.
Measure the current through the resistor by
m _m_e_t _
e_
r in s _
connecting an a _
e _r _
i _
e s_ .
A
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Calculating resistance from ammeter and voltmeter values
Put the resistor or component into a circuit.
Measure the voltage across the resistor, connecting a
tm
e_
t e_ _
r in parallel.
v o_ _l _
__
A
V
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Calculating resistance from ammeter and voltmeter values
Calculate resistance using:
resistance =
voltage
current
A
V
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Calculating resistance from ammeter and voltmeter values
Calculate resistance using:
resistance =
Example:
voltage
current
The current through a resistor is 0.1 amperes when the
voltage across it is 12 volts. Calculate the resistance.
A
resistance =
voltage
12
=
= 120W
current
0.1
V
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Variable resistors
It is often useful to be able to adjust the flow of
current continuously.
For example; controlling the loudness or brightness of
a TV, the heat setting on a toaster, adjusting the
speed of a model train.
To do this we use a v _
_ _l e_ resistor; symbol:
a _r _i _
ab
When the resistance is reduced,
the current i n_ _c _
r e_ _
a_
s _e _.
s
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Intermediate 1
Physics
Practical Electricity

Mains Electricity
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MAINS ELECTRICITY
Safety Note: Mains electricity is dangerous.
Never experiment with mains electricity.
Electricity supplied to houses is called m a_ _i _
n_
s
electricity.
Mains electricity is dangerous because your body can
co
_ n_ d_ u_c_t electricity and mains voltage can cause a
current large enough to k _i _l _
l you.
r e_ if it is wet or damp,
Your body conducts even m _o _
this is the reason why there must be no sockets or
b_
a_
th
switches in a _
_ room.
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MAINS ELECTRICITY
The declared value of mains voltage is _
_0
_ volts
23
p _p _l _
i _
a n_ _c _
e s_ in Britain are designed
All mains a _
to operate with a voltage of 230 volts across them.
This means that the household wiring must be such
that all the appliances are connected in p _a _r _a _l _l _e _l .
This ensures that each appliance receives the same
voltage (230V).
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The three pin plug
Household wiring consists of cables in which there are
h_
r e_ e_ wires.
t_
a _i _n _
s using a three
The cable is connected to the m _
pin plug – one pin for each of the wires.
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The three pin plug
1.The live wire (b _r o_ w_ n_ )
The live wire is the wire connected to the 230 volts supply
from the p _
__
ow
e r_ station. It is coloured
b_
_ n_ . If you touch the brown wire of an appliance
r o_ w
which is connected to the mains you will get a s _
h_
o c_ k_
which can kill you. The switch in an appliance is always
connected to the l _i v_e_ wire so that when the switch
c o_ _
n_
n e_ c_ t_ e_ d_ from
is off, the appliance is dis _
the 230 V supply from the mains.
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The three pin plug
2.The neutral wire (b _
l u_ e_ )
The neutral wire is used to complete the circuit from the
appliance to the mains. If you touch the blue wire in an
appliance you can still get a shock if the appliance is
i _
n s_ and working.
connected to the m a_ _
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The three pin plug
e _
l l_ o_ w_ )
3.The earth wire (g _
_ and y _
r _
e _e n
The earth wire does not normally carry c u
_ _r _
r e_ _
n_
t .
It is a s a_ _f _
e t_ _
y device.
It only carries current if there is a f a_ u_ _
l t_ in the
appliance. All appliances with metal parts which can be
touched must have an e a_ _r t_ h_ wire.
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Double Insulated appliances
Appliances are called "double insulated" if it is impossible
i _
v e_ wire inside it. These appliances do
to touch the l _
t h_ wire and only have a live and
not need an e a
_ _r _
neutral wire. Double insulated appliances have this symbol
on them.
Double insulated symbol. Appliances with this symbol do
not have an earth wire since it is impossible to touch any
a _l parts.
me
_ _t _
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Wiring a plug
All appliances are connected to the mains power supply by
l _
u g_ .
a three pin p _
The plug which connects an appliance to the mains must be
connected as shown:
e_ a_ _r _t _h pin
Earth wire
l l_ o_ w_ )
(Green/ y
_ e
_ _
Live wire
r _o w
n)
(b
_ _
_ _
s _e
Fu
_ _
Neutral wire
l u_ e_ )
(b
_ _
cable grip
n_ e
u _t _r _a _l
_ _
pin
l i_ v_ e_
_
pin
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Wiring a plug
n_
g _e _r ous to operate an appliance if the wires
It is d a_ _
are not connected as shown here. If you are unsure about
a plug’s wiring, do not use it!
Earth wire
l l_ o_ w_ )
(Green/ y
_ e
_ _
e_ a_ _r _t _h
pin
Live wire
r _o w
n)
(b
_ _
_ _
s _e
Fu
_ _
Neutral wire
l u_ e_ )
(b
_ _
cable grip
n_ e
u _t _r _a _l
_ _
pin
l i_ v_ e_
_
pin
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The fuse
The fuse in a plug is a s a_ f_ e
__
_ device to protect
t y
the c a
__
b_
l e_ . It melts if the current in the cable
i g_ h_ . All plugs contain a fuse.
gets too h _
i v_ e_ pin
The fuse is always connected between the l _
i v_ e_ wire of the appliance. This
of the plug and the l _
is so that if something goes wrong, the fuse m _
e_
l t_ s_
and disconnects the live wire of the appliance from the
m_
a_
i n_ s_ power supply.
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Fuse values
Fuse values are usually 3 a m
__
__
p _e _r _e in
p_
e _r _e or 13 a m
domestic appliances.
It is important to use the correct fuse values to avoid too
high a c _
u_
r_
r_
e_
n _.
t The fuse value can be calculated
from the v _
__
o_
l t_ a_ g_ e_ and the p o_ w
e _r rating:
current =
power
voltage
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Fuse values
Example:
Calculate the fuse value required for a mains appliance
with a power of 1150 watts.
current =
power
voltage
=
1150
230
= 5A
13
Therefore a ______
ampere fuse is required.
As a general rule if the power rating is 700 W or more, a
13 ampere (13 A) fuse must be used. If the power rating is
700 W a 3 ampere (3 A) fuse must be used.
less than _____
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Electrical Power
Electrical energy is not free! We have to p _
a_
y for
batteries to get electrical energy from them and we have
to pay for the electrical energy supplied by the
p o_ w
_ _e _r station. The amount of energy used by a
household is measured by the m e
__
__
t e
r at the consumer
unit.
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Electrical Power
w _e _r rating marked on it
Every appliance has a p o
__
(usually in kilowatts kW, or watts W).
The greater the power of the appliance, the more/less
energy it uses in one second (this is because it uses
more/less current).
Also, the longer the time the appliance is used, the
more/less energy it uses. So to save energy and money,
f _
f appliances after use.
always switch o _
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The Consumer Unit
meter
kWh
Scottish Power
Main Fuse
Fusebox for all
household
circuits
Mains electricity comes into a house from
the power station to the consumer unit.
This is where the electricity meter is
Mains electricity comes from the power station into your
house through a service cable which goes to the
e_
t_
e_
r board.
m_
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The Consumer Unit
meter
kWh
Scottish Power
Main Fuse
Fusebox for all
household
circuits
Mains electricity comes into a house from
the power station to the consumer unit.
This is where the electricity meter is
From here the mains supply is split into various domestic
c _i _
r_
c_
u_
i t_ s_ . The lights, power sockets and cooker
e_
p_
a_
r_
a_
t_
e circuits which are wired in
each have s _
a _r _
a_
l l_e_l_ .
p_
The parallel wiring ensures each circuit has the same
230 V voltage needed to operate appliances.
_____
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Mains Protection
Although the fuse in the three pin plug protects the cable
of the appliance, it does not protect any wiring in the
house which comes from the mains supply (wires behind
the w _a _l l_ s_ , etc.).
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Mains Protection
Household wiring is protected at the point where it comes
into the house - at the consumer unit. Every circuit in the
house is protected in the mains consumer unit. In older
houses, a f _
u_
s_
e is used to protect each circuit but in
newer houses and houses which have been recently
rewired, a circuit b _
__
r e
a_
k_
e _r is used.
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The circuit Breaker
A circuit breaker is a very fast s w
_ _i t_ c_ h
_ which opens
and disconnects the circuit when the current gets too
l a_ r_ g_ e
_ . It disconnects the circuit by switching faster
than a f u_ s_ e
_ can by melting.
This means that c _i r_ c_ u_ i_ t_ breakers are much safer
and are gradually replacing fuses altogether.
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Using Extensions
When appliances are plugged into an extension block, the
total current i n
__
c_
r _
e_
a_
s_
e_
s . A kettle carries a
current of about 10 amperes. If four kettles were to be
plugged into an extension block, the total current being
carried would be about
40
____
amperes.
This very high current causes the cables behind the wall
or under the floor, to h e
__
a_
t up and become a f _
i r_ e_
risk. Unfortunately, this is the cause of many house fires.
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Some Safety Rules
DANGER
1. Never use worn or damaged flexes: you could get a
1.
s_
h_
o_
c_
k from an exposed live wire.
2. Avoid the use of extensions if possible: too many
appliances connected to an extension could produce
2.
o v_ e
__
__
r h
e_
a _t _
i n_ g_ at the socket.
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Some Safety Rules
DANGER
3. Always use the correct flex: a flex which is too
t_
h _i n_ could overheat.
4. Always use the correct plug fuse: if the fuse rating is
too high and a f _
a_
u_
l t_ develops, the flex could
overheat.
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Testing for Continuity
Simple conductors can be tested for continuity by trying
to pass a c u_ r_ r_ e_ n
__
t through them in series with some
indicator device (e.g. a b _
u_
l b_ or ammeter).
If there is a current, the series circuit is c o
__
m _p _l _
e _t _
e.
If there is no current, there is a b r_ e_ a_ k
_ in the circuit.
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Testing for Continuity
resistor to limit current
A
indicator
(could be a lamp)
item to be tested
Switches can be tested, after removing them from their
circuit, by using this simple continuity tester. There
should be a current with the switch in the O N
_ position.
There should be n o_ current when the switch is changed
to the OFF position.
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Testing for Continuity
resistor to limit current
A
indicator
(could be a lamp)
item to be tested
The continuity tester must never be used with equipment
connected to the mains!
This simple continuity tester should not be used for
components which are still c o_ n_ n_ e
__
c_
t _
e_
d in their
circuits.
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Using a multimeter
23.04
When using the multimeter to test for
faults, set the multimeter to measure
r_
e _s _
i s_ t_ a_ n_ c_ e_ and place the probes
across the component or circuit element.
item to
be
tested
If the reading is infinity ( a " 1 " shows on the left of
e_
n circuit (this is O.K. if you
the display) there is an o p
__
are testing a switch set to OFF). If there is a reading,
the circuit is c _
o_
m _p _l _
e _t _
e.
Safety Point
You must only test a component if there is no chance of there being any
current in it.
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Intermediate 1
Physics
Practical Electricity
End of Unit
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