Download MEASURING CURRENT - All Saints RC Secondary

ELECTRICAL APPLIANCES
Our home is full of electrical appliances. They
change electricity into a more useful form of
energy.
Some appliances use more energy than others –
their POWER RATING is higher.
Plugs are fitted with a fuse to protect the flex
connected to the appliance. As a general rule a 3A
fuse is fitted for appliances that use up to 700W
and a 13A fuse is fitted for appliances that use
more than 700W.
The table contains some general information:
Appliance
Energy
change
Flex
core
2/3
Power
Fuse (A)
rating (W)
Clock
Table lamp
T.V.
2
2
3
10
100
250
3
3
13
Hairdrier
Iron
Kettle
Fire
3
3
3
3
1000
1200
2000
3000
13
13
13
13
Cooker
3
11500
13
FUSES
Fuses are used to limit the current flowing
through a circuit or into an appliance. A
fuse will melt when the current flowing
through it exceeds the rating of the fuse.
This protects the flex from overheating
which could result in a fire. The circuit is
then broken and the flow of current stops.
HUMAN CONDUCTIVITY
The human body is a conductor of electricity.
Water (moisture) increases the body’s ability
to conduct.
EARTH WIRE
The earth wire is a safety feature which
prevents the metal casing of an appliance
becoming dangerous to touch when a fault
occurs. A large current will flow through
the earth wire and the fuse will melt,
protecting the appliance.
CIRCUIT BREAKER
The circuit breaker is an
absolutely essential device in
the modern world, and one of
the most important safety
mechanisms in your home.
Whenever electrical wiring in
a building has too much
current flowing through it,
these simple machines cut
the power until somebody
can fix the problem.
DOUBLE INSULATION
Double insulated appliances
If you are fitting a plug to a double
insulated appliance (labelled with
the double insulated symbol), you
should not have an earth conductor
in your flex.
3 – PIN PLUG
• Know colours and names of each wire.
• Note how outer insulation is secured by cable grip.
ELECTRIC CHARGE
A current of electricity is a steady
flow of electrons. When electrons
move from one place to another,
round a circuit, they carry
electrical energy from place to
place like marching ants carrying
leaves. Instead of carrying leaves,
electrons carry a tiny amount of
electric charge.
Charge is calculated using:
Q
Q = It
I
t
MEASURING CURRENT 1
LEARNING
INTENTION
To build a series circuit and measure the
current at different points in the circuit.
APPARATUS: power supply, 2 lamps, connecting
leads, switch and ammeter
A
. .
.
C
B
METHOD:
Build circuit and measure current at
points A, B and C
RESULTS:
POSITION
CURRENT (A)
A
B
C
CONCLUSION:
Current is the same at all points in a
series circuit.
IA = IB = IC
MEASURING VOLTAGE
L. INT
To build a series circuit and measure the
voltage across L1, L2 and Vs.
APPARATUS:
power supply, 2 lamps, connecting
leads, switch and voltmeter
+ Vs -
L
L2
1
METHOD:
Build circuit and measure voltage across
L1, L2 and Vs.
RESULTS:
POSITION
VOLTAGE (V)
L1
L2
Vs
CONCLUSION:
The supply voltage equals the sum of
the voltages across the 2 lamps
Vs = V1 + V2
CHANGING THE CURRENT
L. INT.
To investigate what happens to the current when
components called resistors are placed in a circuit.
APPARATUS:
power supply, lamp, resistors with different
values, connecting leads, switch and ammeter.
+
Vs A
METHOD: Build circuit using smallest resistor and measure
corresponding current. Repeat for other resistors.
RESULTS:
RESISTOR
A
RESISTANCE
(  )
10
B
33
C
47
D
100
CONCLUSION:
CURRENT
(mA)
As the resistance increases the
current decreases.
VOLTAGE, CURRENT & RESISTANCE
L. INT.
To find the relationship between voltage, current and
resistance.
APPARATUS:
power supply, resistors, connecting leads,
switch, ohmmeter, voltmeter and ammeter.
+ Vs A

Circuit 1
V
METHOD:
Circuit 2
Measure value of resistors using circuit 1. Put
smallest resistor into circuit 2, measure current and
voltage. Repeat for other resistors.
RESULTS:
Resisto
r
A
R ( )
V (V)
B
C
D
CONCLUSION:
V=IxR
I (A)
V/I
RESISTANCE OF A TORCH BULB
L. INT.
To investigate how the resistance of a torch bulb
varies when the brightness changes
APPARATUS: power supply, bulb, voltmeter, ammeter,
connecting leads.
+ Vs METHOD:
A
Build circuit
Set supply at 2v
Measure current & voltage
V
Record in table & repeat for different voltages
RESULTS:
POWER
SUPPLY (v)
2
4
6
8
10
12
CONCLUSION:
Voltage across Current (A)
bulb (V)
Resistance (
)
0.71
1.4
0.51
RESISTANCE OF A THERMISTOR
A thermistor is a temperature dependent resistor.
L. INT. To find out how the thermistor varies with temperature
APPARATUS: thermistor, ohmmeter, hot & cold water
METHOD: (i) connect thermistor to ohmmeter
(ii) place thermistor in cold water, record resistance
(iii) place thermistor in hot water, record resistance
RESULTS:
Water
Cold
Hot
Resistance (  )
CONCLUSION: As the temperature increases the resistance
RESISTANCE OF AN LDR
An LDR is a light dependent resistor.
L. INT.
To find out how the LDR varies with light
APPARATUS: LDR, ohmmeter,
METHOD:
(i) connect LDR to ohmmeter
(ii) place LDR in the dark, record resistance
(iii) place LDR in the light, record resistance
Resistance (  )
RESULTS:
Light level
Dark
Light
CONCLUSION:
As the light level increases the resistance
decreases.
POWER, ENERGY & TIME
To find the ENERGY used by an electrical appliance we need
to know the POWER RATING of the appliance and the
TIME the appliance is on. The energy can be found by
multiplying the power by the time:-
E = PxT
Where E = energy (J)
P = power (W)
T = time (s)
POWER , CURRENT & VOLTAGE
LEARNING
INTENTION
To investigate the relationship between POWER,
CURRENT & VOLTAGE.
APPARATUS:
power supply, lamps ( different power ratings ),
ammeter, voltmeter & connecting leads.
METHOD:
Build circuit using 24W
lamp. Switch on and
record current and
voltage. Repeat for 36W
lamp.
+
Vs
A
V
RESULTS:
LAMP
RATING
(W)
VOLTAGE
(V)
CURRENT
(A)
CURRENT
X
VOLTAGE
24
12
2
24
36
12
3
36
48
CONCLUSION: By comparing the first and last column we
have shown that the POWER equals the CURRENT multiplied
by the VOLTAGE.
P=IxV
MEASURING CURRENT 2
LEARNING
INTENTION
To build a parallel circuit and measure the
current at different points in the circuit.
APPARATUS:
power supply, 2 bulbs, connecting
leads, switch and ammeter
+ A
B
..
.
C
METHOD:
Build circuit and measure current at
points A, B and C
RESULTS:
POSITION
CURRENT (A)
A
0.24
0.16
B
0.12
0.05
C
0.12
0.11
CONCLUSION: the current at A equals the sum of the
currents at B and C
IA = I B + IC
MEASURING VOLTAGE 2
LEARNING
INTENTION
To build a parallel circuit and measure the
voltage across bulbs 1 & 2 and Vs
APPARATUS:
power supply, 2 bulbs, connecting
leads, switch and voltmeter.
Vs
METHOD:
Build circuit and measure voltage across both
lamps.
RESULTS:
supply voltage
5V
bulb 1
5V
bulb 2
5V
CONCLUSION: Supply voltage equals the voltage
across bulb 1 which equals the voltage across bulb 2
Vs = V1 = V2
FAULT FINDING
Two common electrical faults are ‘open circuits’ and ‘short
circuits’. As every electrical component, from a lamp to a
piece of wire, has a resistance, we can use this knowledge to
identify such faults using an ohmmeter.
OPEN CIRCUIT: the ohmmeter reads infinity as this is the
result of a broken wire or a burnt out
element.
SHORT CIRCUIT: the ohmmeter reads a value that is less
than expected for that component, as
it’s the result of a touching wire.
RESISTANCE IN SERIES
LESRNING
INTENTION
To investigate how the individual resistors
compare with the total resistance in a
series circuit
APPARATUS:
resistors, ohmmeter & connecting leads
METHOD:
measure each resistor individually
connect them in series
measure total resistance
RESULTS
CONCLUSION:
RT = R1 + R2
RESISTANCE IN PARALLEL
LEARNING
INTENTION
To investigate how the individual resistors
compare with the total resistance in a
parallel circuit
APPARATUS:
resistors, ohmmeter & connecting leads
METHOD:
measure each resistor individually
connect them in parallel
measure total resistance
RESULTS
CONCLUSION:
1/RT = 1/R1 + 1/R2
CONSUMER UNIT
The consumer unit
contains the fuses or
circuit breakers that
protect the mains
wiring.
Kilowatt-hour
Electricity companies charge their customers for each
unit of electricity used. 1 unit of electricity is equal to
1Kilowatt-hour (1KWh), which is the amount of
energy a 1KW appliance uses in 1 hour. It can be
calculated using E = P x t. If power is in KW and time
in hrs then E will equal the number of units used.
RING MAIN
•The power sockets in a
house are connected by
means of a ring circuit.
There are 2 paths to
each socket which
means half the current
in 1 path and half in the
other. The cables are
thinner and therefore
cheaper and less heat is
produced in them.
ELECTROMAGNET
An electromagnet is simply a coil of wire
which is wrapped around a soft iron core.
When the wire is connected to a power
source a magnetic field is created and the
iron core acts like a permanent magnet. The
strength of an electromagnet can be
increased by using more turns of wire or a
larger current.
ELECTRIC MOTOR
When a current flows along a
wire which is in a magnetic
field, the wire experiences a
force, causing it to move (see
crocodile physics).
Rotation of motor can be
changed by changing the
direction of current or the
direction of the magnetic
field.
Commutator reverses current; multi-segment commutators give smoother
rotation; brushes allow current to flow into commutator.
Commercial motor