SERIES CIRCUITS
Apparatus needed:
Power supply, mounted pea bulb (2.5 V), 11 Ω rheostat, digital multimeter, leads
A series circuit is one in which all the components are connected in a line, so the circuit is one
complete loop. Connect up the circuit shown here using 2 V on the power supply and the red and
black sockets. Use the two sockets at the same end on the variable resistor (rheostat).
If you have done it right, the bulb should light, and you should be able to alter its brightness a little
by varying the position of the slider on the variable resistor (rheostat).
Now measure the current in the circuit at each of the positions marked 1, 2 and 3. You will have to
do this by breaking the circuit in these three places and connecting a multimeter in the break. You
should use the COM and 10 A sockets on the meter, and the dial should be set opposite 10 A.
1.
Write down your readings here.
Current at position 2 = ………………..
2.
Current at position 1 = ………………..
Current at position 3 = ………………..
What do you notice about the current at each of the three positions?
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3.
What happens if you change the setting of the variable resistor?
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Next take the meters out and reconnect the circuit and then measure the potential differences
(p.d.s or voltages) across each of the components in the circuit. You will have to do this by
connecting a multimeter to either side of the power pack, the bulb and the variable resistor
(rheostat). Use the COM and V sockets on the meter, and the dial should be set opposite 20 V.
4.
Write down your readings here.
P.d. across power supply = ………………..
P.d. across bulb = ………………..
P.d. across variable resistor = ………………..
5.
What do you notice about the p.d. across power supply compared to the p.d.s across the
bulb and variable resistor?
…………………………………………………………………………………………………………….
6.
What happens if you change the setting of the variable resistor?
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PARALLEL CIRCUITS
Apparatus needed:
Power supply, 2 mounted pea bulbs (2.5 V), digital multimeter, leads
A parallel circuit is one in which the components are connected in two or more different branches,
both connected to the powers supply – so there is more than one route around the circuit.
Connect up the circuit shown here using 2 V on the power supply and the red and black sockets.
Now measure the current in the circuit at each of the positions marked 1, 2 and 3. You will have to
do this by breaking the circuit in these three places and connecting a multimeter in the break. You
should use the COM and 10 A sockets on the meter, and the dial should be set opposite 10 A.
1.
Write down your readings here.
Current at position 2 = ………………..
2.
Current at position 1 = ………………..
Current at position 3 = ………………..
What do you notice about the current at each of the three positions?
…………………………………………………………………………………………………………….
Next take the meters out and reconnect the circuit and then measure the potential differences
(p.d.s or voltages) across each of the components in the circuit. You will have to do this by
connecting a multimeter to either side of the power pack, the first bulb, and the second bulb. Use
the COM and V sockets on the meter, and the dial should be set opposite 20 V.
3.
Write down your readings here.
P.d. across power supply = ………………..
P.d. across bulb 1 = ………………..
P.d. across bulb 2 = ………………..
4.
What do you notice about the p.d. across power supply compared to the p.d.s across bulbs 1
and 2?
…………………………………………………………………………………………………………….
5.
the
How does the brightness of the bulbs compare to the series circuit? Why do you think this is
case?
…………………………………………………………………………………………………………….
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TRANSFORMERS 1
Apparatus needed:
Power supply, two 2 m lengths of insulated wire with the ends bared, mounted pea bulb (2.5 V),
two C-cores and clip, two leads, two crocodile clips
1.
Where are transformers used in everyday life?
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To see how a transformer works, carry out the following experiment.
2.
(a)
Wind 20 turns of insulated wire around one of the C-cores, leaving a good length of
wire trailing at each end.
(b)
Connect the ends of the wire to the YELLOW terminals on the power supply and set it
to deliver 1 V (by putting the yellow plugs either side of a figure ‘1’).
(c)
Wind 20 turns of insulated wire around the other C-core and attach the ends to the
small bulb using leads and crocodile clips.
(d)
Turn the power supply on and describe happens as the two C-cores are brought
together.
…………………………………………………………………………………………………….
…………………………………………………………………………………………………….
…………………………………………………………………………………………………….
(e)
Disconnect the wires from the YELLOW terminals and attach them to the RED and
BLACK terminals instead, and repeat the experiment. What happens this time?
…………………………………………………………………………………………………….
…………………………………………………………………………………………………….
(f)
Reconnect the wires to the YELLOW terminals and describe what happens to the bulb
as you (i) decrease and (ii) increase the number of turns on the C-core attached to the
bulb.
…………………………………………………………………………………………………….
…………………………………………………………………………………………………….
3.
What you have just made is a simple transformer. A transformer consists of an iron core with
two separate coils wound around it – the primary coil and the secondary coil. Describe how
a transformer works.
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TRANSFORMERS 2
Apparatus needed:
Power supply, two 60:60 turn pre-wound coils, two C-cores and clip, six leads, two digital
multimeters,
two more digital meters (for current), 12 V light bulb, demountable transformer demonstrations
You have already seen that transformers can make A.C. voltages bigger and smaller. In this
worksheet you will find out a formula for predicting how much bigger or smaller.
1.
Connect up the circuit shown here.
Make sure that the
power supply is set to
4 V (little yellow
plugs either side of
two 2s), that the
meters are set to read
20 V A.C. voltages
(V~ range) and that
you are using only 60
turns on each coil
initially.
2.
Turn the power supply
on and write down the
readings
for
the
primary
(or
input)
voltage (i.e. from the
power supply and for
the secondary (i.e. output) voltage (i.e. from other side of the transformer) in the table.
3.
Then use 120 (= 60 + 60!) turns on the primary coil instead (but still 60 on the secondary)
and record the new readings.
4.
5.
Repeat with 120:120, and finally with 120:60.
Number of
primary turns
(NP)
Number of
secondary
turns (NS)
60
60
120
60
120
120
120
60
Turns ratio
(NS / NP)
Primary
voltage
(VP) / V
Secondary
voltage
(VS) / V
Voltage
ratio
(VS / VP)
Work out the ratios in columns 3 and 6. Can you spot a relationship between VS, VP, NS and
NP?
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