Parallel circuits. Introducing resistance: S3

1. Electricity
3E: Wednesday 24th November

2. Parallel circuits
• In a parallel circuit, current has more than one
path in which to flow around the circuit:
Vsupply
I1
I2 I3
V1
V2 V3
Isupply

3. Experiment
• Aim: Measuring current and voltage in a parallel
circuit
• Set up parallel circuit as shown
• Measure the voltage across the cells (Vsupply) and
the voltage across each lamp (V1, V2, V3, V4)
• Measure the current through each lamp (I1, I2, I3,
I4) and the current drawn from the supply, Isupply

4. Parallel circuits: Conclusions
• The sum of all currents in the parallel
branches in the circuit is equal to the current
drawn from the supply:
Isupply = I1 + I2 + I3 + …
• The voltage across components in parallel are
the same and equal to the supply voltage
Vsupply = V1 = V2 = V3 = …

5. Uses of series circuits
• Torches, old-style Christmas lights…
• Stair lighting uses two or more 2-way switches
in series:
Power Supply
Downstairs switch
Upstairs switch
Lamp
This circuit is in the OFF position
Either switch will turn the light ON

6. Uses of parallel circuits
• New style Christmas lights
• Car lighting circuits:
MM
Ignition
switch
Side & rear
lamps
Headlights
Starter
motor
Wiper
motor

7. Resistance
Learning Objectives:
• Know what resistance is and what units
we measure it in
Starter
Write down as many meanings for the
word resistance as you can in 1 minute!

8. Experimental setup
1. Set up the circuit as shown
2. Turn on the power supply and adjust the voltage across the lamp to 12V
3. Take readings of the voltage and current and record in a table in your workbook
4. Repeat this for several different values (12V down to 0V) of the voltage and
measure both voltage and current through the lamp in your table of results
5. Replace the lamp in the circuit with a resistor and repeat steps 2-4 using a new
table and graph
6. Plot a graph of your results (voltage (V) on y-axis; current (A) on x-axis) for both
the lamp and the resistor

9. Definition of Resistance
• Electrical Resistance of an object is a measure
of its opposition to the passage of an electric
current
• Resistance is measured in Ohms (Ω)
Resistance = voltage / current
R=V/I V=IR I=V/R
George Simon Ohm
German Physicist
V
I R

10. Resistors
• Resistors are simple components used to
control the current and voltage in a circuit

11. Resistive heating
• For a given component, power P = IV where I
is the current through that component and V
is the voltage across that component
• Substituting from V=IR we get:
P = I2
R = V2
/R = IV
Power is measured in Watts

12. Resistors in series
• If we join resistors in series we increase the
resistance of the circuit. The current will
decrease
• The total resistance in series is equal to the
sum of the individual resistances:
Rs = R1 + R2 + R3
where Rs = equivalent series resistance

13. Resistors in parallel
• If we join components in parallel we decrease
the resistance of the circuit. The current will
increase.
• The combined resistance in parallel is calculated
using the formula:
1/RP = 1/R1 + 1/R2 + 1/R3