Current and voltage are measured in amps and volts respectively. Current is measured using an ammeter in series, while voltage is measured using a voltmeter across components. In a series circuit, the total voltage equals the sum of the voltages across components and the current is the same throughout. In a parallel circuit, the total current equals the sum of branch currents and the voltage is the same across all branches. Series circuits are used for things like torches and old Christmas lights, while parallel circuits are common in car lighting systems and new Christmas lights.

1. Electricity
3E: Tuesday 23rd
November

2. Measuring current
• Current (flow of charge) has symbol I, and is
measured in Amperes (A)
• We use an Ammeter to measure current
through a circuit
• Ammeter is connected in line with the other
components – in series
– We ‘break into’ the circuit:

3. Voltage
• Voltage (or potential difference) has symbol
V, and is measured in Volts (V)
• Voltage of a supply is a measure of the energy
given to charges in a circuit:
A voltage of 1V means 1 Joule (J) of electrical energy
is being transferred into other forms every time 1
Coulomb (C) of charge passes through a component
•1 Volt = 1 Joule per Coulomb OR 1V = 1JC-1

4. Measuring voltage
• We use an Voltmeter to measure voltage
across a component:
Vsupply
Vlamp
Ilamp

5. Experiment:
• Aim: Measuring voltage in a series circuit
• Working in pairs:
– Collect 3 cells, 4 lamps, 10 wires, 1 Ammeter and 1
Voltmeter
• A) Measure the Voltage of the supply and the
voltage across each of the lamps in the series
circuit (Experiment A) as per last class
• Write down your results in a table

6. Series circuits: Conclusions
• The voltage of the supply is equal to the sum
of the voltages across all the components in
the circuit:
• Vsupply = V1 + V2 + V3 + …
• When more bulbs are added to the series
circuit the current is reduced
• The current is the same at ALL points within
the series circuit:
• Isupply = I1 = I2 = I3 = …

7. 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

8. 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

9. 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 = …

10. 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

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