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