Electricity

3E: Tuesday 23rd November

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:

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

Measuring voltage

• We use an Voltmeter to measure voltage across a component:

Vsupply

Vlamp

Ilamp

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

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 = …

Parallel circuits• In a parallel circuit, current has more than one

path in which to flow around the circuit:

Vsupply

I1I2 I3

V1V2 V3

Isupply

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

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 = …

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 switchLamp

This circuit is in the OFF positionEither switch will turn the light ON

Uses of parallel circuits• New style Christmas lights• Car lighting circuits:

MM

Ignition switch

Side & rearlamps

Headlights

Startermotor

Wipermotor