Electricity

3E: Wednesday 24th November

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

ResistanceLearning 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!

Experimental setup

1. Set up the circuit as shown2. Turn on the power supply and adjust the voltage across the lamp to 12V3. Take readings of the voltage and current and record in a table in your workbook4. 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 results5. Replace the lamp in the circuit with a resistor and repeat steps 2-4 using a new

table and graph6. Plot a graph of your results (voltage (V) on y-axis; current (A) on x-axis) for both

the lamp and the resistor

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

Resistors

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

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 = I2R = V2/R = IV

Power is measured in Watts

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

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

where RP = equivalent parallel resistance