Electricity

6D: Tuesday 23rd November

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: 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 resistor 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 resistor in your table of results5. Replace the lamp in the circuit with a lamp nd 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 resistor and the lamp

Variable D.C. power supply0-12V

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 getP = I2R = V2/R = IVPower is measured in Watts

Electromotive Force (e.m.f.)

• The e.m.f. of any electrical supply is the number of joules of electrical energy given to each coulomb of electric charge as it passes through the supply

• This is measured in JC-1 or volts

• E.M.F.s can be generated in a variety of ways e.g.: chemical cells, thermocouple, piezo-electric generators, solar cells, electromagnetic generators

Resistors in series: Conservation of energy

• Applying conservation of energy to resistors in series for one coulomb of charge.

• Energy supplied by source = energy converted by circuit componentse.m.f. = IR1 + IR2 + IR3

IRs = IR1 + IR2 +IR3

Rs = R1 + R2 + R3

where Rs = equivalent series resistance

Resistors in parallel:Conservation of charge

• Total charge per second (current) passing through R1, R2, R3 must equal the charge per second (current) supplied by the cell, i.e. passing through RP

• Conservation of charge gives: I = I1 + I2 + I3 (since I=Q/t for each resistor)

E/RP = E/R1 + E/R2 + E/R3 (since I=E/R for each resistor)

1/RP = 1/R1 + 1/R2 + 1/R3

where RP = equivalent parallel resistance