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# 3 e electronics 241110

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Parallel circuits.
Introducing resistance: S3

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### 3 e electronics 241110

1. 1. Electricity 3E: Wednesday 24th November
2. 2. 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
3. 3. 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
4. 4. 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 = …
5. 5. 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
6. 6. Uses of parallel circuits • New style Christmas lights • Car lighting circuits: MM Ignition switch Side & rear lamps Headlights Starter motor Wiper motor
7. 7. Resistance Learning 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!
8. 8. Experimental setup 1. Set up the circuit as shown 2. Turn on the power supply and adjust the voltage across the lamp to 12V 3. Take readings of the voltage and current and record in a table in your workbook 4. 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 results 5. Replace the lamp in the circuit with a resistor and repeat steps 2-4 using a new table and graph 6. Plot a graph of your results (voltage (V) on y-axis; current (A) on x-axis) for both the lamp and the resistor
9. 9. 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
10. 10. Resistors • Resistors are simple components used to control the current and voltage in a circuit
11. 11. 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 = I2 R = V2 /R = IV Power is measured in Watts
12. 12. 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
13. 13. 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