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# 6 d electronics 231110

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Series and parallel circuits. Resistance. Resistors in parallel and series. 6th year Higher Physics introduction to electricity / circuits

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• Note: the potential difference between two points in the external circuit is also measured in volts, but this is concerned with electrical energy being transformed outside the source
• ### 6 d electronics 231110

1. 1. Electricity 6D: Tuesday 23rd November
2. 2. 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 = …
3. 3. 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 = …
4. 4. 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
5. 5. Uses of parallel circuits • New style Christmas lights • Car lighting circuits: MM Ignition switch Side & rear lamps Headlights Starter motor Wiper motor
6. 6. 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!
7. 7. Experimental setup 1. Set up the circuit as shown 2. Turn on the power supply and adjust the voltage across the resistor 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 resistor in your table of results 5. Replace the lamp in the circuit with a lamp nd 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 resistor and the lamp Variable D.C. power supply 0-12V
8. 8. 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
9. 9. Resistors • Resistors are simple components used to control the current and voltage in a circuit
10. 10. 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
11. 11. 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
12. 12. 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 components e.m.f. = IR1 + IR2 + IR3 IRs = IR1 + IR2 +IR3 Rs = R1 + R2 + R3 where Rs = equivalent series resistance
13. 13. Resistors in parallel: Conservation of charge • Total charge per second (current) passing through R1, R2, R3must 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