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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

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

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• 1. Electricity 6D: Tuesday 23 rd November
• 2. Series circuits: Conclusions
• The voltage of the supply is equal to the sum of the voltages across all the components in the circuit:
• V supply = V 1 + V 2 + V 3 + …
• 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:
• I supply = I 1 = I 2 = I 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:
• I supply = I 1 + I 2 + I 3 + …
• The voltage across components in parallel are the same and equal to the supply voltage
• V supply = V 1 = V 2 = V 3 = …
• 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. Uses of parallel circuits
• New style Christmas lights
• Car lighting circuits:
M M Ignition switch Side & rear lamps Headlights Starter motor Wiper motor
• 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. Experimental setup
• Set up the circuit as shown
• Turn on the power supply and adjust the voltage across the resistor to 12V
• Take readings of the voltage and current and record in a table in your workbook
• 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
• Replace the lamp in the circuit with a lamp nd repeat steps 2-4 using a new table and graph
• 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. 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. Resistors
• Resistors are simple components used to control the current and voltage in a circuit
• 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 = I 2 R = V 2 /R = IV
• Power is measured in Watts
• 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. 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. = IR 1 + IR 2 + IR 3
• IR s = IR 1 + IR 2 +IR 3
• R s = R 1 + R 2 + R 3
• where R s = equivalent series resistance
• 13. Resistors in parallel : Conservation of charge
• Total charge per second (current) passing through R 1 , R 2 , R 3 must equal the charge per second (current) supplied by the cell, i.e. passing through R P
• Conservation of charge gives:
• I = I 1 + I 2 + I 3 (since I=Q/t for each resistor)
• E/R P = E/R 1 + E/R 2 + E/R 3 (since I=E/R for each resistor)
• 1/R P = 1/R 1 + 1/R 2 + 1/R 3
• where R P = equivalent parallel resistance