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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

Transcript

  • 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