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Chapter 20

Chapter 20. Electric Circuits. What do you think?. 1) What is electric current? 2) What is Ohm’s Law 3) What is electrical resistance?. Electromotive Force and Current. Electromotive Force and Current. Electromotive Force (emf): maximum potential difference. Electric current (I):

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Chapter 20

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  1. Chapter 20 Electric Circuits

  2. What do you think? 1) What is electric current? 2) What is Ohm’s Law 3) What is electrical resistance?

  3. Electromotive Force and Current

  4. Electromotive Force and Current Electromotive Force (emf): maximum potential difference Electric current (I): flow of charge that passes through the surface at a given time interval SI Unit: C/s = ampere (A)

  5. Conventional Current

  6. Ohm’s Law Ohm’s Law The ratio V/I is a constant, where V is the voltage applied across a piece of material (such as a wire) and I is the current through the material: R is the resistance of the piece of material. SI Unit: Volt/ampere (V/A)= ohm 

  7. Resistance and Resistivity Resistance where = resistivity L = length A = area

  8. Electric Power Electric Power When there is a current I in a circuit as a result of a voltage V, the electric power P delivered to the circuit is P=IV Si Unit: watt (W) P = I2R or

  9. Series Wiring Series wiring: devices are connected in such a way that there is the same electric current through each device. V = V1 + V2 = I(R1 + R2) = IRs Equivalent Resistance Rs = R1 + R2 +R3

  10. Parallel Wiring Series wiring: devices are connected in such a way that there is the same voltage through each device.

  11. Parallel Wiring

  12. Parallel Wiring The remote speaker and the main speaker for the right stereo channel are connected in parallel. At the instant represented on the picture, the ac voltage across the speakers is 6.00 V. The main speaker resistance is 8.00  and the remote speaker resistance is 4.00 . (a) Determine the equivalent resistance. (b) the current supplied by each speaker and the total current.

  13. Parallel Wiring

  14. Parallel Wiring

  15. Circuits Wired Partially in Series and Partially in Parallel Example The figure shows a circuit composed of a 24-V battery and four resistors, whose resistances are 110, 180, 220, 250 . Find (a) the total current supplied by the battery and (b) the voltage between points A and B in the circuit. (p. 603)

  16. Internal Resistance When an external resistance R is connected between the terminals of the battery, the resistance is connected in series with the internal resistance “r” of the battery.

  17. Terminal Voltage Example The figure shows a car battery whose emf is 12.0 V and whose internal resistance is 0.010 . This resistance is relatively large because the battery is old and the terminals are corroded. What is the terminal voltage when the current I drawn from the battery is 10.0 A? (p. 605)

  18. Kirchoff’s Rules Junction Rule: The sum of the magnitudes of the currents directed into a junction equals to the sum of the magnitudes of the currents directed out of the junction.

  19. Kirchoff’s Rules Loop Rule: Across and closed circuit loop, the sum of the potential drop equals to the sum of the potential rises

  20. Using Kirchoff’s Loop Rule Example Find the current I in the circuit. (p. 607)

  21. Using Kirchoff’s Loop Rule Example In a car, the headlights are connected to the battery and would discharge the battery if it were not for the alternator which is run by the engine. The circuit includes an internal resistance of 0.0100  for the car battery and its leads and a resistance of 1.20  for the headlights. The alternator is approximated as an additional 14.00-V battery with an internal resistance of 0.100 . Determinethe currents though the car battery, the headlights, and the alternator. (p. 608)

  22. Capacitors in Parallel In a parallel combination of capacitances, the voltages across each capacitor is the same but the charges on each capacitor are different

  23. Capacitors in Series In a series combination of capacitances, the charges across each capacitor is the same but the voltages on each capacitor are different

  24. RC Circuits Capacitor Charging Capacitor Discharging time constant

  25. Direct and Alternating Current Direct Current when charges move around a circuit in the same direction at all times Alternating Current Alternating Current when charges move first one way and then the opposite way

  26. Alternating Current

  27. Alternating Current Average Power RMS

  28. Alternating Current

  29. The Measurement of Current Galvanometer

  30. The Measurement of Current

  31. The Measurement of Voltage

  32. Safety and the Physiological Effects of Current

  33. What do you think? 1) What is electric current? 2) What is Ohm’s Law 3) What is electrical resistance?

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