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AP Physics III.D

Explore the basics of electric circuits, EMF, current flow, Ohm's Law, resistance, power, series, parallel circuits, and combination circuits. Learn to calculate currents, resistances, and powers in various circuit configurations. Discover the importance of internal resistance and explore the behavior of capacitors in parallel and series arrangements.

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AP Physics III.D

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  1. AP Physics III.D Electric Circuits

  2. 20.1 EMF and Current

  3. The shortcomings of electrostatics

  4. The amazing battery

  5. Current

  6. Ex. The amount of charge that passes through the filament of a certain light bulb is 1.67 C in 2.00 s. Find the a) current and b) the number of electrons that pass through the filament in 1.00 s.

  7. Conventional current • Hypothetical flow of positive charge • Consistent with positive test charge and electric fields • Always goes from higher potential to lower potential

  8. 20.2 Ohm’s Law

  9. An analogy to water

  10. Ohm’s (Georg – a former high school teacher we might note) Law – the ratio of potential difference to current

  11. 20.3 Resistance and Resistivity

  12. Comparison to water again.

  13. Resistivity is a property of a given material. Resistance depends on resistivity and geometry.

  14. Ex. Calculate the resistance per unit length of nichrome wire with radius of 0.321 mm. If a potential difference of 10.0 V is maintained across 1.00 m of a length of nichrome wire, what is the current in the wire?

  15. p. 619: 1-3, 6-7, 9 • a) 0.036 C b) 2.2 EE 17 • 1.3 EE 6 J 9. b) recall Q = mcΔT

  16. 20.4 Electric Power

  17. A couple of derivations

  18. Ex. An electric heater is operated by applying a potential difference of 50.0 V to nichrome wire for a total resistance of 8.00 Ohms. Find the current in the wire and the power dissipated by the heater. How much does it cost to run the heater for 24 hours if the cost per kwh is $0.12?

  19. 20.6 Series Circuits

  20. Series circuit – all devices are connected in such a way that there is the same current through each device. In series wiring the potential difference is divided (potential drop) among the resistors.

  21. Equivalent resistance for a series circuit (note the largest resistor has the greatest effect on the equivalent resistance for a series circuit)

  22. Note: the power in a series circuit can be found from the sum of the powers delivered to the individual resistors, or the power delivered to the equivalent resistance.

  23. Ex. (this one is in the book) For the circuit shown find the a) cur- rent b) power dissipated by each resistor c) the total power de- livered to the resistors by the battery.

  24. p. 620: 21-24, 39-46 • a) 4.4 Ohms b) 2.7 A • $5.0 million per day • 9.0 V • 4.0 EE 1 Ohms • a) 1.6 A b) 14 V, 8.0 V, 1.6 V c) 22 W, 14 W, 2.6 W 46. 140 W

  25. 20.7 Parallel Circuits

  26. Parallel circuit – a circuit wired in such a way that the same voltage is applied across each resistor.

  27. Pondering the equivalent resistance of a parallel circuit.

  28. So show me the formula.

  29. For parallel circuits, the smallest resistor has the greatest effect on the equivalent resistance. This explains a short circuit.

  30. Ex. For the circuit shown find the a) current in each resistor and b) the total power dissipated by the three resistors.

  31. Ex. Three resistors with resistances of 20.0 Ohms, 40.0 Ohms and an unknown resistance are connected in parallel to a battery with a potential difference of 24.0 V. The current in the battery is 3.00 A. Find a) the equivalent resistance b) the unknown resistance and c) the current in the unknown resistor.

  32. Ex. For the circuit shown find a) the potential difference across the battery b) the current in the 12 Ohm resistor c) the current in the unknown resistor d) the equivalent resistance and e) the power dissipated by each resistor.

  33. Summary of Series and Parallel Circuits • Current • Resistance • Potential difference

  34. p. 621: 49-56 • 290 Ohms, 140 Ohms • R/9 • a) yours b) 7.06 A c) 2350 W 56. 5.24 Ohms, 0.76 Ohms

  35. 20.8 Combination Circuits

  36. Ex. What is the current in the 5.00 EE 2 Ohm resistor for the circuit shown?

  37. Brightness (an intelligence component greatly lacking in the students in this class)

  38. 80B2 Use VD and I in device to find n Use total I, 12 V and Rp 48 W 82B4 You draw 600 Ohms 27 J 05B6 Use ideal gas law, V of gas, solve for H You draw Use slope of graph and Part a to find n 80B2, 82B4, Rev. 05B6

  39. 20.9 Internal Resistance

  40. The internal resistance r is connected in series with the external resistance R. r causes the potential difference between the terminals to drop below the maximum EMF. This actual voltage is the terminal voltage.

  41. Ex. For the circuit shown find the a) current drawn from the battery b) the terminal voltage of the battery and c) the current in the 6.0 Ohm resistor.

  42. Capacitors in Circuits 20.9

  43. Capacitors in Parallel

  44. Stored energy for capacitors in parallel

  45. The net effect for capacitors in parallel is an increase in capacitance (Why?)

  46. Capacitors in Series

  47. Summary for capacitors in circuits • Parallel – potential difference is the same across each capacitor, the charge on each capacitor may differ • Series – the charge on each capacitor is the same, but there is a voltage drop across each capacitor

  48. 87B4 (Easy!) 12 Ohms 4.8 V 1.2 V 0.36 W 24 C 88B3 a) 8 A 2 A 20 V 60 micro C 6.0 EE -4 J 90B3 6 Ohms b) 2 A 22 V d) 12 W e) Yours f) 44 W 87B4, 88B3, 90B3

  49. 91B4 4.5 V 0.5 Ohms 1.0 Ohms 9A You draw 97B4.a-b a) i. 6.4 W ii. 58 W You draw 02B3 i. 0.25 A ii.0.33 A i. yours, 0.14 A ii. Yours, yours too Ufiggeritout i. 70 W ii. 17 W 91B4, 97B4.a-b, 02B3

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