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Resistors, Currents and All That Jazz

Chapter 19 – Young & Geller. Resistors, Currents and All That Jazz. September 23. Read the chapter – Important stuff HW assigned. Chapter 19. Current. I. L. - +. A. V. NOTE. Electric Current is DEFINED as the flow of POSITIVE CHARGE.

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Resistors, Currents and All That Jazz

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  1. Chapter 19 – Young & Geller Resistors, Currents and All That Jazz

  2. September 23

  3. Read the chapter – Important stuff HW assigned Chapter 19

  4. Current I L - + A V

  5. NOTE • Electric Current is DEFINED as the flow of POSITIVE CHARGE. • It is really the electrons that move, so the current is actually in the opposite direction to the actual flow of charge. (Thank Franklin!)

  6. Charge is moving so there must be an E in the metal conductor!

  7. Electrons “Bounce Around”

  8. ANOTHER DEFINITION (Average) Current Density

  9. A closed circuit

  10. Ohm • A particular object will resist the flow of current. • It is found that for any conducting object, the current is proportional to the applied voltage. • STATEMENT: DV=IR • R is called the resistance of the object. • An object that allows a current flow of one ampere when one volt is applied to it has a resistance of one OHM.

  11. Ohm’s Law

  12. Resistivity and Resistance

  13. Not Everything Follows Ohm’s Law

  14. Neither does this ……

  15. The Battery + -

  16. The Real Deal

  17. V By the way …. this is called a circuit! A REAL Power Sourceis NOT an ideal battery Internal Resistance EorEmf is an idealized device that does an amount of work E to move a unit charge from one side to another.

  18. A Physical (Real) Battery Internal Resistance

  19. Represents a charge in space Back to Potential Change in potential as one circuits this complete circuit is ZERO!

  20. Consider a “circuit”. This trip around the circuit is the same as a path through space. THE CHANGE IN POTENTIAL FROM “a” AROUND THE CIRCUIT AND BACK TO “a” is ZERO!!

  21. To remember • In a real circuit, we can neglect the resistance of the wires compared to the resistors. • We can therefore consider a wire in a circuit to be an equipotential – the change in potential over its length is slight compared to that in a resistor • A resistor allows current to flow from a high potential to a lower potential. • The energy needed to do this is supplied by the battery.

  22. Series Combinations R1 R2 V1 V2 V i i SERIES Resistors

  23. The rod in the figure is made of two materials. The figure is not drawn to scale. Each conductor has a square cross section 3.00 mm on a side. The first material has a resistivity of 4.00 × 10–3 Ω · m and is 25.0 cm long, while the second material has a resistivity of 6.00 × 10–3 Ω · m and is 40.0 cm long. What is the resistance between the ends of the rod?

  24. R1, I1 R2, I2 V Parallel Combination??

  25. What’s This??? In this Figure, find the equivalent resistance between points (a) F and H and [2.5](b) F and G. [3.13]

  26. (a) Find the equivalent resistance between points a and b in the Figure. (b) A potential difference of 34.0 V is applied between points a and b. Calculate the current in each resistor.

  27. Represents a charge in space Back to Potential Change in potential as one circuits this complete circuit is ZERO!

  28. Consider a “circuit”. This trip around the circuit is the same as a path through space. THE CHANGE IN POTENTIAL FROM “a” AROUND THE CIRCUIT AND BACK TO “a” is ZERO!!

  29. To remember • In a real circuit, we can neglect the resistance of the wires compared to the resistors. • We can therefore consider a wire in a circuit to be an equipotential – the change in potential over its length is slight compared to that in a resistor • A resistor allows current to flow from a high potential to a lower potential. • The energy needed to do this is supplied by the battery.

  30. Some Circuits are HARDER than OTHERS!

  31. NEW LAWS PASSED BY THIS SESSION OF THE FLORIDUH LEGISLATURE. • LOOP EQUATION • The sum of the voltage drops (or rises) as one completely travels through a circuit loop is zero. • Sometimes known as Kirchoff’s loop equation. • NODE EQUATION • The sum of the currents entering (or leaving) a node in a circuit is ZERO

  32. Consider voltage DROPS: -E +ir +iR = 0 or E=ir + iR rise Take a trip around this circuit.

  33. START by assuming a DIRECTION for each Current Let’s write the equations.

  34. In the figure, all the resistors have a resistance of 4.0 W and all the (ideal) batteries have an emf of 4.0 V. What is the current through resistor R?

  35. RC Circuit How Fast ? Initially, no current through the circuit Close switch at (a) and current begins to flow until the capacitor is fully charged. If capacitor is charged and switch is switched to (b) discharge will follow.

  36. What do you think will happen when we close the swutch? Close the Switch I need to use E for E Note RC = (Volts/Amp)(Coul/Volt) = Coul/(Coul/sec) = (1/sec)

  37. Time Constant

  38. Result q=CE(1-e-t/RC)

  39. q=CE(1-e-t/RC) and i=(CE/RC) e-t/RC

  40. Discharging a Capacitor qinitial=CE BIG SURPRISE! (Q=CV) i iR+q/C=0

  41. Power

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