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Understanding Current and Resistance in Physical Resistors

Learn about the behavior of electrons in conducting materials, the impact of current flow and resistance, Ohm's Law, and how charges move in circuits. Practice problems and explanations for better comprehension.

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Understanding Current and Resistance in Physical Resistors

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  1. Current and Resistance 2/2009

  2. EXAMINATION #2 Wednesday March 4, 2009

  3. New Topic:Current and Resistance

  4. Physical Resistors

  5. - - What Happens? “+” “+” REMEMBER, THE ELECTRONS ARE ACTUALLY MOVING THEOTHER WAY! “+” “+”

  6. What’s Moving? Charge!

  7. What is making the charged move?? Battery

  8. KEEP IN MIND • A wire is a conductor • We will assume that the conductor is essentially an equi-potential • It really isn’t. • Electrons are moving in a conductor if a current is flowing. • This means that there must be an electric field in the conductor. • This implies a difference in potential since E=DV/d • We assume that the difference in potential is small and that it can often be neglected. • In this chapter, we will consider this difference and what causes it.

  9. Conducting material DQ,Dt DEFINITION • Current is the motion of POSITIVE CHARGE through a circuit. Physically, it is electrons that move but …

  10. Conducting material DQ,Dt

  11. UNITS • A current of one coulomb per second is defined as ONE AMPERE.

  12. A charged belt, 30 cm wide, travels at 40 m/s between a source of charge and a sphere. The belt carries charge into the sphere at a rate corresponding to 100 µA. Compute the surface charge density on the belt. [8.33e-06] C/m2

  13. Comment on Current Flow

  14. Question ….. A small sphere that carries a charge q is whirled in a circle at the end of an insulating string. The angular frequency of rotation is ω. What average current does this rotating charge represent?

  15. An electric current is given by the expression I(t) = 100 sin(120πt), where I is in amperes and t is in seconds. What is the total charge carried by the current from t = 0 to t = (1/240) s?

  16. ANOTHER DEFINITION (Average) Current Density

  17. The figure represents a section of a circular conductor of non-uniform diameter carrying a current of 5.00 A. The radius of cross section A1 is 0.400 cm. (a) What is the magnitude of the current density across A1? (b) If the current density across A2 is one-fourth the value across A1, what is the radius of the conductor at A2?

  18. 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.

  19. Graph

  20. Ohm’s Law

  21. A DIODE Resistance Varies with Applied Voltage (actually with current)

  22. Let’s look at the atomic level .. • Conduction is via electrons. • They are weak and small and don’t exercise much. • Positive charge is big and strong and doesn’t intimidate easily. • It’s an ugly situation … something like ……

  23. + -

  24. A small sphere that carries a charge q is whirled in a circle at the end of an insulating string. The angular frequency of rotation is ω. What average current does this rotating charge represent? • wq • 2pwq • wq/2p • 4pwq • You need the radius to answer this question

  25. A circular wire (Radius=R) carries a current I and the current density is J=br. Find the value of the total current.

  26. The Current • Electrons are going the opposite way from the current. (WHY?) • They probably follow a path like … Average “drift” speed - vd IN OUT

  27. Notation • vd average drift velocity of the electron • n number of electrons (mobile) per unit volume. • Dt interval of time • Dx average distance the electron moves in time Dt. • Q total amount of CHARGE that goes through a surface of the conductor in time Dt.

  28. The Diagram Often a Vector

  29. We return to the diagram ….. • Consider an electron. • Assume that whenever it “bumps” into something it loses its momentum and comes to rest. • It’s velocity therefore starts at zero, the electric field accelerates it until it has another debilitating collision with something else. • During the time it accelerates, its velocity increases linearly . • The average distance that the electron travels between collisions is called the “mean free path”.

  30. Starting when the electron is at rest: We showed two slides ago: Let n= number of charge carriers per unit volume (mobile electrons) resistivity

  31. Finally

  32. Reference The average drift velocity of an electron is about 10-4 m/s SLOW !

  33. Ponder How can a current go through a resistor and generate heat (Power) without decreasing the current itself?

  34. Loses Energy Gets it back Exit

  35. Conductivity In metals, the bigger the electric field at a point, the bigger the current density. s is the conductivity of the material. r=(1/s) is the resistivity of the material

  36. A conductor of uniform radius 1.20 cm carries a current of 3.00 A produced by an electric field of 120 V/m. What is the resistivity of the material?

  37. Range of r and s

  38. Ye old RESISTANCE Ohm's Law .. but now we know resistivity!

  39. REMEMBER THIS

  40. TemperatureEffect Dr DT

  41. A closed circuit

  42. Power

  43. fini that’s it, Doc Next up: DC Electric Circuits

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