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Chapter 25

Chapter 25. Current and Resistance. An electric field in a conductor causes charges to flow. (See Figure 25.1 at the right.) Classically , electrons are accelerated in direction of field until they hit a nucleus and bounce off, changing direction.

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Chapter 25

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  1. Chapter 25 Current and Resistance

  2. An electric field in a conductor causes charges to flow. (See Figure 25.1 at the right.) • Classically , electrons are accelerated in direction of field until they hit a nucleus and bounce off, changing direction. • Average velocity over this motion is called drift velocity vd. • A current is any motion of charge from one region to another. Current is defined as I = dQ/dt, where Q is amount of charge passing through a given area. Current

  3. A current can be produced by positive or negative charge flow. • Conventional current is treated as a flow of positive charges. • The moving charges in metals are electrons (see figure below). Direction of current flow

  4. Figure 25.3 at the right shows the positive charges moving in the direction of the electric field. Current, drift velocity, and current density

  5. The resistivity of a material is the ratio of the electric field in the material to the current density it causes:  = E/J. • The conductivity is the reciprocal of the resistivity. • Table 25.1 shows the resistivity of various types of materials. Resistivity

  6. Resistivity depends on temperature. See Figure 25.6 at the left. • Table 25.2 shows some temperature coefficients of resistivity. Resistivity and temperature

  7. This resistor has a resistance of 5.7 kΩ with a tolerance of ±10%. Tolerance is given by - No band 20% - Silver band 10% - Gold band 5% Resistors are color-coded for easy identification

  8. Only the resistor in Figure 25.10(a) below obeys Ohm’s law. Ohmic and nonohmic resistors

  9. Q25.1 Two copper wires of different diameter are joined end to end, and a current flows in the wire combination. When electrons move from the larger-diameter wire into the smaller-diameter wire, A. their drift speed increases. B. their drift speed decreases. C. their drift speed stays the same. D. not enough information given to decide

  10. A25.1 Two copper wires of different diameter are joined end to end, and a current flows in the wire combination. When electrons move from the larger-diameter wire into the smaller-diameter wire, A. their drift speed increases. B. their drift speed decreases. C. their drift speed stays the same. D. not enough information given to decide

  11. Q25.2 Electrons in an electric circuit pass through a resistor. The wire has the same diameter on each side of the resistor. Compared to the drift speed of the electrons before entering the resistor, the drift speed of the electrons after leaving the resistor is A. faster. B. slower. C. the same. D. not enough information given to decide

  12. A25.2 Electrons in an electric circuit pass through a resistor. The wire has the same diameter on each side of the resistor. Compared to the drift speed of the electrons before entering the resistor, the drift speed of the electrons after leaving the resistor is A. faster. B. slower. C. the same. D. not enough information given to decide

  13. Q25.3 Electrons in an electric circuit pass through a resistor. The wire has the same diameter on each side of the resistor. Compared to the potential energy of an electron before entering the resistor, the potential energy of an electron after leaving the resistor is A. greater. B. less. C. the same. D. not enough information given to decide

  14. A25.3 Electrons in an electric circuit pass through a resistor. The wire has the same diameter on each side of the resistor. Compared to the potential energy of an electron before entering the resistor, the potential energy of an electron after leaving the resistor is A. greater. B. less. C. the same. D. not enough information given to decide

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