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Chapter 26: Current and Resistance

Fundamentals Of Physics
Pages: 745 - 770

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91 Questions for Chapter 26: Current and Resistance

  1. The magnitude Jof the current density in a certain lab wire with a circular cross section of radius R = 2.00 mmis given by J=(3.00×108)r2, with Jin amperes per square meter and radial distance rin meters. What is the current through the outer section bounded by r = 0.900 Rand r = R ?

    Found on Page 766
  2. Three wires, of the same diameter, are connected in turn between two points maintained at a constant potential difference. Their resistivity and lengths are pand L(wire A),1.2p and 1.2 L(wire B), and 0.9pand L(wire C). Rank the wire according to the rate at which energy is transferred to thermal energy within them, greatest first.

    Found on Page 765
  3. What is the current in a wire of radius R=3.40 mm if the magnitude of the current density is given by (a)Ja=J0r/Rand(b)Jb=J0(1-r/R), in which ris the radial distance and J0=5.50×104A/m2? (c) Which function maximizes the current density near the wire’s surface?

    Found on Page 766
  4. Figure 26-23 gives, for three wires of radius R, the current density J(r) versus radius r, as measured from the center of a circular cross section through the wire. The wires are all made from the same material. Rank the wires according to the magnitude of the electric field (a) at the center, (b) halfway to the surface, and (c) at the surface, greatest first.

    Found on Page 765
  5. Near Earth, the density of protons in the solar wind (a stream of particles from the Sun) is8.70cm-3 , and their speed is 470 km/s. (a) Find the current density of these protons. (b) If Earth’s magnetic field did not deflect the protons, what total current would Earth receive?

    Found on Page 766
  6. How long does it take electrons to get from a car battery to the starting motor? Assume the current is 300 Aand the electrons travel through a copper wire with cross-sectional area0.21cm2and length 0.85 m. The number of charge carriers per unit volume is8.49×1028m-3.

    Found on Page 766
  7. A human being can be electrocuted if a current as small as 50mA passes near the heart. An electrician working with sweaty hands makes good contact with the two conductors he is holding, one in each hand. If his resistance is 2000Ω, what might the fatal voltage be?

    Found on Page 766
  8. A coil is formed by winding 250turnsof insulated 16-gauge copper wire (diameter = 1.3 mm) in a single layer on a cylindrical form of radius 12cm. What is the resistance of the coil? Neglect the thickness of the insulation. (Use Table 26-1.)

    Found on Page 766
  9. Copper and aluminum are being considered for a high-voltage transmission line that must carry a current of 60.0 A. The resistance per unit length is to be0.150Ω/km. The densities of copper and aluminum are 8960and2600kg/m3, respectively. Compute (a) the magnitude Jof the current density and (b) the mass per unit lengthλfor a copper cable and (c) Jfor an aluminum cable (d)for an aluminum cabl

    Found on Page 766
  10. A wire of Nichrome (a nickel–chromium–iron alloy commonly used in heating elements) is 1.0mlong and 1.0mm2in cross-sectional area. It carries a current of 4.0 Awhen a 2.0 Vpotential difference is applied between its ends. Calculate the conductivityof Nichrome.

    Found on Page 766

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