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Q25.10. Why does an electric light bulb nearly always burn out just as you turn on the light, but almost never while the light is shinning? |
Q25.20. A fuse is a device designed to break a circuit, usually by melting when the current exceed a certain value. What characteristic should the material of the fuse have? |
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25.10. A copper wire has a square cross section 2.3 mm on a side. The wire is 4.0 m long and carries a current of 3.6 A. The density of free electrons is 8.5.1028/m3. Find the magnitude of
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25.20. A battery-powered light bulb has a tungsten filament. When the switch connecting the bulb to the battery is first turned on and the temperature of the bulb is 20oC, the current in the bulb is 0.860A. After the bulb has been on for 30s, the current is 0.230A. What is then the temperature of the filament? |
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25.30. The open circuit terminal voltage of a battery is 12.6V. When a resistor R=4.00Ω is connected between the terminals of the battery, the terminal voltage of the battery is 10.4 V. What is the internal resistance of the battery? |
25.40. The following measurements were made on a Thyrite resistor: I(A) 0.50 1.00 2.00 4.00 Vab 2.55 3.11 3.77 4.58 a) Graph Vab as a function of I. b) Does Thyrite obey Ohm's law? How can you tell? c) Graph the resistance R=Vab/I as a function of I.? |
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25.50. |
25.60. The region between two concentric conducting spheres with radii a and b is filled with a conducting material with resistivity ρ. a) Show that the resistance between the spheres is given by R= ρ (1/a - 1/b) / (4π) b) Derive an expression for the current density as a function of radius, in terms of the potential difference Vab between the spheres. c) Show that the result in part (a) reduces to the equation: R=ρL/A (25.10) when the separation L=b-a between the spheres is small. |
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25.70. The open-circuit terminal voltage of a source is 7.86 V, and its short-circuit current is 9.25 A. a) What is the current when a resistor with resistance 2.4Ω is connected to the terminals of the source? The resistor obeys Ohm's law. b) What is the current in the semiconductor device (that does not obey Ohm's law and has the current-voltage relation V=αI+βI2, with α=2.50Ω and β=0.360Ω/A) when connected acros the terminals of this source? c) What is the terminal voltage of the source at the current calculated in part (b)? |
25.80. |
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25.90. |
25.100. |