CHAPTER 25 HOMEWORK
1. (p.654 Ex.6) An electric device draws 5.50A at 110V. (a) If the voltage drops by 10 percent, what will be the current, assuming nothing else changes? (b) If the resistance of the device were reduced by 10 percent, what current would be drawn at 110V?
2. (p.654 Ex.9) A hair dryer draws 7.5A when plugged into a 120V line. (a) What is its resistance? (b) How much charge passes through it in 15min? (Assume direct current.)
3. (p.655 Ex.16) How much would you have to raise the temperature of a copper wire (originally at 20°C) to increase its resistance by 20 percent?
4. (p.656 Ex.35) What is the total amount of energy stored in a 12-V, 90-A.h car battery when it is fully charged?
5. (p.656 Ex.47) Calculate the peak voltage across, and peak current through an 1800-W welder connected to a 450-Vac line.
6. (p.656 Ex.51) A 0.55-mm-diameter copper wire carries a tiny current of 2.5mA. Estimate (a) the electron drift velocity in the wire, (b) the current density, and (c) the electric field.
7*. (p.655 Ex. 25) determine a formula for the total resistance of a spherical shell made of material whose conductivity is s and whose inner and outer radii are r1 and r2. Assume the current flows radially. outward.
8*. (p.656 Ex.41) The current in an electromagnet connected to a 240-V line is 14.5A. At what rate must cooling water pass over the coils if the water temperature is to rise by no more than 6.50°C?
Answer.
9*. (p.656 Ex. 53) At a point high in the Earth's atmosphere. He2+
ions in a concentration of 2.8 x 1012/m3 are moving due
north at a speed of 2.0 x 106m/s. Also, an 8.0 x 1012/m3
concentration of O2- ions is moving due south at a
speed of 7.2 x 106m/s. determine the magnitude and direction of the
current density j at this point.