Page 300 - Schaum's Outline of Theory and Problems of Applied Physics

CHAP. 24] ELECTRIC CURRENT 285

24.11. A 115-V, 1-kW electric oven is mistakenly connected to a 230-V power line that has a 15-A fuse. The oven will
(a) give off less than 1 kW of heat
(b) give off 1 kW of heat
(c) give off more than 1 kW of heat
(d) blow the fuse

24.12. A 230-V, 1-kW electric oven is mistakenly connected to a 115-V power line that also has a 15-A fuse. The oven will
(a) give off less than 1 kW of heat
(b) give off 1 kW of heat
(c) give off more than 1 kW of heat
(d) blow the fuse

24.13. A fully charged 24-V, 100-Ah storage battery has an energy content of
(a) 2.4 kJ (c) 144 kJ
(b) 57.6 kJ (d) 8.64 MJ

Supplementary Problems

24.1. Bends in a pipe slow down the flow of water through it. Do bends in a wire increase its electric resistance?

24.2. How many electrons pass through the filament of a 75-W, 120-V light bulb per second?
24.3. Find the current in a 200- resistor when the potential difference across it is 40 V.

24.4. An electric water heater draws 10 A of current from a 240-V power line. What is its resistance?

2
24.5. The resistance of a 15-m length of iron wire whose cross-sectional area is 0.5 mm is found to be 3.6 . What is
the resistivity of iron?
24.6. Find the resistance of8mof aluminum wire 0.1 mm in diameter. The resistivity of aluminum is 2.6 × 10 −8 ·m.

24.7. How long should a copper wire 0.4 mm in diameter be for it to have a resistance of 10 ? The resistivity of copper
is 1.7 × 10 −8 ·m.

24.8. A resistance thermometer uses a platinum element whose resistance at 20 Cis11 . When the element is placed in a
◦
furnace, its resistance increases to 33 . What is the furnace temperature? The temperature coefficient of resistance
of platinum is 0.0036/ C.
◦
24.9. The diameter of No. 14 wire is 0.06408 in. (a) Find its cross-sectional area in circular mils. (b) The area of No. 10
wire is 10,380 cmil. How does the resistance of a given length of No. 14 wire compare with that of the same length
of No. 10 wire?

24.10. The specific resistance of iron is 72 ·cmil/ft. Find the resistance of 250 ft of iron whose diameter is 1 in.
16
24.11. The temperature coefficient of resistance of carbon is −0.0005/ C. If the resistance of a carbon resistor is 1000
◦
◦
at 0 C, find its resistance at 120 C.
◦
◦
24.12. An iron wire has a resistance of 0.20 at 20 C and a resistance of 0.30 at 110 C. Find the temperature coefficient
◦
of resistance of the iron used in the wire.
24.13. The maximum allowable voltage drop for a 70-m length of a certain cable that carries a current of 40 A is 5 V. What
is the minimum cross-sectional area of the copper wire of the cable in square millimeters?