Chapter 17

4. The current supplied by a battery in a portable device is typically 0.15 A. Find the

number of electrons passing through the device in one hour.

3.4x1021 electrons

7. A 200-km-long high-voltage transmission line 2.0 cm in diameter carries a steady

current of 1 000 A. If the conductor is copper with a free charge density of 8.5 × 1028

electrons per cubic meter, how many years does it take one electron to travel the full

length of the cable?

27 y

10. An electric heater carries a current of 13.5 A when operating at a voltage of 1.20 ×

102 V. What is the resistance of the heater?

R = 8.89 Ω

15. A potential difference of 12 V is found to produce a current of 0.40 A in a 3.2-m

length of wire with a uniform radius of 0.40 cm. What is (a) the resistance of the

wire? (b) The resistivity of the wire?

R = 30 Ω,

19. A wire of initial length L0 and radius r0 has a measured resistance of 1.0 Ω. The

wire is drawn under tensile stress to a new uniform radius of r = 0.25r0. What is the

new resistance of the wire?

R = 256 Ω

28. A wire 3.00 m long and 0.450 mm2 in cross-sectional area has a resistance of

41.0 Ω at 20°C. If its resistance increases to 41.4 Ω at 29.0°C, what is the

temperature coefficient of resistivity?

1.1×10-3 C°-1

33. Suppose your waffle iron is rated at 1.00 kW when connected to a 1.20 × 102 V

source. (a) What current does the waffle iron carry? (b) What is its resistance?

8.33 A, 14.4 Ω

37. The heating element of a coffeemaker operates at 120 V and carries a current of

2.00 A. Assuming the water absorbs all the energy converted by the resistor, calculate

how long it takes to heat 0.500 kg of water from room temperature (23.0°C) to the

boiling point.

11.2 min

12. Suppose you wish to fabricate a uniform wire out of 1.00 g of copper. If the wire is

to have a resistance R = 0.500 Ω, and if all the copper is to be used, what will be (a)

the length and (b) the diameter of the wire?

L = 1.8 m, d = 0.28 mm