6.3 You have a resistor made of an unknown material. You note that at a temperature of 20.0°C its resistance is 529 ȍ. Upon
lowering its temperature to 0°C, the resistance drops to 476 ȍ. (a) Find the temperature coefficient of resistivity of the
material at 20.0°C. (b) Use the table in the text to make a guess as to the material the resistor is made of.
(a) °Cí^1
(b) i. Silver
ii. Carbon
iii. Tungsten
iv. Germanium
v. Silicon6.4 A piece of iron has a resistance of 103 ȍ at 40.0°C. Iron has a temperature coefficient of resistivity of 0.00640°Cí^1 at 20.0°C.
What is the resistance of the iron at 20.0°C?ȍ
6.5 At 20.0°C, a mainly silicon resistor has a resistance of 585 ȍ, and a tungsten resistor has a resistance of 752 ȍ. At this
temperature, the silicon resistor has a temperature coefficient of resistivity of í0.070°Cí^1 and the tungsten resistor has a
coefficient of 0.0050°Cí^1. At what temperature will their resistances be equal?
°C
6.6 A circuit with silver wiring operates at 20.0°C. If you were to replace the silver with copper wiring of the exact same
dimensions, at what temperature would you have to run the circuit in order to have the resistance of the copper circuit be the
same as that of the silver circuit? (Use the resistivities measured at 20.0°C found in the table in the text.)
°C6.7 A conductor of an unknown substance has a resistance of 43.5 ȍ at 40.0° C, and a resistance of 55.4 ȍ at 50.0° C. What is
the resistance of the conductor at 47.0° C? Hint: When applying the equation in the text, the temperature coefficient of
resistivity must be measured at the temperature T 1.ȍ
Section 7 - Electric power
7.1 Instead of using batteries, a certain type of flashlight has a built in hand pump which you can squeeze to generate the power
necessary to light the bulb. Suppose that squeezing the pump at a rate of twice per second generates 16 watts. If the light
bulb has a resistance of 0.60 ohms, and you squeeze twice per second, what will be the potential difference across the light
bulb?
V
7.2 Many 120 V household circuits are rated at 15 A. This means that, if the current in the circuit exceeds this amount, there is a
danger of overheated wires causing a fire. Such circuits are protected by circuit breakers that trip or fuses that blow and
interrupt the current if it exceeds 15 A. (a) Is it safe to plug a 1400 W space heater into such a circuit? (b) Is it safe to plug two
such space heaters into the same circuit, so that the power requirement is 2800 W?
(a) Yes No
(b) Yes No
7.3 How many 60 W light bulbs can you use in a 10 A, 120 V household circuit before tripping the circuit breaker and attempting
to finish this homework problem in total darkness? The number of bulbs, times the individual power, equals the total power for
the circuit.
light bulbs
7.4 A certain appliance running at 240 V consumes 325 W of power. How much energy does it use in one hour?
J
7.5 A simple heater for a cup of water consists of an insulated resistive coil that plugs directly into a 120 V electrical outlet. The
heater is placed into a coffee mug that contains 0.550 kg of water and heats it to boiling (100°C) from room temperature (20°
C) in 180 s. If the specific heat of the water is 4200 J/kg·K over this temperature range, what is the resistance of the coil?
Ignore any changes in the resistance of the coil due to the increase in temperature.ȍ
7.6 In a graphing calculator, there are four AAA batteries, which together provide a potential difference of 1.5 V and a total of
6.0 W·h of energy. If the batteries last 73 hours before they need to be replaced, what was the average current through the
calculator?
A
7.7 A 12 V car battery powers a light bulb. The bulb consumes 60 watts of power. (a) Assuming that the bulb is the only
significant source of resistance in the circuit, find its resistance. (b) How much power would be consumed by a light bulb(^474) Copyright 2007 Kinetic Books Co. Chapter 25 Problems