phys 222, chapter 17 (exam 2)

A 500-W heater carries a current of 4.0 A. How much does it cost to operate the heater for 30 min if electrical energy costs 6.0 cents per kWh? a. 1.5 cents b. 9.0 cents c. 18 cents d. 36 cents

a

A 60-W light bulb is in a socket supplied with 120 V. What is the current in the bulb? a. 0.50 A b. 2.0 A c. 60 A d. 7 200 A

a

A high-voltage transmission line carries 1 000 A at 700 000 V. What is the power carried by the line? a. 700 MW b. 370 MW c. 100 MW d. 70 MW

a

A light bulb, sold as long-lasting, is rated 100 W at 130 V. The "increased" lifetime comes from using it at 120 V. Assuming negligible change in resistance at the different voltage, what is its power consumption at 120 V? a. 85 W b. 92 W c. 100 W d. 108 W

a

A metal wire has a resistance of 25.00 Ω under room temperature conditions of 25°C. When the wire is heated to 85°C the resistance increases by 0.75 Ω. What is the temperature coefficient of resistivity of this metal? a. 5.0 ́ 10-4 (°C)-1 b. 1.3 ́ 10-3 (°C)-1 c. 1.5 ́ 10-3 (°C)-1 d. 2.5 ́ 10-3 (°C)-1

a

A resistor is connected to a battery with negligible internal resistance. If you replace the resistor with one that has twice the resistance, by what factor does the power dissipated in the circuit change? a. 0.50 b. 0.25 c. 4.0 d. 2.0

a

An electric car is designed to run off a bank of 12-V batteries with total energy storage of 3.0 ́ 107 J. If the electric motor draws 6 000 W, what current will be delivered to the motor? a. 500 A b. 400 A c. 200 A d. 100 A

a

How long is a wire made from 100 cm3 of copper if its resistance is 8.5 ohms? The resistivity of copper is 1.7 ́ 10-5 W·m. a. 7.1 m b. 1.7 ́ 102 m c. 2.2 ́ 102 m d. 3.0 ́ 103 m

a

If a lamp has resistance of 120 W when it operates at 100 W, what is the applied voltage? a. 110 V b. 120 V c. 125 V d. 220 V

a

If a light bulb has half the resistance of a 100-W lightbulb, what would be its wattage? Assume both bulbs are attached to the same 120-V circuit. a. 200 W b. 50 W c. 25 W d. More information is needed.

a

Number 10 copper wire (radius = 1.3 mm) is commonly used for electrical installations in homes. What is the voltage drop in 40 m of #10 copper wire if it carries a current of 10 A? (The resistivity of copper is 1.7 ́ 10-8 W×m.) a. 1.3 V b. 0.77 V c. 0.50 V d. 0.13 V

a

Some superconductors are capable of carrying a very large quantity of current. If the measured current is 1.00 ́ 105 A, how many electrons are moving through the superconductor per second? (e = 1.60 ́ 10-19 C) a. 6.25 ́ 1023 b. 3.12 ́ 1022 c. 6.25 ́ 1021 d. 3.12 ́ 1020

a

The resistance of a platinum wire is to be calibrated for low-temperature work. A platinum wire with resistance 1.000 Ω at 20°C is immersed in liquid nitrogen at 77 K (-196°C). If the temperature response of the platinum wire is linear, what is the expected resistance of the platinum wire at -196°C? [aplatinum = 3.92 ́ 10-3 (°C)-1]. a. 0.153 Ω b. 0.232 Ω c. 1.768 Ω d. 1.847 Ω

a

The resistivity of a material is doubled when heated a certain amount. What happens to the resistance of a resistor made of this material when heated the same amount? a. It doubles. b. It quadruples. c. It halves. d. It stays the same.

a

The unit of electric resistance, the ohm, is equivalent to which of the following? a. V/A b. V×m c. A/s d. A/m

a

When current is flowing in a superconductor, which statement is not true? a. A battery is needed to keep the current going. b. Electrical charges are moving. c. The resistance is zero. d. No power is given off in the form of heat.

a

A 0.20-m-long metal rod has a radius of 1.0 cm and a resistance of 3.2 ́ 10-5 W. What is the resistivity of the metal? a. 1.6 ́ 10-8 Ω×m b. 5.0 ́ 10-8 Ω×m c. 16 ́ 10-8 Ω×m d. 160 ́ 10-8 Ω×m

b

A Nichrome wire has a radius of 0.50 mm and a resistivity of 1.5 ́ 10-6 W×m. If the wire carries a current of 0.50 A, what is the potential difference per unit length along this wire? a. 0.003 V/m b. 0.95 V/m c. 1.6 V/m d. 1.9 V/m

b

A copper cable needs to carry a current of 200 A with a power loss of only 3.0 W/m. What is the required radius of the copper cable? (The resistivity of copper is 1.7 ́ 10-8 W×m). a. 0.21 cm b. 0.85 cm c. 3.2 cm d. 4.0 cm

b

A light bulb with a tungsten filament is attached to a source of variable voltage. As the voltage is increased on the bulb, a. the bulb's resistance decreases. b. the bulb's resistance increases. c. the current in the bulb decreases. d. the power dissipated remains constant.

b

A metal wire has a resistance of 10.00 Ω at a temperature of 20°C. If the same wire has a resistance of 10.55 Ω at 90°C, what is the resistance when its temperature is -20°C? a. 0.70 Ω b. 9.69 Ω c. 10.31 Ω d. 13.8 Ω

b

A metallic conductor has a resistivity of 18 ́ 10-6 W×m. What is the resistance of a piece that is 30 m long and has a uniform cross sectional area of 3.0 ́ 10-6 m2 ? a. 0.056 Ω b. 180 Ω c. 160 Ω d. 90 Ω

b

A tungsten wire is used to determine the melting point of indium. The resistance of the tungsten wire is 3.000 W at 20°C and increases to 4.850 W as the indium starts to melt. atungsten = 4.50 ́ 10-3 (°C)-1. What is the melting temperature of indium? a. 132°C b. 157°C c. 351°C d. 731°C

b

A wire has resistance R. A second wire has twice the length, twice the diameter, and twice the resistivity of the first wire. What is its resistance? a. 8 R b. R c. R/4 d. The resistance is not given.

b

An electric car is designed to run off a bank of 12-V batteries with total energy storage of 3.0 ́ 107 J. If the electric motor draws 6 000 W in moving the car at a steady speed of 10 m/s, how far will the car go before it is "out of juice?" a. 25 km b. 50 km c. 100 km d. 150 km

b

An electric clothes dryer draws 15 A at 220 V. If the clothes put into the dryer have a mass of 7.0 kg when wet and 4.0 kg dry, how long does it take to dry the clothes? (Assume all heat energy goes into vaporizing water, Lv = 2.26 ́ 106 J/kg). a. 55 min b. 34 min c. 20 min d. 16 min

b

Carbon has a negative temperature coefficient of resistance of -0.5 ́ 10-3 (°C)-1. What temperature increase would result in a resistance decrease of 1% for a carbon resistor? a. 2°C b. 20°C c. 50°C d. 100°C

b

If a 500-W heater carries a current of 4.00 A, what is the voltage across the ends of the heating element? a. 2 000 V b. 125 V c. 250 V d. 0.008 V

b

If electrical energy costs 5.5 cents per kWh, what does it cost to heat 200 kg water from 15°C to 80°C? (The specific heat of water = 103 cal/kg×°C and 1.0 cal = 4.186 J.) a. 48 cents b. 83 cents c. 16 cents d. 80 cents

b

If the current in a wire is tripled, what effect does this have on the electron drift velocity in the wire? a. It stays the same. b. It triples. c. It decreases by a factor of three. d. It increases by a factor of nine.

b

In a certain material there is a current of 16 A flowing through a surface to the right, and there is an equal amount of positive and negative charge passing through the surface producing the current. How much negative charge passes through the surface? a. 8 C/s toward the right b. 8 C/s toward the left c. 16 C/s toward the right d. 16 C/s toward the left

b

Replacing a wire resistor with another of the same material and length but with three times the diameter will have the effect of changing the resistance by what factor? a. 1/3 b. 1/9 c. 3 d. 9

b

The charge flowing through a light bulb attached to a 12.0-V battery in 14.0 s is 30.0 C. What is the total energy delivered to the filament during this process? a. 12.0 J b. 360 J c. 5040 J d. 168 J

b

The number density of conduction electrons in a metal can be found from the density r of the metal, the mass per mole M of the metal, the number of conduction electrons per metal atom, and Avogadro's number NA. If we assume one conduction electron per atom, which of the following gives the number density of conduction electrons for a given metal? a. NA pM b. NA p/M c. NA M/p d. NA /p M

b

The temperature coefficient of resistivity for a "perfect" ohmic material would be: a. positive and constant. b. zero. c. negative. d. positive and uniformly increasing.

b

The unit of electric current, the ampere, is equivalent to which of the following? a. V×Ω b. V/Ω c. W×m d. V/s

b

When an electric current exists within a conducting wire, which of the following statements describes the condition of any accompanying electric field? a. must be zero b. must be parallel to current flow c. must be anti-parallel (opposite direction) to current flow d. must be perpendicular to current flow

b

Which process will double the power given off by a resistor? a. doubling the current while doubling the resistance b. doubling the current by making a resistance half as big c. doubling the current by doubling the voltage d. doubling the current while making the voltage half as big

b

You measure a 25.0-V potential difference across a 5.00-W resistor. What is the current flowing through it? a. 125 A b. 5.00 A c. 4.00 A d. 1.00 A

b

A 20-Ω platinum wire at 20°C with a temperature coefficient of resistivity of 3.9 ́ 10-3 (°C)-1 will have what resistance at 100°C? a. 14 Ω b. 20 Ω c. 26 Ω d. 28 Ω

c

A certain material is in a room at 27°C. If the absolute temperature (K) of the material is tripled, its resistance doubles. (Water freezes at 273 K.) What is the value for a, the temperature coefficient of resistivity? a. 1 (°C)-1 b. 2 (°C)-1 c. 0.001 7 (°C)-1 d. 0.038 (°C)-1

c

A light bulb has resistance of 240 W when operating at 120 V. Find the current in the light bulb. a. 2.0 A b. 1.0 A c. 0.50 A d. 0.20 A

c

A resistor is made of a material that has a resistivity that is proportional to the current going through it. If the voltage across the resistor is doubled, what happens to the power dissipated by it? a. It doubles. b. It quadruples. c. It increases by a factor of 23/2. d. It increases by a factor of 21/2.

c

A solar panel measures 80 cm ́ 50 cm. In direct sunlight, the panel delivers 3.2 A at 15 V. If the intensity of sunlight is 1 000 W/m2 , what is the efficiency of the solar panel in converting solar energy into electrical energy? a. 24% b. 18% c. 12% d. 6.0%

c

A steam turbine at an electric power plant delivers 4 500 kW of power to an electrical generator which converts 95% of this mechanical energy into electrical energy. What is the current delivered by the generator if it delivers at 3 600 V? a. 0.66 ́ 103 A b. 1.0 ́ 103 A c. 1.2 ́ 103 A d. 5.9 ́ 103 A

c

A water pump draws about 3.8 A when connected to 240 V. What is the cost (with electrical energy at 9 cents per kWh) of running the pump for 10 h? a. 8.0 cents b. 15 cents c. 82 cents d. 95 cents

c

A wire carries a steady current of 0.1 A over a period of 20 s. What total charge passes through the wire in this time interval? a. 200 C b. 20 C c. 2 C d. 0.005 C

c

An 8.00-W resistor is dissipating 100 watts. What are the current through it and the difference of potential across it? a. 12.5 A, 28.3 V b. 3.54 A, 12.5 V c. 3.54 A, 28.3 V d. 28.3 A, 3.54 V

c

By what factor is the resistance of a copper wire changed when its temperature is increased from 20°C to 120°C? The temperature coefficient of resistivity for copper = 3.9 ́ 10-3 (°C)-1 . a. 0.72 b. 1.06 c. 1.39 d. 1.44

c

If a 500-W heater carries a current of 4.00 A, what is the resistance of the heating element? a. 85.7 Ω b. 42.8 Ω c. 31.3 Ω d. 11.2 Ω

c

If a 9.0-V battery, with negligible internal resistance, and an 18-W resistor are connected in series, what is the amount of electrical energy transformed to heat per coulomb of charge that flows through the circuit? a. 0.50 J b. 3.0 J c. 9.0 J d. 72 J

c

If a lamp has a resistance of 120 W when it operates at 100 W, what current does it carry? a. 2.10 A b. 1.2 A c. 0.91 A d. 0.83 A

c

Materials having resistance changes as voltage or current varies are called: a. ohmic. b. inohmic. c. nonohmic. d. deohmic.

c

Superconductivity was discovered by: a. Volta. b. Ohm. c. Onnes. d. Bednorz and Müller.

c

The charge flowing through a light bulb attached to a 12.0-V battery in 14.0 s is 30.0 C.What is the average power supplied by the battery during this process? a. 360 W b. 12.0 W c. 25.7 W d. 5.63 W

c

The current in an electron beam in a cathode-ray tube is measured to be 70 μA. How many electrons hit the screen in 5.0 s? (e = 1.6 ́ 10-19 C) a. 2.2 ́ 1011 electrons b. 8.8 ́ 1013 electrons c. 2.2 ́ 1015 electrons d. 8.8 ́ 1018 electrons

c

The unit for rate of energy transformation, the watt, in an electric circuit is equivalent to which of the following? a. V/s b. A×Ω c. V×A d. V/Ω

c

When a superconductor's temperature drops below the critical temperature, its resistance: a. equals that of a semiconductor of equal dimensions. b. increases by two. c. drops to zero. d. reduces to one half.

c

When you flip a switch to turn on a light, the delay before the light turns on is determined by: a. the number of electron collisions per second in the wire. b. the drift speed of the electrons in the wire. c. the speed of the electric field moving in the wire. d. none of these, since the light comes on instantly.

c

Which is not a force? a. gravity b. electrical force c. voltage d. friction

c

A Nichrome wire has a radius of 0.50 mm and a resistivity of 1.5 ́ 10-6 W×m. What is the resistance per unit length of this wire? a. 0.001 5 Ω/m b. 0.95 Ω/m c. 1.6 Ω/m d. 1.9 Ω/m

d

A flashlight bulb operating at a voltage of 4.5 V has a resistance of 8.0 W. How many electrons pass through the bulb filament per second (e = 1.6 ́ 10-19 C)? a. 3.7 ́ 1016 b. 1.8 ́ 1021 c. 9.4 ́ 1017 d. 3.5 ́ 1018

d

A high voltage transmission line of diameter 2 cm and length 200 km carries a steady current of 1 000 A. If the conductor is copper with a free charge density of 8 ́ 1028 electrons/m3 , how long does it take one electron to travel the full length of the cable? (e = 1.6 ́ 10-19 C) a. 8 ́ 102 s b. 8 ́ 104 s c. 8 ́ 106 s d. 8 ́ 108 s

d

A high-voltage transmission line carries 1 000 A at 700 kV for a distance of 100 miles. If the resistance in the wire is 1 W/mile, what is the power loss due to resistive losses? a. 10 kW b. 100 kW c. 10 MW d. 100 MW

d

A hot water heater operating at 240 V supplies to a quantity of water to warm it to the desired temperature of . What is the cost of the energy if the rate is $0.131/kWh? a. The mass of the water is needed to answer this question. b. The initial temperature of the water is needed to answer this question. c. The mass of the water, the initial temperature of the water, and the time it took to heat the water is needed to answer this question. d. $0.330.

d

A resistor is made of a material that has a resistivity that is proportional to the current going through it. If the voltage across the resistor is doubled, what happens to the current through it? a. It doubles. b. It quadruples. c. It increases by a factor of 2^3/2. d. It increases by a factor of 2^1/2.

d

A superconducting wire's chief characteristic is which of the following? a. an extremely great length b. a large cross sectional area c. an extremely high temperature d. no resistance

d

A wire 1 mm in diameter is connected to one end of a wire of the same material 2 mm in diameter of twice the length. A voltage source is connected to the wires and a current is passed through the wires. If it takes time T for the average conduction electron to traverse the 1-mm wire, how long does it take for such an electron to traverse the 2-mm wire? a. T/4 b. T c. 4T d. 8T

d

An electric toaster requires 1 100 Ω at 110 V. What is the resistance of the heating coil? a. 7.5 Ω b. 9.0 Ω c. 10.0 Ω d. 11.0 Ω

d

Consider some material that has been cooled until it has become a superconductor. If it is cooled even further its resistance will: a. increase. b. decrease. c. stay constant and non-zero. d. None of the above.

d

If a certain resistor obeys Ohm's law, its resistance will change: a. as the voltage across the resistor changes. b. as the current through the resistor changes. c. as the energy given off by the electrons in their collisions changes. d. none of the above, since resistance is a constant for the given resistor.

d

If a metallic wire of cross sectional area 3.0 ́ 10-6 m2 carries a current of 6.0 A and has a mobile charge density of 4.24 ́ 1028 carriers/m3 , what is the average drift velocity of the mobile charge carriers? (charge value = 1.6 ́ 10-19 C) a. 3.4 ́ 103 m/s b. 1.7 ́ 103 m/s c. 1.5 ́ 10-4 m/s d. 2.9 ́ 10-4 m/s

d

Resistor A has twice the resistance of resistor B. When individually connected across a given potential difference, which one dissipates the most power; and when connected in series across the same potential difference, which one dissipates the most power? a. A, A b. A, B c. B, B d. B, A

d

Suppose that a voltage surge produces 140 V for a moment in a 120-V line. What will temporarily be the output of a 100-W light bulb assuming its resistance does not change? a. 109 W b. 118 W c. 127 W d. 136 W

d

Ten coulombs of charge start from the negative terminal of a battery, flow through the battery and then leave the positive terminal through a wire, flow through a resistor and then return to the starting point on this closed circuit. In this complete process, the ten coulombs: a. do positive work on the battery. b. receive heat energy from the resistor. c. have a net loss of potential energy. d. have no net change in potential energy.

d

The heating coil of a hot water heater has a resistance of 20 W and operates at 210 V. How long a time is required to raise the temperature of 200 kg of water from 15°C to 80°C? (The specific heat for water = 103 cal/kg×°C and 1.0 cal = 4.186 J). a. 1.7 h b. 3.8 h c. 5.1 h d. 6.9 h

d

The quantity volt is equivalent to which of the following? a. J×m b. J×C c. C/Ω d. J/C

d

The size of the electric current in a electrical conductor is a function of which of the following? a. velocity of charge carriers b. conductor cross sectional area c. density of charge carriers d. All of the above choices are valid.

d

Two cylindrical resistors are made of the same material and have the same resistance. The resistors, R1 and R2, have different radii, r1 and r2, and different lengths, L1 and L2. Which of the following relative values for radii and lengths would result in equal resistances? a. r1 = r2 and L1 = 2L2 b. 2r1 = r2 and L1 = 2L2 c. r1 = r2 and 4L1 = L2 d. 2r1 = r2 and 4L1 = L2

d

When the voltage across a nonohmic resistor is doubled, the current through it triples. What happens to the power delivered to this resistor? a. This cannot be answered with the information given. b. The power decreases to 2/3 of the original amount. c. The power increases to 1.5 times the original amount. d. The power increases to 6 times the original amount.

d

Which is a unit of power? a. kWh b. W/s c. A×Ω d. J/s

d

Wire A and Wire B are each carrying the same current. If the diameter of Wire A is twice that of Wire B, how does the drift velocity vdA in Wire A compare to that in in Wire B? a. vdA = vdB b. vdA = 2 vdB c. vdA = vdB /2 d. vdA = vdB /4

d