PHY 131 Exam 2

A length L of wire has a voltage V across its ends while a current I runs through the wire. If a voltage 3V were applied to another length of this same wire from the same coil and the same current were found to run through it, what is the length of this second wire?

3L

A circular loop carrying a current of 2.4 A is oriented in a magnetic field of 0.42 T. The loop has an area of 0.33 m2 and is mounted on an axis, perpendicular to the magnetic field, which allows the loop to rotate. What is the torque on the loop when its plane is oriented at a 25° angle to the field? a. 1.4E-1 N⋅m b. 3.0E-1 N⋅m c. 3.3E-1 N⋅m d. 4.4E-2 N⋅m e. 7.0E-1 N⋅m

a

A lightbulb drawing a current of 0.60 A is run for 2.0 hours. How many electrons pass through the bulb during this process? a. 2.7 × 1022 b. 1.4 × 1019 c. 5.4 × 1025 d. 4.3 × 103

a

A metal wire has a resistance of 31 Ω under room temperature conditions of 35°C. When the wire is heated to 81°C the resistance increases by 0.60 Ω. What is the temperature coefficient of resistivity of this metal? a. 4.2E-4 × 10−4 (°C)−1 b. 2.3E-3 × 10−3 (°C)−1 c. 5.5E-3 × 10−3 (°C)−1 d. 6.9E-3 × 10−3 (°C)−1 e. 8.9E-5 × 10−4 (°C)−1

a

A resistor is connected to a battery with negligible internal resistance. If you replace the resistor with one that has quadruple the resistance, by what factor does the power dissipated in the circuit change? a. 1/4 b. 1/16 c. 4 d. 16 e. (square root of 4)

a

A square coil and a triangular coil each have sides of length L. The triangular coil has twice as many turns as the square coil. If a the same current is sent through each coil, when the coils are placed in the same uniform magnetic field making an angle of 30° with the planes of the coils, which coil experiences the greater torque? a. the square coil b. the triangular coil c. Both coils experience the same torque. d. It depends on the magnitude of the magnetic field that the coils are placed in.

a

A superconducting wire carries a current of 6.0 kA. Find the magnetic field at a distance of 1.0 m from the wire. (µ0 = 4π × 10 −7 T⋅m/A) a. 1.2E-3 T b. 3.8E-4 T c. 3.8E-3 T d. 1.9E-3 T e. 7.5E-3 T

a

An electric car is designed to run off a bank of 13.0-V batteries with total energy storage of 3.00 × 107 J. If the electric motor draws 7 150 W, what current will be delivered to the motor? a. 550 A b. 275 A c. 825 A d. 413 A e. 138 A

a

How long is a wire made from 250 cm3 of copper if its resistance is 6.0 ohms? The resistivity of copper is 1.7 × 10-5 Ω·m. a. 9.4 m b. 1.7 × 102 m c. 2.0 × 102 m d. 3.0 × 103 m e. 88 m

a

If I = 2.0 mA and the potential difference, VA - VB = +30 V in the circuit segment shown, determine the charge and polarity of the capacitor. a. 1.5 mC, left plate is positive b. 1.5 mC, right plate is positive c. 0.45 mC, left plate is positive d. 0.45 mC, right plate is positive

a

If I1 goes to the right through R1, I2 goes to the right through R2, and I3 goes to the right through R3, what is the resulting equation resulting from applying Kirchhoff's junction rule at point b? a. I1 + I2 + I3 = 0 b. I1 + I2 - I3 = 0 c. I1 - I2 + I3 = 0 d. I1 - I2 - I3 = 0

a

If R1 < R2 < R3, and if these resistors are connected in parallel in a circuit, which one has the highest current? a. R1 b. R2 c. R3 d. All have the same current.

a

Number 10 copper wire (radius = 1.5 mm) is commonly used for electrical installations in homes. What is the voltage drop in 15 m of #10 copper wire if it carries a current of 12 A? (The resistivity of copper is 1.7 × 10−8 Ω⋅m.) a. 0.43 V b. 0.36 V c. 2.0 V d. 4.3 V e. 0.25 V

a

Three resistors connected in parallel have individual values of 4.0, 6.0, and 10.0 Ω, respectively. If this combination is connected in series with a 12-V battery and a 2.0-Ω resistor, what is the current in the 10.0-Ω resistor? Practice problems for exam #2 Page 14 a. 0.59 A b. 1.0 A c. 11 A d. 16 A e. 0.28 A

a

Three resistors with values of R1, R2 and R3, respectively, are connected in series. Which of the following expresses the total resistance, RT, of the three resistors? a. RT = R1 + R2 + R3 b. RT = (1/R1 + 1/R2 + 1/R3) c. RT = R1 = R2 = R3 d. RT = (1/R1 + 1/R2 + 1/R3) −1

a

Two long parallel wires 44.5 cm apart are carrying currents of 14.3 A and 23.8 A in the same direction. What is the magnitude of the magnetic field halfway between the wires? a. 8.54E-6 T b. 3.42E-5 T c. 1.29E-5 T d. 2.14E-5 T e. 4.27E-6 T

a

Two singly ionized isotopes, X and Y, of the same element move with the same speed perpendicular to a uniform magnetic field. Isotope X follows a path of radius 3.24 cm while isotope Y moves along a path 3.52 cm in radius. What is the ratio of the two isotope masses, mx/my? a. 0.920 b. 1.09 c. 0.847 d. 1.18 e. Not enough information is given.

a

What is the current flowing through the 4-Ω resistor? a. 1 A b. 2 A c. 3 A d. 6 A

a

What is the current through the 2.0-Ω resistor? a. 1.0 A b. 0.50 A c. 1.5 A d. 2.0 A

a

A 0.30-m-long metal rod has a radius of 5.0 cm and a resistance of 2.5 × 10−5 Ω. What is the resistivity of the metal? a. 2.1E-8 Ω⋅m b. 6.5E-7 Ω⋅m c. 1.4E-8 Ω⋅m d. 9.5E-9 Ω⋅m e. 1.3E-7 Ω⋅m

b

A copper cable needs to carry a current of 150 A with a power loss of only 4.0 W/m. What is the required radius of the copper cable? (The resistivity of copper is 1.7 × 10−8 Ω⋅m). a. 0.20 cm b. 0.55 cm c. 2.2 cm d. 1.7 cm e. 0.25 cm

b

A hair dryer draws 1 400 W, a curling iron draws 850 W, and an electric light fixture draws 600 W. If all three of these appliances are operating in parallel on a 120-V circuit, what is the total current drawn? a. 7.92 A b. 23.8 A c. 2.34 A d. 30.8 A e. 11.7 A

b

A proton, mass 1.67 × 10−27 kg and charge +1.6 × 10−19 C, moves in a circular orbit perpendicular to a uniform magnetic field of 0.70 T. Find the time for the proton to make one complete circular orbit. a. 3.0 × 10−8 s b. 9.4 × 10−8 s c. 4.7 × 10−8 s d. 1.5 × 10−8 s e. Not enough information is given.

b

A series RC circuit, which is made from a battery, a switch, a resistor, and a 3.45-μF capacitor, has a time constant of 12.0 ms. If an additional 12.0-μF is added in series to the 3.45-μF capacitor, what is the resulting time constant? a. 41.7 ms b. 9.32 ms c. 1.30 ms d. 15.5 ms e. This cannot be found without the value of the resistance being given.

b

An 8.00-Ω resistor is connected to a battery with internal resistance of 0.250 Ω. If the terminal voltage across the battery is 9.00 V, what is the battery's emf? a. 8.72 V b. 9.28 V c. 0.281 V d. 0.222 V

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 circuits involving resistors and capacitors the quantity 63% often arises. Which of the following is equivalent to this quantity? a. e -2 b. 1 - e -1 c. 1 - e -2 d. e -1

b

Replacing a wire resistor with another of the same material and length but with two times the diameter will have the effect of changing the resistance by what factor? a. 1/2 b. 1/4 c. 2 d. 4

b

The charge flowing through a light bulb attached to a 12.0-V battery in 10.0 s is 35.0 C. What is the total energy delivered to the filament during this process? a. 12.0 J b. 420 J c. 4 200 J d. 120 J e. 770 J

b

The current in a long wire creates a magnetic field in the region around the wire. How is the strength of the field at distance r from the wire center related to the magnitude of the field? a. field directly proportional to r b. field inversely proportional to r c. field directly proportional to r 2 d. field inversely proportional to r 2

b

The magnetic pole of the Earth nearest the geographic North Pole corresponds to which of the following? a. a magnetic north pole b. a magnetic south pole c. a magnetic arctic pole d. a magnetic Antarctic pole

b

Three resistors, each of different value, are used in a circuit with a power source supplying 12 volts. For which of the following resistor combinations is the total power supplied the greatest? a. all three resistors in series b. all three resistors in parallel c. two of the resistors in parallel with the third resistor in series with the parallel pair d. This cannot be found until it is known which resistor is in series with the parallel pair

b

Two resistors of values 7.0 and 15.0 Ω are connected in parallel. This combination in turn is hooked in series with a 3.0-Ω resistor and a 22.3-V battery. What is the current in the 3.0-Ω resistor? a. 22 A b. 2.9 A c. 0.89 A d. 7.4 A e. 170 A

b

Which two resistors are in parallel with each other? a. R and R4 b. R2 and R3 c. R2 and R4 d. R and R1

b

. A proton and a deuteron are moving with equal velocities perpendicular to a uniform magnetic field. A deuteron has the same charge as the proton but has twice its mass. The ratio of the acceleration of the proton to that of the deuteron is: a. 0.5. b. 1. c. 2. d. There is no acceleration in this case.

c

A 1 000-V battery, a 3 000-Ω resistor, and a 0.50-μF capacitor are connected in series with a switch. The time constant for such a circuit, designated by the Greek letter, τ, is defined as the time that the capacitor takes to charge to 63% of its capacity after the switch is closed. What is the current in the circuit at a time interval of τ seconds after the switch has been closed? a. 0.14 A b. 0.21 A c. 0.12 A d. 0.32 A

c

A 10-V-emf battery is connected in series with the following: a 2-µF capacitor, a 2.0-Ω resistor, an ammeter, and a switch, initially open; a voltmeter is connected in parallel across the capacitor. At the instant the switch is closed, what are the current and capacitor voltage readings, respectively? a. zero A, 10 V b. zero A, zero V c. 5 A, zero V d. 5 A, 10 V

c

A 114-m-long wire carrying a current of 3.3 A will be accompanied by a magnetic field of what strength at a distance of 0.026 m from the wire? (magnetic permeability in empty space µ0 = 4π × 10−7 T⋅m/A) a. 8.0E-5 T b. 1.6E-5 T c. 2.5E-5 T d. 1.3E-5 T e. zero

c

A certain material is in a room at 29°C. If the absolute temperature (K) of the material is tripled, its resistance doubles. (Water freezes at 273 K.) What is the value for α, the temperature coefficient of resistivity? a. 0.017 (°C)−1 b. 0.0033 (°C)−1 c. 0.0017 (°C)−1 d. 6.04 (°C)−1

c

A copper wire of length 25.0 cm is in a magnetic field of 0.38 T. If it has a mass of 11.0 g, what is the minimum current through the wire that would cause a magnetic force equal to its weight? a. 12 A b. 0.45 A c. 1.1 A d. 3.0 A e. 0.71 A

c

A current in a solenoid with N turns creates a magnetic field at the center of that loop. The field strength is directly proportional to: a. number of turns in the loop. b. current strength. c. Both choices A and B are valid. d. None of the above are valid.

c

A proton is released such that it has an initial speed of 4.8 × 10 5 m/s from left to right across the page. A magnetic field of 1.2 T is present at an angle of 25° to the horizontal direction (or positive x axis). What is the magnitude of the force experienced by the proton? (qp = 1.6 × 10 −19 C) a. 6.2E-33 N b. 4.3E-33 N c. 3.9E-14 N d. 1.2E-14 N e. 8.4E-14 N

c

A solar panel measures 60 cm × 59 cm. In direct sunlight, the panel delivers 3.1 A at 12 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. 21% b. 16% c. 11% d. 5% e. 8%

c

A superconducting solenoid is to be designed to generate a magnetic field of 6.20 T. If the solenoid winding has 904 turns/m, what is the required current? (µ0 = 4π × 10−7 T⋅m/A) a. 2.7E+3 A b. 1.7E+3 A c. 5.5E+3 A d. 8.7E+2 A e. 1.4E+2 A

c

At the Fermilab accelerator in Weston, Illinois, singly-charged ions with momentum 4.0 × 10 −16 kg⋅m/s are held in a circular orbit of radius 1.00 km by an upward magnetic field. What B field must be used to maintain the ions in this orbit? (qion = 1.60 × 10 −19 C) a. 6.4 T b. 2.6 T c. 2.5 T d. 0.16 T e. 0.40 T

c

Electrical charges and magnetic poles have many similarities, but one difference is: a. opposite magnetic poles repel. b. one magnetic pole cannot create magnetic poles in other materials. c. a magnetic pole cannot be isolated. d. magnetic poles do not produce magnetic fields.

c

If a proton is released at the equator and falls toward the Earth under the influence of gravity, the magnetic force on the proton will be toward the: a. north. b. south. c. east. d. west.

c

Resistors of values R1 = 6.00 Ω, R2 = 4.00 Ω, R3 = 10.0 Ω, and R4 = 7.00 Ω are combined as shown. What is the equivalent resistance for this combination? a. 2.30 Ω b. 3.00 Ω c. 10.7 Ω d. 27.0 Ω

c

The two ends of a 2.00-Ω resistor are connected to a 8.40-V battery. What is the current through the resistor? a. 16.8 A b. 0.238 A c. 4.20 A d. 10.4 A e. 6.40 A

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

Three resistors connected in parallel have the individual voltages labeled ΔV1, ΔV2 and ΔV3, respectively. Which of the following expresses the total voltage ΔVT across the three resistors when connected in this manner? a. ΔVT = ΔV1 + ΔV2 + ΔV3 b. ΔVT = (1/ΔV1 + 1/ΔV2 + 1/ΔV3) c. ΔVT = ΔV1 = ΔV2 = ΔV3 d. ΔVT = (1/ΔV1 + 1/ΔV2 + 1/ΔV3) −1

c

To find the equivalent resistance of the combination of resistors shown below by the method of possibly repeated applications of combining resistors in series and /or combining resistors in parallel, the first step would be which of the following? a. Combine the 6.0 Ω and the 7.0 Ω in series. b. Combine either the 4.0 Ω and the 10.0 Ω in parallel or the 7.0 Ω and the 10.0 Ω in parallel first, since either could be the first step with the other the second step. c. Combine the 4.0 Ω and the 10.0 Ω in series. d. Combine the 6.0 Ω, the 4.0 Ω, and the 10.0 Ω in series in a single step.

c

Two 1.5-V batteries are inserted into a flashlight. The batteries are in series, with their positive terminals in the same direction. One battery has an internal resistance of 0.15 Ω, and the other has an internal resistance of 0.20 Ω. When the switch is closed, a current of 0.56 A passes through the flashlight bulb. 33. What is the resistance of the bulb? a. 0.077 Ω b. 1.6 Ω c. 5.0 Ω d. 5.4 Ω

c

Two long parallel wires 12.0 cm apart carry currents of 2.5 A and 2.8 A in the same direction. Is there any point between the two wires where the magnetic field is zero? a. yes, midway between the wires b. yes, 6.3 cm from the 2.5-A wire c. yes, 5.7 cm from the 2.5-A wire d. yes, 5.7 cm from the 2.8-A wire e. no.

c

What is the current through the 8.0-Ω resistor? a. 1.0 A b. 0.50 A c. 1.5 A d. 2.0 A

c

What is the maximum number of 70-W light bulbs you can connect in parallel in a 120-V home circuit without tripping the 30-A circuit breaker? a. 17 b. 25 c. 51 d. 38 e. 63

c

When a 26.0-Ω resistor is connected across a 4.50-V battery, a current of 480 mA flows. What is the resulting terminal voltage of the battery? a. 29.2 V b. 6.72 V c. 12.5 V d. 54.2 V e. 0.538 V

c

When a light bulb is turned on, its resistance increases until it reaches operating temperature. What happens to the current in the bulb as it is warming up? a. It stays constant. b. It increases. c. It decreases. d. It increases at first and then decreases.

c

A certain capacitor is charged to 10 V and then, at t = 0, allowed to discharge through a certain resistor. There will be a certain time before the voltage across the capacitor reaches 5 V. This time can be decreased for this circuit by increasing: a. the size of the capacitor. b. the size of the resistor. c. the size of the time constant. d. None of the above.

d

A high-voltage transmission line of diameter 2.5 cm and length 250 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. 15.7 × 102 s b. 15.7 × 104 s c. 15.7 × 106 s d. 15.7 × 108 s e. 15.7 × 10-10 s

d

A proton is circling the Earth above the magnetic equator, where Earth's magnetic field is directed horizontally north and has a magnitude of 4.00 × 10-8 T. If the proton is moving at a speed of 2.8 × 107 m/s, how far above the surface of the Earth is the proton? Earth's radius is 6.38 × 106 m, a proton's mass is 1.67 × 10-27 kg, and its charge is 1.6 × 10-19 C. a. 7.31E+6 m b. 1.37E+7 m c. 1.07E+8 m d. 9.26E+5 m

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 23/2. d. It increases by a factor of 21/2.

d

A stationary positive charge +Q is located in a magnetic field B, which is directed toward the right as indicated. The direction of the magnetic force on Q is a. toward the right. b. up. c. down. d. There is no magnetic force

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

In the circuit segment shown if I = 7 mA and Q = 50 µC, what is the potential difference, VA - VB? a. −40 V b. +40 V c. +20 V d. -20 V

d

The heating coil of a hot water heater has a resistance of 19 Ω and operates at 300 V. How long a time is required to raise the temperature of 115 kg of water from 18°C to 75°C? (The specific heat for water = 103 cal/kg⋅°C and 1.0 cal = 4.186 J). a. 1.3 h b. 2.1 h c. 0.51 h d. 1.6 h e. 0.45 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 an 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

Three resistors with values R1, R2 and R3, respectively, are connected in parallel. Which of the following expresses the total resistance, RT, of the three resistors when connected in parallel? a. RT = R1 + R2 + R3 b. RT = (1/R1 + 1/R2 + 1/R3) c. RT = R1 = R2 = R3 d. RT = (1/R1 + 1/R2 + 1/R3) −1

d

Two insulated current-carrying straight wires of equal length are arranged in the lab so that Wire A carries a current northward and Wire B carries a current eastward, the wires crossing at their midpoints separated only by their insulation. Which of the following statements are true? a. The net force on Wire B is southward. b. The net force on Wire A is westward. c. There are no forces in this situation. d. There are forces, but the net force on each wire is zero

d

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

d

Which statement regarding the force of a magnetic field on a current-carrying wire is true? a. The magnetic force on the wire is independent of the direction of the current. b. The force on the wire is directed parallel to both the wire and the magnetic field. c. The force takes its largest value when the magnetic field is parallel to the wire. d. All of the above statements are false

d