Problem set 7, 8, 9, 10 (test 2)
The function of the electric motor is which one of the following conversion processes?
electrical energy to mechanical
If the induced current in a wire loop were such that the flux it produces were in the same direction as the change in external flux causing the current, which of the following conservation laws would end up being violated?
energy
The back emf in an electric motor is its maximum value under which condition?
motor speed is a maximum
In the United States, the value of ω for commercially generated power is ________ in SI units.
377
A soft magnetic material has which property?
It is easy to magnetize
Resistors of values 7 Ω, 15 Ω, and 25 Ω are connected in parallel across a fresh battery. Which resistor dissipates the greatest power?
7 Ω
The force on a charged particle created by its motion in a magnetic field is maximum at what angle between the particle velocity and field?
90 degrees
In applications where electrical shocks may be more likely, such as around water in kitchens and bathrooms, special outlets called GFI's are used. What does GFI stand for?
Ground-fault interrupter
Three resistors connected in parallel each carry currents labeled I1, I2 and I3. Which of the following expresses the value of the total current IT in the combined system?
IT = I1 + I2 + I3
Three resistors connected in series each carry currents labeled I1, I2 and I3. Which of the following expresses the value of the total current IT in the system made up of the three resistors in series?
IT = I1 = I2 = I3
The magnetic pole of the Earth nearest the geographic North Pole corresponds to which of the following?
a magnetic South Pole
A straight wire of length is oriented east-west and is in a magnetic field B pointing north. The wire is moving downward at a constant speed v. Which end of the rod is positively charged?
the east end
A coil is placed in a changing magnetic field and an emf is induced. What happens to the induced emf if the rate of change of magnetic field quadruples?
the emf quadruples
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 magnetic force on the proton to that on the deuteron is:
1
Different units can be used to measure the same physical quantity, differing only by some multiplicative factor. The cgs unit for magnetic field, the gauss, is equal to ________ tesla.
10^-4
A coil is rotated in a magnetic field generating an emf given by emf = NBAω sin θ, where θ = ωt. At which angle θ is the emf one-third of its maximum value?
19.5 degrees
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:
2
When a voltage is generated by rotating a coil in a magnetic field at a constant rate, the period of the voltage equals the time that it takes for the coil to rotate through ________ radians.
2π
The principle or law that says "an induced emf in a circuit loop produces a current whose magnetic field opposes further change of magnetic flux" is credited to
Lens
Household circuits are wired in
Parallel
If R1 < R2 < R3, and if these resistors are connected in parallel in a circuit, which one has the highest current?
R1
If R1 < R2 < R3, and if these resistors are connected in series in a circuit, which one dissipates the greatest power?
R3
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?
RT = (1/R1 + 1/R2 + 1/R3)^−1
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?
RT =R1 +R2 +R3
A battery has an internal resistance Rint. Connecting a wire of negligible resistance between the terminals of the battery draws the maximum current Imax from this battery. If instead a resistor is connected between the terminals, a smaller current is produced. What value resistor placed across the terminals would result in the current produced to be half of Imax?
Rint
Which of the following locations has the smallest (in magnitude) magnetic declination?
The South Carolina- Georgia border
Three resistors, each with resistance R1, are in series in a circuit. They are replaced by one equivalent resistor, R. Comparing this resistor to the first resistor of the initial circuit, which of the following is true?
The current through R equals the current through R1
Three resistors, each with resistance R1, are in parallel in a circuit. They are replaced by one equivalent resistor, R. Compare this resistor to the first resistor of the initial circuit. Which of the following statements is true?
The voltage across R equals the voltage across R1
If one doubles the emfs in a circuit and doubles the resistances in the circuit at the same time, what happens to the currents through the resistors? Assume there are only emfs and resistors in the circuit.
They stay the same
The units T·m2/s are equivalent to
V
If a charged particle is moving in a uniform magnetic field, its path can be
a straight line, a circle, and a helix
Two parallel conductors are carrying currents in the same direction. The currents are non-zero and not necessarily equal. The magnitude of the magnetic field midway between them is 40 μT. If one of the currents then has its direction reversed, what is the resulting magnitude of the magnetic field midway between them?
a value greater than 40 μT
Two parallel conductors are carrying currents in the opposite direction. The currents are non-zero and not necessarily equal. The magnitude of the magnetic field midway between them is 40 μT. If one of the currents then has its direction reversed, what is the resulting magnitude of the magnetic field midway between them?
a value less than 40 μT
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?
all three resistors in parallel
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
east
The term magnetic declination refers to which of the following?
angle between directions to true north and magnetic north
Solenoid No1 has a length L, cross-sectional area A, and N turns. Solenoid No2 has a length 2L, cross- sectional area 2A, and 2N turns. Which solenoid has the greater magnetic field at its center when equal currents are going through them?
both have the same magnetic field
Square coil A has 2N turns and sides of length L, and square coil B has N turns and sides of length 2L. When a current of 2 amps is sent through coil A and a current of 1 amp is sent through coil B, which coil has the greater magnetic moment?
both have the same magnetic moment
The dip angle is
close to or at zero near the equator
The basic function of an electromotive force in a circuit is to do which of the following?
convert some other form of energy into electrical energy
The force exerted on a current-carrying wire located in an external magnetic field is directly proportional to which of the following?
current and field strength
The magnetic field of the Earth is believed responsible for which of the following?
deflection of charged cosmic rays
A circular current loop is placed in an external magnetic field. How is the torque related to the radius of the loop?
directly proportional to radius squared
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?
field inversely proportional to r
A current in a long, straight wire produces a magnetic field. The magnetic field lines
form circles that go around the wire
The right-hand rule allows one to find a property of the interaction of a magnetic field with a charged particle. The right-hand rule applied to moving charges:
gives the direction of the force on a charge moving in a magnetic field
A double loop of wire (making 2 turns) is in the x-y plane centered at the origin. A uniform magnetic field is increasing at a constant rate in the positive z-direction. Viewed from the positive z-axis, in which direction is the induced magnetic field in the loop?
in the negative z-direction
A proton is released such that its initial velocity is from right to left across this page. The proton's path, however, is deflected in a direction toward the bottom edge of the page due to the presence of a uniform magnetic field. What is the direction of this field?
into the page
The magnet moving past an object will produce eddy currents in the object if the object
is a conductor
The "back emf" of a motor refers to a source of voltage that
is biggest when the motor turns fastest
A bar magnet is falling through a loop of wire with constant velocity. The south pole enters first. As the magnet leaves the wire, the induced current (as viewed from above)
is counterclockwise
Household 120-V outlets are made to accept three-pronged plugs. One of the prongs attaches to the "live" wire at 120 V, and another attaches to the "neutral" wire that is connected to ground. What is the round third prong for?
it connects the case of the appliance directly to ground for safety purposes
A circular loop of wire has its radius reduced in half in time Δt. A uniform magnetic field is at an angle of 60° to the plane of the coil, and the magnetic field doubles its intensity in the same time interval Δt. During this interval, what happens to the flux through the coil?
it decreases
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?
it decreases
When an electromagnet has an iron core inserted, what happens to the strength of the magnet?
it increases
A changing current in one loop of wire can induce a current in another loop of wire even though the wires do not touch. The induced current appears almost instantaneously for loops close together. How can this energy exchange happen?
it is done via an electromagnetic wave
A straight wire lies along the y-axis initially carrying a current of 10 A in the positive y-direction. The current decreases and reverses to 10 A in the negative y-direction, the change in current happening at a uniform rate. In the 1st quadrant a square conducting coil has 2 sides parallel to the y-axis and the other 2 sides parallel to the x- axis. The side of the coil nearest and parallel to the straight wire is at a distance equal to the length of one of the sides of the square. As the current is going from 10 A in one direction to 10 A in the other, in which direction is the induced current in this side of the square coil nearest to the straight wire?
it is in the positive y-direction
If the rotating coil used in a generator has its radius doubled, what change results in the amplitude of the voltage produced?
it quadruples
A bar magnet is held above the center of a conducting ring in the horizontal plane. The magnet is dropped so it falls lengthwise toward the center of the ring. Will the falling magnet be attracted toward the ring or be repelled by the ring due to the magnetic interaction of the magnet and the ring?
it will be repelled
Electrical charges and magnetic poles have many similarities, but one difference is
magnetic poles can not be isolated
The basic function of the electric generator is which of the following conversion processes?
mechanical energy to electrical
When a magnetic field causes a charged particle to move in a circular path, the only quantity listed below which the magnetic force changes significantly as the particle goes around in a circle is the particle's
momentum
There is a magnetic force on a particle. It is possible that the particle is
not part of a wire
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
number of turns in the loops and current strength
According to Lenz's law the direction of an induced current in a conductor will be that which tends to produce which of the following effects?
oppose the effect which produces it
A proton moves across the Earth's equator in a northeasterly direction. At this point the Earth's magnetic field has a direction due north and is parallel to the surface. What is the direction of the force acting on the proton at this instant?
out of the earths surface
If an electron is released at the equator and falls toward the Earth under the influence of gravity, the magnetic force on the electron will be toward the:
west
The direction of the force on a current carrying wire located in an external magnetic field is which of the following?
perpendicular to the current and field
A square loop of wire has a current going around it. The magnetic field from the current on each side of the loop results in a magnetic force on the wire on the opposite side. Each resulting magnetic force on the opposite side wire
pushes outward on the loop
Two parallel wires carry a current I in the same direction. Midway between these wires is a third wire, also parallel to the other two, which carries a current 0.5 I, but in the direction opposite from the first two wires. In which direction are the net forces on the outer wires?
since the net forces are zero, there is no direction
The path of a charged particle moving parallel to a uniform magnetic field will be a
straight line
A bar magnet is attached solidly to a frictionless surface and its length is aligned with the x axis. To the right of the first magnet a short distance away is a second bar magnet with its center placed on the x axis and its length perpendicular to the x axis. The second magnet is free to move. Once placed in position at rest, which best describes the initial motion of the second magnet?
the agent will start to rotate
In a mass spectrometer, an ion will have a smaller radius for its circular path if
the charge is greater
Three loops of wire, one circular, one rectangular, and one square, are made from identical lengths of wire. If the loops are in the same increasing magnetic field perpendicular to the plane of the coils, which loop has the greatest induced emf?
the circular one
A current in a solenoid coil creates a magnetic field inside that coil. The field strength is directly proportional to
the current
Three identical bar magnets, each 2 cm long, are placed in a straight line on a frictionless surface with their ends equally spaced with 2 cm between ends. They are constrained so they cannot rotate, but they can slide along the straight line. The left magnet has its north pole nearest the north pole of the middle magnet, and the right magnet has its south pole toward the middle magnet's south pole. Once they are in place and released from rest, which of the following best describes the motion of the magnets?
the end magnets will move away from the center magnet
The operation of an electric motor depends on which of the following effects?
the force acting on a current-carrying wire in a magnetic field
Electricity may be generated by rotating a loop of wire between the poles of a magnet. The induced current is greatest when
the plane of the loop is parallel to the magnetic field
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?
the square coil
Consider two long, straight parallel wires, each carrying a current I. If the currents are flowing in opposite directions:
the two wires will repel each other
A sensitive ammeter is connected to a wire loop and placed within the magnetic field of a strong horseshoe magnet. The ammeter shows a deflection when
the wire is moved perpendicularly to the field
Assume that a uniform magnetic field is directed into this page. If an electron is released with an initial velocity directed from the bottom edge to the top edge of the page, which of the following describes the direction of the resultant force acting on the electron?
to the right
Three resistors connected in series have individual voltages labeled ΔV1, ΔV2 and ΔV3, respectively. Which of the following expresses the value of the total voltage ΔVT taken over the three resistors together?
ΔVT =ΔV1 +ΔV2 +ΔV3
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?
ΔVT =ΔV1 =ΔV2 =ΔV3
