Physics 2 Final Multiple Choice
When a magnet is thrust into a coil of wire, the coil tends to
. repel the magnet as it enters.
Wire #1 (length L) forms a one-turn loop, and a bar magnet is dropped through. Wire #2 (length 2L) forms a two-turn loop, and the same magnet is dropped through. Compare the magnitude of the induced voltages in these two cases.
2) V1< V2 Faraday's law: depends on N (number of loops), so the induced emf is twice as large in the wire with two loops.
A positive charge enters a uniform magnetic field as shown. What is the direction of the magnetic force? (right)
4) upward Using the right-hand rule, you can see that the magnetic force is directed upward. Remember that the magnetic force must be perpendicular to BOTH the B field and the velocity.
Transformers use ac so there will be the required
A. change in magnetic field for operation.
A voltage will be induced in a wire loop when the magnetic field within that loop:
A. changes.
Magnetic field lines about a current-carrying wire
A. circle the wire in closed loops.
The presence of a magnetic field can be detected by using a
A. compass B. magnet C. iron filings D. all of these ALL OF THESE
When you break a bar magnet in half you produce:
A. two bar magnets
A magnetic force cannot act on a electron when it:
At rest Moves parallel to the magnetic field lines
A beam of atoms enters a magnetic field region. What path will the atoms follow?
Atoms are neutral objects whose net charge is zero. Thus, they do not experience a magnetic force.
Like kinds of magnetic poles repel while unlike kinds of magnetic poles:
B. attract
A device that transforms mechanical energy into electrical energy is a
B. generator
Voltage produced by a generator alternates because
B. the changing magnetic field that produces it alternates.
A weak and strong magnet repel each other. The greater repelling force is by the
Both the same
Imagine you are an alien from another planet with infrared eyes. What do you see when you look around the room?
Bright spots where the bodies are and dark elsewhere
Surrounding every moving electron is
C. both of these
A step-up transformer increases
D. neither of these
The force on an electron moving in a magnetic field will be the largest when its direction is
D. perpendicular to the magnetic field direction.
Does this depend on your distance from the mirror?
No
Which resistor has the greatest current going through it? Assume that all the resistors are equal.
R5 The same current must flow through left and right combinations of resistors. On the LEFT, the current splits equally, so I1 = I2. On the RIGHT, more current will go through R5 than R3 + R4 since the branch containing R5 has less resistance
In this list, which electromagnetic wave has the longest wavelength?
Radio waves
Polarization is a property of what kind of waves?
Transverse
What is the equivalent capacitance, Ceq , of the combination below?
a) Ceq = 3/2C The 2 equal capacitors in series add up as inverses, giving 1/2C. These are parallel to the first one, which add up directly. Thus, the total equivalent capacitance is 3/2C.
Moving electric charges will interact with
an electric field and a magnetic field
An object is placed 0.5 m away from a concave mirror of focal length +1.0 m. The image formed by the mirror is
virtual and larger than the object
Assume that the voltage of the battery is 9 V and that the three resistors are identical. What is the potential difference across each resistor?
) 3 V Since the resistors are all equal, the voltage will drop evenly across the 3 resistors, with 1/3 of 9 V across each one. So we get a 3 V drop across each.
A wire loop is being pulled through a uniform magnetic field that suddenly ends. What is the direction of the induced current?
1) clockwise The B field into the page is disappearing in the loop, so it must be compensated by an induced flux also into the page. This can be accomplished by an induced current in the clockwise direction in the wire loop
A wire loop is being pulled away from a current-carrying wire. What is the direction of the induced current in the loop?
1) clockwise The magnetic flux is into the page on the right side of the wire and decreasing due to the fact that the loop is being pulled away. By Lenz's law, the induced B field will oppose this decrease. Thus, the new B field points into the page, which requires an induced clockwise current to produce such a B field.
If a coil is shrinking in a magnetic field pointing into the page, in what direction is the induced current?
1) clockwise The magnetic flux through the loop is decreasing, so the induced B field must try to reinforce it and therefore points in the same direction — into the page. According to the right-hand rule, an induced clockwise current will generate a magnetic field into the page
In order to change the magnetic flux through the loop, what would you have to do?
1) drop the magnet 2) move the magnet upward 3) move the magnet sideways 4) only (1) and (2) 5) all of the above answer: all of the above Moving the magnet in any direction would change the magnetic field through the loop and thus the magnetic flux.
A generator has a coil of wire rotating in a magnetic field. If the rotation rate increases, how is the maximum output voltage of the generator affected?
1) increases The maximum voltage is the leading term that multiplies sin wt and is given by e0 = NBAw. Therefore, if w increases, then e0 must increase as well.
A bar magnet is held above the floor and dropped. In 1, there is nothing between the magnet and the floor. In 2, the magnet falls through a copper loop. How will the magnet in case 2 fall in comparison to case 1?
1) it will fall slower When the magnet is falling from above the loop in 2, the induced current will produce a north pole on top of the loop, which repels the magnet. When the magnet is below the loop, the induced current will produce a north pole on the bottom of the loop, which attracts the south pole of the magnet.
In order to change the magnetic flux through the loop, what would you have to do?
1) tilt the loop 2) change the loop area 3) use thicker wires 4) only (1) and (2) 5) all of the above answer: 4 Since F = BA cos q , changing the area or tilting the loop (which varies the projected area) would change the magnetic flux through the loop.
Two straight wires run parallel to each other, each carrying a current in the direction shown below. The two wires experience a force in which direction?
1) toward each other The current in each wire produces a magnetic field that is felt by the current of the other wire. Using the right-hand rule, we find that each wire experiences a force toward the other wire (i.e., an attractive force) when the currents are parallel (as shown).
A concave mirror with a radius of curvature of 20 cm has a focal length of
10 cm
A beam of atoms enters a magnetic field region. What path will the atoms follow?
2 Atoms are neutral objects whose net charge is zero. Thus, they do not experience a magnetic force.
Two particles of the same mass enter a magnetic field with the same speed and follow the paths shown. Which particle has the bigger charge?
2 The relevant equation for us is: . According to this equation, the bigger the charge, the smaller the radius R= (mv/(qB)
A conducting rod slides on a conducting track in a constant B field directed into the page. What is the direction of the induced current?
2) counterclockwise
If a coil is rotated as shown, in a magnetic field pointing to the left, in what direction is the induced current?
2) counterclockwise As the coil is rotated into the B field, the magnetic flux through it increases. According to Lenz's law, the induced B field has to oppose this increase, thus the new B field points to the right. An induced counterclockwise current produces just such a B field.
What is the direction of the induced current if the B field suddenly increases while the loop is in the region?
2) counterclockwise The increasing B field into the page must be countered by an induced flux out of the page. This can be accomplished by induced current in the counterclockwise direction in the wire loop.
If a north pole moves toward the loop from above the page, in what direction is the induced current?
2) counterclockwise The magnetic field of the moving bar magnet is pointing into the page and getting larger as the magnet moves closer to the loop. Thus the induced magnetic field has to point out of the page. A counterclockwise induced current will give just such an induced magnetic field.
A positive charge enters a uniform magnetic field as shown. What is the direction of the magnetic force? (field to the right, force going up)
2) into the page Using the right-hand rule, you can see that the magnetic force is directed into the page. Remember that the magnetic force must be perpendicular to BOTH the B field and the velocity.
A positive charge moves parallel to a wire. If a current is suddenly turned on, which direction will the force act?
3) + x Using the right-hand rule to determine the magnetic field produced by the wire, we find that at the position of the charge +q (to the left of the wire) the B field points out of the page. Applying the right-hand rule again for the magnetic force on the charge, we find that +q experiences a force in the +x direction
What is the voltage across the lightbulb?
3) 120 V The first transformer has a 2:1 ratio of turns, so the voltage doubles. But the second transformer has a 1:2 ratio, so the voltage is halved again. Therefore, the end result is the same as the original voltage.
If the currents in these wires have the same magnitude, but opposite directions, what is the direction of the magnetic field at point P?
3) direction 3 Using the right-hand rule, we can sketch the B fields due to the two currents. Adding them up as vectors gives a total magnetic field pointing downward.
A proton enters a uniform magnetic field that is perpendicular to the proton's velocity. What happens to the kinetic energy of the proton?
3) it stays the same The velocity of the proton changes direction, but the magnitude (speed) doesn't change. Thus the kinetic energy stays the same.
A wire loop is being pulled through a uniform magnetic field. What is the direction of the induced current?
3) no induced current Since the magnetic field is uniform, the magnetic flux through the loop is not changing. Thus no current is induced.
If a north pole moves toward the loop in the plane of the page, in what direction is the induced current?
3) no induced current Since the magnet is moving parallel to the loop, there is no magnetic flux through the loop. Thus the induced current is zero.
What is the induced current if the wire loop moves in the direction of the yellow arrow? (yellow arrow down)
3) no induced current The magnetic flux through the loop is not changing as it moves parallel to the wire. Therefore, there is no induced current.
If there is a current in the loop in the direction shown, the loop will:
3) rotate clockwise Look at the north pole: here the magnetic field points to the right and the current points out of the page. The right-hand rule says that the force must point up. At the south pole, the same logic leads to a downward force. Thus the loop rotates clockwise
If there is induced current, doesn't that cost energy? Where would that energy come from in case 2?
3) there is less KE in case 2 In both cases, the magnet starts with the same initial gravitational PE. In case 1, all the gravitational PE has been converted into kinetic energy. In case 2, we know the magnet falls slower, thus there is less KE. The difference in energy goes into making the induced current.
A positive charge enters a uniform magnetic field as shown. What is the direction of the magnetic force?
3) zero The charge is moving parallel to the magnetic field, so it does not experience any magnetic force. Remember that the magnetic force is given by: F = vB sin q.
A rectangular current loop is in a uniform magnetic field. What is the direction of the net force on the loop?
3) zero Using the right-hand rule, we find that each of the four wire segments will experience a force outward from the center of the loop. Thus, the forces of the opposing segments cancel, so the net force is zero
A horizontal wire carries a current and is in a vertical magnetic field. What is the direction of the force on the wire?
3) zero current is parallel to magnetic field lines
Wire #1 (length L) forms a one-turn loop, and a bar magnet is dropped through. Wire #2 (length 2L) forms a two-turn loop, and the same magnet is dropped through. Compare the magnitude of the induced currents in these two cases.
3)I1=I2 not equal 0 Faraday's law: says that the induced emf is twice as large in the wire with 2 loops. The current is given by Ohm's law: I = V/R. Since wire #2 is twice as long as wire #1, it has twice the resistance, so the current in both wires is the same.
Given that the intermediate current is 1 A, what is the current through the lightbulb?
4) 2 A Power in = Power out 240 V 1 A = 120 V ??? The unknown current is 2
In what direction would a B field have to point for a beam of electrons moving to the right to go undeflected through a region where there is a uniform electric field pointing vertically upward?
4) out of the page Without a B field, the electrons feel an electric force downward. In order to compensate, the magnetic force has to point upward. Using the right-hand rule and the fact that the electrons are negatively charged leads to a B field pointing out of the page.
A wire loop is in a uniform magnetic field. Current flows in the wire loop, as shown. What does the loop do?
4) rotates There is no magnetic force on the top and bottom legs, since they are parallel to the B field. However, the magnetic force on the right side is into the page, and the magnetic force on the left side is out of the page. Therefore, the entire loop will tend to rotate.
A 6 V battery is connected to one side of a transformer. Compared to the voltage drop across coil A, the voltage across coil B is:
4) zero The voltage across B is zero. Only a changing magnetic flux induces an emf. Batteries can provide only dc current.
A proton beam enters a magnetic field region as shown below. What is the direction of the magnetic field B?
5) - z (into page) The picture shows the force acting in the +y direction. Applying the right-hand rule leads to a B field that points into the page. The B field must be out of the plane because B v and B F.
A horizontal wire carries a current and is in a vertical magnetic field. What is the direction of the force on the wire?
5) out of the page Using the right-hand rule, we see that the magnetic force must point out of the page. Since F must be perpendicular to both I and B, you should realize that F cannot be in the plane of the page at all.
A positive charge enters a uniform magnetic field as shown. What is the direction of the magnetic force?` (up)
5) to the left Using the right-hand rule, you can see that the magnetic force is directed to the left. Remember that the magnetic force must be perpendicular to BOTH the B field and the velocity.
When there is a change in the magnetic field in a closed loop of wire,
A. current is made to flow in the loop of wire. B. a voltage is induced in the wire. C. electromagnetic induction occurs. D. all of these E. none of these ALL OF THESE
Voltage can be induced in a wire by
A. moving a magnet near the wire. B. changing the current in a nearby wire. C. moving the wire near a magnet. D. all of these E. none of these ALL OF THESE
The resistance you feel when pushing a magnet into a coil involves:
A. repulsion by the magnetic field you produce
A step-up transformer in an electrical circuit can step up:
A. voltage
What is the fundamental source of electromagnetic waves?
Accelerating electrons
Your friend says that, if you crank the shaft of a dc motor manually, the motor becomes a dc generator. Do you agree or disagree?:
B. Agree; Your friend is right on!
Power is transmitted at high voltages because the corresponding current in the wires is:
B. low so that overheating of the wires is minimized
Which is the correct way to light the lightbulb with the battery?
C Current can flow only if there is a continuous connection from the negative terminal through the bulb to the positive terminal. This is the case for only Fig. (3).
The source of all magnetism is
C. moving electric charge.
The metal detectors that people walk through at airports operate via
D. Faraday's law.
If your metal car moves over a wide, closed loop of wire embedded in a road surface, will the magnetic field of the Earth within the loop be altered? Will this produce a change of current in the wire?
D. Yes and yes. Traffic lights detect vehicles this way.
You double the voltage across a certain conductor and you observe the current increases three times. What can you conclude
Ohm's law is not obeyed Ohm's law, V = IR, states that the relationship between voltage and current is linear. Thus, for a conductor that obeys Ohm's law, the current must double when you double the voltage
If unpolarized light is incident from the left, in which case will some light get through?
Only the case where there are no adjacent horizontal and vertical polarizers
When you rotate the knob of a light dimmer, what is being changed in the electric circuit?
Power and Current (a and b) The voltage is provided at 120 V from the outside. The light dimmer increases the resistance and therefore decreases the current that flows through the lightbulb.
When watching the Moon over the ocean, you often see a long streak of light on the surface of the water. This occurs because
The ocean is wavy- when water surface changes, the angle of incidence also changes causing different spots on water to reflect the Moon into your eyes
A loop with an AC current produces a changing magnetic field. Two loops have the same area, but one is made of plastic and the other copper. In which of the loops is the induced voltage greater.
Voltage is the same in both
The lightbulbs in the circuits below are identical with the same resistance R. Which circuit produces more light? (brightness power)
a) circuit I In circuit I, the bulbs are in parallel, lowering the total resistance of the circuit. Thus, circuit I will draw a higher current, which leads to more light, because P = IΔV.
Two lightbulbs A and B are connected in series to a constant voltage source. When a wire is connected across B, bulb A will:
a) glow brighter than before Since bulb B is bypassed by the wire, the total resistance of the circuit decreases. This means that the current through bulb A increases.
What happens to the voltage across the resistor R1 when the switch is closed? The voltage will:
a) increase With the switch closed, the addition of R2 to R3 decreases the equivalent resistance, so the current from the battery increases. This will cause an increase in the voltage across R1
Points P and Q are connected to a battery of fixed voltage. As more resistors R are added to the parallel circuit, what happens to the total current in the circuit?
a) increases As we add parallel resistors, the overall resistance of the circuit drops. Since ΔV = IR, and ΔV is held constant by the battery, when resistance decreases, the current must increase.
How does the charge Q1 on the first capacitor (C1) compare to the charge Q2 on the second capacitor (C2)?
b) Q1 > Q2 We already know that the voltage across C1 is 10 V and the voltage across both C2 and C3 is 5 V each. Since Q = CΔV and C is the same for all the capacitors, we have V1 > V2 and therefore Q1 > Q2.
How does the voltage V1 across the first capacitor (C1) compare to the voltage V2 across the second capacitor (C2)?
b) V1 > V2 The voltage across C1 is 10 V. The combined capacitors C2 + C3 are parallel to C1. The voltage across C2 + C3 is also 10 V. Since C2 and C3 are in series, their voltages add. Thus the voltage across C2 and C3 each has to be 5 V, which is less than V1.
What happens to the voltage across the resistor R4 when the switch is closed?
b) decreases We just saw that closing the switch causes an increase in the voltage across R1 (which is VAB). The voltage of the battery is constant, so if VAB increases, then VBC must decrease!
Two space heaters in your living room are operated at 120 V. Heater 1 has twice the resistance of heater 2. Which one will give off more heat?
b) heater 2 Using P = V2 / R, the heater with the smaller resistance will have the larger power output. Thus, heater 2 will give off more heat.
Two lightbulbs operate at 120 V, but one has a power rating of 25 W while the other has a power rating of 100 W. Which one has the greater resistance?
b) the 100 W bulb Since P = V2 / R , the bulb with the lower power rating has to have the higher resistance.
Which of the equations is valid for the circuit below?
c) 2 - I1 - 4 - 2I2 = 0 Eqn. 3 is valid for the left loop: The left battery gives +2V, then there is a drop through a 1Ω resistor with current I1 flowing. Then we go through the middle battery (but from + to - !), which gives -4V. Finally, there is a drop through a 2Ω resistor with current I2.
Current flows through a lightbulb. If a wire is now connected across the bulb, what happens?
c) all the current flows through the wire The current divides based on the ratio of the resistances. If one of the resistances is zero, then ALL of the current will flow through that path
The three lightbulbs in the circuit all have the same resistance of 1 Ω . By how much is the brightness of bulb B greater or smaller than the brightness of bulb A? (brightness power)
d) 1/4 as much We can use P = V2/R to compare the power: PA = (ΔVA)2/RA = (10 V)2/1 Ω = 100 W PB = (ΔVB)2/RB = (5 V)2/1 Ω = 25 W
In the circuit below, what is the current through R1?
d) 2 A The voltage is the same (10 V) across each resistor because they are in parallel. Thus, we can use Ohm's law, ΔV1 = I1R1 to find the current I1 = 2 A.
What is the current in branch P?
d) 6 A The current entering the junction in red is 8 A, so the current leaving must also be 8 A. One exiting branch has 2 A, so the other branch (at P) must have 6 A
In the circuit below, what is the voltage across R1?
d) 8 V The voltage drop across R1 has to be twice as big as the drop across R2. This means that ΔV1 = 8 V and ΔV2 = 4 V. Or else you could find the current I = V/R = (12 V)/(6 Ω) = 2 A, and then use Ohm's law to get voltages.
Two wires, A and B, are made of the same metal and have equal length, but the resistance of wire A is four times the resistance of wire B. How do their diameters compare?
dA = 1/2dB The resistance of wire A is greater because its area is less than wire B. Since area is related to radius (or diameter) squared, the diameter of A must be two times less than the diameter of B.
The lightbulbs in the circuit are identical. When the switch is closed, what happens?
e) nothing changes When the switch is open, the point between the bulbs is at 12 V. But so is the point between the batteries. If there is no potential difference, then no current will flow once the switch is closed!! Thus, nothing changes.
An ammeter A is connected between points a and b in the circuit below, in which the four resistors are identical. The current through the ammeter is:
e) zero Since all resistors are identical, the voltage drops are the same across the upper branch and the lower branch. Thus, the potentials at points a and b are also the same. Therefore, no current flows.
You stand in front of a mirror. How tall does the mirror have to be so that you can see yourself entirely?
half your height
An electromagnet can be made stronger by
increasing the number of turns in a wire, and increasing the current in the coil
The kind of wave produced by a vibrating source is
sound and light
A wire of resistance R is stretched uniformly (keeping its volume constant) until it is twice its original length. What happens to the resistance?
t increases by a factor of 2 Keeping the volume (= area x length) constant means that if the length is doubled, the area is halved. Since , this increases the resistance by a factor of 4.
A loop with an AC current produces a changing magnetic field. Two loops have the same area, but one is made of plastic and the other copper. In which of the loops is the induced current greater?
the copper loopI