PHYS202 Exam 2

A magnetic field increases from 0 to 0.25 T in 1.8s. How many turns of wire are needed in a circular coil 12 cm in diameter to produce an induced emf 6.0

emf = n delta flux/ delta time solve for n

Microwaves travel with the speed of light, c = 3 x 10^8 m/s. At a frequency of 10 GHz these waves cause the water molecules in your burrito to vibrate. What is their wavelength?

3cm (lambda= velocity/frequency)

The moon's internal currents are much weaker; it is much smaller than the earth, has a predominantly solid interior, and spins slowly

Hence the moon's magnetic field is about 104 weaker compare to the earth

electrode beam in cathode ray

If there is no force to deflect the beam, it strikes the center of the screen. • The magnetic force deflects the beam, and creates an image on the screen.

A positive charge enters a uniform magnetic field as shown (into the board). What is the direction of the magnetic force?

to the left (magnetic force must be perpendicular to BOTH the B field and the velocity.)

The Doppler effect has many practical applications

weather radar, speed radar, medical diagnostics, astronomical measurements.

How sound is heard from a radio loudspeaker

- radio loudspeaker is a paper cone that vibrates - air molecules next to the loudspeaker set into vibration produces compressions and rarefactions in air - sound waves reach your ears, setting your eardrums into vibration

What is the voltage across the lightbulb? (plugged into 120V power supply outlet)

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, goes from 120 to 240 to 120 when reaches the lightbulb)

A boat is moored in a fixed location, and waves make it move up and down. If the spacing between wave crests is 20 m and the speed of the waves is 5 m/s, how long does it take the boat to go from the top of a crest to the bottom of a trough ?

2 seconds ( If lambda = 20 m and v = 5 m/s, then T = 4 secs. The time to go from a crest to a trough is only T/2 (half a period), so it takes 2 secs !!)

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.

assume positively charged particle moves in positive x direction and electric field is in the positive y direction. should the magnetic field be in the (a) + z direction (b) - y direction or (c) - z direction, in order to give zero net force?

B should point in + z direction

Ferromagnets are characterized by domains, each have a strong magnetic field, but which are randomly oriented

In the presence of an external magnetic field, the domains align, creating a magnetic field within the material.

Given that the intermediate current is 1 A, what is the current through the lightbulb?

Power in = Power out 240 V x 1 A = 120 V ??? The unknown current is 2 A.

The circulating electrons begin to collide with atmospheric atoms and molecules and ionize it.

Resulting in the emission light knowns as the aurora borealis

Observers A, B, and C listen to a moving source of sound. The location of the wave fronts of the moving source with respect to the observers is shown below. Which of the following is true?

The number of wave fronts hitting observer C per unit time is greatest—thus the observed frequency is highest there.

Do sound waves travel faster in water or in ice?

The speed of sound in ice is nearly 3 times faster than in water. (Speed of sound depends on the inertia of the medium and the restoring force. Because ice and water both consist of water molecules, the inertia is the same for both. However, the force holding the molecules together is greater in ice (because it is a solid), so the restoring force is greater)

lenz's law

an induced current always flows in direction that opposes the change that caused it, a change in field induces a current (magnet moving toward coil induces a field that repels the magnet, moving magnet away from coil induces a field that attracts the magnet)

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:

The voltage across B is zero. Only a changing magnetic flux induces an emf. Batteries can provide only dc current

The earth's magnetic field is caused by currents circulating within its molten, conducting interior.

These currents are stirred by our planet's relatively rapid spin. An equatorial speed of 1,674 km/h or 1,040 mph

When a sound wave passes from air into water, what properties of the wave will change?

Wave speed must change (different (different medium). medium). Frequency does not change (determined by the source). Now, v = fλ and since v has changed and f is constant then λ must also change.

You hear a fire truck with a certain intensity, and you are about 1 mile away. Another person hears the same fire truck with an intensity that is about 10 times less. Roughly, how far is the other person from the fire truck?

about 3 miles (Remember that intensity drops with the inverse square of the distance, so if intensity drops by a factor of 10, the other person must be square root 10 farther away, which is about a factor of 3)

Moving electric charged particles can interact with

and electric and a magnetic field

A weak and strong magnet repel each other. The greater repelling force is by the

both the same (newtons third law)

A wire loop is being pulled through a uniform magnetic field that suddenly ends. What is the direction of the induced current?

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 through a uniform magnetic field that suddenly ends. What is the direction of the induced current?

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)

If a coil is shrinking in a magnetic field pointing into the page, in what direction is the induced current? **

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 righthand rule, an induced clockwise current will generate a magnetic field into the page). X MARKS THE CLOCK

If a coil is rotated as shown, in a magnetic field pointing to the left, in what direction is the induced current?

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?

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?

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.)

motional emf

created in system as rod falls, direction of induced current produces a force that opposed the motion of the rod)

The magnetic force on a moving charged particle can change the particle's

direction (Only an electric force can change the speed of a charged particle. Since the magnetic force acts at right angles to velocity, it can only change the direction of a moving charged particle)

If the currents in these wires have the same magnitude but opposite directions, what is the direction of the magnetic field at point P?

direction 3 (downward in the middle of both opposite currents)

In order to change the magnetic flux through the loop, what would you have to do?

drop the magnet, move the magnet upward, move magnet side to side

In order to change the magnetic flux through the loop, what would you have to do?

drop the magnet, move the magnet upward, move the magnet sideways

If you dip your finger repeatedly onto the surface of still water, you produce waves. The more frequently you dip your finger, the

higher the wave frequency and the shorter the wavelengths

Do sound waves travel faster in water or in ice?

ice (Speed of sound depends on the inertia of the medium and the restoring force, the force holding the molecules together is greater in ice (because it is a solid), so the restoring force is greater)

a rope length L and mass M hangs from ceiling. if the bottom of the rope is given a gentle wiggle a wave will travel to the top of the rope. as the wave travels upward does its speed:

increases (tension increases with height, increasing the speed)

consider a system in which a metal ring is falling out of a region with a magnetic field and into a field free region. according to lenz's law is the induced current in the ring clockwise or counterclockwise?

induced current is counterclockwise

A positive charge enters a uniform magnetic field as shown (to the right). What is the direction of the magnetic force?

into the page

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?

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)

magnetic field shown is due to horizontal current carrying wire. does the current in the wire flow to the left or to the right?

left

Uniform Constant magnetic field. Which loop or loops have magnetic flux that changes with time?

loop two (turns in circular motion not up or down)

The magnets shown in the sketch are dropped from rest through the middle of conducting rings. Notice that the ring on the right has a small break in it, whereas the ring on the left forms a closed loop. As the magnets drop toward the rings, does the magnet on the left have an acceleration that is (a) more than, (b) less than, or (c) the same as that of the magnet on the right?

magnet on left has smaller acceleration, because its going through a complete conducting ring with no breaks, current produces a magnetic field that exerts a repulsive force on the magnet to oppose its motion

Do you expect an echo to return to you more quickly or less quickly on a hot day, as compared to a cold day?

more quickly on a hot day (The speed of sound in a gas increases with temperature. This is because the molecules are bumping into each other faster and more often, so it is easier to propagate the compression wave (sound wave).

If a north pole moves toward the loop in the plane of the page, in what direction is the induced current?

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.)

If a North pole moves toward the loop in the plane of the page, in what direction is the induced current?

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)

A wire loop is being pulled through a uniform magnetic field. What is the direction of the induced current?

no induced current (Since the magnetic field is uniform, the magnetic flux through the loop is not changing. Thus no current is induced)

when switch is closed in the circuit shown the wire between the poles of the horseshoe magnet deflects downward. is the left end of the magnet a north magnetic pole or a south magnetic pole?

north magnetic pole

A horizontal wire carries a current and is in a vertical magnetic field. What is the direction of the force on the wire?

out of the page

What is the direction of the magnetic field at the center (point P) of the square loop of current?

out of the page (Use the right-hand rule for each wire segment to find that each segment has its B field pointing out of the page at point P.)

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?

out of the page (Without a B field, the electrons feel an electric force downwards. In order to compensate, the magnetic force has to point upwards. Using the right-hand rule and the fact that the electrons are negatively charged leads to a B field pointing out of the page.)

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? ***

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)

What is the direction of the magnetic field at the center (point P) of the square loop of current?

out of the page (current moving to the right, use right hand rule)

A wire loop is in a uniform magnetic field. Current flows in the wire loop, as shown. What does the loop do?

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.)

You have a long pipe and a short pipe. Which one has the higher frequency?

the short pipe (A shorter shorter pipe means that the standing wave in the pipe would have a shorter shorter wavelength wavelength. Since the wave speed remains the same, the frequency frequency has to be higher in the short pipe)

motion of bar magnet moving toward coil, as magnet approaches coil the average values of the perpendicular component of the magnetic field increases

this results: magnetic flux through coil increases with time

In order to change the magnetic flux through the loop, what would you have to do?

tilt the loop, change the loop area

In order to change the magnetic flux through the loop, what would you have to do?

tilt the loop, change the loop area (Since Φ = B A cosθ , changing the area or tilting the loop (which varies the projected area) would change the magnetic flux through the loop.)

a positive charge enters a uniform magnetic field as shown (field into the page) what is the direction of the magnetic force (v points up)

to the left

A beam of electrons (which have negative charge) is coming straight toward you. You place a magnet as shown directly above the beam. The magnetic field from the magnet points straight down. Which way will the electron beam deflect?

to the left (since its negative charge it would be opposite of what u get using the right hand rule)

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?

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).

At a football game, the "wave" might circulate through the stands and move around the stadium. In this wave motion, people stand up and sit down as the wave passes. What type of wave would this be characterized as?

transverse wave (The people are moving up and down, and the wave is traveling around the stadium. Thus, the motion of the wave is perpendicular to the oscillation direction of the people, and so this is a transverse wave)

A positive charge enters a uniform magnetic field as shown (into the board). What is the direction of the magnetic force?

upward

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.

v1 < v2 (Faraday's law: depends on N (number of loops) so the induced emf is twice as large in the wire with 2 loops. )

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?

yes and yes (traffic lights detect vehicles this way)

a vertical wire carries a current and is in a vertical magnetic field. what is the direction of the force on the wire?

zero

Consider a wave on a string moving to the right, as shown below. What is the direction of the velocity of a particle at the point labeled A (at top of hill) ?

zero (The velocity of an oscillating particle is (momentarily) zero at its maximum displacement.)

Order of increasing speeds of sound:

• in air (≈ 343 m/s ≈ 770 mph) • in warm air (>343 m/s) • in water (≈ four times speed in air) • in steel (≈ 15 times speed in air)