Physics Exam 3

If both the electric and magnetic fields of an electromagnetic wave double in magnitude, how does the intensity of the wave change? The intensity (a)decreases by a factor of four (b)decreases by a factor of two (c)increases by a factor of two (d)increases by a factor of four (e)increases by a factor of eight.

D. Increases by a factor of four

If the magnetic field in an electromagnetic field is doubled, by what factor does the electric field change?

Double - If B is doubled, E is also doubled since they are proportional to each other

What is electromagnetic induction?

Generation of an emf by moving a conductor through a magnetic field. emf=change in flux/dt

What does Lenz law do?

Give direction of emf; The direction of any magnetic induction effect is such that it opposes the cause of the effect

True or false: Area is not dependent on intensity

If you double the area, you double the power

A solenoid is connected to an ac source. A copper ring and a rubber ring are placed inside the solenoid, with the normal to the plane of each ring parallel to the axis of the solenoid. An induced emf appears

In both rings

How are power and intensity similar?

Intensity is the power that passes perpendicularly through a surface divided by the area of the surface

A conducting bar slides with a velocity v to the right on some conducting rails as shown. A uniform magnetic field is directed perpendicular to the bar. Answer the following two questions: (1) Will the light bulb "turn on" in this case? and (2) How is the magnetic flux through the conducting loop changing, if at all, as the bar slides to the right?

It does turn on, and B will increase Explanation: As more charge is put in more motion, the strength of a magnetic field increases. The light bulb turns on because there is an induced current.

What is the meaning of the minus sign in ℰ = −ΔΦ/Δt?

It ensures the polarity of the induced emf sends the induced current in the proper direction so as to give rise to this opposing magnetic force

How is F = qE related to the types of polarizers?

It is a conductor because electrons feel a force, and the electrons are moving opposite of E

When the switch in CYU Figure 22.2 the drawing is closed, the current in the coil increases to its equilibrium value. While the current is increasing there is an induced current in the metal ring. The ring is free to move. What happens to the ring?

It jumps upward

What is polarization in physics?

It refers to the plane in which the electric field oscillates

A magnetic field has the same direction and the same magnitude B everywhere. A circular area A is bounded by a loop of wire. Which of the following statements is true concerning the magnitude of the magnetic flux that passes through this area?

It's maximum possible value is BA Explanation: When determining flux you take BA cos (phi). You multiply the area of the loop by the uniform magnetic field.

What does it mean when something is linearly polarized?

Its vibrations always occur along one direction

What happens when unpolarized light strikes a polarizer?

Only 1/2 of the incident light is transmitted, the other half being absorbed by the polarizer

If unpolarized light is incident from the left, in which case will some light get through?

Only case 2

The drawing shows a cube. The dashed lines in the drawing are perpendicular to faces 1, 2, and 3 of the cube. Magnetic fields are oriented with respect to these faces as shown, and each of the three fields B1, B2, and B3 has the same magnitude. Note that B2 is parallel to face 2 of the cube. Rank the magnetic fluxes that pass through the faces 1, 2, and 3 of the cube in decreasing order (largest first).

Phi 1, Phi 3, and Phi 2 Explanation: Since we know B and A are constant (not given in the question) when we calculate the cos (20), cos (70), and cos (0) since B2 is perpendicular, we get this order of largest to smallest.

What happens in dichroic polarizers?

Polarization dependent absorption

What generates a current?

The changing field (motion)

In what direction are the electric fields when light is polarized?

The electric fields are all in the same direction (vertical) - The E field is confined and still perpendicular

In what direction are the electric fields when light is unpolarized?

The electric fields it has are in random directions

What is a motional emf (how do you produce it)?

The emf of a battery is produced by chemical reactions, whereas the motional emf is created by the agent that moves the rod through the magnetic field

A circular loop of wire is in a region of spatially uniform magnetic field. The magnetic field is directed into the plane of the figure. If the magnetic field magnitude is increasing...

The induced current in the loop is counterclockwise

What is Lenz's law?

The induced emf resulting from a changing magnetic flux has a polarity that leads to an induced current whose direction is such that the induced magnetic field opposes the original flux change.

The drawing shows two sheets of polarizing material, each of which has its transmission axis aligned vertically. Light is incident from the left, as shown, and can be either completely unpolarized or completely polarized along the vertical direction. In either case, the average intensity of the incident light is the same, and the light exits the polarizer on the right. Which one of the following statements is true concerning the intensity of the exiting light?

The intensity of the exiting light is greater when the incident light is completely polarized

In Example 6, we saw that when the angle between the polarizer and analyzer is 63.4°, the average intensity of the transmitted light drops to 1/10 of the average of the incident unpolarized light. What happens to the light intensity that is not transmitted?

The light intensity that is not transmitted is absorbed by the polarizer and the analyzer. The polarizer absorbs 1/2 of the incident intensity, and the analyzer absorbs 4/10 of the incident intensity.

How is the magnetic flux determined?

The magnetic flux Φ is determined by more than just the magnitude B of the magnetic field and the area A. Φ = BA cos ϕ, where B is the magnitude of the magnetic field, A is the area of the surface through which the magnetic field passes, and ϕ is the angle between the magnetic field and the normal to the surface.

Suppose that a magnetic field is constant everywhere on a flat 1.0-m2 surface and that the magnetic flux through this surface is 2.0 Wb. From these data, which one of the following pieces of information can be determined about the magnetic field?

The magnitude of the component of the field that is perpendicular to the surface

What is the explanation of the magnitudes of the electric field of the wave being proportional (but not equal) to each other, and the electric field and magnetic energies carries by the waving being equal?

The magnitudes of the electric and magnetic fields of the wave are proportional to each other, according to E=cB. The wave carries equal amounts of electric and magnetic energy.

What happens with wire-grid polarizers?

The reflected wave cancels the incoming wave

What is wavelength?

The spatial distance required to capture one complete oscillation

What is period?

The time needed to complete 1 oscillation

What is the explanation behind the wavelength tripling and it reducing the frequency by a factor of three?

The wavelength (λ), frequency (f), and speed (c) of an electromagnetic wave are related according to c=λf, where c is the same for any electromagnetic wave traveling in a vacuum and is independent of λ and f. Since c is constant, λ and f are inversely proportional. When f is reduced by a factor of three, λ increases by a factor of three.

A rectangular loop of wire is attached to a metal rod using rigid, electrically insulating rods so that the distance between the loop and metal rod is constant as the metal rod is rotated uniformly as shown. The metal rod carries a constant current in the direction indicated. Which of the following statements concerning an induced current in the rectangular loop as a result of the current in the metal rod is true?

There is no induced current in the loop. - To have an induced current we need to have a change in flux (concept question 5)

If the electric field in the beam of light is doubled, by what factor does the intensity change?

Times four - If you double E^2 in the equation, you quadruple S

True or false: The direction of the current induced in the circuit is such that the magnetic force acts on the rod to oppose its motion, thereby tending to slow down the rod

True

A photocell (think small solar panel) is sitting in a beam of light. If the area, A, of the photocell is doubled, by what factor does the intensity of light incident upon the photocell change?

Unchanged - If you move the solar panel away from the light, you are changing the property of the light Power = energy/time S = property of light

What is the right hand rule 2?

Using your right hand, point your thumb in the direction of current and curl your fingers. The direction of your fingers is the direction of the field

Malus' law applies to the setup in Figure 24.20, which shows the analyzer rotated through an angle and the polarizer held fixed. Does Malus' law apply when the analyzer is held fixed and the polarizer is rotated?

Yes - You would have linearly polarized light from the first one and to landed on the second polarizer

What happens when you polarize unpolarized light?

You cut the intensity in half and lose 1/2 of light

Eddy currents are electric currents that can arise in a piece of metal when it moves through a region where the magnetic field is not the same everywhere. As the figure shows, for example, a metal sheet moving to the right at a velocity (v) and a magnetic field (B) that is directed perpendicular to the sheet. At the instant represented, the field only extends over the left half of the sheet. An emf is induced that leads to the eddy current indicated. Such eddy currents cause the velocity of the moving sheet to decrease and are used in various devices as a brake to damp out unwanted motion. Does the eddy current in the drawing circulate

clockwise

The velocity of the rod is...

constant

What is an induced current?

current in a circuit due to a changing magnetic field; the coil itself behaves as if it were a source of emf2

A flexible loop of wire lies in a uniform magnetic field of magnitude B directed into the plane of the picture. The loop is pulled as shown, reducing its area. The induced current flows

downward through resistor

Greater speeds and stronger magnetic fields lead to .... emfs for a given length L.

greater

What is the origin of motional emf?

moving electrons through a magnetic field

A conducting bar moves to the left at a constant speed (v) on two conducting rails joined at the left as shown. As a result of the bar moving through a constant magnetic field, a current (I) is induced in the indicated direction. Which one of the following directions is that of the magnetic field?

out of the page Explanation: Point your thumb in the direction of v and then rap your fingers around the wire to get the magnetic field. Since it wraps in a counterclockwise manner, your fingers will be point out at you which is indicative of the answer.

Between the times indicated in parts c and d in the drawing, what is the direction of the magnetic field at the point P for the electromagnetic wave being generated? Is it directed...

out of the plane of the paper (flow of positive charges) - Use RHR 2

The velocity of the rod is.. to a uniform magnetic field.

perpendicular

Work done by the hand is equal to the...

product of the magnitude of the force exerted by the hand, the magnitudes x the rod's displacement, and the cosine of the angle between the force and the displacement

What is the transmission axis?

the line along which light is polarized

What is magnetic flux?

the number of magnetic field lines passing through a surface

CYU Figure 24.2 shows two sheets of polarizing material. The transmission axis of one is vertical, and that of the other makes an angle of 45° with the vertical. Unpolarized light shines on this arrangement first from the left and then from the right. From which direction does at least some light pass through both sheets? (a) From the left (b) From the right (c) From either direction (d) From neither direction. What is the answer when the light is horizontally polarized? What is the answer when the light is vertically polarized?

unpolarized: c horizontally polarized: b vertically polarized: c

Consider the induced emf being generated in Figure 22.4. Suppose that the length of the rod is reduced by a factor of four. For the induced emf to be the same, what should be done? (a)Without changing the speed of the rod, increase the strength of the magnetic field by a factor of four. (b)Without changing the magnetic field, increase the speed of the rod by a factor of four. (c)Increase both the speed of the rod and the strength of the magnetic field by a factor of two. (d)All of the previous three methods may be used.

(d)All of the previous three methods may be used.

What is Faraday's Law?

- The induced emf is directly proportional to the rate of change of flux linkage - It depends on the three factors, B, A, and ϕ, any of which may change

A circular loop of wire is lying flat on a horizontal table, and you are looking down at it. An external magnetic field has a constant direction that is perpendicular to the table, and there is an induced clockwise current in the loop. Is the external field directed upward toward you or downward away from you, and is its magnitude increasing or decreasing? Note that there are two possible answers.

Answer 1: downward and decreasing Answer 2: upward and increasing

What does Malus's law apply to?

Applies to linear polarizer when the incident light is already linearly polarized

What is true in regards to area and emf?

As long as the area is changing, an induced emf and current exist

In the figure, a coil of wire is being stretched. What would be the direction of the induced current if the direction of the external magnetic field in the figure were reversed? (a) Clockwise (b) Counterclockwise

B Explanation: The wire is being stretched decreasing its cross-sectional area and the magnetic flux through it in the process. According to Lenz law, the induced current will have such a direction that will oppose its cause which is by increasing the flux through it. Using the right-hand rule the current in the loop will strengthen the flux through the loop. Therefore the direction of the current in the diagram is correct. According to the same argument given above the current now will strengthen the flux through it by inducing a current in Counterclockwise direction

Why does the peak value of the emf induced in a loop antenna (see Figure 24.5) depend on the frequency of the electromagnetic wave?

Because of Faraday's law, the emf is dependent on how rapidly the magnetic field of the wave is changing and this is determined by the frequency of the wave

You are sitting upright on the beach near a lake on a sunny day, wearing Polaroid sunglasses. When you lie down on your side, facing the lake, the sunglasses don't work as well as they do while you are sitting upright. Why not?

Because of the transmission axis changing - The transmission axis of the Polaroid material is almost horizontal, in the same direction as the polarizer light reflected from the lake

How are electromagnetic fields produced?

By oscillating charges

An electromagnetic wave travels in a vacuum. The wavelength of wave is tripled. How is this accomplished?

By reducing the frequency of the wave by a factor of three

How can polarized light be produced?

By unpolarized light with the aid of polarizing material - ex. Sunglasses

If unpolarized light is incident from the left, and the first polarizer was removed which case (s) would let some light through?

Cases 1 and 2

What angle is phi when the B field and the surface normal are parallel to each other?

Cos (0) = 1

What is the angle (cosine x) when the two transmission axis' are parallel?

Cos (0) = 1 (maximum)

What angle is phi when the B field and the surface normal are perpendicular to each other?

Cos (90) = 0

What is the angle (cosine x) when the two transmission axis' are perpendicular?

Cos (90) = 0 - Which indicates no light is passed through

A conducting rod is free to slide along a pair of conducting rails, in a region where a uniform and constant (in time) magnetic field is directed into the plane of the paper, as CYU Figure 22.3 illustrates. Initially the rod is at rest. There is no friction between the rails and the rod. What happens to the rod after the switch is closed? If any induced emf develops, be sure to account for its effect. (a) The rod accelerates to the right, its velocity increasing without limit. (b) The rod does not move. (c) The rod accelerates to the right for a while and then slows down and comes to a halt. (d) The rod accelerates to the right and eventually reaches a constant velocity at which it continues to move.

D

A coil is placed in a magnetic field, and the normal to the plane of the coil remains parallel to the field. Which one of the following options causes the magnitude of the average emf induced in the coil to be as large as possible? (a) The magnitude of the field is small, and its rate of change is large. (b) The magnitude of the field is large, and its rate of change is small. (c) The magnitude of the field is large, and it does not change.

A

A magnetic field of magnitude B = 0.20 T is reduced to zero in a time interval of Δt = 0.10 s, thereby creating an induced current in a loop of wire. Which one or more of the following choices would cause the same induced current to appear in the same loop of wire? (a) B = 0.40 T and Δt = 0.20 s (b) B = 0.30 T and Δt = 0.10 s (c) B = 0.30 T and Δt = 0.30 s (d) B = 0.10 T and Δt = 0.050 s (e) B = 0.50 T and Δt = 0.40 s

A and D Why?

A circular loop of copper wire is placed next to a long, straight wire. The current (I) in the long, straight wire is increasing. What current does this induce in the circular loop?

A clockwise current

A transmitting antenna is located at the origin of x, y, z axis system and broadcasts an electromagnetic wave whose electric field oscillates along the y-axis. The wave travels along the +x axis. Three possible wire loops are available for use with an LC-tuned circuit to detect this wave...

A loop that lies in the xy plane

In the most common form of lightning, electric charges flow between the ground and a cloud. The flow changes dramatically over short periods of time. Even without directly striking an electrical appliance in your house, a bolt of lightning that strikes nearby can produce a current in the circuits of the appliance. Note that such circuits typically contain coils or loops of wire. Why can the lightning cause the current to appear?

A lightning bolt is a large electric current that changes in time and, thus, produces a magnetic field that also changes in time. When this changing field passes through a coil or loop of wire in an appliance, it can, via Faraday's law, create an induced emf, which can lead to an induced current.

What are the three ways to change flux?

1.) Increase or decrease B field 2.) Changing the Area of the flux 3.) Angle phi

An electromagnetic wave is traveling in a vacuum. The magnitudes of the electric and magnetic fields of the wave are_______________, and the electric and magnetic energies carried by the wave are_________.

1.) Proportional (but not equal) to each other 2.) Equal

In the discussion concerning the figure, we saw that a force of 0.086N from an external agent was required to keep the rod moving at a constant velocity. Suppose that friction is absent and the light bulb is suddenly removed from its socket while the rod is moving. How much force does the external agent then need to apply to the rod to keep it moving at a constant velocity?

0 Newtons (once you take out the light bulb from the circuit, you won't feel resistance)

Steps to determining the polarity of the Induced emf

1. Determine whether the magnetic flux that penetrates a coil is increasing or decreasing. 2. Find what the direction of the induced magnetic field must be so that it can oppose the change in flux by adding to or subtracting from the original field. 3. Having found the direction of the induced magnetic field, use RHR-2 to determine the direction of the induced current. Then the polarity of the induced emf can be assigned because conventional current is directed out of the positive terminal, through the external circuit, and into the negative terminal.

How do you calculate frequency?

1/period (T)

At a certain point in space, the electric and magnetic fields of an electromagnetic wave at a certain instant are given by: E = 6000 V/m (x) B = 2 x 10^-5 T (z) This wave is propagating in the...

Negative y-direction Why?

The drawing shows two sheets of polarizing material, which have their transmission axes crossed. The sheet on the left has its transmission axis aligned vertically. Light is incident from the left, as shown and can be either completely unpolarized or completely polarized along the vertical direction. Which one of the following statements is true concerning the light that exits the polarizer on the right?

No light exits the polarizer on the right for either the unpolarized or the polarized incident light.

Suppose that the coil and the magnet in the figure were each moving with the same velocity relative to the earth. Would there be an induced current in the coil?

No. With both the magnet and coil moving at the same velocity with respect to the earth, there is no relative motion between the magnet and the coil, which is needed for there to be an induced current in the coil.

Two polarizers are oriented such that their transmission axes are perpendicular. A third polarizer is placed between them with its transmission axis at an angle of 45° relative to other poliarizers. Unpolarized light is incident on polarizer #1. The amount of light that passes through polarizer #3 will be...

Non-zero (which indicates that some light will pass through)

True or false: A linear polarizer will reduce the intensity of incident unpolarized by 1/2 and the transmitted light will be linearly polarized in the direction of the polarizers transmission axis

TRUE

True or false: An electromagnetic wave is a transverse wave because the electric and magnetic field are both perpendicular to the direction in which the wave travels

TRUE

True or false: Electromagnetic waves can travel through a vacuum or a material substance, since electric and magnetic fields can exist in either one

TRUE

True or false: Light is a transverse wave

TRUE

True or false: Natural light is unpolarized

TRUE

True or false: The electric field for each direction can be resolved into components perpendicular and parallel to the transmission axis, with the result that the average components perpendicular and parallel to the axis are equal

TRUE

True or false: The force exerted by the hand must be equal in magnitude, but opposite in direction, to the magnetic force exerted on the rod

TRUE

True or false: The induced field is not always opposite to the external field, however, because Lenz's law requires only that it must oppose the change in the flux that generates the emf.

TRUE

True or false: The magnetic flux is proportional to the number of field lines that pass through a surface

TRUE

True or false: The oscillating current (I) in the antenna wires creates a magnetic field (B) at point P that is tangent to a circle centered on the wires. The field is directed as shown when the current is upward and is directed in the opposite direction when the current is downward.

TRUE

True or false: The surface normal is a vector which is perpendicular to that surface

TRUE

True or false: EM wave have both an E field and a B field, and those fields always have the same ration

TRUE c = E/B

What is meant by uniformly in physics?

That all points have the same intensity

What is the angle phi in magnetic flux?

The angle between (B) and the surface normal, not the angle between B and the surface

The rectangular loop of wire is being moved to the right at constant velocity. A constant current (I) flows in the long, straight wire in the direction shown. The current induced in the loop is

zero (there is no induced current) - This is because the movement of the loop is parallel to the direction of the current

The frequency of electromagnetic wave A is twice that of electromagnetic wave B. For these two waves, what is the ratio λA/λB of the wavelengths in a vacuum?

λA/λB = 1/2 because both waves have the same speed as they are directly proportional