Physics Unit 3 Pre-Lecture Q's

Suppose that a magnet and a could were dropped together from the same height at the same time. Would there be an induced current in the coil?

No, because as they fall together, they are not moving relative to each other

Suppose that when you move the north pole of a bar magnet toward a could it induces a positive current in the coil. The strength of the field produced by an electromagnetic can be controlled electronically. Suppose you place a coil near the north pole of an electromagnet and increase the field while keeping everything stationary. Which one of the following will happen?

A positive current will be induced in the coil.

Suppose that the emf from a rod moving in a magnetic field was used to supply the current to illuminate a light bulb in a circuit, and the force needed to keep the rod moving to F. What happens to the force needed drive the motion of the rod if the light bulb is removed?

No force is needed: there is no longer a magnetic force to oppose the motion.

Suppose the current through a light bulb in a circuit is provided by the motional emf of a rod moving in a magnetic field. If you wanted to keep the intensity of the light emitted by the bulb constant when the strength of the magnetic field is doubled, how should the speed of the rod be changed?

The speed of the rod should be halved.

Which of the following does not produce a change in magnetic flux through a square loop of wire

the loop falls through a uniform magnetic field without rotating

suppose a circular coil is placed in a uniform magnetic field B. Which of the following creates the emf of the largest magnitude?

keeping the field at value B and rotating the coil so that the normal to the coil’s loop area rotates 360 relative to the field direction in 2.0 seconds

A changing magnetic field induces an emf of 5 V in a single loop of wire. Suppose the single loop were replaced by one continuous wire wrapped to form a coil of five loops, each with the same size and shape as the original loop, and placed in the same changing magnetic field and with the same orientation as the single loop. What would be the magnitude of the induced emf in the coil?

25 V

A circular coil of N turns rotates at some frequency in a constant magnetic field and produces an emf. Which of the following will double the emf?

any one of the above (double the field, double the frequency, double the area of the coil, double the number of turns of the coil)

A circular loop of wire is positioned so the normal to its area points vertically upward. a bar magnet is dropped from above the loop so that its north pole falls through the loop first. when looking at the loop from above, in which direction is the induced current in the loop just as the north pole of the magnet enters the loop?

counterclockwise

A uniform magnetic field is oriented vertically upward in large rectangular region of space, and the field is zero everywhere else. a circular loops is launched horizontally so that the normal to the loop’s area is also vertical, and the loop travels from a region of zero field into the region of the vertical field. Looking at the loop from the top, in which direction is the induced current?

clockwise

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).

1,3,2

A magnetic field of magnitude B=0.20 T is reduced to zero in a time interval of deltat=0.10 sec, 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?

B=.10T for deltat=0.05s, B=0.40 T and deltat=0.20 s

The drawing shows three flat coils, one square and two rectangular, each being pushed into a region where there is a uniform magnetic field directed into the screen. outside of this region the magnetic field is zero. in each case the magnetic field within the region has the same magnitude, and the could is being pushed at the same velocity v. each coil begins with one side just at the edge of the field region. consider the magnitude of the average emf induced as each coil is pushed from the starting position shown in the drawing until the coil is just completely within the field region. rank the magnitudes of the average emfs in descending order (largest first).

B, A and C (tie)

which of the following will increase the peak emf in an AC generator without changing the frequency

all of the above (replace the magnet with a stronger one - increase the magnetic field strength, replace the coil with one with more turns, increase the coil with one with a larger area.

suppose you reduced the frequency of a generator from 60 Hz to 50 Hz to accommodate a European electrical device. what would you need to do in order to keep the peak emf of the generator the same as it had been when operating at 60 Hz.

increase the magnetic field strength

which of the following are true

all of the above (electromagnetic waves are created by accelerating charges, electromagnetic waves are transverse waves, electromagnetic waves consist of time-varying electric and magnetic fields that are mutually perpendicular, electromagnetic waves move at the speed of light in a vacuum)

who is credited for developing the wave theory of electromagnetism

james Clerk Maxwell

which of the following is true

the electromagnetic spectrum consists of light with a vast range of frequencies of wavelengths, and visible light only makes up a small fraction of this spectrum

what is the wavelength of red light in vacuum that has a frequency of 4.00 × 10^14 Hz

750 nm

consider two electromagnetic waves A and B traveling in vacuum, where the frequency of A is twice that of B. Which of the following is true

the speed of wave a is equal to the speed of wave B.

the speed of light in a vacuum depends only on the electric permittivity of free space

false

if the value of the electric field in an electromagnetic wave were doubled, what would happen to the total energy density of the wave?

the total energy density would quadruple

suppose the electric field in an electromagnetic wave decreases in magnitude. what happens to the value of the wave’s magnetic field

the magnetic field decreases in magnitude.

what are the units of light intensity

watts/meter² (W/m²)

which one of the following statements concerning the energy carried by an electromagnetic wave is true

the energy is carried equally by the electric and magnetic fields

an astronomer observes that the wavelength of light from a distant star is shifted toward the red part of the visible spectrum. which of the following is true?

the distance between the earth and the star in increasing

the figure at the right shows three situations - A,B, and C - in which an observer and a source of electromagnetic waves are moving along the same line. In each case the source emits a wave of the same frequency. the arrows in each situation denote velocity vectors relative to the ground and have magnitudes of either v or 2v. rank the magnitudes of the frequencies of the observed waves in descending order (largest first)

B,A,C

two polarizers are crossed, as shown in the figure. which of the following situations would allow light to be transmitted through both polarizers

none of these would result in the light being transmitted through both polarizers

vertically polarized light with an average intensity of S0 is incident on a polarizer whose transmission axis makes an angle of 30 with respect to the horizontal. what is the average intensity of the light that is transmitted through the polarizer

s0/4

complete the following statement: the polarization direction of an electromagnetic wave is determined by

the direction of the electric field component

the drawing shows two sheets of polarizing material, each of which has its transmission axis aligned vertically. light is incident from the left, as show, 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 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

the radial lines pointing outward away from the wave source and perpendicular to the wave fronts are called

rays

which of the following is true at a very far distance from a wave source

the wave fronts reaching you form flat planes and the rays are parallel to each other

the drawing shows of top view of an object located to the right of a mirror. a single ray of light is shown leaving the object. after reflection from the mirror, through which locations A, B, C or D does the ray pass

C

which of the following is not a property of an image formed by a plane mirror

the image is real

the figure shows a light ray undergoing multiple reflections from a mirrored corridor. the walls of the corridor are either parallel or perpendicular to one another. if the initial angle of incidence is 35 what is the angle of reflection when the ray makes its last reflection

55

a section of the surface of a hollow sphere has a radius of curvature of 0.60 m and both the inside and outside surfaces have a mirror-like finish. what are the focal lengths of the inside and outside surfaces

+0.30m and -0.30m

because of spherical aberration, a parallel ray far from the principal axis does not pass through the focal point upon reflection. how could you change the shape of the mirror, so the ray does passs through F after reflection

open it up by moving the ends of the mirror away from the principal axis, thereby creating a more gentle curvature

it is possible to use a convex spherical mirror to produce an enlarged image

false

suppose you are standing in front of a concave spherical mirror. which of the following image properties are possible

all of the above (your image is upright, inverted, enlarged, reduced, real, virtual)

an object is placed in front of a spherical mirror and its magnification is -6. which of the following properties is true
I. the image is real

II. the image is virtual

III. the image is upright

IV. the image is inverted

V. the image is enlarged

VI. the image is reduced

VII. the image is formed by a concave spherical mirror

VIII. the image is formed by a convex spherical mirror

I, IV, V, VII

what is the greatest distance an image can be located behind a convex spherical mirror

di= -F

a light bulb is connected to the conducting rails as seen below. If you were to increase the speed of the rod in the direction shown, what would happen to the light bulb

it would be brighter

you have three light bulbs: bulb A has a resistance of 240, bulb B has a resistance of 192 and bulb C has a resistance of 144. each of these bulbs is used for the same amount of time in a set up like the one in the drawing. in each case the speed of the rod and the magnetic field strength is the same. rank the setups in descending order, according to how much work the hand in the drawing must do (largest amount of work first)

C,B,A

at the instance shown, the rod encloses a rectangle with the conducting rails. the magnetic field going into the page is B and is perpendicular to the area. what is the chance in magnetic flux through the enclosed rectangle between the initial and final position

deltaphi_B=BL(xf-x0)cos0

suppose the rod is now being pulled towards the lightbulb instead. which of the statements is correct?

the magnetic flux is decreasing, and nature produces a magnetic field into the page

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 loops is clockwise

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

downward through resistor R

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

the is no induced current

a rectangular loops of wire is attached to a metal rod using rigid, electrically insulating rods so that the distance between the loops 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 if the following statements concerning an induced current in the rectangular loops as a result of the current in the metal rod is true

the is no induced current in the loop

in a vacuum one color of red light has a wavelength of 700 nm and one color of violet light has a wavelength of 400 nm. this means that in a vacuum, the red light

none of the above - speed is a constant in a vacuum

at a certain point in space, the electric and magnetic field of an electromagnetic wave at a certain instant are given by E = 6000 V/m and B= 2 × 10^-5 T. the wave is propagating in the

negative y-direction

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

double

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

times four

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

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 the other polarizers. unpolarized light is incident on polarizer #1. the amount of light that passes through polarizer #3 will be

non zero

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

only case two (only non-90)

if unpolarized light is incident from the left, and the first polarizer was removed which cases would let some light through

cases 1 and 2

Five balls labeled A, B, C, D, and E are placed in front of a plane mirror
as shown in the figure. Which ball(s) will the observer, in the position
shown, see reflected in the mirror?

A

You are standing in front of a mirror at the point P shown. There is a
light bulb behind a screen that you cannot see directly. As you look in
the mirror, where does the image of the light bulb appear?

C

You hold a small hand mirror 0.5 m in front of your face and look at
your reflection in full-length mirror 1 m behind you. How far behind the
big mirror does the image of your face appear?

2.0 m

a wave front is a

surface on which all points of the wave are in the same phase of motion