PHY 242 Exam 2

As a train starts from rest and then accelerates down the track, coming toward an observer faster and faster, the speed of the sound waves coming toward the observer will be:

equal to the normal speed of sound in air

An object is placed 40 cm in front of a converging lens of focal length 8.0 cm. The image is:

real, behind the lens

The third harmonic in a tube closed at one end is 735 Hz. If the speed of sound in air is 335 m/s, the length of the tube must be:

34.1 cm

A statement equivalent to Lenz's law is:

Induced currents produce magnetic fields that oppose the change in the magnetic field that induced them in the first place.

A body moves in a simple harmonic motion according to the equation x(t) = (2/π)sin⁡((4π*t) + (π/3)) where the units are in SI. At t=2.0 s, the speed of the body is:

4.00 m/s

A rectangular loop that encloses 200 cm2 is placed in a magnetic field perpendicular to the plane of the loop. The loop has a total resistance 5.0 Ω, and has 4 turns. The magnetic field's magnitude increases uniformly from 10 mT to 25 mT in 5.0 ms. The resulting induced current in the coil is:

48 mA

What is the equivalent capacitance of the circuit shown, if each capacitor has a capacitance C? IMAGE 3

4C/3

A proton (mass = 1.67 x 10 −26 kg , charge = 1.60 x 10 −19 C) is moving at a speed of 8.50 x 10 4 m/s at a point in an evacuated region of space where the electric potential is 35.0 volts. No force but the force of the electric field characterized by the electric potential acts on the proton. The particle travels to a point in the evacuated region of space where the electric potential is 47.0 Volts. What is the speed of the particle at that point in space?

8.36e4 m/s

A 0.28 kg block on a horizontal frictionless surface is attached to an ideal massless spring whose spring constant is 500 N/m. The block is pulled from its equilibrium position at x = 0.00 m to a displacement x = +0.080 m and is released from rest. The block then executes simple harmonic motion along the horizontal x-axis. When the displacement is x = -0.052 m, the acceleration of the block is:

92 m/s^2

For this circuit, which of the following represents a correct way of connecting an ammeter to measure the current through resistor R2? (Indicate all correct answers.) IMAGES 5 & 6

A & D

Q has 50 electric field lines radiating outward and q has 100 field lines converging inward. What is the ratio Q/q?

-1/2

A cubical surface with no charge enclosed and with sides 2.0 m long is oriented with the right and left faces perpendicular to a uniform electric field of magnitude 1.6 x 105 N/C. The net electric flux through this surface is approximately:

0

A sphere of radius 9.0 cm, made of an electrically conducting material, carries a 4.0 x 10-6 C of charge uniformly distributed over the surface of the sphere. The electric field 5.0 cm from the center of the sphere is approximately:

0

A particle with a charge of 1.0 C is 2.0 meters from another particle with a charge of 1.0 C. Find the magnitude of the electric field at the point midway between the two particles.

0 N/C

A 12 volt battery is connected in series with an initially-uncharged 120 microfarad capacitor and a 25 ohm resistor. What is the current through the resistor at the instant the connection is made? IMAGE 4

0.48 A

The simple harmonic motion of an object is described by the graph shown in the figure. Its angular frequency is: IMAGE 1

0.79 rad/s

Two very long, parallel conducting wires carry equal currents in opposite directions. The numbered diagrams show end views of the wires and the resultant force vectors due to the current flow in each wire. Which diagram best represents the direction of the forces? IMAGE 12

1

What is the total current supplied by the battery in the circuit? IMAGE 8

1.5 A

A circular loop of wire 1.0 cm in radius carries a current of 30 A. The magnetic field at the center of the loop is:

1.9 mT

At a given point in space, the electric field is known to be 1200 N/C eastward. What would the force exerted by the electric field on an electron be if the electron was at that point in space? (An electron has a charge of −1.6 × 10 −19 C.)

1.9e-16 N westward

Two sounds differ by 20 dB. This means that the louder sound is _____ times as intense.

100

A circular coil is placed in a magnetic field directed perpendicular to the plane of the coil. The coil has a radius of 15 cm, a total resistance 4.0 Ω, and has 50 turns. The magnetic field's magnitude varies with time as B(t) = 80 sin(50 πt) mT where t is measured in s. The magnitude of the induced current in the coil at t = 20 ms is:

11 mA

Two water waves meet at the same point, one having a displacement above equilibrium of 60 cm and the other having a displacement above equilibrium of 80 cm. At this moment, what is the resulting displacement above equilibrium?

140 cm

Current carrying wires are located along two edges of a cube with the directions of the currents as indicated in the figure. The vector that indicates the resultant magnetic field at the corner of the cube is: IMAGE 11

2

The inhabitants of a planet in another galaxy have their eyes at the exact center of their 4.0 m long bodies. How long must a plane mirror be for such a creature to be able to see all of its body in the mirror?

2.0 m

The magnetic field at a distance of 50 cm from a long, straight wire carrying a current of 5.0 A is approximately

2.0 μT

A jet engine emits a whine of frequency 3000 Hz. When the engine is moving directly away from the observer at half the speed of sound, an observer hears a frequency of:

2000 Hz

The graph that best represents B as a function of r for a wire of radius R carrying a current I uniformly distributed over its cross-sectional area is: IMAGE 10

3

Ampere's Law is valid: A. when all the other specified conditions are satisfied B. when the magnetic field is constant C. when there is a high degree of symmetry in the geometry of the problem D. the current is constant E. when there is no symmetry

A. when all the other specified conditions are satisfied

A ray of light, traveling in air, passes through a flat plate of glass. Refraction occurs. Where does the refraction take place?

At the glass-to-air interface where the light enters the glass, and, at the glass-to-air interface where the light exits the glass

Two identical capacitors A and B are connected in parallel with each other across a battery. If mica (of dielectric constant 5.4) is inserted in B:

B will have the larger charge

Consider a Cartesian coordinate system in which the electric potential due to an unspecified distribution of charge is a function of x only. In a V(x) vs. x graph, the electric field at a particular value of x, is what characteristic of the graph? A. The x-intercept of the curve. B. None of the other answers are correct. C. The y-intercept of the curve. D. The area under the curve from x=0 to x equals the value of x in question. E. The slope of the curve at the value of x in question.

B. None of the other answers are correct.

Object A has a charge q on it, object B has a charge q on it, and object C has a charge 2q on it. These charges are arranged, one each, at the vertices of an equilateral triangle. Which charge has the greatest magnitude of electric force acting on it?

C

The diagram represents the magnetic field lines in a certain region in space. The magnetic field is strongest at: IMAGE 9

C

Which of the following graphs would best represent the electric field of a hollow conductive sphere as a function of distance from its center when it is charged to a potential of 400,000 volts? IMAGE 2

C

Which of the following best represents the ray diagram construction for finding the image of the object shown? The solid dots on either side of the lens locate the two focal points of the lens. IMAGE 15

D

A current may be induced in a coil by: A. holding the coil near a second coil while the electric current in the second coil is increasing. B. moving the coil toward one end of the bar magnet. C. moving one end of a bar magnet through the coil. D. all of the above.

D. all of the above

The index of refraction of a particular medium is almost the same for all wavelengths, hence we often associate a single value, the average value of the index of refraction, to the medium. There is, however, some variation in the value of the index of refraction, with wavelength. As a result the angle of refraction that occurs when light enters or exits such a medium is slightly different for different wavelengths of visible light. What is the name of this phenomenon?

Dispersion

Consider two identical small metal balls, one having a charge of +1 microcoulombs and the other having a charge of +3 microcoulombs. The balls are separated from each other by 15 cm. Each exerts a force F on the other. The balls are brought into momentary contact with each other (assume each ball is on the end of its own non-conducting rod for handling purposes) and then separated such that the distance between them is once again 15 cm. At this point, each exerts a force F' on the other. How does F' compare with F?

F' > F

Two parallel wires carry currents I1 and I2=2I1 in the same direction. which of the following expressions shows the relationship of the forces on the wires?

F1 = F2

Suppose you are asked to find the characteristics of the image of an actual physical object formed by a single lens and you obtain a negative value for the magnification. What does this tell you about the image?

It is inverted

A person is attempting to locate the image of an object of a plane mirror by means of a ray-tracing diagram. The person draws a ray, from the object, which strikes the mirror at right angles to the surface of the mirror. Where does the ray go from there?

It reverses its direction at the surface of the mirror and travels straight back upon itself

A parallel plate capacitor is connected to a battery and charged to voltage V. Leah says that the charge on the plates will decrease if the distance between the plates is increased while they are still connected to the battery. Gertie says that the charge will remain the same. Which one, if either, is correct, and why?

Leah, because the capacitance decreases when the plate separation is increased.

An astronaut is in an all-metal chamber outside the space station when a solar storm results in the deposit of a large positive charge on the station. Which statement is correct?

The astronaut does not need to worry: the charge will remain on the outside surface.

You wish to use a positively charged rod to charge a ball by induction. Which statement is correct?

The ball must be a conductor.

Given a set of equipotential surfaces how does one determine the corresponding set of electric field lines?

The electric field lines are those lines which are everywhere perpendicular to the equipotential surfaces and are directed (as indicated in a diagram by means of arrowheads) from high potential toward low potential.

A single lens is used to form an image of an actual physical object. You calculate the image position q and find that it has a negative value. What does the negative sign tell you about the image?

The image is and virtual, and it is on the same side of the lens as the object.

Consider a capacitor which is charged to 12 volts and then connected across a resistor. Let t1/2 be the time it takes for the voltage to drop down to 6 volts. Suppose the experiment is repeated, but with a pair of capacitors connected in parallel with each other in place of the original capacitor, where each element of the pair has the same capacitance as the original capacitor. How does the new t1/2 compare with the original t1/2?

The new t1/2 is twice the original t1/2.

A positively-charged particle is moving southward in a downward-directed uniform magnetic field. The magnetic force is the only force, if any, acting on the particle. Describe the trajectory of the particle assuming that it remains in the uniform magnetic field, and, that it's elevation does not change.

The particle will travel counterclockwise, as viewed from above, around a horizontal circle.

What are the angles between the direction of travel of light, the electric field, and the associated magnetic field?

They are each at an angle of 90 degrees with respect to each of the other two.

A charge +q is brought to a point a distance r from a charge +Q. The electrostatic potential at distance r from the charge +Q is V. Then, +q is removed and a charge +4q is brought to the same point. The new electrostatic potential at distance r from the charge +Q is:

V

Consider a pair of two identical capacitors. In which case is the capacitance of the pair greater?

When they are connected in parallel with each other.

The diagram below shows a simplified schematic for an adjustable electric heating blanket. The switch may be connected to point W, X, or Y depending on the desired level of warmth. Identify which point the switch should be connected to in order to generate most warmth? IMAGE 7

X

A bar magnet is suspended vertically above a closed coil of wire. The bar magnet is released. It remains vertical as it approaches the coil, falls straight through the coil, and then continues to fall downward, away from the coil. Does the direction of the current induced to flow in the coil as a result of the motion of the magnet reverse itself as the magnet passes through the coil?

Yes

Equipotentials are lines along which

a charge may be moved at constant speed without work against electrical forces.

A circular coil lies in the horizontal plane, having a bar magnet suspended above the coil's center with its south pole pointing down. When the magnet is released and approaches the coil, it induces:

a current in the clockwise direction (when viewed from above)

There are differences and similarities between electrical charges and magnetic poles. One of the differences is that...

a magnetic pole cannot exist alone

Conceptually, Ampere's Law ∮B→⋅dl→=(μo)I reflects that:

an electric current causes a magnetic field

Two isolated conducting spheres (sphere 1 of radius 30 cm and initial charge +6.0 nC and sphere 2 of radius 40 cm and initial charge +2.0 nC) are connected by a conducting wire. Charge will flow in the wire until:

both spheres are at the same electric potential

A capacitor is connected to a battery. When a dielectric is inserted between the plates of the capacitor...

both the capacitance and the charge change

An electromagnetic wave is made up of which of the following oscillating quantities?

electric and magnetic fields

In a loop in a closed circuit, the sum of the currents entering a junction equals the sum of the currents leaving a junction because...

charge is neither created nor destroyed at a junction.

A mass m hanging on a spring with a spring constant k executes simple harmonic motion with a period T. If the same mass is hung from a spring with a spring constant of 2k, the period of oscillation

decreases by a factor of the square root of 2 (2^0.5)

If the area of the plates of a parallel plate capacitor is doubled, the capacitance is...

doubled

The number of electric field lines passing through a unit cross sectional area is indicative of:

field strength

When light is either reflected or refracted, the quantity that does not change in either process is its:

frequency

When the frequency of a source is doubled, the sound produced has:

half of its former wavelength

The magnetic flux through a loop perpendicular to a uniform magnetic field will change:

if the loop is rotated through 180 degrees about an axis through its center and in the plane of the loop

A junior Thomas Edison wants to make a brighter light bulb. He decides to modify the filament. How should the filament of a light bulb be modified in order to make the light bulb produce more light at a given voltage?

increase the diameter only

The electric potential inside a capacitor...

increases linearly from the negative to the positive plate.

The voltage across each capacitor in a set of capacitors in series is...

inversely proportional to its capacitance

A system consists of a mass vibrating at the end of a spring in an ideal simple harmonic motion. The total mechanical energy of the system:

is constant

The electric potential inside a charged solid spherical conductor in equilibrium:

is constant and equal to its value at the surface

A mass-spring system on a horizontal frictionless surface is set in simple harmonic motion with amplitude A. The mass is then doubled and the system is again set into simple harmonic motion with the same amplitude. Which of the following is true about the total mechanical energy of the system due to doubling the mass?

it has not changed

What is the direction of the magnetic field around a wire carrying a current perpendicular into this page?

it is clockwise around the wire in the plane of the page

As an electromagnetic wave travels through free space, its speed can be increased by:

its speed cannot be increased.

A car A moves at speed v toward car B, which is at rest. The frequency of car A's horn is heard by car B to be f. What is the frequency of car A's horn as heard by an observer in car B if car A is at rest and car B moves at speed v toward car A ? (Assume there is no wind)

less than f

When a positive charge is released and moves along an electric field line, it moves to a position of:

lower potential and lower potential energy

A positively charged particle moves in the positive x direction in a uniform magnetic field pointing in the negative z direction. You want to balance the magnetic force with an electric field so that the particle will continue moving along a straight line. The electric field should point in the...

negative y direction

An electron moving in the positive x direction experiences a magnetic force in the positive z direction. If Bx = 0, what is the direction of the magnetic field?

negative y direction

Constant current I is present in a wire located in a region where a uniform magnetic field exists. The wire is perpendicular to the field direction. The magnetic force on the wire is...

perpendicular to the current and to the magnetic field.

The diagram below shows a top view of a glass aquarium filled with water (index of refraction 1.33). A laser pointer is aimed at the front of the aquarium at an angle. When the light from the laser initially hits the front of the aquarium, what is the most likely result? IMAGE 13

some light reflects off the glass at an angle equal to the incident angle, and some light refracts into the water

The speed of sound in air is a function of:

temperature

The battery is disconnected from a series RC circuit after the capacitor is fully charged and is replaced by an open switch. When the switch is closed...

the capacitor does not allow current to pass.

When white light passes through a prism, it breaks up into its component colors as represented in the diagram below. Which statement best describes this phenomenon? IMAGE 14

the component colors have different speeds in the prism

Two strings, one thick and the other thin, are connected to form one long string. As a wave travels along the string and passes the point where the two strings are connected, all of the following change except for:

the frequency of the wave

Charges Q and q, separated by a distance d, produce a potential of 0 V at point P. This means that:

the net work needed to bring a charge from infinity to point P is zero

A positively-charged particle is moving southward in a downward-directed uniform magnetic field. The magnetic force is the only force, if any, acting on the particle. During the next few centimeters of its travel...

the particle keeps on going at the same speed.

The radius of curvature of the path of a charged particle in a uniform magnetic field is directly proportional to:

the particle's momentum

When a microwave filter consisting of vertical parallel metal rods is in the absorbing position, oscillating currents are set up in the rods. At any one instant, the current in each rod has the same magnitude and direction. At that instant:

the rods will try to move together horizontally

For standing waves, the only wave characteristic independent of the specific wave mode is:

the speed of the wave

The purpose of prescription glasses for a near-sighted person is to bring the apparent positions of distant objects:

to the far point of the eye

If the intensity of a sound is increased by a factor of 100, how is the decibel level changed? The new decibel level will be:

twenty units greater

For a standing wave the wavelength must equal;

twice the distance between adjacent nodes

For a standing wave, the wavelength must equal:

twice the distance between adjacent nodes

Which of the following best describes the image for a thin concave lens that forms whenever the magnitude of the object distance is less than that of the lens' focal length?

upright, diminished and virtual

A student is standing 0.5 m from a plane mirror. The resulting image is best described by which of the following?

upright, virtual image, same size

When you stand in front of a convex mirror, the image you see is:

virtual and upright

A 0.5 kg mass is suspended from a massless spring that has a force constant of 79 N/m. The mass is displaced 0.10 m down from its equilibrium position and released. If the downward direction is negative, the displacement of the mass as a function of time is given by:

y(t) = 0.1cos⁡(12.6t−π)

The algebraic sum of the changes of potential around any closed circuit loop is...

zero