Exam 2: 20-23
which of the following statements is false? the magnetic field of a current carrying wire
false: it exists only if the current in the wire is changing -b/c a constant current still produces a magnetic field true: is directed circularly around the wire, decreases inversely with the distance from the wire, exists only if the current in the wire is changing, depends on the magnitude of the current
which of the following cannot produce a magnetic field?
-an electric charge at rest -voltage battery not connected to anything--> doesn't have current hence cannot create electric field -ordinary piece of iron -a piece of any metal
what would happen in the other situations?
-magnetic field gets stronger= increasing the magnetic flux hence the current will go counterclockwise to counteract the increase. -when the coil is moved sideways across the page: causes no change to the flux so no current will be induced
which of the following will not increase a generators voltage output?
-rotating the mag field so that it is more closely parallel to the generators rotation axis =voltage output is directly proportional to magnetic flux
a coil rests in the plane of the page while a magnetic field is directed into the page. a clockwise current is induced?
-when the size of the coil decreases; and when the magnetic field is tilted so it is no longer perpendicular to the page -mag field being in to the page means that the magnetic flux will be decreasing which is the point that the current is induced hence this decrease occurs when the size of the coil decreases or the magnetic field becomes tilted
a 10V, 1A dc current is run through a step up transformer that has 10 turns on the input side and 20 tons on the output side what is the output?
0V and 0A -dc doesn't produce a chaining flux so there is no current or voltage that will be induced in the secondary coil --> if it had been ac then with step up here is twice as any loops in the secondary coil so double the voltage and cut the current in half
which statements about the force on a charged particle in a magnetic field are true?
a magnetic force is exerted only if the particle is moving; a stationary charged particle does not experience a force in the magnetic field. therefore the particle must be moving to experience a force. the force is max when the particle is moving perpendicular to the field, not parallel to the field, since the force is perpendicular to the motion of the particle it acts as a centripetal force changing the particle's direction but not its KE> that is since the force is perpendicular to the motion it does no work on the particle. the direction of the force is always perpendicular to the direction of motion and also to the magnetic field
the alternating electric current at a wall outlet is most commonly produced?
a rotating coil that is immersed in a magnetic field, (generators are the rotating coils_
if all else is the same for which surface would the radiation pressure form light be the greatest?
a white surface, pressure is related to the momentum of light. with black object all of the light is absorbed so the radiation pressure is proportional to incident momentum. with colored some of the lift reflected thus increasing the change in momentum of the light and increasing the pressure. for white , the max amt of light is reflected off the surface creating the greatest change in momentum of the light and the greatest radiation pressure
which of the following types of electromagnetic radiation travels the fastest?
all em waves travel at the same speed
which of the following travel at the same speed as light?
all em waves travel at the same speed but d) ultrasonic waves are not em waves so only one that does not travel at the speed of light
a nonconducting plastic hoop is held in a magnetic field that points out of the page. as the strength of the field increases
an induced emf will be produced but no current, since the flux through the loop is increasing, an emf will be produced. however since plastic is not a conductor, no current will be induced the emf is produced regardless of whether there is a conducting path for current or not
a square loop moves to the right from an area where B=0 completely through a region containing a uniform magnetic field directed into the page and then out to B=0 after point L. A current is induced in the loop?
as it passes line J and L; when it enters and exits because that is where the change in the magnetic field is occurring from B=0 to B=constant and then B=constant to B=0; the movement doesn't do anything but the change in magnetic field does which leads to a change in magnetic flux hence induces a current
a proton moves through space, it creates
both an electric and magnetic field; electric fields are created by charged objects whether the charges are moving or not. Magnetic fields are created by moving charged objects. since the proton is charged and moving it creates both and electric and magnetic field
if the intensity of the em wave doubles?
both the magnetic field and the electric field must increase by a factor of the sq root of 2. intensity is proportional to the product of the amplitudes with the field amplitudes proportional to each other, if the intensity doubles then each field must increase by same factor sq root 2/
which of the following can a transformer accomplish
changing both the current and voltage, power is conserved across a transformer, where power is the product of voltage and current. when a transformer increases voltage it must proportionately decrease the current
a current in a wire points into the page as shown at the right. in which direction is the magnetic field at point A (choose below)
downward; because current is the thumb and since into the page the current will be going in the clockwise direction -In which direction is B, which is right underneath the current? down and the direction of the current is to the left at point B
if the earth-sun distance were doubled, the intensity of radiation from the sun that reaches the earth's surface would
drop to 1/4; radiation doesn't decrease linearly with distance instead it decreases as the square of distance
two loops of wire are moving in the vicinity of a very long straight wire carrying a steady current. find the direction of the induced current in each loop.
for C (with velocity upwards) -zero, because the loop remains in the same distance from the wire so the mag flux through the wire remains constant and no current is induced because it is parallel for d (moving left to right; so away from current, distance increasing) -clockwise, the magnetic field of the D direction is pointing into the page as the loop moves away the magnetic flux decreases so a clockwise current is induced in the loop to oppose the change in flux
in empty space, which quantity is always larger for x-ray radiation than for a radio wave?
frequency, x rays have much smaller wavelengths than radio waves but they have a higher frequency than the radio waves
a proton enters a uniform magnetic field that is perpendicular to the proton's velocity. what happens to the KE of the proton?
it stays the same, force does not do work on the object, since the magnetic force is perpendicular to the velocity of the proton the force acts as a centripetal force, changing the proton's direction but not doing any work and thus not changing its KE
if electrons in a wire vibrate up and down 1000 times per second they will create an em wave having?
it would have a frequency of 1000 Hz
Indicate which of the following will produce a magnetic field?
magnet earth moving electric charge electric current= because it is a flow of electrons
a long straight wire carries a current I. a small loop of wire rests in the plane of the page . which of the following will not induce a current in the loop>
moving the loop in a direction parallel to the wire. -a current will be induced in the loop whenever the magnetic flux through the loop is changed. increasing the current in the wire will increase the magnetic field produced by the wire and therefore the magnetic flux in the loop. rotating the loop changes the angle between the loop and the wire and will change the flux. since the magnetic field strength decreases with distance from the wire moving the loop away from eh wire, either with or without rotation, will decrease the flux in the wire. if the loop is moved parallel to the wire, then the flux through the loop will not change so no current will be created in the loop.
the radius of an atom is not the order of 10^-10 m. in comparison the wavelength of visible light is
much larger, visible light has a wavelength of 10^-7 to 10^-8 which is over a thousand times larger than the size of an atom
an electromagnetic wave is traveling straight down toward the center of the earth. at a certain moment in time the electric field points west. in which direction does the magnetic field point at this moment?
north; the direction of the magnetic field is perpendicular to the direction the wave is traveling and perpendicular to the electric field so only north and south are possible answers. the right hand rule can be used to determine the direction is correct by pointing fingers in direction of the field and bending them in the direction of the magnetic field where thumb reps the wave
for a charge particle a constant magnetic field can be used to change?
only the direction of the particle's velocity
two separate but nearby coils are mounted along the same axis. a power supply controls the flow of current in the first coil and thus the magnetic field it produces. the second coil is connected only to an ammeter. the ammeter will indicate that current is flowing in the second coil
only when the current in the first coil changes -of constant then there is no change and hence the second coil will not produce and induced current
when a charged particle moves parallel to the direction of a magnetic field, the particle travels in a ? down and
straight path -usually when charged particle is moving perpendicular it experiences a force with a velocity, v but the charge particle moving parallel to the direction of the magnetic field. When moving parallel then the velocity is constant and there will be no deflections in the path of the charge particle.
when you swipe a credit card the machine sometimes fails to read the card, what can you do differently?
swipe the card more quickly so that the induced emf is higher, because there is a greater rate of change of magnetic flux that results
which of the following is true about all series ac circuits?
the current at any point in the circuit is always the same as the current at any other point in the circuit -current is also out of phase with the voltage across and inductor and the voltage across the capacitor. the current and voltage across a resistor is always in phase because the resistor does not change the phase in the current
two parallel wires are vertical. the one on the left carries a 10 A current up. the other carries a 5A down. compare the magnitude of the force that each wire exerts on the other.
the two wires exert the same force on each other, the force that one wire exerts on a second must be equal in magnitude but opposite direction according to Newtons 3rd law
which of the following statements about the force on a charged particle due to a magnetic field are not valid?
they are all valid: it depends on the particle's charge, velocity and it depends on the strength of the external magnetic field, it acts at right angles to the direction of the particle's motion
a wire carries directly away from you. which way do the magnetic field lines produced by this wire point>
they make circles around the wire--> always make circles -dont confuse with electric fields _-> which point away and toward point charges
a laptop computer's charger unit converts 120 V from a wall power outlet to the lower voltage required by the laptop. inside the charger's plastic case is a diode or rectifier that changes ac to dc plus a
transformer, because need something that converts high voltage input into low voltage output without power loss. -generator: mech into electrical -motor: electrical to mech
which of the following about transformers is false?
transformers work using ac current or dc current, dc current has no changing flux which is the property that a transformer depends on hence transformers have ac circuits
in a vacuum, what is the difference between a radio wave and an x ray?
wavelength and frequency -all em waves have the same velocity (speed of light number) in a vacuum, the velocity is the product of the wavelength and frequency. since the X-rays and radio waves have different wavelengths but the same speed, they will also have different frequencies
when a generator is used to produce electric current, the resulting electric energy originates from which source?
whatever rotates the generator's axle; a generator converts mechanical energy into electrical energy. the generators magnetic field remains unchanged as the generator operates, so energy is not being pulled from the magnetic field. resistance in the coils removes the electrical energy from the system in the form of heat- it does not provide the energy. in order for the generator to work, an external force is necessary to rotate the generator's axle. this external force does work, which is converted to electrical energy
starting in 2009, tv stations in the US switched to digital signals. to watch todays Digital broadcast TV, could you use a pre-2009 TV antenna meant for analog.
yes, Digital signals are broadcast with the same carrier frequencies so your old antenna will be fine
a wire loop moves at constant velocity without rotation through a constant magnetic field. the induced current in the loop will be?
zero; if the loop is moving through a constant field without rotation then the flux through the loop will remain constant and no current will be induced
