chapter 9: magnetism and electromagnetic induction

RAT: the source of all magnetism is

a) tiny bits of iron

b) tiny domains of aligned atoms

c) small loadstones

d) the motion of electrons

d) the motion of electrons

RAT: surrounding moving electric charges are

a) electric fields

b) magnetic fields

c) both of these

d) neither of these

c) both of these

RAT: a magnetic force acts most strongly on a current-carrying wire when it is

a) parallel to the magnetic field

b) perpendicular to the magnetic field

c) either of both of these

d) neither of these

b) perpendicular to the magnetic field

RAT: a magnetic force acting on a beam of electrons can change

a) only the direction of the beam

b) only the energy of the electrons

c) both of these

d) neither of these

a) only the direction of the beam

RAT: when you move a bar magnet to and fro, first thrusting it into, and then withdrawing it from, a coil of wire, you induce

a) direct current

b) alternating current

c) neither dc nor ac

d) alternating voltage only, not current

b) alternating current

RAT: the principle underlying the operation of an electric motor is that

a) electric and magnetic fields repel each other

b) a current-carrying wire experiences force in a magnetic field

c) like magnetic poles both attract and repel each other

d) ac voltage is induced by a changing magnetic field

b) a current-carrying wire experiences force in a magnetic field

RAT: the essential physics concept in the operation of an electric generator is

a) coulomb’s law

b) ohm’s law

c) faraday’s law

d) newton’s second law

c) faraday’s law

RAT: a transformer works in accordance with

a) coulomb’s law

b) ohm’s law

c) faraday’s law

d) newton’s second law

c) faraday’s law

RAT: a step-up transformer in an electric circuit can step up

a) voltage

b) energy

c) both

d) neither

a) voltage

RAT: electricity and magnetism connect to form

a) mass

b) energy

c) ultra-high frequency sound

d) light

d) light

true or false

magnetic force is similar to an electrical force in that a magnet can both attract and repel without touching

true

strength of a magnet’s interaction depends on the ______ between the magnets

distance

whereas electric charges produce electrical forces, regions called _______ give rise to magnetic force

magnetic poles

if you suspend a bar magnet at its center by a piece of string, you’ve got a _______

compass

true or false

not all magnets have a north and south pole

false; All magnets have both a north and south pole (they can’t exist without one another)

electric charges can be isolated, what about magnetic poles?

no they cannot be isolated; a north magnetic pole never exists without the presence of a south pole, and vice versa

what is like the head and tail of a coin?

the north and south poles; they don’t exist without the other

true or false

you can continue breaking the pieces of a magnet in half and never isolate a single pole

true because atoms themselves are magnets

checkpoint:

does every magnet necessaringly have a north and a south pole?

yes, like every coin has a head and a tail

the space around the magnet is energized by a ___________

magnetic field

how is the shape of the magnetic field revealed?

they spread out from one pole and return to the other pole

the direction of the field outside the magnet is, by convention, from the _____ pole to the ____ pole

north to the south pole

where the lines are closer together on a field, the field is

a) stronger

b) weaker

a) stronger

is the magnetic field stronger at the poles?

yes; the lines are tighter near there

a magnetic field is produced by a motion of?

a) gravitational force

b) voltage

c) electric charge

d) current

c) electric charge

where is the motion of the common bar magnet from?

the electrons of the atoms that make up the magnet; these electrons are constantly in motion

what are the two kinds of electron motion that produce magnetism?

electrno spin and electron revolution

in most common magnets, ______ is the main contributor to magnetism

a) electron spin

b) electron revolution

a) electron spin

true or false

every spinning electron is a tiny magnet

true

a pair of electrons spinning in the same direction creates a

a) weaker magnet

b) stronger magnet

b) stronger magnet

a pair of electrons spinning in opposite directions creates a

a) weaker magnet

b) stronger magnet

a) weaker magnet because they cancel and work against each other

are most substances magnets?

no, in most atoms the various fields cancel one another because the electrons spin in opposite directions

do the fields cancel each other in these substances?

iron, nickel, and cobalt

no; therefore, these substances are magnets

made from alloys of rare-earth elements, these are the _______ permanent magnets made

a) weakest

b) strongest

b) strongest

is the term rare earth mean it is actually rare earth?

no; these metals are not particularly rare or precious and are about as abundant as tin or lead

the magnetic field of an individual iron atom is so strong that ineractions among adjacent atoms cause large clusters of them to ________

a) be perpendicular to one another

b) repel each other

c) line up with one another

c) line up with one another

these clusters of aligned iron atoms are called?

magnetic domains

in a magnetic domain, is each domain perfectly magnetized and made up of billion of aligned atoms?

yes!

is every piece of iron a magnet?

no; because the domains in ordinary iron are not aligned

in a common iron nail, how are the domains?

a) perfectly aligned

b) random

c) unaligned

b) random

how can you induce alignment on a domain that is randomly oriented?

use a magnet

true or false

the domains align themselves much as electric charges in a piece of paper align themselves (become polarized) in the presence of a charged rod

true

how can permanent magnets be made?

by placing pieces of iron or similar magnetic materials in a strong magnetic field

soft or hard iron is easier to magnetize than steel?

soft iron

true or false

stroking motion aligns the domain

true

Checkpoint:

Why doesn’t a magnet pick up a penny or a piece of wood?

a penny and a piece of wood have no magnetic domains or even magnetic atoms that can be induced into alignment

Checkpoint:

How can a magnet attract a piece of iron that is not magnetized?

like the compass of a needle, domains in the unmagnetized piece of iron are induced into alignment by the magnetic field of the magnet. one domain pole is attracted to the magnet and the other domain pole is repelled. Does this mean the net force is zero? No, because the force is slightly greater on the domain pole closet the magnet than it is on the farther pole. That’s why there is a net attraction. In this way, a magnet atttracts unmagnetized pieces of iron

________ produces a magnetic field

a moving charge

does a current of charge produce a magnetic field?

yes

the magnetic field about the current-carrying wire makes up a pattern of _______ circles

concentric circles; when the current reverses direction, the compass needles turn around, showing that the direction of the magnetic field changes also

if the wire is bent into a loop, the magnetic field lines become bunched up inside the loop. if te wire is bent into another loop that overlaps the first, the concentration of magnetic field lines inside the loops is _______

a) halved

b) doubled

c) tripled

d) quadrupled

b) doubled

it follows that the magnetic field intensity in this region is _____ as the number of loops is _______

a) increased; increased

b) increased; decreased

c) decreased; increased

d) decreased; decreased

a) increased; increased

if a piece of iron is placed in a current-carrying coil of wire, the alignment of magnetic domains in the iron produces a particularly strong magnet known as an _________

electromagnet

how can the strength of an electrmagnet be increased?

simply by increasing the current through the coil

true or false

strong electromagnets are used to control charged-particle beams in high-energy accelerators

true; they also levitate and propel high-speed trains

how is levitation accomplished?

levitation is accomplished by magnetic coils that run along a guideway

the coils _____ large magnets on the train’s undercarriage

a) attract

b) repel

b) repel

When trains are levitated a few centimeters, power supplied to the coils propels the train by continuously what?

a) producing direct electric current fed to the coils, which increases their magnetic polarity

b) alternating the electric current fed to the coils, which alternates their magnetic polarity

b

the alternating pulls and pushes produce a forward thrust

superconductors have the interesting property of _______ magnetic fields

expelling

because magnetic fields cannot penetrate the surface of a superconductor, magnets _____

a) attract

b) repel

c) levitate

c) levitate

true or false

a charged particle at rest does not interact with a static magnetic field

true

if the charged particle moves in a magnetic field, the magnetic character of a charge in motion becomes evident: how?

the charged particle experiences a deflecting force

the force is greatest when the particle moves in a direction _____ to the magnetic field lines

a) parallel

b) perpendicular

b) perpendicular

at other angles, the force is less, and it becomes zero when the particle moves _____ to the field lines

a) parallel

b) perpendicular

a) parallel

in any case, the direction of the force is always ________ to the magnetic field lines and the velocity of the charged particle

perpendicular

when a moving charge crosses through a magnetic field:

a) charge is deflected

b) no deflection occurs

a) charge is deflected

when a moving charge travels parallel to the field

a) charge is deflected

b) no deflection occurs

b) no deflection occurs

true or false

the force that acts on a moving charged particle, such as an eectron in an electron beam, does not act along the line that joins the sources of interaction. Instead, it acts perpendicularly both to the magnetic field and to the elctron beam

true

are charge particles from outer space deflected by Earth’s magnetic field?

yes; otherwise the harmful cosmic rays bombarding earth’s surface would be much more intense

if the particles are deflected while moving inside a wire, the wire is also ________

deflected

if we reverse the direction of current, the deflecting force acts in the _______ direction

a) same

b) opposite

b) opposite

force is strongest when the current is ______ to the magnetic field lines

perpendicular

true or false

the direction of force is not along the magnetic field lines or along the direction of current

true

true or false

the force is perpendicular to both field lines and current. it is a sideways force—perpendicular to the wire

true

Checkpoint

What law of physics tells you that if a magnet produces a force on a current-carrying wire, then a current-carrying wire must produce a force on a magnet?

Newton’s third law, which applies to all forces in nature

the simplest meter to detect electric current is a ______

magnetic compass

the next simplest meter to a magnetic compass is?

a compass in a coil of wires

true or false

when an electric current passes through the coil, each loop produces its own effect on the needle, so even a very small current can be detected

true

such a current-indicating instrument is called?

galvanometer

the coils turn against a spring, so the greater the current in its windings, the greater its _______

deflection

if we change the design of the galvanometer slightly so that the deflection makes a complete turn rather than partial rotation, we have an?

electric motor

-cyclic fashion to produce continous rotation

-used to run clocks, operate gadgets, and lift heavy loads

-changes direction each time the coil makes a half rotation

what does this describe?

motor

when a current passes through the loop, it flows in a _____ directions in the upper and lower sides of the loop

a) same

b) opposite

b) opposite

unlike a galvanometer, the current in a motor is _____ during each half revolution by means of stationary contracts on the shaft

a) doubled

b) reversed

c) continued

b) reversed

why do the currents in the loops alternate in motors?

alternates so that the forces in teh upper and lower regions do not change directions as the loop rotates; the rotation is continuous as long as current is supplied

many loops of wire are wound about an iron cylinder, called an _____, which then rotates when the wire carries current

armature

Checkpoint:

What is the major similarity between a galvanometer and a simple electric motor? what is the major difference?

they both use coils positioned in a magnetic field

the major difference is that the maximum coil rotation in a galvanomter is half a turn, whereas the coil in a motor (which is wrapped on an armature) rotates through many complete turns

the only current-prudcing devices were _____ which produced small currents by dissolving meteals in acids

volatic cells

faraday and henry both discovered _______—that electric current could be produced in a wire simply by moving a magnet into or out of a coil of wire

electromagnetic induction

true or false

faraday and henry discovered that voltage is caused, or induced, by the relative motion between a wire and a magnetic field

true

true or false

the greater the number of loops of wire moving in a magnetic field, the lesser induced voltage

false; the greater the number of loops moving in a magnetic field, the greater induced voltage

pushing a magnet into a coil with twice as many loops induces _____ as much voltage

twice as much voltage

is it more difficult to push a magnet into a coil made up of more or less loops?

more because newton’s third law! we basically have to work against this back force to induce more voltage

very slow motion produces

a) greater voltage

b) hardly any voltage

b

very rapid motion results in what voltage

a) greater voltage

b) hardly any voltage

a) greater voltage

what is faraday’s law?

voltage induced = number of loops x (change in magnetic field/time)

true or false

the amount of current produced by electromagnetic induction doesn’t depend on the resistance of the coil and the circuit that it connects, as well as the induced voltage

false; it does matter!

what are the three ways in which voltage can be induced in a loop of a wire?

by moving the loop near a magnet and by moving a magnet near the loop;

you can also change a current in a nearby loop

all three methods to induce voltage possess the same essential ingredient. what is that?

a changing magnetic field in the loop

when a magnet is repeatedly plunged into and back out of a coil of wire, the direction of the induced voltage

a) doubles

b) alternates

c) reverses

d) stops

b) alternates