Ch. 24 Magnetism
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Magnetic domains normally occur in
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iron
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The source of all magnetism is
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moving electric charge
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Magnetic field lines about a current-carrying wire
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circle the wire in closed loops.
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Moving electric charges will interact with
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an electric field or a magnetic field.
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If a steady magnetic field exerts a force on a moving charge, that force is directed
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at right angles to the direction of the motion.
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Do both electrical forces and magnetic forces depend on motion?
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only the magnetic force requires motion
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Does every magnet necessarily have a north and a south pole?
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yes. poles always occur in pairs
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If magnets wont stick to your stainless steel refrigerator door, the door is probably a mixture of steel and nickel.
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but magnets will stick to stainless steel when it is made with chromium instead of nickel
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like poles repel each other
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opposite poles attract
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how can a magnet attract a piece of iron that is not magnetized
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domains in the unmagnetized piece of iron are induced into alignment by the magnetic field of the nearby magnet.
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what law of physics tells you that if a current carrying wire produces a force on a magnet, then a magnet must produce a force on a current carrying wire?
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newtons third law, which applies to ALL forces in nature
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what is the major similarity between a galvanometer and a simple electric motor? what is the major difference?
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in both, coils are positioned in a magnetic field. a force produces rotation when current passes through the coils. the major difference is that the maximum rotation of the coil in a galvanometer is one half turn, whereas, in a motor, the coil (wrapped on an armature) rotates trough many complete turns - accomplishd by alternating the current with each half turn of the armature
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Magnetic force
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between magnets, it is the attraction of unlike magnetic poles for each other and the repulsion between like magnetic poles. Between a magnetic field and a moving charged particle, it is a deflecting force due to the motion of the particle: the deflecting force is perpendicular to the velocity of the particle and perpendicular to the magnetic field lines. this force is greatest when the charged particle moves perpendicular to the field lines and is smallest (zero) when it moves parallel to the field lines.
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Magnetic field
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the region of magnetic influence around a magnetic pole or a moving charged particle
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magnetic domains
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clustered regions of aligned magnetic atoms. when these regions themselves are aligned with one another, the substance containing them is a magnet
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electromagnet
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a magnet whose field is produced by an electric current. it is usually in the form of a wire coil with a piece of iron inside the coil.
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cosmic rays
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high speed particles that travel throughout the universe
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by whom, and in what setting, was the relationship between electricity and magnetism discovered?
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Hans Christian Oersted in a high school classroom noted how a current affects a magnet, thus relating electricity and magnetism.
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what is the source of magnetic force?
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moving electrons
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in what way are magnetic poles very different from electric charges?
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magnetic poles cannot be isolated; electric charges can
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what produces a magnetic field?
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the motion of electric charges produces a magnetic field
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what is a magnetic domain?
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a magnetic domain is a culster of aligned atoms
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why is iron magnetic and wood is not?
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iron has magnetic domains, wood does not
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we learned that the direction of the electric field about a point charge is radial to the charge. what is the direction of the magnetic field surrounding a current carrying wire?
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the m agnetic field takes the form of concentric circles about the current carrying wire.
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why is the magnetic field strength greater inside a current carying loop of wire than about a straight section of wire?
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inside the loop the lines are more concentrated
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why are the magnetic fields of superconducting magnets often stronger than those of conventional magnets
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greater electron flow produces greater magnetic field strength
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in what direction relative to magnetic field does a charged particle move in order to experience maximum deflecting force? minimum deflecting force?
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force is maximum when motion is perpendicular to the field; minimum when parallel to the field
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what relative direction between a magnetic field and a current carrying wire results in the greatest force?
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force is maximum when the current is perpendicular to the field
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what is a galvanometer called when it has been calibrated to read current? when it has been calibrated to read voltage?
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when calibrated for current, a galvanometer is an ammeter; when calibrated for voltage, a voltmeter
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is it correct to say that an electric motor extends the physics that underlies a galvanometer?
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yes. a motor is a sphopisticaed galvanometer.
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what are magnetic pole reversals?
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magnetic pole reversals are reversals of the north and south poles, common throughout earths history
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all atoms have moving electric charges. why aren't all materials magnetic then?
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how the charge moves dictates the direction of its magnetic field (a magnetic field is a vector quantity)
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if you place a chunk of iron near the north pole of a magnet, attraction will occur. why will attraction also occur if you place the same iron near the south pole of the magnet?
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attraction will occur because the magnet induces opposite polarity i a nearby piece of iron. north will induce south and south will induce north. this is similar to charge induction, where a balloon will stick to a wall whether the balloon is negative or positive.
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why isnt it advisable to make a horseshoe magnet from flexible material?
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the poles of the magnet attract each other and cause the magnet to bend, even enough for the poles to touch if the material is flexible enough
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what is different about the magnetic poles of common refrigerator magnets and those of common bar magnets
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refrigerator magnets have narrow strips of alternating north and south poles. these magnets are strong enough to hold sheets of paper against the door, but they have a very short range b/c the north and south poles cancel a short distance from the magnetic surface.
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why will a magnet attract an ordinary nail or paper clip but not a wooden pencil?
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a magnet will induce the magnetic domains of a nail or paper clip into alignment. opposite poles in the magnet and the iron object are then closest to each other and attraction resutls. a wooden pencil, does not have magnetic domains that will interact with a magnet
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will either pole of a magnet attract a paper clip? Explain what is happening inside the attracted paperclip.
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the domains in the paper clip are induced into alignment in a manner similar to the electrical charge polarization in an insulator when a charged object is brought nearby. either pole of a magnet will induce alignment of domains in the paper clip
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by what mechanism do the iron filings shown about the magnet align with the magnetic field?
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the mechanism of alignment involves two factors: first each filing is turned into a tiny magnet by the magnetic field of the bar magnet, which induces domain alignment in the filing. second, a pair of equal and opposite torques act on each filing whenever it is not parallel to the magnetic field lines. these torques rotate the filings into alignment with the field lines like little compass needles.
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we know that a compass points northward because earth is a giant magnet. will the northward pointing needle point northward when the compass is brought to the southern hemisphere.
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yes. the compass aligns with earths magnetic field which extends from the magnetic pole in the southern hemisphere to the magnetic pole in the northern hemisphere.
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magnet A has twice the magnetic field strength of magnet B (at equal distance) and, at a certain distance, it pulls on magnet B with a force of 50 N. with how much force, then, does magnet B pull on magnet A.
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Magnets A and B are pulling equally on each other (newtons third law). if A pulls on B with 50 Newtons, then B also pulls on A with 50 newtons.
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a loudspeaker consists of a cone attached to a current carrying coil located in a magnetic field. what is the relationship between vibrations in the current and vibrations of the cone?
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the vibrations in both the coil and the speaker cone have identical freq at any instant
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can an electron at rest in a stationary magnetic field be set into motion by the magnetic field? what about the electron placed at rest in an electric field?
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an electron must be moving across magnetic field lines in order to feel a magnetic force. so an electron at rest in a stationary magnetic field will feel no force to set it in motion. in an electric field, however, an electron can be accelerated whether or not it is already moving.
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two charged particles are projected into a magnetic field that is perpendicular to their velocities. if the particles are deflected in opposite directions, what does this tell you about them?
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if the particles enter the field moving int he same direction and are deflected in opposite directions (say, one left and one right) the charges must be of opposite sign.
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in a mass spec, ions are directed into a magnetic field where they curve and strike a detector. if a variety of singly ionized atoms travel at the same speed through the magnetic field, would you expect them to all be deflected by the same amount or would diff ions be bent different amounts? defend your answer
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singly charged ions traveling with the same speed thru the same magnetic field experience the same magnetic force. the extent of their deflections then depends on their accelerations which in turn depend on their respective masses. the least massive ions are deflected the most, and the most massive ions are deflected the least.
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if you had two bars of iron - one magnetized and one unmagnetized - and no other materials at hand, how could you determine which bar was the magnet
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to determine only by their interactions with each other which of two bars is a magnet, place the end of bar 1 at the midpoint of bar 2 (like making a "T"). if there is an attraction then bar 1 is the magnet. if there isnt, then bar 2 is the magnet.
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will a pair of parallel current carrying wires exert forces on each other?
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Yes each will experience a force because each is in the magnetic field generated by the other. interestingly, currents in the same direction attract and currents in opposite directions repel.
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in preparation for an MRI scan, why are patients advised to remove eyeglasses, watches, jewelry, and other metal objects?
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currents are induced in metals by the changing magnetic field of the MRI device.
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a "dip needle" is a small magnet mounted on a horizontal axis so that it can swivel up or down (like a compass on its side). where on earth will a dip needle point most nearly vertically? where on earth will it point most nearly horizontally?
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the dip needle will point most nearly vertically near the earths magnetic poles, where the field points toward or away from the poles. it will point most nearly horizontally near the equator.
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what is the net magnetic force on a compass needle? by what mechanism does a compass needle align with a magnetic field?
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the net force on a compass needle is zero b/c its north and south poles are pulled in opposite directions with equal forces in the earths magnetic field. when the needle is not aligned with the magnetic field of the earth then a pair of torques is produced. this pair of equal strength torques is called a "couple", rotates the needle into alignment with the earths magnetic field.
