11.1 magnets

electric doorbell

-a classic electric doorbell uses a magnet and a spring to drive a piece of iron into two chimes "ding dong" -when you press the doorbell button, you close an electric circuit and the resulting electric current pushes the iron magnetically into the first chime, "ding" and when you release the button your open the circuit, stopping the current and its magnetism so that the spring can push the iron back into the second chime "dong" -electric currents can produce magnetic forces. Electric currents are magnetic. More specifically, moving electric charge produces a magnetic field.

compasses

-a simple permanent magnet with one north magnetic pole and one south magnetic pole. -aids in navigation since Earth itself has a magnetic dipole and that dipole affects the orientation of the needle - the needle's north magnetic pole tends to point northward. -you can guess what must be located near Earth's north geographic pole-a south magnetic pole. Attraction from that south magnetic pole is what draws the compass's north magnetic pole toward the north. --> however, Earth's magnetic poles are actually located far beneath its surface and aren't perfectly aligned with the geographic poles. Magnetically active materials in everything from distant mountains to nearby buildings assert their own magnetic influences on the compass needle. --> overall the compass needle is responding to the influences of countless magnetic poles.

an electromagnet

-currents produce magnetic fields -direction of current determines direction of magnet -not permanent: current is off --> magnet is hone

electromagnet

-first observed that current in a wire caused a nearby compass needle to rotate. -when you use iron powder to disclose the magnetic flux lines surrounding a long, straight, current carrying wire, those flux lines circle the wire like concentric rings, growing more widely operated as the distance from the wire increases. -the wire is an electromagnet, a device that becomes magnetic when it carries an electric current. Because an electromagnet has no true magnetic poles, the magnetic flux lines can't stretch from north to south pole. Instead each flux line of an electromagnet is a closed loop. -Since the flux lines are packed tightest near the surface of the current-carrying wire, thats where the magnetic field is strongest. A piece of iron is pulled toward increasing magnetic field, we see that the wire will attract iron to it whenever its carrying a current.

magnetic field

-hard to sum up all the separating influence--> easier to view compass needle as interacting with something local--a magnetic field, an attribute of space that exerts a magneostatic force on a pole. -the compass needle responds to the local magnetic field, a field that's created by all the surrounding magnetic poles. -as with an electric field, the magnetic field here appears to be acting merely as an intermediary; various poles produce the magnetic field and this magnetic field affects our compass needle. -magnetic field can be created by things other than pole.

magnets and magnetic fields

-like poles repel, opposite poles attracts -only a few materials show strong magnetic effects (ferromagnetic materials: iron, cobalt, nickel and gadolinium) -there are no magnetic monopoles, i.e. when cutting a magnet the magnet is not operated into south and north, but two new magnets are obtained. (electric charges are difference, since a single electric charge does exist) -magnets are surrounded by a magnetic field, charges are surrounded by an electric field. .

Iron Fillings

-magnetic fields seem abstract,--> if you sprinkle iron filing into the field you can see them. -like tiny compass needle the iron particles magnetize along the local magnetic field and then stick together, north pole to south pole, in long strings that delineate the magnetic field! -

permanent magnet

-not all permanent magnets are as simple as that -depending on hw they were magnetized they can have their north and south poles located in unexpected places and even have more than one pair of poles. -plastic sheet magnets are good example of multiple-pole magnets: each has a repeating pattern of alternate poles along its length. Patterns vary but most have poles that form alternating parallel stripes.

ferromagnetic material

-ordinary steel is this material. It is actively and unavoidably magnetic on the size scale of atoms. If you try the above trick with a plastic or aluminum surface, the button magnet won't stick. That's because steel develops a strong magnetic polarization. -electrons, protons and neutrons-> all have magnetic dipoles particularly the electrons and the atoms they from often display this magnetism--> most isolated atoms have significant magnetic dipoles. -although most atoms are instrinsically magnetic, most materials are not--> bc another round of pairing and canceling occurs when atoms assemble into materials. This second round of cancellation is usually so effective that it completely eliminated magnitism at the atomic scale.

magnetic dipole

-pairs of magnetic pole are available in nature. These pairs consist of equal north and south poles, spatially separated from one another. -since the two opposite poles have equal magnitudes, they sum to zero and the magnetic dipole has zero net magnetic force

magnetic monopoles

-particles that carry pure north or south magnetic poles have never been found. -such pure magnetic particles may not even exist in our universe. -explains why there is no magnetic spark

an aside: the earth as a magent

-the earth has a magnetic field. Earth acts like a huge magnet in which the south pole of the earth's magnet is north (north pole of a compass needle points towards it) -magnetic poles are not at geographic poles, magnetic north pole is in Northern Canada. It moves with about 64 miles/ year toward northern Russia. Deviation between turn north (rotation axis) and magnetic north is called magnetic declination -the angle that the earth's magnetic field makes with the horizontal at any point is referred to as the angle of din

magnetic field lines

-the force one magnet exerts on another can be described as the interaction between on magnet and the magnetic field of the other -can draw magnetic field lines -the direction of the magnetic field is tangent to ta line at any point -the number of lines per unit area is proportional to the magnitude (strength of the field) -outside a magnet, lines point from the North to the South pole

the Fridge: iron and steel

-the magnet is attracted to the fridge, however if you flip the button magnet over, thinking that it will now be repelled by the fridge--> wrong. Although the fridge is clearly magnetic, its magnetism somehow responds to the button magnets so that the two always attract. -the steel in a drive is composed of countless microscopic magnets, each with a matched north a south pole. Normally those individual magnetic dipoles are oriented semi-randomly (a) so the fridge exhibits no overall magnetism. When you bring one pole of a button magnet near the fridge its tiny magnets evolve in size, shape and orientation. those tiny magnets reorient to attract it In soft magnetic material this reorientation is only temporary (b). Overall opposite poles shift closer to the button magnet's pole and like poles shift father from the button magnet's pole. --> Steel develops a magnetic polarization and consequently attracts the pole of the button magnet.

contunued

-the magnetic field around a current carrying wire is fairly weak, however a practical doorbell winds that wire into a coil to concentrate and strengthen its field. -the flux lines outside the coil resemble those outside a button magnet of similar dimension. It as though the coil as a north pole at one end and a south pole at the other. Because there are no true poles present however, the flux lines don't end anywhere. Instead they continue right through the middle of the coil and form complete loops. -when current flows through the coil, nearby iron finds itself magnetized along the local magnetic field and then pulled toward increasing field-toward the tightly packed flux lines at the coil's end. Since the flux lines continue right into the coil and grow even more tightly packed inside, the iron will be pulled inward toward the very center of the coil. -that's how the doorbell works. When you press the doorbell button, current flows through a coil of wire and the resulting magnetic field yanks an iron rod into the center of that coil. About the time the rod reaches the center, par of it hits the first chime. When you then open the switch, stopping the current and its magnetism, a spring pushes the iron rod back out of the coil and hits the second chime. (a) the magnetic field around a loop of current carrying wire points up through the loop and down around the outside of the loop. The magnetic field arrow passing through each black dot indicates the magnitude and direction of the force a north test pole would experience at the dot's location. (b) the field produced by a two-pole button magnet is almost identical to that. -while current is flowing through the coil and the iron rod is inside it, the two objects act as a single powerful electromagnet. The magnetic field surrounding the pair is the sum of the coil's modest magnetic field and the magnetized iron's much strong field. In effect the current in the coil magnetizes the iron and the iron creates most of the surrounding magnetic field. practical electromagnets, which control switches and valves in your furnace or air conditioner and can lift cars at the scrap yard, generally use urn or related materials to dramatically enhance the magnetic field produced by a current in a coil of wire.

magnetostatic force/magnetic field

-the magnetic field at a given location measure the magneto static force that a unit of pure north pole would experience if it were placed at that point--> equal to the pole times the magnetic field at the pole's position -magnetostatic force=pole x magnetic field where the magneto static force is in the direction of the magnetic field -a negative amount of pole (a south pole) experiences a force opposite the magnetic field -SI unit of magnetic field is the new per ampere meter (tesla) -Earth magnetic field is relatively weak: .00005 T in a roughly northward direction. Earth's field pushes the compass needle's north pole northward and south pole southward. Unless the compass needle is perfectly aligned with that field, it experiences a torque and undergoes angular acceleration -since its mount allows the needle to rotate only horizontally and it experience little friction as it does, the needle son settles down with its north pole pointing roughly northward--> the needle minimizes its magneto static PE by pointing along the direction of the local magnetic field and is thus in stable equilibrium when oriented that way. -b/c Earth's magnetic field is so uniform in vicinity of the compass, its northward push on the needle's north pole exactly balances its southward push on the needle's south pole and the needle experience zero net force. Ig you bring the compass near a button magnet, the local magnetic field will not be uniform and the needle may experience a net force. The magnetic field gets stronger near one of the button's poles and the compass needle will experience a net force toward or away from that pole, depending on which way it is oriented. -when the needle is aligned with a nonuniform field-its north pole pointing in the same direction as the local field-the force on tis two opposite poles won't balance and it will experience a net force in the direction of increasing field. ---> if its aligned against the field, it will experience a net force in the direction of the decreasing field.. ---> in practice, as you bring the compass near your button magnet, its needle will first pivot into alignment with the local field and then find itself pulled toward increasing field, toward the nearest pole of the button magnet. The same thing happens when your bring two button magnets together; each pivots into alignment with the other's magnetic field and the two then leap at each other. -a piece of steel exhibits similar behavior when you hold it near a button magnet: it becomes magnetized along the direction of the local magnetic field and then finds itself pulled toward increasing field, toward the button magnet's nearest pole. That's ho the button magnet hold a note to the fridge.

some materials that can be permanently magnetized

-these are permanent magnets -they are usually made of steel, or other materials that can retain the orientation of its magnetic domains for a long time.

magnetic field produced from current through a wire loop

-we use right hand rule to find direction of a field. Align thumb with current (positive charges)--> fingers point in direction of field (north to south) -the higher the current and the more coils we have, the stronger the field (magnet)

Connections between electricity and magnetism

1. moving electric charge produces a magnetic field.

few materials

few avoid this total cancellation and thus manage to remain magnetic at the atomic scale. The most important of these is the ferromagnets, include ordinary steel and iron. -if you examine a small region of ferromagnetic steel, you'll find it is composed of many microscopic regions or magnetic domains that are naturally magnetic and cannot be demagnetized (a) Within a single domain, all the atomic-scale magnetic dipoles are aligned and together they give the overall domain a substantial net magnetic dipole. -while common steel always has these magnetic domains, magnetic interactions orient nearby domains so that their magnetic dipoles oppose one another and cancel. The microscopic magnets balance on another so well that the steel appear nonmagnetic. -when you bring a strong magnetic pole near steel (b) the individual domains grow or shrink, depending on which way they're orientated magnetically. The steel undergoes magnetization and becomes magnetized (c). The atoms themselves don't move during this process; the change is purely a reorientation of the atomic scale magnetic dipoles. Domains that attract your button magnet's pole grow while those that repel it shrink, and it sticks to the fridge.

electromagnetic

if the flux lines you're following doesn't end at a pole, what created its magnetic field? the answer reveals a deep connection between magnetism and electricity. Some magnetic fields aren't produced by magnetic poles at all; they're produced by electricity. -

EX: you place a long tell wrench in the 1-T field near a strong magnet. The field magnetizes the wrench, and it develops a north pole of 1000 Axm at its near end and an equal south pole at its distant end. Only the near end of the wrench is in the 1-T field and experiences a magnetic force. What force does the field exert on the wrench and its north pole?

it exerts almost 1000N in the direction of the field. --> The force exerted on the wrench's north pole is equal to its 1000 Axm pole times the 1-T magnetic field. Since 1 T- 1 N/Axm, that product is 1000 N and points in the direction of the field.

EX: if you touch the north pole of a permanent magnet to one end of a steel paper clip, the clip's other end will become magnetic. What pole will that other end have?

it will have a north pole. the paper clip will become magnetically polarized with its south pole nearest the permanent magnet's north pole. the other end of the paper clip will have a north pole and will be able to polarize other paper clips. These polarized clips attract one another strong enough to cling together in a long chain

EX if you bring the north pole of a large strong magnet near the north pole of a small weak magnet that you are holding in place, what will happen to that small magnet?

its magnetic poles will interchange. Even though the small magnet can't move, its magnetic poles can. When the repulsion between the two north poles becomes strong enough, the poles of the small magnet will interchange and it will then present its south pole to the north pole of the large magnet. You will have premaritally reversed the small magnet's poles.

magnetic poles

north poles carry positive amounts of magnetic pole while south poles carry negative amounts. -the manetostatic forces between two poles grow weaker as they move apart and are inversely proportional to the square of the distance between them

soft magnetic material

one that demagnitzes itself easily when all nearby poles are removed -chemically pure iron, which has few flaws is a soft magnetic material -easy to magnetize and easy to demagnetize

hard magnetic material

one that does not demagnetize itself easily and that tend to retain whatever domain structure is imposed on it by its most recent exposure to strong nearby poles. (c) (i.e. button magnet) --> like steel, the material in the button magnet is ferromagnetic, unlike steel the button magnet's domains do not shrink or grow easily. During its manufacture the button magnet was magnetized by exposing it to such strong magnetic influences that its domains rearranged to give it permanent magnetic poles. It now has a north and south pole. Unless you expose the button to extremely strong magnetic influences or heat it or pound it, it will retain its present magnetization --> a permanent magnet -abilily to remember its magnetization can be useful for saving information. Once magnetized in a particular manner so as to represent a piece of info, the material will retain its magnetization and the associated info until something magnetizes it differently. Info retention in hard magnetic materials form the basis for most magnetic recording and storage, like credit card.

Coulomb's law for magnetic poles

the forces between magnetic poles are promotional to the amount of each pole and inversely proportional to the square of their separation force=permeability of free space x pole₁ x pole₂ ÷ 4π x (distance between poles)² OR F=µ₀× p₁×p₂ ÷ 4πr² the permeability of free space is 4π x 10^-7 N/A² -the force that pole 1 exerts on pole 2 is equal in amount but oppositely directly from the force that pole 2 exerts on pole 1.

EX in MRIs a patient is immersed in an intense magnetic field.that field is created entirely without permanent magnets or iron. How is that possible?

the magnetics field is created by the current in a coil of wire. MRI requires a magnetic field that is intense, uniform, and spacious enough for a patient to fit inside. The best way to create a colossal field is with a current carrying coil. The field is so enormous that its flux lines extend far from the magnett and can attract iron or steel objects from across the room.

EX you lock the needle of your compass and move its north pole near the north pole of a powerful button magnet. will the needle experience a magneto static force toward strong field or weak field?

the needle will experience a force toward weak magnetic field (away from the button magnet) With its magnetic poles aligned opposite to the button's magnetic field, the compass needle experiences a force toward weaker magnetic field. Actually, if you continue to push the needle closer to the button magnet, you can accidentally re-magnetize the needle; its pole will permanently interchange and it will subsequently point south rather than north!

EX if you sprinkle iron fillings onto the magnetic strip of a credit card, a pattern of tiny iron bridges will from. Where are the magnetic poles relative to those bridges

the poles are at the ends of the bridges. The iron filings follow the magnetic flux lines, which extend from north poles to south poles. Thus, one end of each bridge is a north pole and the other end is a south pole.

you have a disk shaped permanent magnet. the top surfaces is its north pole and the bottom surface is its south pole. if you crack the magnet into two half circles, the two halve will push apart. Why?

the top surfaces of both halves are still north poles, and the bottoms surfaces are still south poles. The two tops repel, as do the bottoms. This puzzling phenomenon, in which a shattered permanent magnet opposes attempts to erasable it, is an illustration of the PE contained in a permanent magnet. The magnet is a collection of many tinier magnets all aligned with their north poles together and their south poles together. Like poles repel one another, so the tiny magnets are difficult to hold together. Given a change, the magnet will push apart into fragments. Very strong permanent magnets release so much PE when they break that they practically explode when cracked.

magnetic flux lines

these strands map the magnetic field in an interesting way, first at each point on a strand, the strand points along the local magnetic field. Second, the strands are most tightly pack where the local magnetic field is strongest--> the strands follow along the local magnetic field direction and have a density proportional to that local field. The line highlighted by the strands are the magnetic flux lines -flux lines are helpful when exploring a magnetic field, and in a large area--> ins teach you can hold a compass in your hand and walk in the direction its needle is point--the direction of the magnetic field. The path you'll follow in this compass-guided walk is a magnetic flux line. If you repeat this trip from many different starting points, you'll explore the whole magnetic field, flux line by flux line. -Since a magnetic field tends to point away from north poles, and toward south poles, these tours will typically take you from north poles to south poles. In fact, for the permanent magnets, every magnetic flux line begins at a north pole and ends at a south pole--> flux lines never start at or end on anything other than a magnetic pole. While flux lines remerge in all directions from a north pole and coverage form all directions on a south pole; thats it; flux lines never begin or end in empty space.

demagnetized

when you remove the button magnet from the fridge, the steel returns to its original nonmagnetic state. The demagnetization process isn't perfect because some of the domains get stuck. -although magnetic forces within the steel favor a complete return to apparent non magnetism, chemical forces can make it hard for the domain to grow or shrink. Adjacent domains are separated by domain walls, boundary surface between one direction of magnetic orientation and another. These domain walls must move if the domains are to change size. However flaws in the steel can interact with a domino wall and keep it from moving--> fails to demagnetize itself completely. To remove the last bit of residual magnetism from steel you must help the domain walls move typically with heat or technical shock.