MagnetismA Strangely Attractive Topic BY WASAY ZIA +92-314-9030305
History #1 Term comes from the ancient Greek city ofMagnesia, at which many natural magnets werefound. We now refer to these natural magnetsas lodestones (also spelled loadstone; lode meansto lead or to attract) which contain magnetite, anatural magnetic material Fe3O4. Pliny the Elder (23-79 AD Roman) wrote ofa hill near the river Indus that was madeentirely of a stone that attracted iron.
History #2 Chinese as early as 121 AD knew that aniron rod which had been brought near one ofthese natural magnets would acquire andretain the magnetic property…and that such arod when suspended from a string would alignitself in a north-south direction. Use of magnets to aid in navigation can betraced back to at least the eleventh century.
Basically, we knew the phenomenon existed and we learned useful applications for it. We did not understand it.
Finally, the Science Not until 1819 was a connection between electrical andmagnetic phenomena shown. Danish scientist Hans ChristianOersted observed that a compass needle in the vicinity of awire carrying electrical current was deflected! In 1831, Michael Faraday discovered that a momentarycurrent existed in a circuit when the current in a nearbycircuit was started or stopped Shortly thereafter, he discovered that motion of amagnet toward or away from a circuit could produce the sameeffect.
Let This Be a Lesson! Joseph Henry (first Director of theSmithsonian Institution) failed to publish whathe had discovered 6-12 months before Faraday
The Connection is MadeSUMMARY: Oersted showed that magnetic effectscould be produced by moving electrical charges;Faraday and Henry showed that electric currentscould be produced by moving magnets
A Sheep in a Cow Suit? All magnetic phenomena result from forces between electric charges in motion.
Looking in More Detail Ampere first suggested in 1820 that magnetic properties of matter were due to tiny atomic currents All atoms exhibit magnetic effects Medium in which charges are moving has profound effects on observed magnetic forces
For most of our discussions, we will assume the medium is empty space,which is a reasonable approximation of air in this context.
Top Ten List What We Will Learn About Magnetism1. There are North Poles and South Poles.2. Like poles repel, unlike poles attract.3. Magnetic forces attract only magnetic materials.4. Magnetic forces act at a distance.5. While magnetized, temporary magnets act like permanentmagnets.
Top Ten continued6. A coil of wire with an electric current flowing through it becomesa magnet.7. Putting iron inside a current-carrying coil increases the strengthof the electromagnet.8. A changing magnetic field induces an electric current in aconductor.
Top Ten Continued9. A charged particle experiences no magnetic force whenmoving parallel to a magnetic field, but when it is movingperpendicular to the field it experiences a force perpendicularto both the field and the direction of motion.10. A current-carrying wire in a perpendicular magnetic fieldexperiences a force in a direction perpendicular to both thewire and the field.
For Every North, There is a SouthEvery magnet has at least one north pole and one south pole. Byconvention, we say that the magnetic field lines leave the North endof a magnet and enter the South end of a magnet. If you take a bar magnet and break it into two pieces, each piece willagain have a North pole and a South pole. If you take one of thosepieces and break it into two, each of the smaller pieces will have aNorth pole and a South pole. No matter how small the pieces of themagnet become, each piece will have a North pole and a South pole. S N S N S N
No Monopoles AllowedIt has not been shown to be possible to end up with a singleNorth pole or a single South pole, which is a monopole ("mono"means one or single, thus one pole). S NNote: Some theorists believe that magnetic monopoles mayhave been made in the early Universe. So far, none have beendetected.
Magnets Have Magnetic Fields We will say that a moving charge sets up in the space around it a magnetic field, and it is the magnetic field which exerts a force on any other charge moving through it. Magnetic fields are vector quantities….that is, they have a magnitude and a direction!
Defining Magnetic Field Direction Magnetic Field vectors as written as B Direction of magnetic field at any point is defined as the direction of motion of a charged particle on which the magnetic field would not exert a force. Magnitude of the B-vector is proportional to the force acting on the moving charge, magnitude of the moving charge, the magnitude of its velocity, and the angle between v and the B-field. Unit is the Tesla or the Gauss (1 T = 10,000 G).
The Concept of “Fields” Michael Faraday realized that ...A magnet has a‘magnetic field’distributed throughoutthe surrounding space
Magnetic Field LinesMagnetic field lines describe the structure of magnetic fieldsin three dimensions.They are defined as follows. If at anypoint on such a line we place an ideal compass needle, free toturn in any direction (unlike the usual compass needle, whichstays horizontal) then the needle will always point along thefield line.Field lines converge where the magnetic force is strong, andspread out where it is weak. For instance, in a compact barmagnet or "dipole," field lines spread out from one pole andconverge towards the other, and of course, the magneticforce is strongest near the poles where they come together.
Action at a Distance ExplainedAlthough two magnetsmay not be touching,they still interactthrough theirmagnetic fields.This explains the‘action at a distance’,say of a compass.
Force on the Charge Right Hand Rule! Put your fingers in the direction of motion of the charge, curl them in the direction of the magnetic field. Your thumb now points in the direction of the magnetic force acting on the charge. This force will bend the path of the moving charge appropriately.
Watch the Bending Fingers! QuickTime™ and a GIF decompressor are needed to see this picture.
Cyclotron Developed in 1931 by E. O. Lawrence andM. S. Livingston at UC Berkeley Uses electric fields to accelerate andmagnetic fields to guide particles at very highspeeds
How a Cyclotron Works Pair of metal chambersshaped like a pillbox cutalong one of its diameters(cleverly referred to as “D”s)and slightly separated Ds connected toalternating current Ions injected near gap Ions are accelerated as long as they remain “instep” with alternating electric field
Magnetic Force on Current-Carrying WireSince moving charges experience a force in a magnetic field, a current-carrying wire will experience such a force, since a current consists ofmoving charges. This property is at the heart of a number of devices.
Electric MotorAn electric motor, is amachine which convertselectrical energy intomechanical (rotational orkinetic) energy.A current is passedthrough a loop which isimmersed in a magneticfield. A force exists onthe top leg of the loopwhich pulls the loop outof the paper, while aforce on the bottom legof the loop pushes the The net effect of these forces isloop into the paper. to rotate the loop.
Electromagnet (Magnetism from Electricity)An electromagnet is simply a coil of wires which, when acurrent is passed through, generate a magnetic field, asbelow.
Magnetic Properties of MatterIn other words….materials which producemagnetic fields with no apparent circulation ofcharge.All substances - solid, gas, and liquid - react tothe presence of a magnetic field on some level.Remember why?How much they react causes them to be put intoseveral material “types”.
Magnet - isms Ferromagnetism - When a ferromagnetic material isplaced near a magnet, it will be attracted toward the region ofgreater magnetic field. This is what we are most familiar with whenour magnet picks up a bunch of paperclips. Iron, cobalt, nickel,gadolinium, dysprosium and alloys containing these elements exhibitferromagnetism because of the way the electron spins within oneatom interact with those of nearby atoms. They will alignthemselves, creating magnetic domains forming a permanent magnet. If a piece of iron is placed within a strong magnetic field, thedomains in line with the field will grow in size as the domainsperpendicular to the field will shrink in size.
Making a Magnet from a Ferromagnetic Material • domains in which the magnetic fields of individual atoms align • orientation of the magnetic fields of the domains is random • no net magnetic field. • when an external magnetic field is applied, the magnetic fields of the individual domains line up in the direction of the external field • this causes the external magnetic field to be enhanced
A Ferromagnet in the Middle If we look at a solenoid, but rather than air, wrap it around a nice iron core. What happens to the change in flux for a given current? Can you see why ferromagnetic materials are often put in the middle of current- carrying coils?
More Magnet - isms Diamagnetism - When a diamagnetic material is placednear a magnet, it will be repelled from the region of greatermagnetic field, just opposite to a ferromagnetic material. It isexhibited by all common materials, but is very weak. People andfrogs are diamagnetic. Metals such as bismuth, copper, gold,silver and lead, as well as many nonmetals such as water and mostorganic compounds are diamagnetic.
More Magnet - isms Paramagnetism - When a paramagnetic material isplaced near a magnet, it will be attracted to the region of greatermagnetic field, like a ferromagnetic material. The difference isthat the attraction is weak. It is exhibited by materialscontaining transition elements, rare earth elements and actinideelements. Liquid oxygen and aluminum are examples ofparamagnetic materials.