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Magnetism A Strangely Attractive Topic
History #1 • Term comes from the ancient Greek city of Magnesia, at which many natural magnets were found. We now refer to these natural magnets as lodestones (also spelled loadstone; lode means to lead or to attract) which contain magnetite, a natural magnetic material Fe3O4. • Pliny the Elder (23-79 AD Roman) wrote of a hill near the river Indus that was made entirely of a stone that attracted iron.
History #2 • Chinese as early as 121 AD knew that an iron rod which had been brought near one of these natural magnets would acquire and retain the magnetic property…and that such a rod when suspended from a string would align itself in a north-south direction. • Use of magnets to aid in navigation can be traced 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 and magnetic phenomena shown. Danish scientist Hans Christian Oersted observed that a compass needle in the vicinity of a wire carrying electrical current was deflected! • In 1831, Michael Faraday discovered that a momentary current existed in a circuit when the current in a nearby circuit was started or stopped • Shortly thereafter, he discovered that motion of a magnet toward or away from a circuit could produce the same effect.
Let This Be a Lesson! • Joseph Henry (first Director of the Smithsonian Institution) failed to publish what he had discovered 6-12 months before Faraday
The Connection is Made SUMMARY: Oersted showed that magnetic effects could be produced by moving electrical charges; Faraday and Henry showed that electric currents could be produced by moving magnets So....
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 Magnetism 1. 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 permanent magnets.
Top Ten continued 6. A coil of wire with an electric current flowing through it becomes a magnet. 7. Putting iron inside a current-carrying coil increases the strength of the electromagnet. 8. A changing magnetic field induces an electric current in a conductor.
Top Ten Continued 9. A charged particle experiences no magnetic force when moving parallel to a magnetic field, but when it is moving perpendicular to the field it experiences a force perpendicular to both the field and the direction of motion. 10. A current-carrying wire in a perpendicular magnetic field experiences a force in a direction perpendicular to both the wire and the field.
For Every North, There is a South Every magnet has at least one north pole and one south pole. By convention, we say that the magnetic field lines leave the North end of a magnet and enter the South end of a magnet. If you take a bar magnet and break it into two pieces, each piece will again have a North pole and a South pole. If you take one of those pieces and break it into two, each of the smaller pieces will have a North pole and a South pole. No matter how small the pieces of the magnet become, each piece will have a North pole and a South pole. S N S N S N
No Monopoles Allowed It has not been shown to be possible to end up with a single North pole or a single South pole, which is a monopole ("mono" means one or single, thus one pole). Note: Some theorists believe that magnetic monopoles may have been made in the early Universe. So far, none have been detected. S N
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).
Famous, continued Gauss
The Concept of “Fields” Michael Faraday realized that ... A magnet has a ‘magnetic field’ distributed throughout the surrounding space
Magnetic Field Lines Magnetic field lines describe the structure of magnetic fields in three dimensions.They are defined as follows. If at any point on such a line we place an ideal compass needle, free to turn in any direction (unlike the usual compass needle, which stays horizontal) then the needle will always point along the field line. Field lines converge where the magnetic force is strong, and spread out where it is weak. For instance, in a compact bar magnet or "dipole," field lines spread out from one pole and converge towards the other, and of course, the magnetic force is strongest near the poles where they come together.
Action at a Distance Explained Although two magnets may not be touching, they still interact through their magnetic 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.
Cyclotron • Developed in 1931 by E. O. Lawrence and M. S. Livingston at UC Berkeley • Uses electric fields to accelerate and magnetic fields to guide particles at very high speeds
How a Cyclotron Works • Pair of metal chambers shaped like a pillbox cut along one of its diameters (cleverly referred to as “D”s) and slightly separated • Ds connected to alternating current • Ions injected near gap • Ions are accelerated as long as they remain “in step” with alternating electric field
Magnetic Force on Current-Carrying Wire Since moving charges experience a force in a magnetic field, a current-carrying wire will experience such a force, since a current consists of moving charges. This property is at the heart of a number of devices.
Electric Motor An electric motor, is a machine which converts electrical energy into mechanical (rotational or kinetic) energy. A current is passed through a loop which is immersed in a magnetic field. A force exists on the top leg of the loop which pulls the loop out of the paper, while a force on the bottom leg of the loop pushes the loop into the paper. The net effect of these forces is to rotate the loop.
Electromagnet (Magnetism from Electricity) An electromagnet is simply a coil of wires which, when a current is passed through, generate a magnetic field, as below.
Magnetic Properties of Matter In other words….materials which produce magnetic fields with no apparent circulation of charge. All substances - solid, gas, and liquid - react to the presence of a magnetic field on some level. Remember why? How much they react causes them to be put into several material “types”.
Magnet - isms • Ferromagnetism - When a ferromagnetic material is placed near a magnet, it will be attracted toward the region of greater magnetic field. This is what we are most familiar with when our magnet picks up a bunch of paperclips. Iron, cobalt, nickel, gadolinium, dysprosium and alloys containing these elements exhibit ferromagnetism because of the way the electron spins within one atom interact with those of nearby atoms. They will align themselves, creating magnetic domains forming a permanent magnet. If a piece of iron is placed within a strong magnetic field, the domains in line with the field will grow in size as the domains perpendicular 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 placed near a magnet, it will be repelled from the region of greater magnetic field, just opposite to a ferromagnetic material. It is exhibited by all common materials, but is very weak. People and frogs are diamagnetic. Metals such as bismuth, copper, gold, silver and lead, as well as many nonmetals such as water and most organic compounds are diamagnetic.
More Magnet - isms • Paramagnetism - When a paramagnetic material is placed near a magnet, it will be attracted to the region of greater magnetic field, like a ferromagnetic material. The difference is that the attraction is weak. It is exhibited by materials containing transition elements, rare earth elements and actinide elements. Liquid oxygen and aluminum are examples of paramagnetic materials.
