2. 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.
3. 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.
4. Basically, we knew the phenomenon existed and
we learned useful applications for it.
We did not understand it.
5. 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.
6. 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
7. 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
8. A Sheep in a Cow Suit?
All magnetic phenomena result from forces between electric charges in motion.
9. 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
10. For most of our discussions, we will assume the medium is empty space, which is a reasonable approximation of air in this context.
11. Top Ten List
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.
What We Will Learn About Magnetism
12. 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.
13. 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.
14. 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
15. 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
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16. 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!
17. 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).
22. 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.
28. 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.
