Shri. Belheshwar Vidyamandir, Belhe Std. 8th Subject- Scicence WEL-COME Shri. Belheshwar Vidyamandir, Belhe Std. 8th Subject- Scicence

Sayali Dilip Dhumal B.Sc.B.Ed (Physics) Present By Sayali Dilip Dhumal B.Sc.B.Ed (Physics)

A Strangely Attractive Topic Magnetism A Strangely Attractive Topic

MAGNETISM History of Magnetism Bar Magnets Magnetic Dipoles

History of Magnetism The first known magnets were naturally occurring lodestones, a type of iron ore called magnetite (Fe3O4). People of ancient Greece and China discovered that a lodestone would always align itself in a longitudinal direction if it was allowed to rotate freely. This property of lodestones allowed for the creation of compasses two thousand years ago, which was the first known use of the magnet. In 1263 Pierre de Maricourt mapped the magnetic field of a lodestone with a compass. He discovered that a magnet had two magnetic poles North and South poles. In the 1600's William Gilbert, physician of Queen Elizabeth I, concluded that Earth itself is a giant magnet. In 1820 the Danish physicist Hans Christian Ørsted discovered an electric current flowing through a wire can cause a compass needle to deflect, showing that magnetism and electricity were related.

History (cont.) In 1830 Michael Faraday (British) and Joseph Henry (American) independently discovered that a changing magnetic field produced a current in a coil of wire. Faraday, who was perhaps the greatest experimentalist of all time, came up with the idea of electric and magnetic “fields.” He also invented the dynamo (a generator), made major contributions to chemistry, and invented one of the first electric motors In the 19th century James Clerk Maxwell, a Scottish physicist and one of the great theoreticians of all times, mathematically unified the electric and magnetic forces. He also proposed that light was electromagnetic radiation. In the late 19th century Pierre Curie discovered that magnets loose their magnetism above a certain temperature that later became known as the Curie point. In the 1900's scientists discover superconductivity. Superconductors are materials that have a zero resistance to a current flowing through them when they are a very low temperature. They also exclude magnetic field lines (the Meissner effect) which makes magnetic levitation possible.

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.

So.... 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....

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 S N 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).

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.

Field Lines Around a Magnet

Field Lines Around a Doughnut Magnet

Field Lines Around a Bar Magnet

Thank You…