Magnetism

1. Magnetism

2. S Bar Magnet N S N Magnetism Since ancient times, certain materials, called magnets, have been known to have the property of attracting tiny pieces of metal. This attractive property is called magnetism.

3. Iron filings N S W N S N E S N Bar magnet Compass Magnetic Poles The strength of a magnet is concentrated at the ends, called north and south “poles” of the magnet. A suspended magnet: N-seeking end and S-seeking end are N and S poles.

4. Cutting a magnet in halfwill not isolate a singlenorth or south.  One magnet becomes two,then four, and so on. This process will never end: it cannot be cut to reveal a single north or single south pole. MAGNETS: A magnet has polarity-it has a north and a south pole.

5. N S S N N S S N N S Magnetic Attraction-Repulsion Magnetic Forces: Like Poles Repel Unlike Poles Attract

6. Magnetism is induced by aligning areas called domains within a magnetic field.

7. Magnetism originates in the motion of the electrons in iron atom.   Spinning electrons act like tiny magnets. Cancellation of this effect occurs in most materials. Iron, nickel, cobalt are exceptions.

8. INDUCED MAGNETISM Some metals can be turned into temporary magnets by bringing them close to a magnet

9. N S Magnetic Field Lines We can describe magnetic field lines by imagining a tiny compass placed at nearby points. The direction of the magnetic field B at any point is the same as the direction indicated by this compass. Field B is strongwhere lines are dense and weak where lines are sparse.

10. N N N S Field Lines Between Magnets Unlike poles Attraction Leave N and enter S Repulsion Like poles

11. B-field of bar magnet is similarto the Earth's magnetic field.B-field lines leave north face,enter at south face.

12. E + + ^ B v v Origin of Magnetic Fields Recall that the strength of an electric field E was defined as the electric force per unit charge. Since no isolated magnetic pole has ever been found, we can’t define the magnetic field Bin terms of the magnetic force per unit north pole. We will see instead that magnetic fields result from charges in motion—not from stationary charge or poles. This fact will be covered later.

13. Just like magnets produce magnetic fields, current carrying wires can also produce magnetic fields.

14. We learned last time that a magnetic fields exert forces on moving charges F = qvB A magnetic field bending an electron beam

15. Iron filings I I B B Magnetic Field of a Long Wire When a current I passes through a long straight wire, the magnetic field B is circular as is shown by the pattern of iron filings below and has the indicated direction. The right-hand thumb rule: Grasp wire with right hand; point thumb in direction of I. Fingers wrap wire in direction of the circular B-field.

16. ELECTROMAGNETIC INDUCTION An electric field can produce a magnetic field. The reverse is also true: amagnetic fieldcan give rise to anelectric field. A conductor that is caused to move relative to a magnetic field generates an electric field.

17. Moving the wire and keeping the magnet stationary is the same as moving the magnet and keeping the wire stationary.

18. Flux through coil changes because bar magnet is moved up and down.    Michael Faraday  (1791-1867) Michael Faraday discovered that when a conductor cuts magnetic flux lines, an emf(voltage)is produced.