Magnetism

1. Magnetism April 2014

2. engaging questions • Where do you find magnets? • What is the use of magnets in everyday life?

3. magnetic field lines • Sketch magnetic field lines on your notebook that you will observe for a bar magnet, a disk magnet, two bar magnets placed end to end, north pole to north pole and north pole to south pole.

4. History • The term magnetism stems from certain rocks called lodestones. • They were found in the region of Magnesia in Greece. • In the 12th century, the Chinese used them for navigating ships. • In the 18th century, the French physicist Charles Coulomb studied the forces between lodestones.

5. magnetic poles N S north pole south pole Like poles repel; opposite poles attract.

6. electric charges and magnetic poles • Magnetic poles behave similarly to electric charges in some ways, but there is a very important difference. • Electric charges can be isolated, but magnetic poles cannot. • Negatively charged electrons and positively charged protons are entities by themselves. An electron or a proton moves by itself. But a magnetic north pole cannot exist without the presence of a south pole, and vice versa. The north and south poles of a magnet are like the head and tail of the same coin.

7. A magnet pole cannot exist by itself.

8. magnetic field • the space around a magnet, in which a magnetic force is exerted, is filled with a magnetic field. • The shape of the field is revealed by magnetic field lines. • The direction of the field outside the magnet is defined as the direction in which the north pole of the compass needle points. • Where the lines are closer together, the field strength is greater.

10. magnetic field

12. Magnetism: • Magnetic field: Symbol is B • extends through space, similar to E field. • “B fields”caused by moving electric charges • no “magnetic charge” * • true even for permanent magnets • Magnetic forces: • force exerted on moving electric charges • as point charge or as electric current • •Field lines: • used to represent the field

13. the nature of a magnetic field • Magnetism is very much related to electricity. • Just as an electric charge is surrounded by an electric field, the same charge is also surrounded by a magnetic field if it is moving. • This is due to the “distortion” in the electric field caused by motion. • It was explained by Albert Einstein in 1905 in his theory of special relativity.

14. Charges in motion have associated with them both an electric and a magnetic field. • A magnetic field is produced by the motion of electric charges.

15. Draw magnetic field lines around a magnet. N S

16. Draw magnetic filed lines around the magnets. N S S N

17. Draw magnetic filed lines around the magnets. N S N S

18. Draw magnetic filed lines around the magnets. N S N S

19. Draw magnetic filed lines around the magnets.

20. magnetic domains magnetic domains The magnetic fields of individual iron atoms are so strong that interactions among adjacent iron atoms cause large clusters of them to line up with one another. These clusters of aligned atoms are called magneticdomains. Each domain is perfectly magnetized. A magnet aligns in the individual filings.

21. electric currents and magnetic fields • A moving charge produces a magnetic field. An electric current also produces a magnetic field.

22. electromagnets A current-carrying coil of wire with many loops is an electromagnet. Magnetic field lines about a current – carrying wire crowd up when the wire is bent into a loop.

23. electromagnets

24. Review questions • What do electric charges have in common with magnetic poles? • They form field lines; likes repel and opposites attract; they are caused by movement of electrons.

25. Review questions • What is the shape of the magnetic field that surrounds a current-carrying wire? • A circle around the wire • If a current-carrying wire is bent into a loop, why is the magnetic field stronger inside the loop than outside? • Because many magnetic field lines are concentrated on the small area inside the loop