Magnetism

1. Magnetism Chapter 21

2. Magnets and Magnetic Fields Section 21.1

3. History of Magnets Cretan shepherd named Magnes was herding sheep in an area of Northern Greece called Magnesia about 4,000 years ago. The nails in his shoes and the metal tip of his staff became firmly stuck to a large, black rock.

4. History Continued� Stories of magnetism date back to the first century B.C. in the writings of Lucretius and Pliny the Elder (23-79 AD). Archimedes is purported to have used loadstones to remove nails from enemy ships thus sinking them. The Chinese developed the mariner�s compass some 4500 years ago.

5. History Continued�. William Gilbert in 1600 was the first to realize that the Earth was a giant magnet and that magnets could be made by beating wrought iron.

6. Three types of magnetism Ferromagnetism � Everyday or �Normal� Magnetism Diamagnetism Paramagnetism

7. Magnetic Forces Magnetic force � the force a magnet exerts on another magnet, on iron or another metal, or on moving charges Electricity and Magnetism go hand in hand. Thus, magnetic force is part of electromagnetic force. Electricity can be used to make magnets, and magnets can be used to make electricity.

8. Magnetic Force, ctd. On what do you think magnetic force depends? Is there magnetic force when two magnetic objects are far apart?

9. Magnetic Force, ctd. Magnetic force is strongest at the poles. All magnets have two poles (North & South) The direction of the force between two magnets depends on how the poles face one another. Like poles repel, opposites attract. (Shocker.)

10. Magnetic Fields Magnetic Fields � surround a magnet and cause magnetic forces. Strongest near the poles Will attract or repel other magnets that enter the field

11. Magnetic Fields, ctd. Magnetic Field lines begin at the north pole of the magnet and go toward the south pole. (p. 631)

12. Magnetic Fields, ctd. Earth is essentially a really big magnet. There is a magnetic field all around it, called the magnetosphere. Compasses point to the magnetic north pole, which isn�t exactly the geographic north pole.

15. Northern Lights

16. Magnetic Field On The Sun

17. Magnetic Field Strength SI Unit � Tesla (T) Typical Strengths of Magnets Earth�s Magnetic Field � 4 x 10-5 T Refrigerator Magnet � 0.03 T Junkyard Magnet � 1.0 T Neodymium Magnet � 1.25 T MRI � 2-3 T Strongest Sustained Magnetic Field � 45 T Person � 3 x 10-13 T

18. Magnetic Materials A material�s magnetic field depends upon its atomic makeup. In a few materials, such as iron, nickel, and cobalt, the electrons inside make up a strong magnetic field. The fields combine to make magnetic domains. Magnetic Domain: region that has a large number of atoms with aligned magnetic fields. Domain Animation

19. Magnetic Materials, ctd. A material with magnetic domains is called a ferromagnetic material. These materials are naturally magnetized. When a ferromagnetic material is magnetized, most of its magnetic domains are aligned.

20. Magnetic Materials, ctd. Anything can become magnetic. This depends upon the structure and setup of the atoms that make up the material. Levitating Frog To magnetize the material, the magnetic domains must be aligned. Some materials� domains are more easily aligned than others. (ex. metals)

21. Magnetic Materials, ctd. Nonmagnetized materials: Just because something CAN be a magnet doesn�t mean that it is. The domains have to be aligned. Magnetized materials: How might you go about magnetizing a ferromagnetic material that isn�t magnetized?

22. Destroying a Magnet How might you go about demagnetizing a magnet? Hitting it hard Heating it

23. Electromagnetism Section 21.2

24. Electricity and Magnetism Relationship discovered in 1820 by Danish scientist Hans Christian Oersted Electricity and Magnetism are different parts of one force called the electromagnetic force. Moving electric charges create a magnetic field

25. Solenoids & Electromagnets When a wire carrying a current is looped, the magnetic fields combine to make the wire act like a bar magnet. The field through the center is very strong. Loop it more, it gets stronger

26. Solenoids & Electromagnets A coil of current-carrying wire that causes a magnetic field is called a solenoid. Place a ferromagnetic material in the middle of the coil�you have an electromagnet. (This increases the strength of the magnetic field.)

27. Solenoids & Electromagnets Changing the current in the wire controls the strength and direction of the magnetic field. The current can also be used to turn the magnetic field on and off. Scientists manipulate current and magnets all the time when making devices we use every day.

28. Solenoids & Electromagnets The strength of an electromagnet depends on: The current in the solenoid (more current ? more strength) The number of loops in the coil (more loops ? more strength) The type of material in the core

29. Electromagnetic Devices Electromagnets can take electrical energy (from the moving electrons in current) and change it into motion that can do work Electromagnetic devices therefore change electrical energy into mechanical energy. Galvanometers Electric motors loudspeakers

30. Galvanometers Measure small amounts of current using a solenoid Used in gas gauges whereby a sensor in the gas tank reduces current as the tank gets low.

31. Electric Motors Use an electromagnet to turn an axle When current flows, one side is pushed and the other is pulled, which rotates the axle Current continues to change direction, which causes it to spin