Magnetism

1. Magnetism Natural Attraction without pheromones

2. History of Magnets • More than 2000 years ago, rocks called lodestones were found in the region of Magnesia in Greece. • In the 12th century, the Chinese used them for navigating ships.

3. What are magnets? • Most materials • Have paired up electrons moving in opposite directions. • The field created by one moving charge is canceled by the other. • No magnetic field is created.

4. What are magnets? • Any charges in motion produce a magnetic field. • Some materials like Iron, Nickel, or Cobalt • Have a single electron or paired electrons spinning in the same directions. • The magnetic field created by one electron is not canceled by the other. • An atomic sized magnet is created.

5. Why is Fe magnetic and Al not? • What makes a good magnet? • Every spinning electron is a tiny magnet. • A pair of electrons spinning in the same direction is a stronger magnet. • A pair of electrons spinning in opposite directions work against one another; the magnetic fields cancel. • Fe has 4 unpaired electrons spinning in the same direction. • Cobalt has 3. • Nickel has 2. • Aluminum has one unpaired electron.

6. Temporary Magnets • What happens when you place a magnet next to a nail? • This is because the magnet causes the nail to become polarized; the nail becomes a magnet. • This is temporary; if you pull the magnet away, the nail loses its magnetism.

7. Permanent Magnets • Permanent magnets are produced in the same manner as the nail; however, due to the microscopic structure of the material, the magnetism becomes more permanent. • Most permanent magnets are made of ALNICO, an iron alloy containing 8% Aluminum, 14% Nickel, and 3% Cobalt. • Some rare earth elements, such as neodymium and gadolinium, produce strong permanent magnets.

8. Magnetic Domains • In magnetic materials, neighboring atoms pair up to form large groups of atoms whose net spins are aligned. • These groups are called domains. • When a piece of iron is not a magnet, the domains point in random directions.

9. Magnetic Domains • If the non-magnetized iron is placed in a strong magnetic field, the domains will line up in the direction of the field. • In temporary magnets, the domains will return to their random orientation after the field is removed. • In permanent magnets, the domains will remain aligned. • 1 domain = 1 quadrillion (1015) atoms

10. Magnetic Poles • All magnets have two regions “poles” that produce magnetic forces. • They are named like this because if you take a magnet and suspend it from the middle (so that it can swing freely), it will rotate until the north pole of the magnet points north and the south pole points south. • Like poles repel. • Opposite poles attract.

11. No less than two. • North and South cannot be separated. • If a magnet is broken, poles aren’t separated; two smaller magnets are formed.

12. Magnetic Fields • You have probably noticed that forces between the magnets (both attraction and repulsion), are felt not only when the magnets are touching each other, but also when they are held apart. • In the same way that gravity can be described by a gravitational field, magnetic forces can be described by the magnetic fields around magnets.

13. Magnetic Fields • Force lines around a magnet • Always flow from N to S • Circular in path

14. Magnetic Field Demo • What kinds of magnetic fields are produced by pairs of bar magnets?

15. MagneticField Lines • The shape of the magnetic field is revealed by magnetic field lines.

16. Magnetic Field Lines • Magnetic field lines are the same as electric field lines in that both are stronger when lines are drawn closer together. • So the magnetic field is stronger at the poles • The magnetic field lines have arrows going from north to south. • Magnetic field lines do not cross because the magnetic field cannot go in two directions at once.

17. Common Uses of Magnets • Magnetic recording media: VHS tapes, audio cassettes, floppy disks, hard disks. • Credit, debit, and ATM cards • Common television and computer monitors • Speakers and Microphones • Electric motors and generators • Compasses • Magnets can pick up magnetic items (iron nails, staples, tacks, paper clips) that are either too small, too hard to reach, or too thin for fingers to hold. Some screwdrivers are magnetized for this purpose. • Magnets can be used in scrap and salvage operations to separate magnetic metals (iron, steel, and nickel) from non-magnetic metals (aluminum, non-ferrous alloys, etc.). • Magnetic levitation transport, or maglev, is a form of transportation that suspends, guides and propels vehicles (especially trains). The maximum recorded speed of a maglev train is 361 mph.

18. How to demagnetize a magnet • Heating a magnet past its Curie temperature - the molecular motion destroys the alignment of the magnetic domains. • 768°C for Iron • Hammering or jarring – the mechanical disturbance tends to randomize the magnetic domains. • Placing the magnet in an alternating magnetic field.

19. Ferrofluid • a mixture of tiny iron particles covered with a liquid coating that are then added to water or oil. • Used in car suspensions, cancer detection, loud speakers • video

20. Earth’s Magnetic Field • Earth is a huge magnet. • This is possibly due to the molten Iron core. • The magnetic field around Earth is called the Magnetosphere

21. Earth’s Magnetic Field • Magnetic north pole is different than geographical north pole. • There is about a 25 ̊difference from geographic north pole to magnetic north pole, this is called magnetic declination • In addition, the north pole of a magnet is attracted to earth’s north pole because that is the magnetic south pole. • The south pole of a magnet is attracted to the earth’s south pole because that is the magnetic north pole.

22. Magnetosphere • Extends several tens of thousands of km into space. • Protects Earth from solar winds.

23. Dynamo Theory • The dynamo theory proposes a mechanism by which a celestial body such as the Earth generates a magnetic field. • In the case of the Earth, the magnetic field is induced and constantly maintained by the convection of liquid iron in the outer core.

24. Magnetic field of Earth is not stable • The magnetic poles of Earth wander up to 15 km every year. • Based upon the study of lava flows throughout the world, Earth's magnetic field reverses at intervals, ranging from tens of thousands to many millions of years, with an average interval of approximately 250,000 years. • The last reversal is theorized to have occurred 780,000 years ago.

25. After a being put into a strong magnetic field a temporary magnet has the following configuration. If the field if turned off, which pole is the north pole of the magnet? • Left pole • Right pole • middle • No poles [Default] [MC Any] [MC All]

27. Auroras • Charged particles from the sun become trapped in Earth’s magnetic field. • This occurs near the magnetic poles. • These charged particles collide with electrons of the atoms in our atmosphere and transfer their energy. • The colors of the lights are determined by the type of gases in the atmosphere. • O2 releases green light; N2 releases red light • aurora borealis (northern lights); aurora australis (southern lights)

28. Animal Migration • Some animal species do have the ability to detect the magnetic field, & they use it to make their migrations. • Bats and sea turtles use magnetic information to find their way. • We're not 100 percent sure how animals detect the magnetic field, but small particles of magnetite have been found in the brains of some species. Those particles may be reacting to the magnetic field and activating nerves in such a way as to send orientation information to the animal's brain.

29. Bacteria & Magnets • Some bacteria have a chain of magnetite as part of their internal structure • They use this magnetite to find their way in swamps • Bacteria in the northern hemisphere have magnetite that are opposite in polarity than the bacteria with magnetite in the southern hemisphere.

30. Animal Migration Video

31. Below show the domains of a magnet, which pole is the north pole of the magnet? • Left pole • Right pole • middle • No poles [Default] [MC Any] [MC All]

32. We can determine the magnetic field by measuring the force on a moving charge: The SI unit of magnetic field is the Tesla (T). Dimensional analysis: 1 T = 1 N·s / (C·m) = 1 V ·s / m2 Sometimes we use a unit called a Gauss (G): 1 T = 104 G The earth’s magnetic field is about 0.5 G B q v Magnetic field Units

33. .. ... .. ... .. ... .. ... x x x x x x x x x x x x x x x x x x x x B out of the page B into the page Understanding the magnetic force requires us to work in THREE dimensions. So let’s use a new notation to depict the forces in the TWO dimensional world.

34. The tail of an arrow. The tip of an arrow. . x The dots The x’s What should you use-- dots or x’s?

35. Magnetic Forces1. Forces on moving charges2. Forces on currents in wires or fluids • A charged particle in a static (not changing with time) magnetic field will experience a magnetic force only if the particle is moving. • q is charge,v is velocity, B is the magnetic field B, angle q, F is the magnitude of the force on the charge is: |F| = | q v B sinq |= q vB Or B = F magnetic qv

36. WARNING: cross-product (NOT simple multiplication!) Magnetic Force DON’T FORGET: Forces have directions! This force acts in the direction perpendicular to the plane defined by the vectors v and B as indicated by the right-hand rule!

37. A particle with a charge of 8 C is moving at 2,000 m/s when it enters a magnetic field perpendicular to its direction of motion that has a magnitude of 4 Tesla. What is the magnitude of the force exerted on the particle? • 64,000 N • 1000 N • 62.5 N • 4000 N [Default] [MC Any] [MC All]

38. Right Hand Rule • Draw vectors v and B with their tails at the location of the charge q. • Point fingers of right hand along velocity vector v. • Curl fingers towards Magnetic field vector B. • Thumb points in direction of magnetic force F on q, perpendicular to both v and B.

39. A current-carrying wire is placed in a magnetic field, as shown below. Indicate the direction of the force exerted on the wire. • up • down • right • left [Default] [MC Any] [MC All]

40. I1 I2 b Force on parallel wires • Each of two parallel wires with current I, experiences an attractive magnetic force that diminishes as one over the distance separating the wires: F  I1 I2 L / b. • L = length • We use this proportionality to define the unit of current • The force on wire 2 is equal to current in wire 2 times magnetic field from wire 1 times length of wire 2. • Magnetic field generated by a current diminishes as one over distance from wire (1/d)

41. I2 I1 r B Magnetic Field from a Wire • The magnetic field lines from a current form circles around a straight wire with the direction given by another “right hand rule” (thumb in direction of current, finger curl around current indicating direction of magnetic field).

42. The magnetic force of a straight 1.5 m segment of wire carrying a current of 9 A is 3.0 N. What is the magnitude of the component of the magnetic field that is perpendicular to the wire? • 40.5 T • 2 T • .22 T • 4.5 T [Default] [MC Any] [MC All]

43. A loudspeaker is an electromechanical device which converts an electricalsignal into sound. Loudspeakers are used in numerous applications from hearing aids to air raid sirens Loudspeakers are the most variable elements in any audio system, and may be responsible for marked audible differences between otherwise identical systems. Example Microphone Audio Circuit Speaker

44. The Main Components

45. Magnetic Forces due to Electric Current • Current is charges in motion • Causes force on magnet • Example: Compass near wire with current current wire Top View Side View

46. I B Fields of Current Distributions By winding wires in various geometries, we can produce different magnetic fields. For example, a current loop ( to plane, radius r, current emerging from plane at top of loop): Magnetic field at center of loop B = m0 I / (2r) Magnetic field far from loop: B  I·(Area of loop) / r3

47. Solenoids If we stack several current loops together we end up with a solenoid: In the limit of a very long solenoid, the magnetic field inside is very uniform: B=m0 n I n = number of windings per unit length, I = current in windings B 0 outside windings

48. Galvanometer • Current in coil is finite, due to non-zero resistance of coil • Magnetic field produces torque on current in coil. • Needle swings until magnetic torque is balanced by torsion of spring

49. A current-carrying wire is placed in a magnetic field, as in the diagram below. Find the direction of the Force applied to the wire. • up • down • right • Out of the page [Default] [MC Any] [MC All]

50. A current-carrying wire is placed in a magnetic field, as in the diagram below. Find the direction of the Force applied to the wire. • up • down • right • Out of the page [Default] [MC Any] [MC All]