MAGNETISM

1. MAGNETISM Mole A-Train Kenny Johnny Wang

2. _ + WHAT IS MAGNETISM? A Magnet can be compared to a electric dipole, with field lines exiting one side and coming back into the opposite side. All magnets have a north end and a south end, field lines exit from the north and enter the south. The difference between an electric field and a magnetic field, is that while an electric field effects all charges, a magnetic field only effects charges when they are in motion. Magnetic field lines of a bar magnet

3. Force Due to Magnetic Field The difference between an electric and a magnetic field is that where a electric fields apply a force on any charged particle, a magnetic field only applies a force on a charge in motion. The force felt by the charge in motion is given by the formula: F = q v b sin (  ) F = Force q = Charge on the particle v = velocity of the particle b = magnetic field magnitude  = angle between the velocity vector and the magnetic field

4. EFFECTS OF MAGNETISM The force on a particle with a charge of q moving with velocity v is given by the vector equation F = qv · B, where B is the direction and magnitude (a vector) of the magnetic field. When the charged particle is moving parallel to the magnetic field, the force on it is zero. But when the particle is moving in any other direction, there is a net force on the particle.See next slide No net force  Please Remember: F = qv * B q is in coulombs, v in m/s, and B is in teslas.

5. EFFECTS OF MAGNETISM (CONT’D) As said in the previous slide, there is a net force if the particle moves in a direction not parallel to the magnetic field. The force on the particle is perpendicular to both v and B. But how do you figure out in which direction? See next slide ?

6. EFFECTS OF MAGNETISM (CONT’D) It is by something know as the “Right hand rule.” First hold out your right hand. Point your fingers in the direction of the velocity times the sign of the charge. Curl them towards the direction of the magnetic field. Stick out your thumb, and that will be the direction of the force on the particle.   Remember (AGAIN):F = qv * B Yellow line = F = Line pointing towards you  = Line pointing away from you Red = + charge Blue = - charge Purple = v Black line = B

7. EFFECTS OF MAGNETISM (CONT’D) In the last slide we mentioned the of the right hand rule. This is also true for an wire or other object with an electrical current flow through it, since a current is nothing more than a flow of positive (negative) charges. To use the right hand rule, simply replace v with the direction of the flow of the current. An equivalent equation for this is F = iL*B, where L is the length of the wire in meters that lies in the magnetic field. Try solving the problem on the right (the magnetic field is uniform). N i  S

8. EFFECTS OF MAGNETISM (CONT’D) ANSWER: N Now, let’s try this. Let’s say its length is 25 cm, and the current going through it is 5 A, but we don’t know the magnitude of the magnetic field, but we measure the force on the bar to be 10 N. What is the field strength?           i  ANSWER #2: 10 N = (5A)(0.25m)*B (10 N)/[(5A)(0.25m)] = B B = 8 N/A*m = 8 T S

9. MONOPOLE MAGNETS DON’T EXIST Magnets always have both north and south poles. The reason why monopole magnets don’t exist is because the north and south poles are created by the alignment of the molecules inside. They are all aligned in the direction of the north pole, so even if a magnet is broken in half, the alignment of the molecules has not changed, so there is the north and south poles still exist.

10. UNITS USED IN MAGNETISM ampere (A or amp): The ampere is the SI base unit of electrical currents. One ampere is the current that would create, between two infinitely long parallel wires with negligible cross section place one meter apart in a perfect vacuum, a force of 0.2 micro newtons between each other per meter of length. All other electrical units are all defined in terms of the ampere. The unit is known informally as the amp, but A is its official symbol. coulomb (C) The SI unit of electric charge. One coulomb is the amount of charge accumulates in one second by a current of one ampere. Since electricity a flow of electrons, one coulomb represents the charge of approximately 6.241 506 x 1018 electrons. C = s·A

11. UNITS (CONT’D) tesla (T): The tesla is the SI unit of flux density (or field intensity) for magnetic fields. A tesla is the field intensity required to generate one newton of force per ampere of current per meter of conductor. A magnetic field of one tesla is very powerful magnetic field. Sometimes it may be convenient to use the gauss, which is equal to 1/10,000 of a tesla. The tesla is probably the most important unit used in magnetism. T = N/A·m = kg/(A·s2)·m

12. QUANTUM MECHANICS EXPLANATION OF MAGNETISM Magnetic fields are due to the flow of electron, also called an electrical current. This is how one can explain the intrinsic magnetic properties of electrons called spin angular momentum and orbital angular momentum.

13. Orbital Angular Momentum Orbital momentum is just as it sounds; an electron orbits the nucleus of an atom and from this it carries orbital angular momentum. Conceptually you can think about it this way but this is not actually what happens. What actually occurs is very complicated and can only be explained with quantum mechanics.

14. Spin Angular Momentum To understand this, you must understand that electrons have a property called spin, which is kind of conceptually analogous to the spin of a spinning top. This gives it spin angular momentum. Basically, along with orbital angular momentum, spin angular momentum gives it a vector quantity, meaning it moves with direction. Here is a site that shows in a more complex way how the direction of the spin would affect the magnetic field.:Spin and Win

15. Magnets A bar magnet is made using these properties of electrons and atoms. If all of the magnetic poles due to these properties are lined up in a solid, a magnet is formed. To make a magnet out of a metal, on can metal it, and expose it to a magnetic field as it becomes a solid again, causing the poles to line up and form a permanent magnet.

16. PERMANENT MAGNETS Because of its intrinsic properties, the atoms of a metal become tiny magnets with two poles, a dipole, and when groups of these dipoles that are pointing in the same direction come together they make what is known as a domain. If the power of a magnetic field is strong enough it can align the domains resulting in the overall magnetism of the material. Lets melt this down, and bring in a magnetic field. Temperature Now, when we let the solid cool down, and take away the external magnetic field, we have formed a magnet in the same direction as the magnetic field from earlier Melting point Bar Magnet Domains

17. MAGNETIC DOMAINS Magnetic domains are groups of atoms that have the same magnetic alignment. Think of them as super-tiny magnets. In a non-magnetized piece of iron, these domains have random magnetic field alignments, and cancel each other out. But in a magnet, they are all aligned in one direction, producing a net magnetic field. The pictures bellow show domains with little disbursements of arrows pointing in various directions. Usually only iron, nickel, or cobalt can have domains that align. Domain Demo This is what happens when a magnet gets too close to the grid of domains, it aligns the arrows with the magnets field.

18. SOURCES OF MAGNETIC FIELDS There are many sources of magnetic fields, not just from a bar magnet, and many of them will be described in this presentation.

19. BAR MAGNETS Bar magnets are metal bars that have magnetic properties. The magnetic field produced by a bar magnet flows from the north end to the south end. It is a permanent magnet. The Earth can be considered a bar magnet as well. It has a pair of geographical poles and magnetical poles. See next slide Bar magnet demo

20. THE EARTH AS A BAR MAGNET Therefore, Earth is a gigantic magnet. However, the magnetic poles of the earth are offset from the geographic ones by 11.5°. Interesting enough, the true magnetic north pole of the Earth is in fact closer to the south pole rather than the north pole. Although the north & south poles of a compass point to there respective directions when used for direction finding, all magnetic poles are attractive to the opposite pole; the north pole of a compass must point towards a south pole and vice versa. So in truth, it’s more like a 168.5° offset. Scientists suggest that the magnetic poles are moving further apart from the geographic poles at a slow rate each year, they also predict that in the future the magnetic flow will be disrupted, forcing it to switch directions.

21. THE EARTH AS A BAR MAGNET (CONT’D) The fact that it has a magnetic field is very important, because blocks out harmful solar radiation. Also, we know that it flipped around because of sea floor spreading. Magma escapes out of the rift between plates and cool. Tiny bits of magnets gets permanently aligned one way or another due to the magnetic field of the Earth during creation. Afterwards, when the sea floor moves out, the bits of magnets retain their alignment for millions of years. Scientists have found that different strips of sea floor, corresponding to a uniquely different time period, have alignments opposite than, say, a nearby strip of sea floor. This can only be explain by that the Earth’s magnetic field “flips” occasionally.

22. Electromagnets Anything with an electrical current running through it has a magnetic field.You can easily find the direction of the magnetic field produced by a current flowing through the wire with the “Right Hand Rule”.To use the “Right Hand Rule” with an electromagnet, take your right hand and make a fist. After this, point your thumb in the direction of the current. Your thumb represents the current, and your fingers represent the direction of the magnetic field. If you flip your hand over, then you’re reversing the direction of the current.

23. Electromagnets (cont’d) • The magnetic field lines around an electrically charged wire form concentric rings. • You can also use a similar “Right Hand Rule” with solenoids. • Take your right hand and create a fist, and point your thumb up. Your thumb represents the direction of the magnetic field in the solenoid, and your fingers represent the direction of the current in the wire loops.

24. Properties of Solenoids • Solenoids are one the most common forms of electromagnets. • Solenoids consist of a tightly wrapped coil of wire around a core (usually iron). When a charge is applied to the coil, a magnetic field is produced. • As the coil becomes more tightly wrapped, the magnetic field becomes more concentrated inside the coil and less concentrated outside of it.

25. Solenoids (cont’d) • If the direction of the current in the coil is reversed, the North and South poles become reversed as well. • The magnetic field lines produced by a tightly wrapped solenoid look very similar to those produced by a bar magnet. • The magnetic field produced by a current flowing through a wire is perpendicular to the direction of the current.

26. Question #1 Find the direction of the magnetic field. It may help to use the Right Hand Rule, here. i Answer: The easiest way to do problems like this is to use the Right Hand Rule. Take your right hand, and orient your thumb in the direction of the current. Then, curl your fingers as if you were making a first. The magnetic field follows your fingers.

27. + + + + + + + Question #2 Find the direction of the magnetic field in the solenoid. Hint: The Right Hand Rule works here, too. i Out of the page Into the page Answer:    Once again, a Right Hand Rule applies to these problems as well. Open your right hand, and curl your fingers in the direction of the current. Then, extend your thumb outwards; it should point in the direction of the magnetic field.

28. Faraday and Magnetic Flux Magnetic flux is a measure of the amount of magnetic field lines going through an area of a Gaussian surface. As a bar magnet nears the surface the flux increases, and as it goes further away, the flux decreases • Michael Faraday discovered that a changing magnetic flux in a wire can create an electric current. One simple example of this is a magnet moving in and out of a wire loop.

29. Electromagnetic Induction • Any change in magnetic flux can create a current, such as a wire moving in a magnetic field, or a magnet moving through a wire loop. • If there is no change in magnetic flux, then no current can be produced, even if there is a very strong magnetic flux present.

30. Electromagnetic Induction (cont’d) • The practical application of this is in an electric generator, where an electric current is said to be induced in a wire that is experiencing a change in magnetic flux. In a simple generator, a wire loop is placed between two magnetic poles, and is then rotated by an external force. This creates a change in magnetic flux, which in turn creates an alternating electric current. http://www.micro.magnet.fsu.edu/electromag/java/faraday/ (More Ahead)

31. Simple Electric Generator (AC) Electric generators transform a torque into a current. As a rotating wire in a generator moves from a straight angle to a 90 degree angle relative to the magnetic field, the current increases to a maximum. When it then moves from a 90 degree angle to a straight angle, the induced electric current moves to zero. When the rotating wire continues to move after reaching a straight angle, it begins to create a current flowing in the opposite direction. When the wire is once again at a 90 degree angle, this current is at a maximum, and when the wire is back to it’s starting position, the current is zero. This cycle repeats every time the wire makes a complete revolution, in a periodic manner.

32. AC Electric Generators (cont’d.) • The simplest form of an electric generator is called an alternating current (or AC) generator. • The current produced by an AC generator switches directions every time the wire inside of it is rotated to make a half turn. In standard generators in the United States, the generator has a frequency of 60Hz, which means the current switches direction 120 times every second! A graph of the current output from an AC generator produces a sinusoidal curve due to the periodic nature of the generator’s rotation. Animation of an AC generator.

33. Electric Motors • The same principles that allow an electric generator to function also work to allow an electric motor to function. • Electric motors are quite similar to electric generators, but work in the reverse fashion, generating a torque from an electric current. • In a simple AC electric motor, a current is fed into a wire rotor placed within the field of a magnet. • Remember that you can use the Right Hand Rule to determine the direction of the force due to the magnetic field on a current flowing through a wire. • Animation of a DC motor. In a DC motor running from an AC current, there is a mechanism called the commutator that switches the contacts from which the rotor is getting current from when the current switches directions, producing direct current in the rotor. Second animation.

34. AC Electric Motors (cont’d) When a current is fed into the wire rotor of a motor in a magnetic field, a force is felt on the two wires that do not line up with the magnetic field. They provide a torque on the wire loop and turn the loop. As the loop reaches a half turn, the current changes direction, and the torque continues in the same direction. This happens many times per second, causing the rotor to constantly turn. The turning of the rotor provides torque which can be harnessed to do work.

35. FLUX INTENSITY Flux intensity is the number of magnetic field lines in a given area. As in the previous slide, tesla is SI measurement of this. Some known magnetic fields and their flux intensity in teslas are: Earth’s magnetic field: 5 x 10-5 T Small bar magnet: 0.01 T Strongest laboratory electromagnetic: 20 T Surface of neutron star: 108 T NEUTRON STAR

36. CURIE POINT The curie point is reached when a magnet is heated up so much that it wants to forget its magnetic behaviors. The loss is only temporary; the magnet will regain its characteristics as soon as it is returned to the temperature at which it originally had magnetic properties. Here are a few sites which show the curie point of iron, nickel, and : curie point of iron curie point of nickel curie point of dysprosium 1 2 3 4

37. IS LEVITATION POSSIBLE? Yes! Through the power of diametic plate strategically placed around an object. The reason any living creature has the ability to be levitated is because everything has the potential to be magnetic. We all have domains in our body, but ours are almost always randomly oriented. Magnetic levitation is also sometimes used by high speed “bullet” trains. Click on the links below to see a frog being levitated: http://theory.uwinnipeg.ca/mod_tech/node83.html

38. APPLICATIONS IN SCIENCE Besides being used in motors and generators, there are many applications for magnets in scientific or medical devices. For example, magnets are used in MRI (magnetic resonance) scans, which are used to help diagnose medical conditions. Additionally, high-powered electromagnets are used in particle accelerators. Particle accelerators are huge machines used to accelerate subatomic particles to nearly the speed of light. Scientists study the interactions of different particles being smashed into each other at these high speeds.

39. HISTORY OF MAGNETS The first magnets were naturally occurring lodestones, sometimes referred now as magnetite, that were magnetized piece of iron ore. 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 ability of the lodestone 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 concluded that the earth itself is a giant magnet.

40. HISTORY CONTINUED In 1820, Hans Christian discovered an electric current flowing through a wire can cause a compass needle to rotate, showing that magnetism and electricity were related. In 1830 Michael Faraday and Joseph Henry discovered that a changing magnetic field produced a current in a coil of wire. Pierre Curie discovered that magnets loose their magnetism above a certain temperature which became known as the Curie point. In the 1960's and 1970's scientists developed superconducting materials. Superconductors are materials that have an extremely low resistance to a current flowing through them, usually at a very low temperature.

42. Credits How speakers work http://www.geo.umn.edu/orgs/irm/bestiary/index.html Bestiary of magnetic minerals http://sprott.physics.wisc.edu/demobook/chapter5.htm History of magnets http://www.webmineral.com/data/Magnetite.shtml Magnetite http://pupgg.princeton.edu/~phys104/2000/lectures/lecture4/sld001.htm Slide show http://www.physics.umd.edu/deptinfo/facilities/lecdem/demolst.htm Best ever site for pictures, simple explanations, etc. http://www.trifield.com/magnetic_fields.htm Another good site for how magnets work http://bell.mma.edu/~mdickins/TechPhys2/lectures3.html Equations and such http://schools.moe.edu.sg/xinmin/lessons/physics/default.htm see also

43. Credits http://www.micro.magnet.fsu.edu/electromag/java/index.html Main Index http://www.micro.magnet.fsu.edu/electromag/java/detector/ How a metal detector works http://www.micro.magnet.fsu.edu/electromag/java/compass/ How a compass is oriented magnetically http://www.micro.magnet.fsu.edu/electromag/java/faraday2/ How Faraday did his current experiment http://www.micro.magnet.fsu.edu/electromag/java/harddrive/ How a hard drive works http://www.micro.magnet.fsu.edu/electromag/java/magneticlines/ How magnet lines is working http://www.micro.magnet.fsu.edu/electromag/java/magneticlines2/ How two magnets repel and attract http://www.micro.magnet.fsu.edu/electromag/java/nmr/populations/index.html Nuclear spin up/down http://www.micro.magnet.fsu.edu/electromag/java/pulsedmagnet/ Pulsed magnets http://www.micro.magnet.fsu.edu/electromag/java/speaker/

44. Credits http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/elemag.html http://library.thinkquest.org/16600/intermediate/magnetism.shtml http://www-geology.ucdavis.edu/~gel161/sp98_burgmann/magnetics/magnetics.html http://www.micro.magnet.fsu.edu/electromag/java/index.html http://webphysics.davidson.edu/Applets/BField/Solenoid.html http://www.ameslab.gov/News/Inquiry/spring96/spin.html http://cfi.lbl.gov/~budinger/medTechdocs/MRI.html http://www.wondermagnet.com/dev/images/dipole1.jpg