Basics of Magnetism: Magnetic Forces and Fields

1. Magnetism

2. MAGNETISM A force of attraction or repulsion due to an arrangement of electrons. Magnets exhibit both attractive and repulsive forces. These are exhibited between the north and south poles of a magnet.

3. Magnetic Forces… Magnetic forces are produced by north and south poles. Like poles repel; opposite poles attract North and south poles always occur in pairs

4. Source of Magnetism Magnetic Fields • A magnetic fieldis produced by the motion of electric charge • In magnetic substances such as iron, the magnetic fields created by spinning electrons do not cancel each other out; large clusters of magnetic atoms align to form magnetic domains • In nonmagnetic substances, electron pairs within the atoms spin in opposite directions; there is no net magnetic field.

6. Unmagnetized Iron Slightly Magnetized Iron Strongly Magnetized Iron Magnetic Domains Domain: clusters of aligned iron atoms

8. Magnetic Domains S S N N When a magnet is broken into two pieces, each piece is an equally strong magnet.

9. Magnetic Field of a Bar Magnet

11. Properties of Magnets • Rule for magnetic poles: Like poles repel and unlike poles attract each other. • Magnetic Materials: • Natural magnets are naturally occurring substances that have magnetic properties Ex. Magnetite

12. Certain other substances can be made into magnets through magnetic induction • Temporary magnets: • Easy to magnetize • Lose magnetism quickly • Ex. Soft Iron • Permanent magnets: • Difficult to magnetize • Tends to stay magnetized • Ex. Alnico

13. Question 1

14. Pop Quiz... What is the biggest magnet you can think of? Hint: you're on it right now!

16. MAGNETOSPHERE Region around Earth that is influenced by Earth’s magnetic field

17. Magnetosphere The magnetosphere effectively shields the Earth from most of the direct solar wind through deflection.

19. “Northern Lights” At the poles, some of the charged particles descend into the earth’s atmosphere, ionizing the atoms of the atmosphere and causing them to fluoresce producing the Northern Lights or Aurora borealis.

21. Will a compass lead you directly to the geographic North Pole???

22. How a compass works A standard naming system for the poles of magnets is important. Historically, the terms north and south reflect awareness of the relationship between magnets and the earth's magnetic field. A freely suspended magnet will eventually orient itself north-to-south, because of its attraction to the north and south magnetic poles of the earth. The end of a magnet that points toward the Earth's geographic North Pole is labeled as the north pole of the magnet; correspondingly, the end that points south is the south pole of the magnet. The Earth's current geographic north is thus actually its magnetic south. Confounding the situation further, it is known that the Earth's magnetic field has reversed itself in the past, so this system of naming is likely to be backward at some time in the future

23. Magnetic South Pole

24. Magnetic Variation A compass does not point directly to the north pole but to the magnetic South pole. This difference is called magnetic variation. Magnetic declination-the discrepancy between true north and magnetic South

25. Solidified magma shows orientation of Earth’s magnetic field.

28. Rocks generally belong to two groups according to their magnetic properties. One group has so-called normal polarity, characterized by the magnetic minerals in the rock having the same polarity as that of the Earth's present magnetic field. This would result in the north end of the rock's "compass needle" pointing toward magnetic north. The other group, however, has reversed polarity, indicated by a polarity alignment opposite to that of the Earth's present magnetic field. In this case, the north end of the rock's compass needle would point south. How could this be? This answer lies in the magnetite in volcanic rock. Grains of magnetite -- behaving like little magnets -- can align themselves with the orientation of the Earth's magnetic field. When magma (molten rock containing minerals and gases) cools to form solid volcanic rock, the alignment of the magnetite grains is "locked in," recording the Earth's magnetic orientation or polarity (normal or reversed) at the time of cooling.

29. Researchers have reported the earth’s magnetic field reverses itself about once every half-million years. (In the last 1 hundred years it has decreased by 5 %, at that rate it will reverse itself in 2000 years)

30. Earth as a Magnet • Earth acts as a magnet and is surrounded by a magnetic field called the magnetosphere ..Possible theories are… • Movement of charges in conducting liquid core (rotational or looping effect) • Rapid Rotation of the Earth (1 & 2: Dynamo theory) • Convection currents in the Earth (heat release-volcanoes)

31. Electic Currents and Magnetic Fields The origins of magnetic fields were a mystery until Oersted’s 1819 discovery that an electric current generates a magnetic field. He noticed the deflection of compass needles during a demonstration with electricity An electric current flowing through a wire gives rise to a magnetic field The shape of a magnetic field are concentric circles around the current carrying wire ( the direction depends on the direction of the current) • http://hyperphysics.phyastr.gsu.edu/hbase/magnetic/galvan.html

32. Electric Currents and Magnetic Fields We said a moving charge creates a magnetic field. This means that every current-carrying wire is surrounded by a magnetic field!! There are several ways to demonstrate this is true … One is by surrounding a wire with compasses and then passing a current through the wire … the compasses line up with the magnetic field which circles around the wire.

33. Right hand Rule The direction of a magnetic field around a current carrying wire. -thumb- in direction of current -fingers- in direction of magnetic field

34. Current and Compass • If a current-carrying wire runs directly over a magnetic compass, the needle of the compass will • Not be affected by the current • Point in a direction perpendicular to the wire • Point in a direction parallel to the wire • Tend to point directly to the wire

35. Magnetic Field of a Current-Carrying Wire

36. Solenoid If the wire is bent into a loop, the field can be built up so that it circles through the middle of the loop. If the wire is bent into another loop, overlapping the first the concentration of magnetic field lines inside the double loop is twice as much as in the single loop. The magnetic intensity in this region is increased as number of loops are increased. This is called a Solenoid. If a piece of iron is placed in a current-carrying coil of wire with many loops, the magnetic domains in the iron are induced into alignment, which further increases the magnetic filed intensity. This is known as an Electromagnet.

37. A Simple Electromagnet!!! Question: What do you think happens if we increase the number of coils in the wire?

38. Electromagnetism • the number of loops • the amount of current • the material in the core galvanometer- a device for measuring currents

39. A charged particle moving through magnetic field experiences a deflecting force. Therefore a current-carrying wire experiences a force in a magnetic field. If you grasp the wire with the right hand in such a way that the fingers point in the direction of the current, and then curl in the direction of the magnetic field then the thumb points the direction of the force.

40. Right Hand Rule Another • Used to determine the direction of the force exerted on a current-carrying wire in a magnetic field • fingers- in direction of current. (the flow of positives) • Curl fingers in the direction of the magnetic field (B) • Thumb- shows the direction of the force (FB) exerted on the wire 2 techniques to find the FB • the reason for a magnetic force on a current carrying wire is the interaction between the Bwire and the B that the wire was put in

41. The Force betweenTwo Parallel Wires Parallel wires carrying current in the same direction attract each other. Parallel wires carrying current in opposite directions repel each other. http://www.physics247.com/physics-homework-help/magnetic-fields-forces.php

42. What is the path of the charge placed in the given Magnetic field? A charged particle moving in a plane perpendicular to a magnetic field will move in a circular orbit with the magnetic force playing the role of centripetal force. The direction of the force is given by the right-hand rule. Equating the centripetal force with the magnetic force and solving for R the radius of the circular path we get mv^2 / R = q v B and R = m v / q B

43. Summary (1)

44. Summary (2)

45. How a Speaker Works When the electrical current flowing through the voice coil changes direction, the coil's polar orientation reverses. This changes the magnetic forces between the voice coil and the permanent magnet, moving the coil and attached diaphragm back and forth. When the coil moves, it pushes and pulls on the speaker cone. This vibrates the air in front of the speaker, creating sound waves. The electrical audio signal can also be interpreted as a wave. The frequency and amplitude of this wave, which represents the original sound wave, dictates the rate and distance that the voice coil moves. This, in turn, determines the frequency and amplitude of the sound waves produced by the diaphragm. Electrical energy is converted to sound from mechanical waves. Waves begin to vibrate on electrical signal going through 2 coils. Since the electrical signal is constantly changing, the deflecting force is also changing. Therefore, coil will vibrate.

46. Diagram of a Speaker

47. Galvanometer • The figures show a galvanometer or current meter. The magnetic field is arranged so that it is always perpendicular to the coil as the coil pivots on low-friction bearings. • A spiral spring produces angle-dependent Force that is opposed by the magnetic field induced Force. Therefore, flowing current through the coil produces a rotation and pointer deflection that is proportional to the current.

48. Question 2

49. DC Electric Motor Are devices that change electrical energy into mechanical energy The current through the loop passes through a commutator switch, which reverses the current as the loop approaches the equilibrium position. http://www.physclips.unsw.edu.au/jw/electricmotors.html

50. D C Motor