1. Magnetism

2. History
Term comes from the ancient Greek city of Magnesia, at which many natural magnets were found. We now refer to these natural magnets as lodestones (also spelled loadstone; lode means to lead or to attract) which contain magnetite, a natural magnetic material Fe3O4.
Pliny the Elder (23-79 AD Roman) wrote of a hill near the river Indus that was made entirely of a stone that attracted iron.

3. History
Chinese as early as 121 AD knew that an iron rod which had been brought near one of these natural magnets would acquire and retain the magnetic property…and that such a rod when suspended from a string would align itself in a north-south direction.
Use of magnets to aid in navigation can be traced back to at least the eleventh century.

4. Force of attraction or repulsion due to electron arrangement.
What is magnetism?
Force of attraction or repulsion due to electron arrangement.
Magnetic forces are the strongest at the poles.
Magnets have two poles: North and South.

5. Magnetic poles and fields
Magnets always have two poles.
If a magnet is cut it will still have two poles.
Like magnetic poles repel.
Unlike magnetic poles attract.
A magnetic field is the area in which magnetic forces act.

6. N S No Monopoles Allowed
It has not been shown to be possible to end up with a single North pole or a single South pole, which is a monopole ("mono" means one or single, thus one pole).  N S

7. Atoms themselves have magnetic properties due
to the spin of the atom’s electrons.
Groups of atoms join so that their magnetic fields
are all going in the same direction
These areas of atoms are called “domains”

8. When an unmagnetized substance is placed in a magnetic
field, the substance can become magnetized.
This happens when the spinning electrons line up in the
same direction.

9. An unmagnetized substance looks like this…

10. While a magnetized substance looks like this…

11. How to break a magnet: 1. Drop it 2. Heat it
This causes the domains to become random again!

12. Earth’s magnetic properties
Gilbert was a scientist that showed that the Earth behaves as a magnet
The Earth’s magnetic field is strongest at the poles
Earth’s magnetic field is called the magnetosphere
Geographic North
Pole
Magnetic South
Pole
Magnetic North
Pole
Geographic South
Pole

13. We use the Earth’s magnetic field to find direction.
The needle of a compass always points toward the magnetic south pole. We call this direction “North” (remember, opposites attract )

14. Other sources of magnetism
Magnetic fields of the Earth and other planets are related to their cores
The sun is a source of a magnetic field

15. The Earth’s magnetic field extends far into space
The Earth’s magnetic field extends far into space. It is called the “magnetosphere.”
When the magnetic particles from the sun, called “solar wind”, strike this magnetosphere, we see a phenomenon called…

16. The Aurora Borealis in the Northern Hemisphere
And the Aurora Australis in the Southern Hemisphere

17. Magnets Have Magnetic Fields
We will say that a moving charge sets up in the space around it a magnetic field,
and
it is the magnetic field which exerts a force on any other charge moving through it.
Magnetic fields are vector quantities….that is, they have a magnitude and a direction!

18. Defining Magnetic Field Direction
Magnetic Field vectors as written as B
Direction of magnetic field at any point is defined as the direction of motion of a charged particle on which the magnetic field would not exert a force.
Magnitude of the B-vector is proportional to the force acting on the moving charge, magnitude of the moving charge, the magnitude of its velocity, and the angle between v and the B-field. Unit is the Tesla or the Gauss (1 T = 10,000 G).

19. The Concept of “Fields”
Michael Faraday realized that ...
A magnet has a ‘magnetic field’
distributed throughout
the surrounding space

20. Magnetic Field Lines
Magnetic field lines describe the structure of magnetic fields in three dimensions.They are defined as follows. If at any point on such a line we place an ideal compass needle, free to turn in any direction (unlike the usual compass needle, which stays horizontal) then the needle will always point along the field line.
Field lines converge where the magnetic force is strong, and spread out where it is weak. For instance, in a compact bar magnet or "dipole," field lines spread out from one pole and converge towards the other, and of course, the magnetic force is strongest near the poles where they come together.

21. Field Lines Around a Magnet

22. Field Lines Around a Doughnut Magnet

23. Field Lines Around a Bar Magnet

24. Field Lines Around a Magnetic Sphere

25. Field Lines of Repelling Bars

26. Field Lines of Attracting Bars

27. Action at a Distance Explained
Although two magnets may not be touching, they still interact through their
magnetic fields. This explains the ‘action at a distance’, say of a compass.

28. Electric Current and Magnetism
The electrons moving around the nucleus carry electric charge.
Moving charge makes electric current so the electrons around the nucleus create currents within an atom.
These currents create the magnetic fields that determine the magnetic properties of atoms.

29. Creating Magnetic Fields
A single moving charge does create a magnetic field in the space around it, but since both the charge and the constant are very small, we usually don’t have to worry about these effects - except in two cases:
1. magnetic materials (permanent magnets) and
2. currents (electromagnets).

30. Creating Magnetic Fields: 1. Permanent Magnets
Materials are made up of atoms that have electrons moving around the central nucleus. These moving electrons create magnetic fields. However, in almost all materials the magnetic fields created by the orbiting electrons tend to cancel - except in some materials like iron. This is the basis of making magnets out of iron bars.

31. Magnetism
Ferromagnetism - When a ferromagnetic material is placed near a magnet, it will be attracted toward the region of greater magnetic field.  This is what we are most familiar with when our magnet picks up a bunch of paperclips.  Iron, cobalt, nickel, gadolinium, dysprosium and alloys containing these elements exhibit ferromagnetism because of the way the electron spins within one atom interact with those of nearby atoms.   They will align themselves, creating magnetic domains forming a permanent magnet.  

32. Creating Magnetic Fields 2. Electromagnets
If we have a series of charges moving (which means a current), then we can generate an appreciable magnetic field (electromagentism).
The Connection is Made: Oersted showed that magnetic effects could be produced by moving electrical charges; Faraday and Henry showed that electric currents could be produced by moving magnets
So....

33. The true nature of magnetism
The magnetic field of a coil is identical to the field of a disk-shaped permanent magnet.

34. All magnetic phenomena result from forces between electric charges in motion.

35. Electromagnets Temporary magnets. Magnetism can be turned on and off.
Made by wrapping wire around a piece of iron and sending a current through the wire.
Magnetic field is in the same direction as the electric current.

36. Increasing the strength of electromagnets
Increasing the voltage or current.
Increasing the number of loops of wire.

37. The field of a straight wire
A magnetic compass shows the presence and direction of the magnetic field around a straight length of current-carrying wire.

38. Force on the Charge Right Hand Rule of Thumb
When your fingers curl in the direction of current, your thumb points toward the magnet’s north pole (in the direction of the magnetic force acting on the charge).

39. When a current is run through a cylindrical coil of wire, a solenoid, it produces a magnetic field like the magnetic field of a bar magnet.

40. Coils are used in electromagnets, speakers, electric motors, electric guitars, and almost every kind of electric appliance that has moving parts.

41. Magnetic force on a moving charge
A magnetic field that has a strength of 1 tesla (1 T) creates a force of 1 newton (1 N) on a charge of 1 coulomb (1 C) moving at 1 meter per second.
This relationship is how the unit of magnetic field is defined.

42. Magnetic field near a wire
The field of a straight wire is proportional to the current in the wire and inversely proportional to the radius from the wire.
Current (amps)
B = 2x10-7 I r
Magnetic field
(T)
Radius (m)

43. Magnetic fields in a coil
The magnetic field at the center of a coil comes from the whole circumference of the coil.
No. of turns of
wire
Magnetic
field
(T)
B = 2p x10-7 NI r
Current
(amps)
Radius
of coil (m)

44. Using electromagnets – Alarm Bells
8J Using electromagnets – alarm bells
Using electromagnets – Alarm Bells
Describe what is happening here.

45. Using electromagnets –Loudspeakers
Loudspeaker has a membrane but oscillations are created by variations in electrical current, which cause an electromagnet to be pulled towards and away from a second, permanent magnet.
One of the magnets is attached to the cone of the speaker. The strength of the electromagnet changes and these oscillations cause the membrane of the loudspeaker to vibrate with the same frequency as the oscillations in the electrical current. Therefore the cone vibrates and makes a sound. 45

46. Using electromagnets – Relay
A relay is a device that uses a small current to operate a switch to control a high current.
The switch is closed, current flows through the coil and the iron. When core becomes magnetised attracts the iron armature.
The top of the armature is pulled towards the electromagnet.
The armature turns on the pivot, and the bottom part moves to the right, pushing the contacts together.
The second circuit is now complete, and the large current will make the motor turn.

47. Using electromagnets The Circuit Breaker
Circuit breakers are safety devices built into mains electricity circuits. They are designed to switch off the current if something goes wrong.
Often when something goes wrong with an electric circuit, a large current will flow.
When a high current flows, the strength of the electromagnet increases.
The iron rocker is attracted to the electromagnet more strongly.
The iron rocker is pulled down by the electromagnet.
The switch contacts are separated.
The circuit is broken.
The current is switched off.
The springy piece of metal stops the rocker from moving back up.

48. Using electromagnets – Electric Meters
Since magnetic force on a wire depends on the current, can use this effect to design a meter to measure current.
Current
Needle moves
Current
21-Nov-18 48

49. A galvanometer measures the direction and relative strength of an electric current from the magnetic field it produces. A coil of wire wrapped around an iron core becomes an electromagnet that rotates in the field of a permanent magnet. The rotation moves pointer on a scale.

50. Electric Motors
An electrical motor is an electromagnetic device that converts electrical energy into mechanical energy.
A motor has two working parts - a stationary magnet called a field magnet and a cylindrical, movable electromagnet called an armature.
The armature is on an axle and rotates in the magnetic field of the field magnet.
The axle is used to do work.

51. The principle of the electric motor
To keep the disk spinning, the external magnet must be reversed as soon as magnet (B) passes by.
Once the magnet has been reversed, magnet (B) will now be repelled and magnet (C) will be attracted.
As a result of the push-pull, the disk continues to rotate counterclockwise.

52. Commutation
The process of reversing the current in the electromagnet is called commutation and the switch that makes it happen is called a commutator.

53. Electric Motors
All types of electric motors have three key components:
A rotating element (rotor) with magnets.
A stationary magnet that surrounds the rotor.
A commutator that switches the electromagnets from north to south at the right place to keep the rotor spinning.

54. Generators: electromagnetic induction, faraday law, lenz law alternator, dynamo
Aplications dynamos and alternators.
In cars, bycicles microphone magnetic recording.
Transformers transmision of electrical power
Car speedometer Metal detector.

55. Induced Current
The next part of the story is that a changing magnetic field produces an electric current in a loop surrounding the field
A moving magnet induces electric current to flow.
called electromagnetic induction.

56. Magnetic flux
A moving magnet induces current in a coil only if the magnetic field of the magnet passes through the coil.
If the magnetic field does not change, such as when the magnet is stationary, the current is zero.

57. Faraday's Law
Faraday’s law says the current in a coil is proportional to the rate at which the magnetic field passing through the coil (the flux) changes.
Consider a coil of wire rotating between two magnets

58. FARADAY’S LAW Changing magnetic flux produces an emf.
Or Changing B-Field produces E-Field.
The rate of change of magnetic flux is required.

59. LENZ’S LAW
The polarity of the emf induced by a changing flux will produce a current that generates a magnetic field opposing the flux change that produced it

60. If the magnetic field is increasing, the induced current is in one direction.
If the field is decreasing, the induced current is in the opposite direction.

61. The Electromagnetic Connection
A changing magnetic field produces an electric field, and a changing electric field produces a magnetic field.
Electric and Magnetic fields can produce forces on charges
An accelerating charge produces electromagnetic waves (radiation)
Both electric and magnetic fields can transport energy
Electric field energy used in electrical circuits, e.g., released in lightning
Magnetic field carries energy through transformer, for example

62. Generators
A generator is a device that uses induction to convert mechanical energy into electrical energy.
Because the magnet near the coil alternates from north to south as the disk spins,
the direction of the current reverses every time a magnet passes the coil.
This creates an alternating current.

63. Generators
Use of a cylindrical iron core and by shaping the pole pieces
this produces an approximately uniform field in the narrow air gap
the arrangement of coils and core is known as the armature

64. Alternator - A Simple AC Generator
Wires connected to the rotating coil would get twisted
Therefore we use circular slip rings with sliding contacts called brushes

65. Alternator - A Simple AC Generator
Therefore this arrangement produces a sinusoidal output as shown below

66. Dynamo - A Simple DC Generator
Replace the two slip rings with a single split slip ring called a commutator
this is arranged so that connections to the coil are reversed as the voltage from the coil changes polarity
hence the voltage across the brushes is of a single polarity

67. Alternator - A Simple AC Generator
Use of a commutator

68. Electric motors vs. generators
Electric motors convert electricity to mechanical energy in order to do work
Generators convert mechanical energy to electrical energy

69. Mutual inductance
A changing flux in one element induces an emf in another.
Multiple inductors can exhibit combined self and mutual inductance.

70. Induced current

71. Transformers
There is a relationship between voltages and turns for device like this. Made of primary and secondary coils.
This uses electromagnetic induction, similar to a generator.

72. Transformers Used to increase or decrease voltage
Two types of transformers:
“Step-up” (increase voltage)
“Step-down”(decrease voltage)

74. Transformers
Transformers are extremely useful because they efficiently change voltage and current, while providing the same total power.

75. Dynamic Microphone Coil of wire moves in a magnetic field. Robust.
Requires no power.
Side view of dynamic mic diaphragm A with coil of wire B attached.