1. Magnetism & Magnetic Fields
Preliminary Course Physics
Module 8.3
Electrical Energy in the Home

2. MAGNETISM
A property possessed by certain materials (nickel, iron, cobalt) that allows them to attract small pieces of iron such as iron filings.

3. Magnetic Fields
a “region of influence” surrounding a magnet or an electromagnet in which other magnetic materials experience a force.
Symbol : B
Unit : Tesla (T)
Basically there are two types of magnet:
Permanent magnets which are made to retain their magnetism for a very long time. They are generally made of steel which is difficult to magnetise but is also difficult to demagnetise, so once made into a magnet it will stay magnetised for a long time. Another type of permanent magnet you often see in school labs is the ceramic magnetic made of ferrite. Permanent magnets can come in many different sizes and shapes, bars, horseshoes, the flat disc magnets which are used on the back of fridge magnets for example.
The other type are electromagnets. These are temporary magnets which are only magnetised while a current flows in a coil. They are made of iron which is easy to magnetise and demagnetise and so can be switched on and off by the current. Electromagnets are used for lifting large ferrous metal objects, such as old cars or sheets of steel. Again they can come in many different shapes and sizes.

4. The Source of Magnetic Fields - Permanent Magnets.
Since electrons are charges in motion, they produce magnetic fields.
In other materials such as iron, cobalt, and nickel, the electrons are oriented in such a ways as to impart magnetic properties to the atomic structure.
These atoms are grouped in a tiny region called the magnetic domain.

5. Magnetic Domains
(A) In an unmagnetized piece of iron, the magnetic domains have random arrangement that cancels any overall magnetic effect. (B) When an external magnetic field is applied to the iron, the magnetic domains are realigned, and those parallel to the field grow in size at the expense of the other domains, and the iron is magnetized

6. No Magnetic Monopoles!
A bar magnet cut into halves always makes new, complete magnets each with both a north and a south pole. The poles always come in pairs, and the separation of a pair into single poles, called monopoles, has never been accomplished.
Field lines are really just a pictorial way of describing field strength and direction. Individual lines do not really exist. A line through any particular point just shows how a free north pole would move if it were released at that point. Which point you choose determines the actual line you draw.
Why do poles exist in the first place.
We picture a magnetised piece of iron being made of many small magnets or domains. Recall a domain is a region of the metal's crystal structure which is magnetised in a particular direction. These domains are lined up north to south in the magnetised iron, so the field lines are contained inside the material. It is only near the ends where not every domain magnet is balanced by those around it so some of the field escapes into the surrounding space.
Thus the magnetic force we experience (outside the magnet) is concentrated at the ends (poles).

7. Earth’s Magnetic Field
Magnetic N pole
GeographicN pole
The earth's magnetic field. Note that the magnetic north pole and the geographic North Pole are not in the same place. Note also that the magnetic north pole acts as if the south pole of a huge bar magnet were inside the earth. You know that is must be a magnetic south pole since the north end of a magnetic compass is attracted to it and opposite poles attract. S N
Axis of rotation

8. Origin of Earth’s Magnetic Field
The Earth’s magnetic field is thought to originate with moving charges.
The core is probably composed of iron and nickel, which flows as the Earth rotates, creating electrical currents that result in the Earth’s magnetic field.
* Earth’s magnetic field is dealt with in
more detail in Module 8.4 The Cosmic
Engine

9. Oersted’s Discovery Oersted discovered that a compass needle below a
wire:
(i) pointed north when there was no current,
(ii) moved at right angles to wire when a current flowed one way, and
(iii) moved at right angles in the opposite direction when the current was reversed

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

11. Right-hand Grip Rule Use:
right-hand rule of thumb to determine the direction of a magnetic field around a conventional current and

12. Current Loops
A current-carrying wire that is formed into a loop has perpendicular, circular field lines that pass through the inside of the loop in the same direction.
This has the effect of concentrating the field lines, which increases the magnetic field intensity.
Since the field lines pass through the loop in the same direction, the loop has a north and south pole.

13. Many loops of wire formed into a cylindrical coil is called a solenoid.
When a current is in the solenoid a magnetic field around the solenoid is created that acts like a magnetic field and is called an electromagnet
it produces a magnetic field like the magnetic field of a bar magnet

14. Applications of Electromagnets
Electric Meters
The strength of the magnetic field produced by an electromagnet is proportional to the electric current in the electromagnet.
A galvanometer measures electrical current by measuring the magnetic field.
A galvanometer can measure current, potential difference, and resistance.
Electromagnetic Switches
A relay is an electromagnetic switch device that makes possible the use of low voltage control current to switch a larger, high voltage circuit on and off.

15. Galvanometer
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

16. Relay Circuit
A schematic of a relay circuit. The mercury vial turns as changes in the temperature expand or contract the coil, moving the mercury and making or breaking contact with the relay circuit. When the mercury moves to close to the relay circuit, a small current activates the electromagnet, which closes the contacts on the large-current circuit

17. Telephone
(A) Sound waves are converted into a changing electrical current in a telephone.
(B) Changing electrical current can be changed to sound waves in a speaker by the action of an electromagnet pushing and pulling on a permanent magnet. The permanent magnet is attached to a stiff paper cone or some other material that makes sound waves as it moves in and out

18. Dissection of a Loudspeaker

19. How a Loudspeaker works
A speaker takes an electrical signal and translates it back into physical vibrations to create sound waves.
Speakers do this with one or more drivers.
The main parts of a driver are:
* Cone * Surround (or basket) * Voice coil

20. Steps in Making a Loudspeaker Work
1. The amplifier constantly switches the electrical signal, so the charge on the red wire fluctuates between a positive charge and a negative charge.
2. Current going through the speaker voice coil moves one way and then reverses and flows the other way.
3. The polarity of the electromagnetic field associated with the voice coil reverses (many times a second).
4. The electromagnet and the permanent magnet interact with each other on each reversal.
5. The magnetic forces between the voice coil and the permanent magnet reverses (every time the electrical signal switches).
6.The coil (and diaphragm) is pushed back and forth rapidly.

21. Loudspeaker in Action

22. Magnetic Recording
Magnetic recording is a backbone technology of the electronic age. It is a fundamental way for permanently storing information.
magnetic tape (in the form of compact cassettes) is a popular way of distributing music.
video tape is used widely both in the broadcast industry and at home to store material for later viewing
magnetic recording is used on computer floppy disks, hard disks and magnetic tape as the main method for data storage

23. Cassette Tape
Consists of a thin plastic base material, and bonded to this base is a coating of ferric oxide powder. The oxide is normally mixed with a binder to attach it to the plastic, and it also includes some sort of dry lubricant to avoid wearing out the recorder.
When exposed to a magnetic field, the ferromagnetic material is permanently magnetized by the field

24. Placing a Magnetic Record on a Tape
An audio signal is sent through the coil of wire to create a magnetic field in the iron core of the record head.
Magnetic flux forms a fringe pattern to bridge the gap in the core
Magnetic flux magnetises the oxide on the tape.
The oxide permanently "remembers" the flux it sees.
Tape machine ecord head (about the size of a flatteneed pea)
*Two – one to record each stereo channel on the tape

25. Computer Hard Drive Hard disks: were invented in the 1950s.
have a hard platter (a high-precision aluminum or glass disk) that holds the magnetic medium
can spin underneath the read/write head at speeds up to 272 kph!
have extremely small magnetic domains compared to a cassette tape's

26. Multiple Platters
In order to increase the amount of information the drive can store, most hard disks have multiple platters. This drive has three platters and six read/write heads:

27. Hard Disk Data Storage
Data is stored on the surface of a platter in sectors and tracks. Tracks are concentric circles, and sectors are pie-shaped wedges on a track