1. Physics Unit 6 - Magnetism1

2. Magnetism General term for properties and interaction of magnets
Very closely related to electricity
Magnetism is caused by moving electric charges

3. Producing a Magnet Electromagnets
Electrons moving through a wire produce a magnetic field
Magnetic materials
Electrons move around in an atom produce a magnetic field in a specific direction
Non-magnetic materials have lots of atoms producing magnetic fields in all random directions
Magnetic materials have atoms where fields align

4. Magnetic Domains
Magnetic domain - Group of atoms with aligned magnetic poles
In non-magnetic objects, magnetic domains are randomly aligned
A permanent magnet or a magnetized object has aligned magnetic domains
domain

5. Magnetization
Magnetic objects can be made into a magnet when their magnetic domains line up
Rubbing an iron bar with a magnet or passing it through an electromagnetic field makes most of the domains line up.
The field very close to another magnet can magnetize another object (EX: paper clips stuck to a magnet also act as a magnet)
If magnetic domain remains aligned in the object, it is called a permanent magnet

6. Magnetic Poles
Similar to charges, there are two types of poles: North and South
Like poles repel (same as with charges)
Opposite poles attract (same as with charges)

7. Magnetic Poles
There is no such thing as an individual magnetic “charge”
Every north pole needs a south
pole, and vice versa
A broken magnet creates new poles
Each magnet has both a N and a S pole

8. Magnetic Field A magnet is surrounded by a magnetic field
This field is what exerts a force on other magnets or other magnetic materials
Magnets produce a force without needing to touch
Magnetic field lines are used to represent the magnetic field in the area around a magnet

9. Magnetic Field Lines Direction: Strength: Lines point from N to S pole
This is the direction of force on another magnet’s north pole.
Another N is forced away from N pole and toward S pole.
Strength:
Field is strongest where lines are closest together
This happens at the poles of the magnets

10. Earth’s Magnetic Field
Earth is surrounded by magnetic field
Caused by the rotation of its molten iron-nickel core
Earth acts as if a giant bar magnet were thrust through its center
Magnetic poles close to but slightly off from geographic north and south poles (where Earth spins on its axis)

11. Compasses
A compass has a narrow needle-shaped magnet that freely rotates on a small pin
The red side of the magnet has been called the north pole, since it points north in direction
Since opposites attract, this means Earth’s geographic north pole is actually a magnetic south pole!

12. Magnetic Field from Currents
Remember, currents are moving charges. Moving charges produce magnetic fields
The magnetic field around a
single wire is in co-centric
loops around the wire
This is not terribly useful,
but when the wire is wrapped
in solenoid (coil) the magnetic
field acts like a bar magnet

13. Electromagnets
Electromagnet – strong, temporary magnet formed by current passing through a wire coil
Doubling the number of coils or doubling the current will double the strength of the magnet
Two ends are different – N and S poles (like other magnets)
Can easily be turned on and off
Used in electric motors,
speakers, and junkyard
magnets

14. Electric Motor Device to convert electric energy to kinetic energy
Electromagnet rotates between poles of a magnet
It alternates being repelled and attracted by magnets around it, causing it to keep rotating
Ex. Blender, toy car, fan

15. Inside real electric motor
assembled motor
field magnet
(stationary parts)
brushes & wires to battery
armature & commutator
(rotating parts)

16. Speaker Device to convert electrical energy into sound energy
Wire coil acts as electromagnet, moving back and forth as it repels and attracts a permanent magnet
The coil makes a paper cone vibrate, producing sound

17. Galvanometer
Device that measures small electric current by running it through an electromagnet that deflects a needle

18. Electromagnetic Induction
Electromagnetic induction – a current caused by a changing magnetic field
Caused by relative motion between a wire coil and a magnet
Reverse of electromagnet
Electromagnet uses a current to produce a magnet
Electromagnetic induction uses a magnet to induce a current

19. Electromagnetic Induction
Generators are electric motors in reverse.
Microphones are speakers in reverse
Generators convert kinetic energy into electrical energy
Microphones convert sound energy into electrical energy
By rotating the armature, a current is produced in the coil as it rotates between the fixed magnet
Sound vibrates the paper cone, causing the coil to move in and out relative to the fixed magnet and a current is produced in the coil

20. Electric Motor vs. Electric Generator
Electrical energy output
Permanent Magnet
Electromagnet
Permanent Magnet
MOTOR
Commutator
Source of mechanical energy
Electromagnet
Source of electrical energy
Commutator
Mechanical energy output

21. Electric Generator PE of lake water is converted to KE
Hydroelectric Dam
PE of lake water is converted to KE
KE turns the generator shaft which creates electrical energy

22. More on Induction
“The Forever Flashlight™ uses induction and electromagnetic energy to eliminate the need for batteries.”
Shaking the flashlight moves a magnet that induces a current in a stationary coil, which is then stored in a capacitor.

23. DC & AC Direct Current (DC) Current flows in one direction
EX: batteries
Alternating Current (AC)
Current reverses its direction at regular intervals
EX: electrical outlets
7th 12-5

24. Transformers Increases or decreases AC voltage
Primary coil AC produces a magnetic field that induces AC in the secondary coil
Voltage ratio = ratio of turns in each coil
8th 12-5

25. Transformers Increases AC voltage
Step-Up Tranformer
Increases AC voltage
More turns in output (secondary) coil
Used at power plants
Step-Down Tranformer
Decreases AC voltage
More turns in input (primary) coil
Used into households and into devices
8th 12-5

26. Transformers
Examples
8th 12-5

27. Other Applications of Magnetism
Magnetic strips – store and read information from the arrangement of tiny magnetized particles
Traffic light sensors – use electromagnetic induction to sense changes in magnetic field in the presence of large metal object (car)
8th 12-5