1. Magnetism

2. Historical
Early magnets called lodestones, naturally occurring iron ore magnetite
Named magnets by Greeks since they were found in the region Magnesia
Important for early navigation
Coulomb showed magnetic force has similar relationship to electric and gravitational forces

3. Historical
Oersted showed electric current and magnetism related in 1820
He found electric current created magnetic field which would deflect a compass needle

4. Magnetic Poles Like electric charges, there are two types
Called north and south because of the way they react to the earth’s magnetic field
Unlike electrical charges, can’t have isolated N or S pole; always have both
like electrical charges, opposite poles attract, like poles repel

5. Magnetic Poles
Forces between poles are directly proportional to strength of the poles, inversely proportional to the square of distance between them.
Magnets can be large: a planet, or small: a spinning electron

6. Magnetic Fields
Magnetic forces act over a distance by means of a field, like electric and gravitational forces
Magnetic field is represented by lines of force like electric field
Field lines can be seen by using bits of iron sprinkled on paper placed near magnet

8. Magnetic Fields
Direction of field is direction of force on N pole placed in field
Magnetic field lines shown as arrows going out of N poles, into S poles

9. Cause of Magnetism
Magnetic fields created by moving electrical charges
In common magnets, moving charges are spinning electrons
Movement of charges in wires (electric current) creates electromagnets

10. Ferromagnetism
In most materials, magnetic effects due to motion of electrons cancel out
Some materials have several spinning electrons without an opposite spinning partner
Called ferromagnetic from the Latin word for iron
Fe, Co, Ni, Dy, Pa, Nd

11. Magnetic Domains
Since each atom of ferromagnetic material is magnet, groups of atoms will line up together -- called domains
In unmagnetized iron, domains are randomly oriented
When magnet is brought near, domains become aligned together, iron becomes magnetized

12. Magnetic Domains
Domains can be rearranged and magnetism lost by heating or vibration

13. Electric Current and Magnetism
For straight wire with current, magnetic field lines form concentric circles around wire
Changing direction of current changes direction of magnetic field
If wire is bent into loop, magnetic field is concentrated inside loop

14. Electric Current and Magnetism
If more loops are added, magnetic field strength increases with each loop in coil
Long coil called a solenoid
If iron rod is placed inside solenoid, magnetic field is concentrated inside iron and electromagnet results

15. Magnetic Field of Loop

16. Magnetism and Moving Charges
Charged particle moving in magnetic field will experience deflecting force due to interaction of two magnetic fields
Force is greatest when motion is perpendicular to external magnetic field, zero when parallel
Direction of force is perpendicular to both velocity and external magnetic field

18. Magnetism and Moving Charges
Electrons in TV sets are aimed to screen using magnetic fields
Earth’s magnetic field deflects incoming charged particles from sun and outer space
Current carrying wires in magnetic fields experience similar sideways force because of charges moving in wires

19. Magnetism and Moving Charges
Two nearby current carrying wires will exert forces on each other because each creates a magnetic field
So current carrying wire will deflect compass and will be deflected by strong external magnetic field

21. Meters and Motors
Analog (not digital) meters use coil of wire with magnets on each side to measure current: galvanometer, ammeter, or voltmeter
Current through coil creates magnetic field; coil will try to line up with field of magnets
If coil is allowed to rotate, motor can be made

22. Galvanometer

23. Meters and Motors
As coil becomes aligned with magnets, current direction changes through commutator and brushes
In large motors, permanent magnets replace with electromagnets
Armature is rotating coil in motor

24. Earth’s Magnetic Field
Thought to be caused by convection currents in liquid interior of planet
Geologic evidence shows magnetic field has moved around and reversed polarity
Magnetic poles don’t line up with geographic poles

25. Earth’s Magnetic Field
Difference between magnetic north and true north called magnetic declination
Since north poles of magnets and compasses point to north, Earth’s north magnetic pole is really a south type pole; south pole is really north type pole

26. North Magnetic Pole