MAGNETISM

 

I.              Demo – Iron filing, plastic, and magnet

 

II.           CHECK QUESTION: How do the field line of magnetic, electric, and gravity fields differ? [ magnetic field line form closed loops.  Electric field line emanate from positive charges to negative charges. Gravity field lines emanate only from mass].

 

III.         Magnetic Domains

A.  magnetism is a result of moving charges. Every moving charge has a magnetic field around it.

Magnetic field
 

curent
 
 

 

 

 

 

 

 


B.   The magnetic field is a relativistic Òside effectÓ or Ô distortionÓ in the electric field of a moving charge.

 

C.   Not all material can become magnetic

 

D.  DEMO: paperclip in magnet, metal bars through horseshoe

 

E.   Electrons spin in all materials. Some elements have more  unpaired electrons than others. Electrons occur in pairs but if there are not enough electrons then some will be unpaired. Unpaired electrons= magnetic. This is because paired electrons spin in opposite direction and their magnetic fields will cancel each other.

 

F.   Elements that have these unpaired electrons have magnetic domains. These are like small tiny magnets. If these small magnets are all arranged the element becomes more magnetic. That is why you can cause magnetic induction like I did in the paperclip. By placing the magnet in a strong magnetic field I made all the magnetic domains line up and become magnetic. Some materials can hold this arrangement well others canÕt and demagnetize quickly.

 

G.   

 

 

 

 


H.  magnets become demagnetized when the magnetic fields are no longer lined up by:

a)    heat

b)   dropping

 

I.     CHECK QUESTION: Given  that charges are either positive or negative, are magnets either north or south? [ no, every magnet has both a north pole and south pole just as every coin has two sides]

 

IV.         Show Video

 

 

 

MAGNETISM AND ELECTRICITY

 

 

I.              DEMOS

 

A.  Bare wire, power supply, and compass- show needle deflection

B.   Nail wrapped with wire = electromagnet

 

*** as you double or triple or more the number of wires next to each other the magnetic fields add together to give an electromagnet.

*** uses – junk yards, solenoids, high- speed passenger trains

 

C.   Monitor+ magnet – show deflection of electrical signal using magnet. MANETISM IS CAUSING MOTION!

 

The magnetic force on a moving charge is always perpendicular there fore it is never in the same direction as a motion and does no work. This means that it only changes direction of motion.

 

D.  Large meter, coil of wire, and magnet – show how moving magnet inside coil of wire causes current

 

II.           The earthÕs magnetism

1)   the earth is like a big magnet. The magnetic poles however wander through the years and even reverse. The reasons for this is not totally understood but geological rocks have recorded information of these magnetic poles and reversals.

 

2)   The magnetic field of the earth protects us from cosmic rays that are deflected by it. This is why people are not allowed to work in highflying crafts for too many hours.

 

3)   Van Allen radiation belts

These are two lunar shaped shells composed of charge particles trapped in the earthÕs magnetic filed a few hundred to at least 50,000 km above the earths surface. Protons and electrons are found in both the outer part and inner part of the shells. As ions and electrons dip into the atmosphere at the pole a beautiful fluorescent like glow appears= Aurora Borealis (north) Australis Borealis (south)

 

4)   Earths magnetism and living creatures

 

i)     people believe the magnetic field has had an effect on evolution:

a) One theory says that when life was passing through its earliest phases, the magnetic field of the earth was strong enough to hold off cosmic and solar radiations that were violent enough to destroy life. During pole reversals, cosmic radiation on the earthÕs surface was increased causing an increase in the occurrence of mutations.

 

b) Evidence that supports this is the increase in life changes and the dates of the magnetic pole reversals.

 

5)   Homing pigeons have magnetite magnets in their skulls to aide in navigation.

 

6)   Bacteria south of the equator build the same single domain magnets as their counter parts north of the equator do, but the magnetite grains are aligned in the opposite direction to coincide with the oppositely- directed magnetic field in the southern hemisphere.

 

7)   Magnetic domains have also been found in the abdomen of some bees.

 

8)   So far humans have not been found to have any type of magnetic materials in their bodies.

 

9)   Magnetism has been used, however, to examine parts of the human body called MRI ( magnetic resonance imaging)

 

a)    An external alternating magnetic fields is applied to a part of the body of a patient. Due to the environment of neighboring atoms, slight differences in the natural frequencies of protons are detected by a Ô magnetic echo

 

b)   The resonant signals from the protons of normal cells differ slightly for cancerous tissue and are picked up by a sensitive magnetometer.

 

 

 

 

 

 

MOTORS AND GENERATORS

 

 

I.              GENERATORS VS MOTORS

a.    DEMO – hand generator into motor

 

b.    Generator is a device that converts mechanical energy into electrical energy.

 

c.    A motor is a device that coverts electrical energy into mechanical energy.

 

d.    In either case you donÕt get something from nothing. Energy is just converted.

 

e.    With the advent of the generator, the task at hand was to design a way to move coils of wire past magnetic fields, or to move magnetic fields past coils of wire.

 

f.     Turbines were placed beneath water falls and steam from boiling water was used to keep turbine blades turning as electricity was generated in large amounts—enter the industrial revolution!

 

 

II.           Transformers

 

a.    Transformers are used to increase of decease current. PG&E wants a lot of to be sent to its customers, but not every customer needs all the current. With a transformer high current can be sent and then Ôstepped downÕ so that danger is reduced. This allows less voltage to be needed and keeps the price of electricity down.

 

b.    Basically, a transformer has two coiled wires side by side, one has more turns that the other. As current moves through one current, creating a magnetic fields, the magnetic field induced causes current to be induced in the second coil. The number of turns in each coil determines whether the current is stepped up or down.

A.  Primary voltage        = Secondary voltage

# of primary turns          # of secondary turns

 

Vp   = Vs

Np       Ns

                          

 

B.   If you want the secondary voltage to go up the # of turns must be more. If you want the secondary voltage to go down, the # of turns must be less.

 

C.   Voltage and current are inversely proportional because energy must be the same due to conservation energy = power= VI

SOÉÉ.

 

D.  This means if you have more turns in the secondary coil the voltage goes up but the current goes down, you have stepped down the current = MORE TURNS = STEP UP TRANSFORMER LESS TURNS = STEP DOWN TRANSFORMER

 

E.   Transformers are also used in toy electric trains and portable radios.

 

 

c.    CHECK QUESTION:

Remember when I charged the rod with the fur and made the pith ball dance? What kind of field is induced by the making rod? [ magnetic field] What kind of field in turn does this magnetic filed induce? [ electric field] and does this make a new field? [ yes, electric]

And so on and so on and so onÉ..= EM radiation = let there be light!!!