Magnetism and E&M Induction
Hewitt Lecture
Paper clips fall to table top.
Why?
Gravitational attraction
Is the ceiling pulling up?
Yes
How come they arenʼt moving up toward the ceiling?
More ground than ceiling so they go down.
Bring in a big magnet and pick them up.
See the magnetic force is much greater than the electric force 1 x 1039
stronger!
Now show a smaller magnet- a compass.
Show a Compass that is open (not enclosed in a housing) so everyone can
see it (one that pivots on a stand so everyone can view it from the whole
room).
Which way is it pointing?
North
Guess what the whole world is?
A big magnet
And guess what surrounds the earth?
A magnetic field
And what does that compass line up with?
The earthʼs magnetic field
What is the source of the magnetism?
150 years ago, Hans Christian Oersted, in a high school classroom from
Denmark, was showing the students that there was no relationship
between magnetism and electricity. The physics books had them as two
separate subjects- electricity and magnetism. He would demonstrate that
there was no relationship by orienting a current carrying wire over the
magnet (compass) in a particular orientation (perpendicular to it) and even
though the wire was carrying a current, nothing happened. But after class
one day, a student came up and oriented it another way, parallel to the
wire, and look what happened. . . the compass moved. Yikes, what is going
on?
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No Current!
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Current
(compass rotates 900)
It turns out there is an interaction between a current carrying wire and a
magnet. To make a long story short, the source of all magnetism is moving
electric charges. When electrons move through the wire they create a
magnetic field that is in concentric loops around the wire.
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No Current!
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Current
If I orient a wire through a piece of paper and put compasses on it, and
there is no current through the wire, all the compasses (which are little
magnets), are oriented north. When I put a current through the wire, they
reorient in concentric circles around the wire.
Show magnetic field lines of a strong horseshoe magnet using iron filings.
Back to the current in the wire: the stronger the current in the current
carrying wire, the stronger the magnetic field.
If I loop the wire upon itself. Where is the magnetic field going to be
concentrated? Out far away from the coil? Inside the coil?
Inside the coil, of course!
If I put a piece of iron inside the coil of wire, guess what the iron will turn
into?
A magnet!
Guess what I call this?
An electro-magnet because it needs electrical current to work.
Show a well made electro-magnet with an iron core, or make one with a
battery and wire. Where is the magnetic field concentrated? Inside or
outside?
Inside.
Try to pick up paper clips without having a battery hooked up to the electromagnet.
It wonʼt work.
What do I need to pick up the paper clips?
I must have a current moving through the coil of wire.
Turn on current and it will work.
Back to a regular magnet (not electro-magnet): You want to mess up a
magnet? Drop the magnet and it will lose its magnetism.
Why?
The domains move from the one way orientation.
Remember- the source of magnetism is moving charges!
Where is the moving charges in a natural magnet?
The atom is made of charges and it is these moving charges that cause the
magnetism. Most atoms have charges going opposite ways so the
magnetic effect is canceled out. But in one, they are oriented in the same
way. What material is this?
Iron!!
It is ferromagnetic.
Put a wire in the magnetic field of a permanent magnet.
The wire is copper. Is it not attracted to the magnet because it is not
ferromagnetic like the iron. But, put a current through it and it moves. It
moves down in one orientation, it moves up if we reverse the wire.
I wonder if we can harness that jumping motion?
Is this a big deal?
You bet, this is how electric motors work!!
Whenever you see an electric motor- the simple thing going on is that it is
nothing more than a current carrying wire being deflected in a permanent
magnetic field. Thatʼs it!
If I take the wire and double back it on itself what is going to happen? One
wire will get pushed up, the other wire will get pushed down; the whole
thing will twist!
Show a big meter and explain that a meter is nothing more than a coil of
wire in a permanent magnetic field. When current flows through the wire, it
feels a force and the coil moves, the needle on the meter moves!
The direction the needle moves (right or left) depends on which way the
current flows. The amount the needle moves depends on the amount of the
current.
Is there anyway to get the needle on this meter to go all the way around?
No, because the current is going one way and it will always feel a force that
keeps it from going all the way around.
However, if I could somehow switch the current, then it could keep
spinning, around and around! That is what a motor does, it reverses the
direction of the current every half turn!
Demo: Show an electric motor and explain how a commutator reverses
the direction of the current every half turn. This causes the direction of the
force to change so it is always pushing in the same direction.
Show an electric motor working.
If it werenʼt for devices like electric motors our world would be so different
today.
Most motors rotate at 60 Hz, 60 times per second.
Letʼs switch gears now. . .
If you can go from electricity and get magnetism, can you go from
magnetism and get electricity?
Is nature symmetric?
1831
Michael Faraday and Joseph Henry found the same effect at the same
time.
At the time people were able to get electricity from batteries because of
Volta- an Italian- but then Faraday and Henry discovered a new way to get
electricity!
Demo: Magnet, Wire, meter. . .watch this
At first move the wire through the magnet very slowly and askDo you see anything?
No
Now move it quickly. Do you see something?
Yes
Wow, there is no battery nearby and what are we generating?
Electric current!
Just by moving a wire through a magnetic field.
Faraday goes to bed that night and wondered- what if I use two wires?
More voltage
What about three wires?
Even more voltage
How about a thousand wires?
Even crazy more voltage
Eventually he came up with what is called Faradayʼs Law
Change a magnetic field in a closed conducting loop of wire and you will
induce a voltage in that loop and you get current. Increase the number of
loops, increase the voltage!
This is how we make electricity.
Rotate a loop in a magnetic field and you produce electricity. It is that
simple.
When you learn this at first you think these phenomena are different
1. You run an electric current through a magnetic field and you get a
deflection of a wire.
2. You move a wire through an magnetic field you get electricity.
It is really one in the same thing.
Motor Effect and Generator Effect are one and the same thing.
Show how they are the same thing.
Show how to light up a lamp by turning a crank.
What is going on.
Closed loop of wire surrounded by magnetic field.
Rotate a wire through the magnetic field- you get.
Demo
Aluminum (not iron)- is shot up using an electromagnetic propulsion
machine.
Why did the Al fly up?
Why did the Al temporarily become a magnetic?
Show how it levitates.
It is a closed loop of Al in a changing magnetic field (the magnetic field is
due to alternating current).
Figure out the force pushing it up.
Coil and magnet. ... . .. connected to a coil with a compass (magnet).
Move either the coil or the magnet and you can see the compass move.
Bury in the road a coil of wire in the road.
The magnetic field of the earth.
If you could change the magnetic field of the earth . . .
When your iron car goes over the loop of wire, it changes the earthʼs
magnetic field and induces a current in the loop- the light changes.
Talk about tape recorders and/or hard drive.
Write: The coil reorients the little pieces of iron.
Read: Then you play them back and it reads.
Question to leave you with:
1. What causes EMI in the aluminum?
2. Why does it get harder to crank the coil when the light is lit.