Generator Principles and Equations Explained

[Rockazella]

The latest assignment in my high school physics class is to study and teach the class about some part of the electrical distridution system. My part happens to be, How does a generator work and what are the principles behind it?
I have sort of a basic lesson plan, but I want to make sure I'm not missing anything.

Would anyone be willing to tell me all the major principals and equations that deal with the workings of a generator? Please keep in mind this is only a high school level class, so all I need are the fundamental principals.
Thanks

 

[BoulderHead]

Originally posted by Rockazella
Would anyone be willing to tell me all the major principals and equations that deal with the workings of a generator? Please keep in mind this is only a high school level class, so all I need are the fundamental principals.
Thanks

Would you first be willing to tell what you have compiled so far?

 

[Rockazella]

Well I was planing to start it off with linking mechanical power to electric power. The mechanical power you apply to the generator (P=W/t) equals the electric power (P=IV) you get out. Of course that's only if you don't account for losses due to friction and such.

Then I was going to explain how acording to Farad's Law, any change in the magnetic enviornment of a coil of wire will produce a voltage in that wire. (This is an area where I would like to get more detailed, hopefully with you're help)

Next I would talk about how the rotational motion of the coil in the generator causes an AC. I will back this up with a demonstration of me cranking a little generator hooked up to an oscilloscope.


What I would like to know is:
First off, do I have anything wrong there?
Second, how could I elaborate on any of those?
Last, I should talk about 3 stage ac power. I havn't been able to find a ton on it, so could you explain that?

 

[BoulderHead]

Remember that this is a High School class and don’t go overboard would be my recommendation to you. I would concentrate on making my presentation lean more towards the interesting side than the informative. For example you might talk about something they will have all likely noticed which is that you can use one magnet to cause another to move physically, then comment that perhaps some genius may have had the idea that the reverse might also be true, that a physical movement might ‘cause’ an electromagnetic field to be produced…blah, blah, blah.

You could go on to explain or even demonstrate with a loop of wire, horseshoe magnet, and a very sensitive ammeter how you can whip the wire across the ‘lines of force’ and create a current flow (or better yet your oscilloscope idea). This, after all is just a very small scale version of essentially what the power companies do…blah, blah, blah.

I wouldn’t even bother trying to get into the 3-phase thing at all…too much boring detail I think. Instead you might at this point attempt a simple explanation of the relationship between Power, Voltage, and Current. Tell them how you can play around with the values of I and E while keeping a constant P. This is what the power companies do when they step up the voltage to extremely high levels in order to deliver power with minimal line loss to some distant city…blah, blah, blah.

If you keep them entertained with a little ‘shock and awe’ without all the equations and math I think your teacher will give you a favorable nod. One last demonstration that will help with this is to obtain a nice sized permanent magnet motor to pass around the class. Have them spin the shaft with the two output/input wires disconnected then with the wires connected. You can comment, and they can feel, that connecting the wires is like increasing the load demanded from the power station and demands a lot more work be done.

Good luck, and as for the blah, blah, blah above you can add as much or as little to insure your presentation fits within the time allowed. Oh, and do rehearse it before giving it to the class so that you will know how long it will actually take and to help you look more comfortable.

 

[enigma]

The demo my physics prof gave when first talking about electromagnetism was:

He got a pretty powerful 'C' magnet, and rested a copper wire in between it. He then hooked up a battery to the wire (provided the current) and got it to leap off the table. If you have access to the magnet, you could do the same, and point out that it works in reverse also. If you move the magnet near a wire, it generates a current, and that is how the power company makes power.

 

[Rockazella]

you know where I can get this magnet?

 

[Integral]

A very good demo is to take 2 small DC motors, wire them together with about 10' of wire. Spinning one should cause the other to spin. Hand a motor to different students and let them feel a generator in action. Points out that a motor is the same as a generator. One uses electricy to create motion the other uses motion to create electricty.

 

[enigma]

I don't know where to get a magnet like the one used. Maybe call up a physics dept. at a local college or CC, and take one out on loan?

Integral's generator idea should teach the same lesson, though.

 

[drag]

Maybe you can use energy as your primary
guide. You can say - look I supplied mechanical
energy which moving the magnet created a
changing magnetic flux in the coil which
according to Faraday's law (like you said)
transformed the mechanical (which you can show
with the kinetic energy equation) energy of the
magnet into energy of AC voltage/current which
can then be transformed into some other energy in
some device(heat/light/mechanical movement/whatever).

Live long and prosper.

 

[Rockazella]

Thanks a lot for the responses. I will definately use a few of those demonstrations.

Recently when I went back over my lesson plan, a question came into my mind and it may come up while I'm teaching. It has to do with realting generators to motors.
What does the electromagnetic field look like around the coils in the motor? For it to work, both a north and south field must be created. However I don't get how a wire carrying electrons (current) will create both fields. Can anyone help me out here?

 

[enigma]

If you point your right thumb in the direction of the current, your fingers curl in the direction of north. There is no set 'position' where south is.

That is why the wire is coiled around a magnet. If you put a wire in a coil, the direction that the field 'points' is one direction on the inside of the coil, and the other direction outside the coil. An object placed inside the coil will then pick up that field.

I hope that makes sense...

 

[mcleodnine]

A simple permanent magnet motor has a pair of coils (insulated wire wrapped around a ferrous mass) each positioned opposite the other across the shaft, which rotates on a bearing surface. On one end of the shaft is a pair of contacts which connect to the coil. A current applied to those contacts creates a magnetic filed in the coil and pushes (pulls?) the coil away from the permanent magnet. when it rotates halfway around the coil is energized in the opposite direction as the current is reversed and we complete a cycle. Lather rinse repeat.

That's probably an oversimplified explanation and I trust someone more qualified could do a better job explaining it.

 

[Rockazella]

If you point your right thumb in the direction of the current, your fingers curl in the direction of north. There is no set 'position' where south is.

Curl in the direction of north? I guess I don't understand how curled fingers point in a direction. Are you saying north circles around the wire in a direction?

 

[enigma]

Originally posted by Rockazella
Curl in the direction of north? I guess I don't understand how curled fingers point in a direction. Are you saying north circles around the wire in a direction?

Sort of.

For a single wire, there is no set 'north'. You will be going 'north' if you follow it like your hand curls, and going 'south' if you go the other way.

If you make a loop out of the wire, however... you'll see that the 'north's and the south's all add up in the same direction. It's kind of hard to explain without pictures...

http://www.npaci.edu/successes/images/field.gif

The wire loop I'm talking about would be passing through the tight field lines in the center.

EDIT: In that picture, if blue is north and yellow is south, then the current would be moving into the screen on the right side, and out of the screen on the left side.

 

[Rockazella]

Cool picture!

Although I still don't see how it matches what your describing. Judging by the picture, it would seem there is a very well defined north and south pole projecting from opposite sides of the wire.

Sorry I don't want to be bothersome. I'm probly just not realising something. How does that picture show the rotating north field that you described?

 

[enigma]

I'm probably going to have to leave it to someone else to explain it better. Electromagnetism is not my area of expertise...

The field flows from north to south. When a wire is coiled, whether it's in space or around an electromagnet, it sets up a field. If you travel through the loop one way, you're going from north to south (from blue to yellow). The other way is south to north.

It's sort of like east and west on a compass. There is no west or east 'place', there is only a west or east 'direction'.