Students enjoyed doing it, even though it was challenging making the coil just right so it would spin easily. They persisted and enjoyed that wonderful eureka moment when it actually worked.
Motor Speed
One group wanted to figure out how fast the armature was spinning. Because of small imperfections inherent to hand-made parts, we found that the armatures would bounce, ever so slightly, with each rotation. So the students recorded the sound using their laptop, and then counted rotations off the recorded sound wave. I think they came up with about 10 rotations per second.
I need to check if there’s a free phone app we can use that will show the sound waveform more efficiently — Pocket WavePad seems like it might work.
Accidental arc welding
At the end, some students wanted to figure out just how fast they could get the motor running. Disdaining the online instructions, they went for more power, hooking up all the batteries they could scavenge from the other groups before I had to make them stop. They did manage to weld the insulating varnish on the coil wire to the paperclip contact before the end.
All the sparks did lead to a discussion of how arc welding works, however, which I was able to tie into the maths of transformers; cheaper arc welders (like this one) take in 20 Amps at 120 Volts, and output 70 Amps at 22 Volts.
This group did not want to stop, so I gave them permission to pass on P.E. for that one day, with a note that said they were doing some “remedial” physics. They got a kick out of that.
Magnetic fields and electric fields are directly related:
We saw before that a current moving through a wire creates a magnetic field.*
The opposite is also true. A moving magnetic field induces an electric field (and thus a current) in a wire (from Faraday’s Law).
Electromagnetic Induction
Moving a magnet through a wire coil creates an electric current. The faster you move the magnet, or the more coils of wire you have, the greater the current. Moving the magnet back and forth will switch the direction of the current back and forth as well, creating an alternating current.
So, simply moving a magnet next to a wire, or through a coil of wire, will induce a voltage in the wire.
You can create more voltage by:
Faster motion.
More coils of wire.
Notes:
Relative Motion: moving the coil of wire around the magnet would create the same voltage as moving the magnet through the coil.
Moving the magnet creates the same current as moving the coil.Moving the coil creates the same current as moving the magnet.
Conservation of Energy: By moving the magnet, you convert the kinetic energy of the motion to electrical energy.
So the greater the voltage you want to create, the more energy it will take to move the magnet. If you have a lot of coils and are creating a large voltage, the magnet is going to be harder to push through the coil because the induced electrical field induces a magnetic field that opposes the original magnet.
Current Direction: Moving the magnet back and forth will switch the direction of the current back and forth as well, creating an alternating current.
Faraday’s Law (37.2)
The voltage induced in a coil depends on the number of loops and the rate at which the magnetic field changes (as well as the resistance of the coil material — better conductors will permit a greater voltage).
Electrical Generators
Diagram showing the parts of a simple motor.
If we make a coil like the ones we used to make the motors, but took out the battery from the circuit then the motor would not move. However, if we rotated the coil ourselves, mechanically, as it sat over the magnetic field, we would create a current in the wire. This is how generators create electricity.
Generators (mostly) create electrical currents by rotating wire coils inside magnetic fields.
Alternating Current Generators
A generator adapted from our motor would only have a current as long as the copper wire from the exposed side of the coil was completing the circuit. When the insulated side was touching the paperclip there would be no current. Electrical power plants are set up so that the rotating coil (also called an armature) is always in electrical contact, but when the coil goes through the second half of its rotation the current is reversed in direction. This switching back and forth of the direction of the current in the wires is how alternating currents are created.
Power Plants
Electrical power plants turn their wire coils (armatures) in any number of ways.
Wind turbines and hydroelectric turbines use wind and water respectively to spin their wire coils directly.
Hydroelectric power plant diagram. Via the USGS.
Most other power plants produce heat energy to boil water, which creates steam, which is piped past the turbines that turn the coils.
Coal, oil and natural gas plants burn these fossil fuels to produce the heat;
nuclear power plants use the heat generated from radioactive decay to produce the steam that turns the turbines;
Diagram of a nuclear power plant. Image via the TVA.
on the other hand, solar panels don’t use turbines or any moving parts to produce electricity.
Motors versus Generators (37.4)
Motors and generators are essentially the same, except that motors use electricity to produce mechanical energy, while generators use mechanical energy to produce electricity.
Notes
* edited March 22nd, 2012 at 10:01 pm, in response to feedback in this comment.
Moving charges create magnetic fields. Currents moving through a wire are moving electric charges (electrons). Therefore, current-carrying wires generate a magnetic field around them.
Bending wires into a loop will create will create a magnetic field through the loop.
The more loops you have the stronger the magnetic field.
The magnets created by pushing a current through loops of wire is called an electromagnet.
If you use superconducting wire, you can create an extremely powerful superconducting magnet that can be used in magnetic levitation (maglev) trains.
Magnetic field through a coil of wire (with a current running through it). The more loops of the wire, the stronger the electric field.
Magnets Deflect the Movement of Charged Particles (36.6)
Moving charged particles create magnetic fields. So if a moving charged particle encounters a magnetic field the two magnetic fields will interact and the motion of the particle will be deflected.
A charged particle is deflected from it's forward motion when a magnetic field is turned on.
Note that the deflection only occurs when the particle is not traveling parallel to the magnetic field lines.
The big, old styled TV’s use this to shoot electrons at the screen to make the picture.
The Earth’s magnetic field deflects the charged particles ejected by the Sun, protecting the planet.
The Earth's magnetic field protects us from the solar wind. Image from NASA.
Interaction Between Magnets and Currents in Wires
Just like charged particles are deflected when they run into a magnetic field, charges running through a wire will create a magnetic field that will interact with external magnets to cause the wire to move.
The force on the wire is perpendicular both to the direction of the current and the lines of the magnetic field.
If you reverse the current in the wire (send it the other way) the force will be in the opposite direction.
The force resulting from a current is at right angles to the magnetic field and the current; the "right hand rule" is an easy way to remember this. Image adapted from User:Acdx on Wikimedia Commons.
You can use this principle to create a galvanometer, which is a device that detects electric currents, or to build motors.
A simple electric motor.
Earth’s Magnetic Field
Convection currents in the Earth’s molten, metalic outer core create the Earth’s magnetic field.
Because the pattern of convection changes over time, the Earth’s magnetic field:
Is not located at the north pole (axis of rotation).
Wanders: it moves a little each year.
Flips so its poles reverse every 800,000 years or so.
The location of the magnetic north pole changes with time. Image via the National Forest Service.
As lava cools, the magnetic minerals in it orient themselves with the Earth’s magnetic field. One way of telling how old basalt rocks on the seafloor are is by looking at the direction of their magnetic field. Since the Africa and the Americas are moving apart, slowly over millions of years, there is a suture in the Earth’s crust in the middle of the Atlantic ocean where new seafloor is made from erupting, under-sea volcanos. As a result, there are magnetic stripes all along the Atlantic Ocean (and all the other oceans too) that have recorded each time the Earth’s magnetic polarity has reversed.
Magnetic striping in oceanic crust. Image from the USGS.
This is a really simple electric motor that only requires some wire, a battery, and a magnet. Simon Quellen Field has a wonderfully detailed description of how to build the motor, and some elegant tips on how you can make the motor run faster.
My middle-schoolers quite enjoyed building one of these, and I’m planning on having my high-school physics students also try it; only a couple of them claim to have done it before. It should be a good way to tie together electricity and magnetism.
(Evil Mad Scientist has an even simpler motor, but, given that the risk that their homopolar motor is quite capable of launching a drywall nail across the room, I think I’d suggest not trying that one without extremely close supervision.)
Although it’s a bit trickier, another great way of demonstrating electromagnetic induction is to build a simple alternating current generator that runs a small light bulb.
Jeffery Tomich had a good article last month on the leakage from the coal ash pond at a coal burning power plant near to our school. While the leakage appears to pose no real risk to us, it is a serious environmental issue at a local site that a number of students drive by on the way to school.
I’ve annotated the following excerpt from the article based on the questions my students asked when we talked about the it.
Since Since 1992, a coal ash pond next to the Ameren power plant here has been … hemorrhaging up to 35 gallons a minute [into the local groundwater].
…
At many [other] sites, trace metals in coal ash including lead, mercury, arsenic and selenium have been found in groundwater at levels that exceed drinking water standards.
…
In 2007, a U.S. Environmental Protection Agency report identified 63 sites in 26 states where the water was contaminated by heavy metals from coal ash dumps. That was more than a year before an estimated 5.4 million cubic yards of coal ash sludge escaped an impoundment in Kingston, Tenn. The sludge spread across 300 acres, and 3 million cubic yards spilled into a river.
…
The waste is created from burning coal to create electricity. At Labadie’s ash ponds, it’s composed of fly ash, a fine, talc-like powder that’s captured by filters in the plant’s stacks to reduce pollutants released into the air, and bottom ash, a coarser material that falls to the bottom of coal boilers.
…
a report prepared by Robert Criss, a Washington University professor, identified several dozen private wells along the bluffs near Labadie Bottoms that could be at risk of contamination. Contaminants could infiltrate from shallow alluvial soils to the deeper Ozark aquifer [(see also USGS, 2009)] tapped by residents for drinking water, according to the report.
…
Ameren believes the leaks don’t pose an environmental threat. But because of ongoing concerns, and because the EPA has asked the utility to monitor them, Ameren will make repairs to the ash pond by the end of the year
Most power plants create electricity by spinning a magnet while it’s inside a coil of wire. That how coal power plants do it, it’s how hydroelectric power plants do it, it’s how wind plants do it, it’s even how nuclear power plants do it; solar power panels don’t do it this way, however. The coal and nuclear plants, for example, boil water to create steam which spins the turbine that rotates the magnet.
In theory, you can use any type of power source to spin the turbine, including people power. On bicycles, you can use them to power your lights. But because you’re now using some of your mechanical energy to create electricity, it will slow you down a bit. Newer, hub dynamos, however, are apparently quite efficient.
So, in theory, you could use any type of animal to generate electricity. Including, for example, using bugs to charge your iPod.
I love how he holds up the voltmeter 34 seconds into the video to prove that his device works.
This video from NASA (via physorg.com) includes a nice little section showing the movement of charged particles (cosmic rays) through the Sun’s magnetic field. What’s really neat, is that the Voyager spacecraft (now 33 years old) have discovered magnetic bubbles at the edge of the solar system that make the particles dance a little. It’s a wonderful application of the basic principles of electricity and magnetism.
The Shiloh National Battlefield is only a couple hours east of Memphis (or west of Nashville), and its proximity to Corinth, MS, and a state park with a hydroelectric dam, make it an excellent place for an immersion trip during the cycle when we study the U.S. Civil War and electromagnetism. Two years ago, on a couple beautiful, sunny days in the middle of spring (early April), almost on the anniversary of the battle, we made the trip.
Paleozoic (?) (250-550 million years ago) fossils from Pickwick Landing State Park.
We drove over on a Tuesday morning, and since our very nice cabins at Pickwick Landing State Park were not quite ready yet, we ate the lunch we’d brought with us at a picnic shelter on the park grounds. The choice of picnic shelter number 6 was serendipitous, because not only was it beautifully located, but just down the hill, at the edge of the water, is an excellent outcrop of fossiliferous limestone.
The next morning we hiked along the Confederate line of advance during the Battle of Shiloh.
Reenacting the Confederate skirmish line at Shiloh.Confederate or Union?
It was a relatively long hike, but useful in that it allowed students a feel at least for the scale of the battle, and the conditions the soldiers endured. There was also a nice museum at the end, with an interesting video and an excellent demonstration from one of the park rangers (you need to book an appointment ahead of time).
Finally, on Thursday morning, on our way back to Memphis, we stopped at the Civil War Interpretive Center in Corinth, Mississippi. The museum is excellent, especially the Stream of American History, which is abstract enough that it makes a great puzzle for students to figure out.
Stream of American History.
The map below shows the locations of the stops, and has links to the posts about each stop.
