Last week, my middle schoolers did a set of experiments on electricity and magnetism. They answered the questions:
How does the voltage across each light bulb change as you add more and more bulbs to a parallel circuit?
How does the voltage across each light bulb change as you add more and more bulbs to a series circuit?
How does the number of coils of wire wrapped around a nail affect it’s magnetism (as measured by the number of paperclips it can pick up)?
How does the amount of salt mixed into water affect its conductivity?
An electromagnetic nail lifts two paperclips.Students measure the conductivity of a salt water solution.
Each question is designed so that students have something to measure and will be able to use those measurements to make predictions. For example, once they’ve measured the voltage across four bulbs in series, they should be able to predict the voltage across the bulbs in a series of ten.
Some of the experiments, like the nail electromagnet, should have simple linear trends, with students choosing the advanced option having to find an equation to fit their data for the predictions. And I’ll challenge the students in Algebra II to find the equations for the inverse relationships–I’ve already asked their math teacher (Mr. Schmidt) to help them out if they need it.
This has also provided the opportunity for them to apply what they’ve just learned about drawing circuit diagrams (we use this set of symbols).
Circuit diagrams of bulbs in parallel. The voltage difference across each bulb is also noted.
Ice melts around an embedded leaf, taking the pattern of the leaf.
Darker colored objects absorb more light than lighter colored objects. Darker objects reflect less light; they have a lower albedo. So a deep brown leaf embedded in the ice will absorb more heat than the clear ice around it, warming up the leaf and melting the ice in contact with it. The result, is melting ice with shape and pattern of the leaf. It’s rather neat.
NASA’s Heliophysics (physics of the sun) website has an excellent collection of videos that would link quite nicely with physics discussions of the physics of light (electromagnetism) and the Earth’s magnetic field (as well as the action of charged particles in a magnetic field.
They also have awesome solar videos, like this one of coronal rain.
Natural phenomena like this are great for students to analyze because they require the integration of multiple concepts to explain.
Astronaut Don Pettit makes water droplets orbit a knitting needle. Instead of gravity, the attractive force that holds the water droplets in orbit is generated by the static electric charge on the knitting needle and on the water droplet. This works because gravity and electromagnetic forces follow similar rules (inverse square laws).
Just in time for our physics test — on electromagnetism — the Sun has had a Coronal Mass Ejection of charged particles that is heading toward the Earth.
[The Coronal Mass Ejection] is moving at almost 1,400 miles per second, and could reach Earth’s magnetosphere – the magnetic envelope that surrounds Earth — as early as tomorrow, Jan 24 at 9 AM ET (plus or minus 7 hours). This has the potential to provide good auroral displays, possibly at lower latitudes than normal.
The Earth's magnetic field deflects charged particles around the planet, although some do get redirected down toward the poles, making the arouras. (Image from NASA's Spaceplace).
A Coronal Mass Ejection has about 100 billion tons of electrons, protons and other particles (NASA Cosmicopia, 2011), usually ionized, that would bombard the Earth and the atmosphere if we weren’t protected by the Earth’s magnetic field.
Most of the ions are deflected around the Earth but some get focused down toward the poles. At the poles, these ions hit nitrogen and oxygen molecules (that make up 98% of the atmosphere), exciting many of them. Excited atoms and molecules give off light. The light shows created are called the auroras.
Aroura australis, as seen from the International Space Station.
I like the second video they post because, at the end, there is a splatter of interference from all the charged particles affecting the detector.
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.
