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.
Calculating the forces between two charged particles (electric force), two magnets (the magnetic force), and two masses (the gravitational force) require remarkably similar equations. But, while electricity and magnetism are directly related (that’s why it’s called electromagnetism), gravity is its own fundamental force. Yet they all depend (inversely) on the square of the distance between the two objects creating the force, so they’re all said to obey some form of the inverse square law.
Gravitational Force (Fg)
The force exerted by two masses on one another is:
where:
G is the gravitational constant (6.67300 × 10-11 m3 kg-1 s-2
m1 and m2 are the masses of the two objects attracting one another.
d is the distance between the two objects.
Electrical Force (Fe)
The force exerted by two electrically charged objects on one another (like a proton and an electron), is:
where:
K is the electrical constant, sometimes called Coulumn’s constant (8.9876 × 109 N m2 C-2
q1 and q2 are the sizes of the charges (in Coulumbs) of the two objects attracting one another.
d is the distance between the two objects.
Magnetic Force (Fm)
The force exerted by two magnets on one another, is:
YouTube user derDon1234 has compiled an interesting video montage of historical events over the last 100 years. derDon1234 makes some interesting choices about what to show — condensed into 10 minutes — but it’s a valuable perspective, with some fascinating and poignant video. It’s worth a look.
Magnetic field of a bar magnet; shown by the alignment of iron filings. Image by Black and Davis (1913) via Wikipedia.
What creates magnetic fields? (36.3)
Magnetic fields are created by moving electric charges. At the atomic level:
In atoms, most of the magnetic field comes from the spinning of electrons (remember electrons have a negative charge).
In most elements, the magnetic fields of electrons pair up and cancel each other out
Only certain elements, which have a few electrons that don’t pair up, can form magnets:
Iron (Fe), Nickel (Ni) and Cobalt (Co) are the common magnetic elements.
Iron is the most powerful magnetic element. It has 4 electrons whose magnetism are not canceled out (because of their arrangement in their electron shells)
Some rare earth elements are also naturally magnetic.
Magnetic Domains (36.4)
Each iron atom has a very small magnetic field, but when a bunch of them line up they add to each other to create a stronger field.
A region with a bunch of lined up atoms is called a magnetic domain.
Microscopic view of the grains that make up a magnet. Each grain has a magnetic field that, if oriented in the same direction, makes for a strong magnet. Each magnetic grain is called a magnetic domain. (Image by Gorchy (2005) via Wikipedia).
If all the magnetic domains line up you have a strong magnet. If they’re all randomly arranged, you don’t have a magnet at all; they cancel each other out and it’s unmagnetized).
Unmagnetized iron (left) and magnetized iron (domains aligned) (right). Adapted from image by Theresa knott at en.wikibooks.
Magnetic Poles (36.1)
Magnets have two poles: a north-seeking pole, and a south seeking pole; they align with the Earth’s magnetic field.
Like poles repel and opposite poles attract.
The force between the poles depends on the strength of the poles (p) and the distance (d) between them:
Note how similar this equation is to the force between two charges (Coloumb’s Law; Fc), and the force between two masses (gravitational force; Fg).
Electric fields come from charged particles, which can be separated, but north and south magnetic poles belong to each domain (and even each atom) so they cannot be separated.
If you break a bar magnet in half you don’t get a separate north and south poles, you just get two magnets, each with it’s own north and south pole.
Magnetic Fields (36.2)
Magnets that are free to move will align themselves with the magnetic fields around them.
The region around the magnet that is affected by the magnetic force is filled with a magnetic field. In theory the magnetic force goes on forever, but is only strong in a relatively small region.
Compasses, which have magnets that are free to move, will align themselves with the magnetic fields around them. When you’re away from other magnets and electronic devices, compasses align with the Earth’s magnetic field.
So how do you create a magnet?
In an unmagnetized piece of iron, the magnetic domains are arranged randomly. If you place it in a strong magnetic field, the domains will align with the strong magnetic field and the iron will become magnetic.
Softer iron alloys will align easier, and stay aligned to make strong, permanent magnets.
Any metal with iron in it (like steel cans or filing cabinets) will gradually align their magnetic domains with the Earth’s magnetic field if they not moved for a long time, but the Earth’s field is not strong enough to make a permanent magnet.
Stroking a piece of iron with a magnet will also align the domains.
Touching a magnet to a paperclip or iron nail will align its magnetic domains and create a temporary magnet, which is why you can use a magnet to hold up a chain of nails. This induced magnetism will only last a short time and eventually (within seconds or minutes) of removing the magnet, the nails will lose their magnetism.
Dropping or heating a permanent magnet will shift some of the domains out of alignment, reducing the strength of the magnet.
Planet Money recounts the story of the seminal document that, in 1979, sparked the transformation of China’s economy into capitalism.
A key thing to note: the document was a contract, which assigned property rights to individuals (families actually) rather than the collective. And even though the contract could not be legally binding in communist China, the signers had to be confident enough that it would be respected — by each other at the least.
The result of the change was a 5 fold increase in the amount of food produced by the farm.
Despite the risks, they decided they had to try this experiment — and to write it down as a formal contract, so everyone would be bound to it. By the light of an oil lamp, Yen Hongchang wrote out the contract.
The farmers agreed to divide up the land among the families. Each family agreed to turn over some of what they grew to the government, and to the collective. And, crucially, the farmers agreed that families that grew enough food would get to keep some for themselves.
The contract also recognized the risks the farmers were taking. If any of the farmers were sent to prison or executed, it said, the others in the group would care for their children until age 18.
