Electric and Magnetic Fields and Lenz's Law
[Shopping List: cow magnet; sphere and disk neodymium magnets; large wire coil; galvanometer, amp-meter or multi-meter; wires with and without alligator clips; batteries; small compasses; aluminum tracks; copper pipes; steel ball bearings; shaker flashlights; thick sheet or block of aluminum or copper]
Electricity and Magnetic Fields
- Hold a length of wire close to a small compass. Does the compass needle move at all?
- Press one end of the wire against one terminal of a battery, and again move it close to the compass. Does the needle move this time?
- Now press each end of the wire to the terminals of the battery, forming an electric circuit, and once again move it close to the compass. Does the needle move now?
Magnets and Electric Fields (Electricity)
- Connect the large wire coil to a multi-meter or galvanometer using connector wires with alligator clips.
- Move a cow magnet back and forth in and around the coil while observing the meter. Does the needle move?
- Repeat 2 with the neodymium magnet, which is much stronger.
- Hold the magnet- either inside or outside of the coil- but do not move it. What happens to the needle on the meter?
- Look closely at the inside of the shaker flashlight. Turn it on (you may have to shake it a few times first). How does it work without any battery?
Lenz's Law
- Holding a 12" long copper pipe vertically, drop a steel ball bearing into the top end and observe how long it takes to fall through the pipe.
- Repeat the experiment, this time dropping a neodymium sphere or disk magnet into the pipe. How long does it take to fall through the pipe now?
- Hold the long aluminum track horizontally, and place a steel ball bearing onto the track. Practice rolling the ball back and forth by lifting the ends to observe the ball's speed.
- Now replace the steel ball with a neodymium magnet sphere and tip it to show that the magnet does not stick to the aluminum. Replace it on the track and roll it back and forth. How fast does the magnet roll?
- Carefully raise one end of the track higher and higher to observe the speed of the magnet. If you're careful, you should be able to hold the track nearly vertical while the magnet slowly falls barely even touching the track.
What's Happening: Electric and magnetic fields are not only similar in their behavior (both magnets and electric charges can be attracted or repelled by other magnets/charges), they are intricately linked together. A moving magnet creates or induces its own electric field, and a moving electric charge (or electric current flowing in a wire, which is just many moving electrons moving through the wire) induces its own magnetic field. The key word is moving, there must be motion to induce a field. A metal wire does not produce a magnetic field (actually it will if you move it, but the field is much too weak to move the compass needle), but when connected to a battery an electric current (electricity) flows through the wire (i.e. electrons are moving) which produces a magnetic field around the wire that can be detected by the compass. If the battery terminals are reversed, the direction of the current flow also reverses, and the induced magnetic field points in the opposite direction.
Similarly, when a magnet is moved near a coil of wire, it induces an electric field which- just like the electric field created by a battery- causes electrons or electricity to flow in the wire. In this experiment the amount off electricity created was very small (much too small to light a light bulb for example), but the multi-meter is sensitive enough to measure it. When the magnet is not moving however, no matter how close it may be to the wire, no electricity is produced. Also notice that the needle on the meter changes direction as you change the direction that the magnet is moving, i.e. the flow of electricity changes direction also. Finally, the stronger the magnet- or the faster you move it, or the closer it is to the conductor-, the stronger the induced electric field will be. The shaker flashlight uses exactly this mechanism to produce electricity (as do most electrical generators). Each time you shake the flashlight a small but powerful magnet moves back and forth through a coil or wire, and this small amount of electricity is stored in a capacitor until there is enough to operate the special light bulb, called an LED, which uses very little electricity). It usually takes 2-3 minutes of shaking to operate the bulb for 20-30 minutes. In this project we used a neodymium magnet (actually neodymium-iron-boron), sometimes called a rare-earth magnet, because it is very powerful.
So if a moving magnet induces its own electric field which can then make electricity flow in a nearby wire (or any good electrical conductor), wouldn't that electricity (which is just moving electrons) induce its own magnetic field? The answer is yes, it does. So when we roll a powerful magnet down the aluminum track (which is a very good electrical conductor), as long as the magnet is moving it will induce electrical currents in the aluminum (called Eddy currents) which will induce their own magnetic field, essentially creating a magnet in the aluminum, even though aluminum is not normally a magnetic material (that's why a stationary magnet does not stick to aluminum). The same is true for the copper pipe, which is also a very good conductor of electricity. But this only works while the ball magnet is moving- again the reason why a stationary magnet does not stick to aluminum or copper.
The scientist who discovered this was named Lenz, and he also showed that the induced magnetic field always points in the opposite direction from the field of the original magnet which created it. In other words, if the sphere or disk magnet is falling down the copper tube with its south pole pointed down, the magnetic field (or magnet) induced in the copper will point south pole up. Anyone who has played with two magnets knows that when you point the south pole of one magnet towards the south pole of a second magnet (or north pole to north pole), they repel or push away from each other. The magnetic field of the second magnet creates a force which pushes on the first magnet. In our experiment the original magnet is falling down the tube (due to the force of gravity), so the induced magnetic field in the copper creates an opposite force that pushes back up, against gravity. Because some of the energy is lost in the copper due to its electrical resistance, the induced force will always be weaker than the force (in this case gravity) that created it, so the falling magnet slows down- in our experiment it slows down a LOT- but never actually stops (or reverses direction). [If we repeated this experiment with a superconductor (a material which has zero electrical resistance), there would be no loss of energy and the induced magnetic field would be strong enough to stop the falling magnet, or levitate it.]
Variation: You can actually feel the induced magnetic force by simply moving a powerful magnet very close to a good conductor. The closer you get, the more powerful the force, but don't actually touch the conductor, or friction will affect your results.
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