Create magnetism with electricity
Introduction
When electric charges move they create magnetism.
Material
Assembly
Strip the insulation off about 2 cm (1 inch) off both ends of the wire. For wire with plastic insulation use a wire stripper. For magnet wire fold over the sandpaper so that the sandy sides touch each other then pull the end of the wire through the sandpaper to remove the enamel coating. Do this several times rotating the wire between each pull until all the insulation is removed and the wire is shiny.
Screw the nut onto the bolt.
Wrap electrical tape (or other tape) around the bolt to protect the wire from being cut by the sharp edges of the threads.
Wind the wire onto the bolt, leave a fist width, about 10 cm (4 inches), of wire free at each end.
You must always wind the wire in the same
direction.
One way to do this is to look at the head of the bolt as you wind and
always wind clockwise.
Put tape over the wire windings to hold them in place.
Bend the ends of the wire back over themselves and fold them into a sandwich of aluminum foil, we'll call this a pad.
Put the rubber band around the battery passing over the positive and negative ends.
Slide one of the aluminum foil pads between the rubber band and the negative terminal of the battery.
To Do and Notice
Put a paper clip onto the table try to pick it up with the steel bolt. Usually the bolt will not attract the paper clip.
If the bolt attracts the paper clip then the bolt has been magnetized, demagnetize it by striking it sharply onto a solid surface such as a cinder block. Do this until it will not attract the paper clip.(It is also possible that the paper clip itself could be magnetized, rap it with a hard object to demagnetize it.)
Attach the two pads of the wire to the ends of the
battery.
Now the steel bolt will attract the paper clip.
You have made an electromagnet.
Detach one of the pads from the battery. The paper clips may fall off.
If the paper clip remains attached then you have magnetized the steel bolt and maybe even magnetized the paper clip.
If the bolt has retained magnetization after you disconnect it, reversing the connection to the battery will magnetize the bolt in the opposite direction, this will usually repel all attached paper clips.
Make sure you disconnect one of the pads after each experiment to avoid depleting the battery.
Reverse the leads, note that the electromagnet still works, it will still pick up paper clips.
Put the bolt horizontal on a table top lined up east west (Use the compass to find north). Bring a compass near one end of the bolt. If you get the compass close enough the magnet in the compass will be attracted to the steel of the bolt. Move the compass just far enough away that this doesn't happen. Now connect the wires to the battery. Notice that one end of the compass is attracted to the electromagnet. The red (north pole) of the compass will be attracted to the south pole of the electromagnet.
Move the compass to the opposite end of the electromagnet and notice that there is an opposite magnetic pole at each end of the bolt.
Reverse the connections to the battery and notice that the magnetic poles switch ends of the electromagnet.
What's Going On?
When electric current flows through a wire, the wire becomes surrounded with a magnetic field. Each bit of current carrying wire is surrounded by concentric circles of magnetic field, centered on the wire.
When you wind the wire into a circle all of the magnetic fields surrounding each bit of wire line up through the center of the coil creating a large magnetic field with a north magnetic pole at one end of the wire and a south magnetic pole at the other.
The iron atoms in the bolt are all small magnets. Usually these magnets point in different directions and so cancel each other out, the bolt is not a magnet. When the bolt is placed inside the coil, the iron atom magnets line up with the magnetic field of the coils making it much stronger.
When the magnetic field of the coil is removed the atoms in pure iron go back to being randomly arranged and the bolt loses its magnetization. However the iron atoms in a bolt with impurities such as those used to make steel may remain aligned. When this happens we say that the bolt has become magnetized. Striking the bolt sharply adds vibrational energy which randomizes the alignment of the iron atoms removing the magnetization.
Going Further
Try the experiment with a brass, aluminum, or plastic bolt. Notice how much strength is added to the electromagnet by the steel bolt.
You can also quantify the strength of your electromagnet by adding plastic paper clips to the metal paper clip until it falls off your electromagnet.
You can also measure the strength of an electromagnet by holding up a steel paper clip held away from the steel bolt of the electromagnet by layers of paper stock such as 3 x 5 cards. Stronger magnets will hold the paper clip up through a greater thickness of card stock.
So What?
Scrap steel yards use electromagnets to lift cars and then to drop them into shredding machines. These electromagnets are made of pure iron so that when the electric current is turned off the iron core of the electromagnet loses its magnetization almost completely. This allows the car to drop off the electromagnet.
Etc.
Joseph Henry, an American Science teacher, invented the strong electromagnet when he wrapped coper wire with silk, thus giving it an insulation layer. He could thus wind many turns of wire into a small space creating a strong magnet. Before Henry people had to keep an air gap between the coils of wire to prevent short circuits.
Math Root
The relationship between the electric current and the direction of the magnetic field is inherently three dimensional. It follows a right hand rule. If you point the thumb of your right hand along the direction of flow of positive charges through a wire, then wrap the fingers of your right hand around the wire, your fingers will point in the direction of the magnetic field. The magnetic field is defined to point out of a north pole and into a south pole.
The strength of an electromagnet is proportional to the electric current that flows in the wire, it is also proportional to the number of turns of wire in the loop.
Advanced Physics
Electricity and magnetism are related by relativity. If you are stationary and look at a stationary charge it will be surrounded by an electric field. But if you and the charge are in relative motion a portion of the electric field will be transformed into a magnetic field.
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Scientific Explorations with Paul Doherty |
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4 March 2003 |