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A compass allows us to observe the direction of a magnetic field: compass needles are just little magnets that are free to rotate. Normally, compasses respond to Earth’s magnetic field, orienting themselves parallel to magnetic field lines. If we create a magnetic field that is stronger than Earth's field—for example, by using electric currents—a compass needle will orient itself parallel to the new field.
Caution! Working with electricity can really heat things up! Always be careful with hot wires. And be sure that you don't leave the clip leads connected too long‚ because the electric current will rapidly drain the battery and may cause it to overheat. A few seconds should be long enough to make good observations.
Constructing the stand:
Make one of your own design or follow the steps below to build a stand out of PVC. Note that it's not necessary to use PVC cement; friction fit will be ample to hold the stand together.
Making a square coil:
Putting the whole thing together:
Observe the compass needles when there is no current passing through the wire. Rotate the cardboard platform. What happens to the compass needles? They will point north, orienting themselves so that they are parallel to Earth’s magnetic field. (Note: A few of your compasses may point south! Inexpensive compasses that are exposed to a strong magnet will sometimes become magnetized in the reverse direction. It’s nothing to worry about, though—just keep in mind which end of each compass points north.)
Now attach the clip leads to the battery terminals. Watch what happens to the compass needles as current passes through the coil. (CAUTION: Don't leave the battery connected for more than several seconds at a time—the electric current can rapidly drain the battery and may cause it to overheat.) If the electrical current is large enough, each compass will point in a direction tangent to a circle centered on the vertical coil wires that they surround.
Rotate the cardboard platform again. What happens to the compass needles this time? The compasses will continue to point in a direction tangent to a circle centered on the vertical coil wires.
Switch the clip leads to the other terminals of the battery. What happens? The compass needles will reverse direction when the electrical current reverses direction.
Compass needles line up with magnetic fields. Since Earth is a magnet, a compass will normally line up with Earth’s magnetic field. Because opposite magnetic poles attract, the magnetic north pole of the compass points toward the magnetic south pole of the Earth. (The magnetic south pole of the Earth is located in northern Canada—that is not a misprint. Indeed, the magnetic south pole of the Earth is near the geographic north pole. To make things even more confusing, mapmakers call this the north magnetic pole.)
Electric current passing through a wire creates a magnetic field. In the vertical side of the coil, there are four wires and the current is the same size and moving in the same direction in all of them. Thus the compasses are sensing the magnetic field produced by a current four times larger than if they were surrounded by just one of the wires. This allows the use of a D-cell battery rather than a larger and more expensive battery that would be necessary to produce an equivalent current in a single wire.
The electric current passing through the vertical side of the coil creates a magnetic field that is stronger than Earth’s field (in a region close to the wires). You can visualize the shape of this new field as a set of concentric circles surrounding the vertical coil wires. The closer to the vertical coil wires you are, the stronger the magnetic field. The compass needles align themselves with the total magnetic field at each point, the sum of Earth’s field and that of the wire. Since the magnetic field from the vertical coil wires is significantly larger than that from Earth, each needle ends up pointing essentially in the direction of the magnetic field of the wire.
When you reverse the current, the direction of the magnetic field also reverses, and the needles dutifully follow it.
To find the direction of the magnetic field made by an electrical current, you can use a technique called the right-hand rule.
Place your right hand with the thumb parallel to the wire carrying the current. Point your thumb in the direction of the electrical current in the wire. (Remember: The electric current flows from the plus side of the battery through the wire to the minus side.) Wrap your fingers around the wire. Your fingers will now point in the direction of the magnetic field around the wire. If there are compasses near the wire, they will point in the same direction as your fingers. (See diagram below; click to enlarge.)
Note that what actually moves in the wire are electrons flowing from the negative side of the battery to the positive side. Electrical engineers and scientists think of “current” as a flow of positive charges that produces the same effect as that produced by the flow of negative charges in the opposite direction.
Iron filings trace out magnetic field lines in three dimensions.
Magnetic lines stop here.
Ding dong! This investigation shows how your doorbell works.
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Attribution: Exploratorium Teacher Institute