GED Science Practice Test: Electromagnetism And Generators

It is most important to understand the interactions of electricity and magnetism.  All electricity, with the exception of solar cells, is generated through the interaction of electricity and magnetism.  Furthermore, motors, like the motors of cars, household appliances, etc., work because of those interactions. We will begin by looking at a simple interaction of electricity and magnetism.

When an electrical current flows through a wire, that current actually creates a magnetic field around the wire.  This creation of a magnetic field from a flowing electrical current is called electromagnetism.  This phenomenon was discovered by accident in 1820 by Hans Christian Oersted.  He noticed that when he ran a current through a wire, that it caused a nearby compass needle to deflect.


An electromagnet is a temporary magnetic field that is created from the flow of electrical current.  The magnetic field that is generated is relatively weak, but it can be strengthened both by coiling the wire to concentrate the generated magnetic field, and by placing a ferromagnetic material within the coil of wire. The following diagram shows a simple electromagnet:


An electromagnet is different from a regular magnet in more than just appearance.  Since the magnetic field of an electromagnet is created from the flow of electrical current, the magnetic field disappears if the electrical current disappears.  Thus, the magnetic field of an electromagnet is temporary.  This is useful in a number of applications.  One way that electromagnets are used is in car junkyards.  In junkyards, unworking cars need to be easily moved around.  With an electromagnet, you can pick up a car (made largely of steel, a ferromagnetic material), move it to a new location, turn off the electromagnet, and drop the car in its new location.  With a permanent magnet, this could not be done as the magnet would permanently hold on to the car and not be able to drop it in its new location!

Another device that takes advantage of the fact that electrical currents create magnetic fields is a motor.  If an electrical current creates a magnetic field, and that magnetic field is placed in the opposite direction to an existing magnet, motion can be generated as a result.  Remember that like charges on a magnet repel each other.  If you place two magnets close to each other, with their north ends facing each other, they will generate motion and push away from each other.  In a motor, you can purposefully place the north end of an electromagnet, and the north end of a permanent magnet in proximity to each other to create motion.  When the electrical current is turned on, the electromagnet is created, and the North end of the electromagnet pushes against the North end of the permanent magnet.  When the electrical current is turned off, the motion ceases, because the electromagnet is no longer active.  The following diagram shows a simple electrical motor made from a battery, some wire, and a permanent magnet:


This device is a motor, because the coil of wire on top of the magnet will spin when the circuit is completed.  The following picture shows a similar electrical motor.  You can see the spin of the coil of wire by the blur in the photo:



In the section on motors, we learned that if you can put the north end of a permanent magnet next to the north end of an electromagnet, you can create motion.  In order to determine where the north end of an electromagnet is, you could go through a trial and error process, and check it with a compass.  However, when building a complex motor, you would want to anticipate ahead of time where the north end of the electromagnet would be. The direction of the magnetic field generated by an electrical current can be determined by using the “right hand rule” as shown in the diagram below:


This rule says that if you point your thumb in the direction of conventional electrical current flow (positive to negative), your fingers will curl in the direction of the magnetic field.  The tip of your fingers will point north.

Not only do flowing electrical currents produce magnetic fields, but moving magnetic fields can create electrical currents.  The following diagram shows a set-up in which a magnet moved into and out of a coil of wire will generate an electric current in that loop of wire.  A galvanometer is a device that measures electricity:


The creation of an electrical current from moving magnetic fields is called electromagnetic induction.

If you have ever used an emergency flashlight, either the kind that you shake to activate, or the kind that press a lever repeatedly to activate, you’ve experienced electromagnetic induction.  The motion of shaking the flashlight or pressing the lever moved a magnet relative to a coil of wire.  That coil of wire was connected to a light bulb that illuminated from your action.  In fact, some of these flashlights have a transparent cover so you can see the mechanism inside.  You can often see the coil of copper wire inside of these flashlights!

Electromagnetic induction is also responsible for how power plants generate electricity.  Consider a hydroelectric power plant, or a hydropower plant or dam, for a moment.  In a hydropower plant, the force of falling water spins a mechanism that has a coil of wire on it.  That coil of wire is made to spin next to a magnet.  This movement, though involving a wire moving next to a stationary magnet, is equivalent to a magnet moving next to a stationary wire.  As a result, electricity begins to flow in the electrical wire.  A device that takes mechanical energy and converts that to magnetic energy and then to electrical energy is called a generator.   The following diagram shows a very simplified generator:


Notice that motors and generators are the exact opposite.  Motors use electrical energy to create magnetic energy, which creates motion.  Generators take motion to create magnetic energy to create electrical energy.



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