Electricity and Magnetism
Investigation 3 – Concept Day
Electricity and Magnetism: Investigation 3
In this Investigation, we will introduce the concept of electromagnetism. In the lab, you will have the opportunity to build a small electromagnet.
We also wish to accentuate the importance of electromagnetism in our modern world. We will describe and demonstrate how both an electric motor and electric generator work.
Finally, we will illustrate the setup of the experiment in the Investigation 3 lab and go through a simple calculation of magnetic field strength.
In this slide, two applications of electromagnetism are shown. On the left, a large electromagnet picks up scrap metal in a factory. The magnetic field is produced by applying a strong electric current. As one may imagine, electromagnets of such immense strength require very powerful currents of electricity.
On the right is a cutaway photograph of an electric motor. An electric motor is essentially a coil of wires in a magnetic field. In this cutaway section, the magnet can be seen to completely encase the coil. When a current is applied to the coil in the magnetic field, the coil will turn. A shaft runs through and is attached to the coil. At the shaft’s left end, a fan blade is attached to help cool the motor as it turns. The extension of the shaft to the right can be used to do work. It may, for example, turn a water pump, spin a clothes dryer, or lift a load.
This slide shows a very simple desktop electric motor. It contains only the most basic parts of an electric motor so that it may help you visualize clearly how it works.
A coil of coated wire is supported by two bent paperclips (secured to the desk with tape) over a circular magnet. The coating of the area of the coiled wire that touches the paperclips has been removed so that electrical contact is made between the coil and paperclips.
Finally, three D-cell batteries in series are attached with alligator clips the each of the paper clips to complete a circuit.
This slide shows an enormous electric generator. An electric generator works the exact opposite of an electric motor. It has a coil surrounded by fixed magnets like an electric motor. However, instead of applying a current to the coil, causing it to turn, the coil is mechanically turned, which causes an electrical current!
The coil can be turned by many different means. In very old telephones (see below), for example, one could turn the coil within a magnetic field with a hand crank to produce enough electricity to place a call.
A similar device called a dynamo is still used on bicycles to generate electrical current to power headlights and taillights for night riding.
Bicycle dynamos, like the one shown in the video above, work with a mechanism that converts the mechanical energy of the spinning bicycle wheel into electrical energy. The diagram below illustrates the inner components of a basic bicycle dynamo. Once again, like the old telephone magneto above and the turbines we discussed in the CELL Introduction and will again mention below, the dynamo works by converting mechanical energy into electrical energy through the interaction between magnets and wire coils. The application of electromagnetism to a countless variety of machines was one of the major advances that changed and modernized industry over the past century.
On a much, much larger scale, giant turbines can spin the coil of massive electrical generators. The mechanical energy to turn the turbines themselves can be provided by steam (from coal or nuclear heat), water flow (hydroelectricity), or even wind. The electricity produced by any of these forms of mechanical energy can then be purchased as electrical current by consumers to run appliances, heat homes, and so on.
This slide shows the same setup as that seen earlier for an electric motor in slide ELECT-3-3, except that instead of having the apparatus connected to batteries to turn the coil in a magnetic field, as in the case of the electric motor, it is now hooked up to a multimeter to measure voltage. In this configuration, it is essentially a simple generator.
As can be seen, manually turning the wire coil (applying mechanical energy), even slightly, in a magnetic field causes a small amount of voltage (electrical energy) to be detected by the multimeter.
This slide illustrates yet another use for electromagnetism. The center photograph shows a patient entering an MRI. MRI stands for Magnetic Resonance Imaging. Perhaps you may have experienced an MRI if you suffered a broken arm or ankle or some other trauma.
The illustration on the left shows a whole-body MRI. Unlike x-ray, MRIs image not only the bones of the skeleton but soft tissue as well. In this image, tissue such as arm and leg muscles, the heart, liver, and brain can clearly be seen.
The image at the lower right shows an MRI profile of the human head. The brain can clearly be seen in great detail. One can even see the grey and white matter of the cerebrum. The resolution of a good MRI allows scientists and physicians to see detail as small as a square millimeter.
Finally, the two images on the upper right show a normal brain on the left and the MRI image of a patient with a stroke on the right. Obviously, MRI is an extremely valuable tool for doctors to diagnose a whole host of pathologic conditions.
This slide illustrates the experimental setup of an electromagnet that you will construct in the Investigation 3 lab. An electromagnet’s strength can be determined by knowing the current applied and the length and number of turns of the wire coil as will be further discussed on the next slide.
This final slide shows how to calculate the strength of the magnetic field of an electromagnet. You will perform similar calculations with data you obtain in Investigation 3 lab.
The formula for magnetic field is as follows:
On the lower right of this slide is an experimental setup of a small electromagnet. On the lower left is a calculation using the magnetic field formula and the experimental data.
Notice that the determined field strength in this example is about 4.3 x 10-3 T (Tesla). The images obtained with the MRI on a previous slide use magnetic fields at least a thousand times greater than this. Modern MRIs include magnets that range from 3.0 T to 7.0 T.
How strong is 7 T? In the Investigation 3 lab, you will build an electromagnet using batteries, a metal nail, and a coil of wire. With a magnetic field strength of about 4.0 x 10-3 T, you may be able to pick up a few small metal paperclips. On the other hand, if you were able to make a similar electromagnet but with 7.0 T field strength, you would be able to pick up a piece of metal that weighs more than a very heavy bowling ball!