Electromagnets

The initial investigation into the correct shaping of the wire to achieve the desired field shape begins the engineering design process. During the activity, students proceed with the design process, making revisions to try to achieve the strongest electromagnet possible so as to pick up the most steel. Electromagnets are important inclusions to many electrical engineering designs and also supplement the work in other engineering disciplines such as mechanical and aerospace.

After this lesson, students should be able to:

• Describe how the magnetic field of an electric current can be used to create a magnetic field similar to that of a permanent magnet.
• Build an electromagnet.
• Identify the properties of an electromagnet that affect its strength.

An understanding of the magnetic field produced by an electric current and the general properties of an electric current.

Legacy Cycle Information

This lesson concludes the legacy cycle with the last iteration of Research and Revise, the summative quiz in Test Your Mettle, and the final share out of designs in Go Public. Students understand the fundamentals of magnetic fields and have a design of their own that will achieve the desired goal; they conclude by sharing their designs with other students.

Figure 1. The magnetic field of a permanent magnet.

Lecture Information

An electromagnet takes advantage of the induced magnetic field of an electric current to mimic a permanent magnet with the advantage that it can be shut off. An electromagnet requires a continuous supply of electrical energy in order to maintain a magnetic field, which differs from a permanent magnet that doesn't require power. In an electromagnet, the battery's chemical energy transforms to electrical energy in the wire. Additionally, electromagnetic energy in the nail transforms to heat energy. Because the magnetic fields obey the principle of superposition, lining up several wires with the currents all pointing in the same direction results in the net magnetic field of the four wires acting together being a larger loop around all the wires, as illustrated in Figure 3. In order to have the same shape and direction of loops as in a permanent magnet (see Figure 1), one needs a collection of wires with currents all going one way near a collection of wires with currents all pointing in the opposite direction of the first group. This can be achieved by looping the wire, with the overall effect illustrated in Figure 4.

Figure 2. (left) Induced magnetic field of an electric current going into the page. Figure 3. (right) Super position of induced magnetic field of four electric currents going into the page.

The strength of this induced magnetic field, or electromagnet, is dependent on several factors. First, any induced magnetic field is proportional to the strength of the current inducing it. If the current in the electromagnet is increased, then the strength of its magnetic field increases. Next, the number of loops forming the electromagnet affects its strength as a magnet because the overlying fields support each other. If there are more loops and they are wound tightly together, the magnet is stronger. Finally, if the loops are formed around a material that is itself magnetic, the magnetic field induced by the current in turn induces a magnetic field in the material, further accentuating the overall strength of the electromagnet. If the core is made of iron, nickel, or cobalt, the electromagnet is strengthened.

Figure 4. Magnetic field made by loops of current carrying wire attracts metal on the right.CopyrightCopyright 2006 Abuelodelanada, Wikimedia Commons http://commons.wikimedia.org/wiki/File:Bobina_corriente.jpg

As you discovered in the activities with permanent magnets, in order to pick up only steel, you need a magnetic field shaped like this (draw the magnetic field lines of a permanent magnet or show the image), but in order to put the steel back down, you need a magnetic field you can turn off, which is an induced magnetic field and looks like this (draw the magnetic field lines of an induced magnetic field or show the image). As an engineer, how can you make this (indicate the induced magnetic field) look like this (indicate the permanent magnetic field)? (Ask the students to think about possible solutions and discuss their ideas. Do not completely give away the solution, but provide the second paragraph as guidance if no workable solutions are provided.)

To help you narrow down a solution, consider what happens when you have multiple magnetic fields. If the loops curl in the same direction, then the fields help each other and become stronger. If the fields curl in opposite directions, then the fields cancel each other. If you line up four wires so that all the currents go into the board, then the magnetic fields all curl the same way and you get this total magnetic field (draw the net magnetic field from superimposing the individual fields). This total field, especially if the wires are put close enough together, just looks like one big loop around all the wires. That's half of the loops you need to get the same effect as a permanent magnet.

(Begin the Building an Electromagnet associated activity.)

Unit Quiz: For unit summative assessment, administer the 10-question Clean Up This Mess Quiz (PDF) to assess students' understandings of permanent magnetic fields, induced magnetic fields and electromagnets. Administer the quiz after the construction of the electromagnet in Building an Electromagnet or after the presentations in Building an Electromagnet. Review students' answers to gauge their depth of comprehension.