Electricity and Magnetic Fields

This lesson covers induced magnetic fields and builds into the design of electromagnets, which are science topics that many types of engineers must understand to do their work, such as designing electric motors and unlocking car doors. Students are asked to think about how they might use this information as if they are engineers challenged to design a device to separate steel from aluminum for a recycler.

After this lesson, students should be able to:

• Identify an electric current as a source of a magnetic field.
• Describe the magnetic field lines around a wire carrying an electric current.

An understanding of the shape and nature of magnetic fields of permanent magnets and how to find magnetic field lines. An understanding of the flow of electricity in circuits.

Legacy Cycle Information

This lesson continues the Review and Revise step. Students have identified a magnetic field as a solution to separating steel from aluminum but still have additional work to do in properly designing and controlling that magnetic field. This lesson and accompanying activity provides them the knowledge of induced magnetic fields necessary to completely designing an electromagnet.

Lecture information

In the activity, students are investigating a concept called magnetic induction, although that term is not used until the summary of the lesson. Electric fields and magnetic fields are closely linked. Magnetic fields affect moving electrons, a principle that underlies iron's, nickel's, and cobalt's easy attraction to magnets. The reverse is also true: moving electrons create magnetic fields.The net movement of electrons in a simple circuit is sufficient to generate a magnetic field. The stronger the electric current, the stronger the magnetic field induced is. If there is no electric current, then there is no magnetic field.

The magnetic field is perpendicular to the direction of the current or the wire carrying it. This requires three-dimensional thinking in order to conceptualize it and also makes for a slightly elaborate lab setup. The magnetic field wraps around the wire in concentric circles, with the field growing weaker the further from the wire it is observed. The direction of the wrapping, or curl, follows what is called the "right hand rule"—if the thumb of one's right hand points along the wire in the direction of current flow, taken to be from the positive to negative terminals of the battery, then the magnetic field wraps around the wire in the direction of the fingers of the right hand.

Engineers use these principles of electricity and magnetism in many different applications. A few examples are electric motors, solenoids that unlock car doors, and electromagnets for applications such as a separating trash as described in this unit.An electromagnet picks up and moves scrap steel.CopyrightCopyright 1941 Alfred T. Palmer Wikimedia Commons http://commons.wikimedia.org/wiki/File:Allegheny_Ludlum_Steel_Corp_Scrap_Piles.jpg

(Have a magnet and a pile of aluminum and steel—materials from Magnet or Not? work well—available for a brief class demonstration.) Based on what we've looked at, a magnet will successfully separate steel from aluminum because the iron in the steel is affected by the magnetic field but the aluminum is not. (Use the magnet to pick up some of the steel from the aluminum.) So you have picked up the steel and separated it from the aluminum. You still have a problem. What must you still do to finish your job? (Briefly discuss student ideas, guiding them toward the need to remove the steel from the magnet.)

To summarize, you use the magnet to pick up the steel, then you move the magnet and steel to your new steel pile. Now, you must remove the steel from the magnet. And we're not just talking a single piece of (say whatever steel material you are holding attached to the magnet). We're talking possibly large amounts of scrap metal. How are you going to get the steel off of the magnet? (Listen to and discuss student ideas. Make sure they understand that this is not just a matter of pulling the steel scrap off—in order to pick up large amounts of scrap the magnet must be strong, which prevents simply pulling the steel off. Additionally, that requires manual labor, a solution already deemed not cost-effective. Direct them toward the need to turn off the magnet.)

If we're going to turn off the magnet, we need a magnet made from something we can turn off. What options do we have of things we can turn off? (Listen to and briefly discuss student ideas. Quickly direct them to the option of electricity.) If we can find a magnetic field around an electric current, we can turn it off by turning off the electric current. (Begin the Circuits and Magnetic Fields associated activity.)

(Conduct this closure after students have completed the accompanying activity.) We have one problem left. The magnetic field created by the electric current, or the induced magnetic field, does not match the magnetic field of a permanent magnet. We need to design the shape of the wire so that its induced magnetic field closely matches the magnetic field of a permanent magnet. But first, you've got to check to see how well you know all these magnetic fields. We'll discuss this in the next activity. (Hand out the Magnetic Field Check (PDF).)

Lesson Concepts Formative Assessment: Administer the six-question Magnetic Field Check (PDF), giving students no additional aids other than the advice "use the right hand rule." Review their answers to gauge their comprehension before moving on.