Circuits and Magnetic Fields

Electromagnetic induction is a fundamental principle underlying motor design and many disciplines of both electrical and mechanical engineering. The study of electromagnetic fields is also a fundamental discipline of electrical engineering. Students think about how they might use an electromagnet as engineers to design a device to separate steel from aluminum for a recycler.

After this activity, students should be able to:

• Describe the shape of the induced magnetic field of an electrical current.
• Describe how current in a particular direction yields a curving magnetic field according to the right hand rule.

An understanding of the movement of charge in an electrical circuit and of the effect of different electrical properties on current strength.

In the previous activity, you detected and drew the magnetic field of a permanent magnet. What important tool did you use? (Make sure students identify the compass.)

Now, you will find the magnetic field around an electric current. To produce the electric current, you will need a battery, wire and light bulb. Hook up your circuit, and find the magnetic field with your compass following the procedures I will describe next.

Setup

• Make copies of the Induced Magnetic Field Worksheet (PDF), one per group.
• Have ready the required materials for each group.

1. Divide the class into groups of three students each.
2. Present the Introduction/Motivation section.
3. Hand out the worksheets and direct students to mark 30 points spread out across the paper.
4. Provide students with the activity materials and let them to investigate on their own.
5. After students have identified that there is no magnetic field detected in the same plane as the current, inform them that the magnetic field is actually in a different dimension, so the setup needs to have the current going somewhere else besides on along the table top, then walk them through the setup as shown in Figure 1.
6. Have students diagram the magnetic field by putting the compass at different points on the paper and drawing an arrow to show the direction of the compass needle until all points have been catalogued.Figure 1. Induced magnetic field detection setup.CopyrightCopyright Todd France

Upon completion of the field diagrams, have several groups draw their diagrams on the classroom board for comparing and contrasting. Point out the circular pattern of the arrows and inform students of the right-hand rule using the drawing provided at the top of this document.

If time permits, have students reverse the terminal connections, causing the current to flow up through the paper rather than down. Have them observe the change in the magnetic field and verify the right-hand rule.

The light bulb and holder are provided so the students have evidence that current is flowing through the wire and to keep the wire from growing too hot. Its resistance, however, may slow the current enough that it cannot produce a strong enough magnetic field for the compass to detect. During step 4, it is okay if students use the light bulb in the circuit, but once the setup shown in figure 1 is built in step 5, it is best to leave the light bulb out (to achieve a strong enough current) and only have students connect the wire briefly when they are directly observing the compass. Be careful as the wire will get hot.

During step 4, students will likely have both the compass and the wire laid flat on the table. Because the compass cannot turn in the direction of the field, which is into and out of the table, they should see no effect of the current on the magnetic field. If they insist they have observed an effect, have them turn the current on and off by disconnecting the wire from the battery. If there were a magnetic field due to the current, disconnecting the wire would cause a movement of the compass needle, but there will be no movement. Any magnetic field observed will be due to the earth's magnetic field; all compass needles should point north at this point unless students lift the wire off the table so it is not flat anymore.

During step 5, it is important that students maintain a consistent direction of current in all groups. It is important that end of the wire held vertically be the end connected to the positive terminal and that the end going through the paper and between the books be connected to the negative terminal. Otherwise, students will observe the reverse field direction from the other groups.

When students are measuring the magnetic field, it is possible that the end of the field pointing south does not register a significant movement in the compass. Teachers will have to explain to students that the field is in fact pointing the opposite way of the needle but the needle does not rotate because the field is cancelled by the earth's magnetic field.