Purpose: To gain a better understanding of how solenoids work and to explore the uses of solenoids.

Procedural overview: Students construct their own simple solenoids and conduct experiments to measure the solenoids’ properties.

Approximate class time: 30-50 minutes.

Materials:

• Activity Guide for Students: Solo for Solenoid
• Copper wire
• Wire cutters
• Empty toilet paper tubes
• Empty paper towel tubes
• Empty kitchen spice bottles
• Large glass or plastic test tubes
• Small corks that will freely slide inside the test tubes
• Small steel or iron nails
• Magnetic compasses small enough to fit inside the tubes/bottles
• Bar magnets small enough to fit inside the tubes/bottles
• Other strong magnets of assorted shapes and sizes
• Steel rods that can fit inside the tubes/bottles
• Voltmeter (electrical multimeter)
• 1.5 volt batteries (any size, AA to D) and holders for them
• Paper clips
• Meter sticks
• Water

Notes to the teacher:

Note that the research in this week’s article, “Origami outfits help bots retool,” could be implemented with the resources that a high school student can find — a metal cube, heat-shrink material, a remote control provided by moving around magnet solenoids and a heating pad under the table. Encourage your students to pursue a research idea even if they do not immediately have access to professional laboratories.

Some good sources for small magnetic compasses, magnets, battery holders and electrical multimeters are: Home Science Tools, American Science and Surplus and Educational Innovations.

Students can work in small groups and each group should have an identical set of supplies. Encourage students to be creative with their own designs — there is no one right way to build or use a solenoid!

Procedure and questions for students, with possible answers:

1. Make a solenoid: Cut a piece of wire 1 meter in length and strip the insulation off the ends. Wrap the wire around a paper, glass or plastic tube, with all the loops of wire going the same way around the tube. What is the diameter of the tube and the number of loops in the coil around the tube? You can keep the solenoid coil on the tube, or pull it off if it will keep its shape.

Answers will vary.

2. Hold a small magnetic compass level. Which way is north? Put the compass a short distance inside the wire-wrapped tube and keep the compass level. Does the direction of the compass needle change or stay the same?

The compass should still point to north inside the tube.

3. Connect the ends of the solenoid wire to one 1.5-volt battery. What happens to the compass needle? Which way does it point? What does that tell you? Which direction can you point the solenoid to make the change as large as possible? Disconnect the battery after a few seconds or it will start to overheat.

If the solenoid is oriented perpendicular to north, the compass needle will point to north when the battery is disconnected, and point away from north one way down the length of the solenoid when the battery is connected. That demonstrates that current flowing through the solenoid coil creates a magnetic field inside the coil pointing in one direction.

4. Connect the ends of the solenoid wire to one 1.5-volt battery in the opposite direction. What happens to the compass needle? Which way does it point? What does that tell you? Disconnect the battery after a few seconds or it will start to overheat.

When the battery is reversed, the compass needle should point the opposite direction down the length of the solenoid. That demonstrates that current flowing in the opposite direction through the solenoid coil creates a magnetic field inside the coil pointing in the opposite direction.

5. When the solenoid is on, use the same or a similar compass inside the solenoid coil versus in various positions outside the coil. What do you learn about the strength of the magnetic field? Disconnect the battery after a few seconds or it will start to overheat.

The magnetic field is strongest inside the coil, and outside the coil it gets rapidly weaker as you get farther from the coil in any direction.

6. Try connecting two or more batteries to the solenoid. Try batteries in series or in parallel. Check the magnetic field strength at various positions inside and outside the solenoid, and notice how much the compass needle deflects in each case. What do you learn? Remember to disconnect the batteries after a few seconds.

Increasing the current flowing through the coil, especially by using batteries in parallel, should increase the magnetic field strength.

7. Make an identical solenoid but use a much longer piece of wire. How long is the wire and how many loops does it make around the tube? Using a magnetic compass, what do you notice about the effect of the number of loops on magnetic field strength?

The more loops there are, the stronger the magnetic field strength should be.

8. Make a similar solenoid but use a tube with a smaller diameter, and the same number of coils as in one of your previous solenoids. What is the tube diameter and how many loops are around the tube? Using a magnetic compass, what do you notice about the effect of the tube diameter on magnetic field strength?

For the same number of loops, the smaller the diameter, the stronger the magnetic field strength should be (inside and close to the ends of the solenoid).

9. Especially for the smaller diameter solenoid, what happens to the magnetic field strength if you put a steel rod through the center of the solenoid?

Iron in the steel rod should help to concentrate the magnetic field, creating an electromagnet.

10. Connect a straight piece of wire to a battery for a few seconds and then disconnect it. Now connect a solenoid to the battery for a few seconds and then disconnect it. Which case creates a larger spark when you disconnect from the battery?

The solenoid should create more of a spark than the straight wire, since so much energy is stored in the solenoid’s magnetic field.

11. Connect a solenoid to a multimeter or voltmeter instead of a battery. How much voltage does the solenoid have? How much current?

The solenoid should have zero voltage and zero current.

12. With the solenoid still connected to a multimeter or voltmeter, pass a bar magnet back and forth through the coil very quickly. What happens to the voltage and current on the meter?

The voltage and current readings should show fluctuations when the magnet is moving.

13. Can you use a solenoid to make a nearby object (other than a compass needle) move when the solenoid is connected or disconnected from a battery? The stronger you can make the solenoid, the better for this purpose.

A small but strong magnet or a paper clip near the end of the solenoid may move when the solenoid is turned on or off.

14. Can you make a solenoid coil move when the solenoid is connected or disconnected from a battery? Make the solenoid as light as possible by removing the cardboard/glass/plastic tube and just keeping the coiled wire shape.

If a strong magnet is held fixed near the end of the solenoid coil, the solenoid coil may flex toward or away from the magnet when it is turned on or off.

15. Fill a test tube most of the way with water. Stick a steel or iron nail through a small cork, and let it float freely on top of the water inside the test tube. Wrap the test tube with as many loops as possible to make a solenoid. What happens when you connect the solenoid to a battery, and then disconnect it?

If the solenoid is strong enough, turning it on can pull the floating nail and cork downward in the water toward the center of the coil. Turning it off lets the nail and cork bob back up to the top of the water.

16. What have you learned about how solenoids create magnetic fields?

The magnetic field runs down the length of the solenoid inside the coil, is strongest at the solenoid’s center and weaker outside. The direction of the magnetic field depends on the direction current runs through the solenoid from the connected battery. Larger currents, more wire loops and smaller solenoid diameters create stronger magnetic fields.

17. What have you learned about how solenoids can be used to control motion?

Students have used several configurations of solenoids to cause motion of the solenoid or a nearby object. When the solenoid is turned on, it produces a magnetic field, and if a magnetic object is close, it will interact with the magnetic field produced by the solenoid.

18. Now that you know more about the properties of solenoids, what are some of their applications?

Solenoids can produce quick and powerful linear motion and are used in inductors, electromagnets and antennas. Some applications include power car locks and other simple locking devices, medical clamping equipment, dishwasher cycle switch mechanisms and air conditioning units.