GO IN DEPTH

# Magnets 2: How Strong is Your Magnet?

#### Materials

Per student or group, for Motivation activity:

• Paper clip
• Small bar or horseshoe magnet

Per pair, for Development activity:

• Bar magnet
• Clothespin
• Masking tape (1 long strip and 21 one-inch pieces)
• 1/2 manila folder
• 12 oz paper cup
• 20 paper clips
• Pencil

One of each for whole class demonstration for Assessment activity:

• A ring stand and a clamp
• 4 to 6 ceramic magnets
• Paint, masking tape, or correction fluid
• Fishing line or string

OR

One of each for whole class demonstration for Assessment activity:

• Pencil with a steel eraser holder (make sure the magnets attract the holder)
• Two donut magnets that fit onto the pencil

### Purpose

To experimentally measure the strength of a magnet and graph how the strength changes as the distance from the magnet increases, and as the barrier (masking tape) is built between the magnet and an iron object.

### Context

This lesson is the second in a two-lesson series on magnets. Students should carry out many investigations in order to familiarize themselves with the pushes and pulls of magnets. By exploring magnets, students are indirectly introduced to the idea that there are forces that occur on earth which cannot be seen. This idea can then be developed into an understanding that objects, such as the earth or electrically charged objects, can pull on other objects. It is important that students get a sense of electric and magnetic force fields (as well as gravity) and of some simple relations between magnetic and electric currents (Benchmarks for Science Literacy, p. 93.) In later years, students will learn about gravitational force and how electric currents and magnets can exert a force as well.

Students at this level should have already discovered that magnets can make an object move without touching it. What they may not have paid attention to is the type of materials that move and do not move when near a magnet.

In Magnets 1: Magnetic Pick-ups, students looked at various objects, made predictions about whether they were magnetic, and then tested their predictions. This exploration was an introductory activity to magnets and magnetism.

In Magnets 2: How Strong is Your Magnet?, students will discover how barriers and increased distance can vary the strength of a magnet. The main notion to convey in this lesson is that forces can act from a distance. A magnetic field (the pull of the magnet) will pass through materials like tape with almost no effect. The tape does not block the attraction of the magnet for the paper clip. Rather, each piece of tape removes the paper clip from the surface of the magnet by one more small increment of distance, equal to the thickness of the tape. The tape is just a convenient way to move the clip and the magnet apart bit by bit. The distance between the magnet and the clips, not the tape itself, lessens the attraction of the magnet. We also can describe this in terms of the magnetic field: as you move farther from the magnet’s pole, the field becomes weaker and weaker.

### Motivation

Begin the lesson by asking students to predict whether or not a magnet will exert a force on a paper clip and why. Use this discussion as an opportunity to review what students know about magnets.

You can show that the strength of the magnetic field decreases the farther you move away from the magnet by a simple demonstration. Tie one end of a thread to a paper clip and tape the other end to the surface of a table. Hold a magnet above the paper clip. You can hold the clip up in the air (and keep the string taut) as long as the magnet is fairly close to the paper clip. If you move the magnet too far away from the clip, the strength of the magnetic field decreases, and the paper clip falls.

Allow students to try the same experiment.

Ask them to estimate how far away they can move before the clip falls.

• How could we measure this systematically?
• How strong is the magnet?
• How far away will it work?
• Can a small magnet attract a paper clip from across the room? From across your desk?
• How can you find out how strong your magnet is?
• How can you quantify the strength of your magnet?

### Development

Divide the class into pairs. Distribute the student sheet, How Strong is Your Magnet?, to each student. Distribute, or have students collect, the necessary materials for this activity. (See Materials.)

Review this procedure with students prior to the activity.

2. Clamp your magnet to the clothespin. Tape the clothespin to the bottom of the cup as shown in the sketch.
3. Pull out one end of the paperclip to form a hook. Touch the hook to the magnet. It should stick to one pole of your magnet.
4. Take turns with your partner and carefully add paper clips to the hook, one by one. Count the total number of paper clips that you can hang onto the hook before the weight becomes too much for the magnet to hold and the paper clips fall.
5. Write this number of paper clips on your data sheet on the line for zero pieces of tape.
6. Next, stick three pieces of masking tape (labeled #1, #2, and #3) on the bottom of your magnet. (See the picture on the student activity sheet.) Now repeat your experiment and see how many paper clips you can hang on the hook. Make sure the hook touches the tape, not the magnet itself. Write your findings on your data sheet.
8. Keep adding pieces of tape, three at a time, repeat the experiment, and write down what you find. As you add more and more layers of tape, what do you notice about the number of paperclips on the hook? Is the magnet able to hold more, the same, or fewer clips? Do you think that the tape is causing this? Why?

Questions to ask during the activity:

• How many paper clips can you hold without the masking tape?
• As you begin adding layers of tape, what happens? Why?
• Is the masking tape a magnet? How do you know? If not, why are the paper clips attracted to it?
• What happens to the strength of the magnet as you add more layers of tape? Why?
• Is the magnetic attraction blocked by the tape, or is it just that the tape adds distance between the magnet and the paper clip?
• What does the graph tell us?

Use your findings to create a class chart showing the results of the experiment. Make a large graph on newsprint paper or the chalkboard. Average student findings and graph the class findings. The x-axis (horizontal) is for the distance from the magnet (that is, the number of layers of tape beginning with zero); the y-axis (vertical) is for the strength of the magnet (number of paper clips it can hold).

### Assessment

To summarize the lesson and assess student understanding, ask students to reflect back on the How Strong is Your Magnet? activity and respond to these questions in their science journal:

• Describe what you observed in the demonstration.
• How does this demonstration prove that forces can act at a distance?

Or, to assess student understanding in a novel context, use one of the following demonstrations from the Exploratorium website. (Choose the demonstration that best suits the level of your students and the amount of experience they have had with magnets.) You can conduct the demonstration for the whole class.

For younger students, use the demonstration described in Magnetic Forces.

For older students, you may prefer to use the demonstration described in Strange Attractor.

Regardless of which demonstration you choose, have students answer these questions in their science journal:

• Describe what you observed in the demonstration.
• How does this demonstration prove that forces can act at a distance?

### Extensions

Encourage students to test a variety of other materials to see if they serve as insulators (foil, brown paper bag, waxed paper, cotton, etc.). Students can follow the procedure described in the Development activity above.