The Magnet and the Paper-Clip Chain
Students explore magnets at a station. They hang a bar magnet from a string and bring a steel paper clip near the bottom; it sticks. They then hang a second clip from the first, then a third, forming a chain of three clips below the magnet. When they add a fourth clip, it falls off. A student says, "The clips at the bottom of the chain are weaker magnets than the top clip." Another says, "The magnet's force just gets used up as it passes through the clips." The teacher asks them to test whether the chain length changes with different magnets and different distances.
1. Design a test to find out whether the number of clips a magnet can hold depends on the strength of the magnet. Identify the variable you change, the variables you keep the same, and what you measure.
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Change (independent variable): the magnet used - for example, a strong magnet and a weak magnet (or magnets of different sizes).
Keep the same (controlled variables): the type and size of paper clips, the way the chain is hung (same string, same starting point), and the order in which clips are added.
Measure (dependent variable): the greatest number of clips the chain holds before one falls off.
Reasoning: If the strong magnet holds more clips than the weak one under identical conditions, then magnet strength affects chain length. Repeating each trial several times makes the count reliable, since a clip near the limit may fall off by chance.
A strong answer adds that clips should be added one at a time and allowed to settle before adding the next.
2. The students notice that the third clip in the chain can pick up a small steel tack, but the first clip can pick up two tacks. Does this support the claim that "the magnet's force is used up as it passes through the clips"? Explain.
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Not exactly. The observation shows the force gets weaker farther from the magnet, not that it is consumed.
Evidence: the first clip held two tacks and the third held one. If the force were 'used up,' the third clip would hold nothing. Instead, each clip still acts as a magnet, just a weaker one.
Better explanation: the magnet temporarily magnetizes each clip, and that effect fades with distance from the original magnet. The chain is like a relay - each clip passes along a weaker version of the pull.
A strong answer suggests a confirming test: slide the chain sideways so the bottom clip touches a tack, and check whether the tack stays stuck when the chain is lifted. If the bottom clip can still attract, the force is present, just weaker.
3. One group finds that a chain of three clips holds, but when they use a thicker clip as the first link, only two clips hold. They conclude the thicker clip is a weaker magnet. Give a better explanation and a test to check it.
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Better explanation: the thicker clip is heavier, so the same magnetic pull may not be enough to support the same number of clips below it. The chain length depends on both the strength of the pull and the weight being held, not on the clip being a 'weaker magnet.'
Test: compare chains using clips of the same thickness but different weights (for example, add a small washer to the bottom), or compare the thick clip against a thin clip of the same weight. If the thick clip holds fewer clips only because it is heavier, matching the weights should make the difference shrink or vanish.
A strong answer names the confound clearly: thickness and weight changed together, so the test must separate them by holding one constant.