The Crane Challenge Electromagnet
Teams in a fifth-grade class build electromagnets for a 'crane challenge': lift 25 steel paper clips in one pick-up. Each team gets one 6 cm iron nail, insulated copper wire, one 1.5-volt D-cell, tape, and identical paper clips. Team A wraps the wire neatly in one direction and connects it to a single D-cell. They count clips for different numbers of wraps: 10 wraps lifted 3 clips; 20 wraps lifted 7 clips; 30 wraps lifted 11 clips; 40 wraps lifted 15 clips. Each trial starts with a fresh battery and the same nail. Team B uses 40 wraps and the same nail but lifts only 2 clips. The class must decide whether more wraps always means a stronger magnet, and Team B wants to know what went wrong before the 20-minute building time runs out.
1. Identify the manipulated (independent) variable, the responding (dependent) variable, and two variables Team A kept constant in this investigation.
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The manipulated variable is the number of wire wraps around the nail, because that is what the team deliberately changed: 10, 20, 30, then 40.
The responding variable is the number of paper clips lifted, because that is the measurement that changes as a result - 3, 7, 11, and 15 clips.
Controlled variables include: the same iron nail, the same type of insulated wire, the same number and type of paper clips, a fresh 1.5-volt D-cell in each trial, and the same pick-up method (touching the nail tip to the pile once and lifting).
Controlling these matters because the experiment is only fair if the number of wraps is the only difference between trials. If Team A used a bigger battery at 40 wraps, they could not tell whether the extra clips came from more wraps or from more power.
2. Predict how many wraps Team A would need to lift 25 clips, and justify your prediction with the data. Then explain why the pattern cannot continue forever.
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The data show a steady pattern: each additional 10 wraps adds about 4 clips (3 → 7 → 11 → 15). That is roughly 0.4 clips per wrap.
Continuing the pattern: 50 wraps ≈ 19 clips, 60 wraps ≈ 23 clips, 70 wraps ≈ 27 clips. So Team A needs about 65 wraps to reach 25 clips - between 60 and 70. A good answer states the prediction plus the reasoning, not just the number.
The pattern cannot continue forever because the battery supplies a limited amount of current. More wire also adds resistance and makes the coil longer, and the iron nail can only become so magnetic before it stops gaining strength. Eventually the battery drains and the coil heats up. More wraps help up to a point, then the gains shrink.
3. Team B used 40 wraps but lifted only 2 clips, while Team A lifted 15. Give two explanations and one test that would help Team B find the problem.
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Explanation 1: the coil is wound in conflicting directions. If the wire reverses direction partway up the nail, the magnetic fields of the two sections oppose each other instead of adding together, which weakens the magnet even with 40 wraps.
Explanation 2: the battery or the connection is weak. An old or partly used D-cell, or a loose or corroded connection, delivers less current. Current is what creates the magnetic field in the coil, so fewer amps means fewer clips. A break in the wire's insulation that lets bare copper touch the nail can also short out wraps.
Test: Team B should touch the bare wire ends directly to the battery terminals while holding the coil steady, and compare with the same nail rewound neatly in one direction. If the magnet strengthens, the coil was the problem; if it stays weak, the battery is the likely cause, and they should try a fresh cell.