Electromagnet Trials with a Wire Change
Students build an electromagnet by wrapping insulated copper wire around an iron nail and connecting it to a battery. They count the paper clips the electromagnet can lift. Trial A uses 20 turns, one 1.5 V battery, and 0.50 mm wire: 4 clips. Trial B uses 40 turns, one 1.5 V battery, and 0.50 mm wire: 9 clips. Trial C uses 40 turns and two 1.5 V batteries in series, but the group has run out of 0.50 mm wire and uses 0.25 mm wire instead: 15 clips. The group concludes, 'Doubling the voltage more than doubles the strength.' The teacher asks whether the evidence supports that conclusion and what the group should do next.
1. Identify the confounding variable in Trial C and explain how it could affect the conclusion.
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The confounding variable is the wire diameter. Trial C uses 0.25 mm wire, while Trials A and B use 0.50 mm wire. A thinner wire has more resistance for the same length, so at the same voltage it allows less current. Electromagnet strength depends on current, number of turns, and the core. Because the wire changed, the current in Trial C is not simply the result of doubling the voltage.
The 15 clips cannot be attributed to voltage alone. The thinner wire may have reduced the current, so the true effect of doubling voltage could be different from what the group observed. The conclusion is not supported because more than one variable changed.
2. Redesign the experiment to isolate the effect of voltage. List controlled variables and a prediction.
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Keep the iron nail, wire gauge and length, number of turns, temperature, and paper-clip type the same. Use 40 turns of 0.50 mm wire for both trials. Change only the voltage: one 1.5 V battery versus two 1.5 V batteries in series. If possible, measure current with an ammeter and repeat each trial several times.
Prediction: with resistance and turns held constant, doubling the voltage should increase the current, so the electromagnet should lift more clips. The increase may not be exactly double because the iron core can saturate and the battery has internal resistance. Comparing controlled trials, not changing wire thickness, is what allows a valid conclusion about voltage.
3. A student says replacing the iron nail with a plastic rod will make the electromagnet stronger because plastic is an insulator. Evaluate this statement.
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The statement is incorrect. Electromagnet strength is increased by a ferromagnetic core such as iron. The iron nail becomes magnetized and its magnetic domains align with the field from the coil, greatly strengthening the magnetic field. Plastic is not ferromagnetic, so it would not magnetize and would not concentrate the field in the same way. The electromagnet would be weaker with a plastic core.
Being an insulator describes electrical conductivity, not magnetic behavior. The wire's insulation already prevents short circuits between turns. The relevant property for the core is ferromagnetism, not whether the material conducts electricity.