Electromagnet Strength with Turns and Batteries
Students build an electromagnet by wrapping insulated copper wire around an iron nail and connecting the ends to a 1.5 V battery. They count how many steel paper clips the nail can lift. With 20 turns, it lifts 4 clips; with 40 turns, 7 clips; with 60 turns, 11 clips. They then connect the same 60-turn electromagnet to two 1.5 V batteries in series and it lifts 18 clips. A student claims, 'Adding turns always doubles the strength, so 80 turns will lift 22 clips.' Another student says the data do not support that rule. The group also notices the wire and battery get warm after being connected for a minute. They want to improve the electromagnet and make a safe test plan.
1. Describe the pattern between number of turns and number of clips lifted for the 1.5 V tests. Is the relationship doubling? Use the data.
Read the worked answer
For the 1.5 V tests, the clips lifted are 4, 7, and 11 for 20, 40, and 60 turns. The strength increases with more turns, but it does not double each time. Doubling from 20 to 40 turns would predict 8 clips, but the result is 7. Going from 40 to 60 turns adds 4 clips, not a doubling from 7 to 14. The increases are roughly 3 clips per 20 turns at first, then 4 clips, so the relationship is closer to a steady increase than to doubling. The data support 'more turns can make the electromagnet stronger' but not 'adding turns always doubles the strength.'
2. Evaluate the claim that 80 turns will lift 22 clips. What additional evidence would make the prediction stronger?
Read the worked answer
The prediction is not well supported. It assumes a doubling rule that the 20-, 40-, and 60-turn data already contradict. It also ignores the two-battery result: changing current from one battery to two raised the 60-turn result from 11 to 18 clips, so current matters as well as turns. An 80-turn prediction also assumes the nail does not become magnetically saturated and that the added wire does not change resistance or current much. To make the prediction stronger, the students should actually test 80 turns with the same nail, wire type, battery voltage, and clip type, repeat the trials, and average the counts. They should also record current so they can tell whether turns alone caused the change.
3. Explain why the wire and battery get warm and why this matters for the design.
Read the worked answer
The wire has electrical resistance, so some electrical energy is converted into thermal energy as current flows. The battery also has internal resistance and can warm up. More turns mean more wire, and higher current means more heating, so the electromagnet can become warm quickly. This matters because heat can soften or melt insulation, create a short circuit, or drain the battery, and it means not all the electrical energy is becoming magnetic energy. A safer design would limit how long the circuit is connected, use wire rated for the current, avoid connecting the battery directly across a low-resistance path, and compare designs using the same safe connection time.