Scrap Yard Crane: What Makes an Electromagnet Stronger?

Field Trip Notes: The Magnet Crane

1Mr. Desmond Achebe's eighth-grade class visited a metal recycling yard. A tall crane swung a large round disk over a pile of scrap. When the operator switched on the current, a heap of steel cans and car parts jumped up and stuck to the disk. When she switched it off, the scrap dropped into a bin. She said the big disk can lift well over a ton of steel at one time. The disk is an electromagnet: a coil of wire wrapped around an iron core that becomes magnetic when electric current flows through the wire.

2Back at school, students listed questions. Some asked why the crane is painted yellow or whether recycling is worth the cost. Mr. Achebe explained that a testable question names something to change and something to measure, and can be answered by collecting data in an investigation.

3Each group built a small model of the crane by wrapping insulated wire around a large iron bolt and connecting it to 1.5-volt battery cells, a switch and an ammeter, which measures current in amperes (A). The coils can get warm, so the groups switched the current off between trials. To test their electromagnets, they counted how many small steel paper clips each one could lift when it touched the pile. More clips lifted meant a stronger pull, so the clip count gave each group a simple way to compare their electromagnets.

4In Test 1, the class changed the number of turns of wire. They used the same bolt, the same kind of paper clips and the same pile, and they adjusted the circuit so the ammeter read 1.0 A on every run. In Test 2, they kept 30 turns and changed the current. In Test 3, they kept 30 turns and 1.0 A, and placed a steel washer on a force sensor at different distances below the bolt to measure the pull on it. Every result in the tables is the average of three trials.

5Later, Ivy Nakashima tried a different kind of force. She rubbed a balloon on a wool scarf for 5 seconds, and it pulled tiny bits of paper up from 2 cm away. After she rubbed it for 20 seconds, it pulled them up from 5 cm away.