The Kitchen Stir Plate: Weighing a Pull Across a Gap

A Stirrer With No Spoon

1Imani Castellanos, an eighth grader, wanted a way to stir hot cocoa and iced tea without a spoon. She built a stir plate: a small fan motor with a disc magnet glued to its top, inside a plastic case with a thin plastic lid. A stir bar, a small magnet sealed in smooth plastic, sits at the bottom of the mug. When the motor turns, the stir bar spins in the drink, even though only the drink and the mug touch it. Her science teacher, Mr. Feliks Oyama, explained that the space around a magnet is a magnetic field, and that a piece of steel or another magnet placed in the field feels a force.

2In Test 1, Imani set a full 420 g steel can of beans on a digital kitchen scale with a plastic platform and pressed zero. She clamped the disc magnet to a wooden stand above the can, facing down, and used plastic spacers to set the gap between the magnet and the can. When something pulls up on the can, the scale shows a reading below zero; a reading of −150 g means an upward pull about equal to the weight of 150 g. She took three readings at each gap and averaged them. Before she began, she clamped a wooden disc the same size as the magnet 2 mm above the can, and the scale read 0 g.

3In Test 2, Imani put the stir bar in a glass mug of water on the stir plate. She stacked thin cardboard sheets between the lid and the mug to change the gap between the disc magnet and the stir bar. At each gap she turned the speed dial up from 1 to 5 and wrote down the highest setting at which the bar kept spinning smoothly. At a 10 mm gap she also tried a ceramic mug and a plastic cup. In both, the bar spun smoothly up to setting 4, just as in the glass mug.

4Imani's cousin Bashir thought the motor simply shook the mug and rattled the bar around. So Imani pulled the disc magnet off the motor and ran it at setting 5 with a 6 mm gap. The case buzzed and shook as much as before, but the stir bar lay still at the bottom of the mug.

5Later, Bashir spilled a pile of salt and pepper on the counter. Imani rubbed a plastic spoon on a wool sweater and slowly lowered it over the pile. At 3 cm above the pile, nothing moved. At 2 cm, pepper flakes began to jump up and stick to the spoon. At 1 cm, many more pepper flakes and a few salt grains jumped up. Pepper flakes are usually lighter than salt grains of about the same size. A spoon that had not been rubbed lifted nothing at 1 cm, and the rubbed spoon did not pull on a steel paper clip.