The Slippery Fridge Door: Questions From a Kitchen Magnet Log

Why Won't the Magnets Stick?

1Last month the Brennan family replaced their old refrigerator. The new one has a shiny stainless steel front and painted gray steel sides. When eighth grader Ottavia Brennan moved the family's photo magnets to the new door, they slid straight down to the floor. Yet the same magnets held tightly to the painted sides. Her grandfather, Ilir Brennan, a retired machinist who lives with the family, told her that magnets do not pull equally hard on every metal, or even on every kind of steel. Instead of guessing why, he asked her to turn what she saw into questions she could test.

2Ottavia hooked a small spring scale, marked in newtons (N), to a round magnet. She set the magnet flat against a surface, pulled the scale slowly and straight out, level with the floor, and read the force at the moment the magnet let go. Every surface stood upright: Grandpa held the skillet and the baking sheet, and later he clamped the steel plates in a wooden rack. She used two magnets, a small round one and a large round one, and she slid paper recipe cards between the magnet and the surface to make gaps. Each card is about 0.3 mm thick. She did three pulls for each setup and wrote down the average.

3Over one weekend she tried many setups and wrote them in her log in the order she did them (Table 1). Some trials changed the magnet, some changed the surface, and some changed the number of cards. Grandpa pointed out that the cast-iron skillet is about 5 mm thick and has no paint, while the fridge's side panel is a thin sheet of steel, less than 1 mm thick, under a coat of paint.

4Next, Grandpa brought home scrap plates of plain, unpainted steel from his old shop, all the same size but of different thicknesses. Ottavia tested each plate with the small magnet and no cards (Table 2).

5The kitchen gave her more to wonder about. When Grandpa switched his hand mixer from low to high, the beaters turned faster through the thick cookie dough. He said the high setting sends a larger current through the coil of wire inside the motor, where the coil and a set of magnets push on each other. Later, Ottavia tore a strip of plastic wrap off the roll to cover a bowl of soup that was cooling on the counter. Before she could use it, the wrap clung to her sleeve, and puffed-rice crumbs jumped up onto it from a few centimeters away.