Practical: Using a magnet
A magnet attracts iron, nickel, cobalt and steel objects: nails, paper clips, pins, sewing needles. Copper, aluminium, silver, plastic, wood, paper, rubber, water and chalk are not attracted. Objects of stainless steel (some spoons) may be attracted weakly or not at all, so each should be tested. A magnet’s pull falls quickly with distance: at 2–3 cm it can still hold a clip, farther away it cannot. The magnetic field passes through paper, cloth and thin plastic, so a clip tied by thread to the table can be held “in mid-air” below a magnet: it does not fall because the magnetic pull balances its weight. Magnets are used to separate scrap: iron and steel items from aluminium and plastic; big magnetic cranes lift steel and iron scrap. These cranes often use electromagnets, which release the load when the current is switched off. A heavy blow or heating can weaken a magnet. Safety: keep magnets away from small children and pets and from heat; do not put small magnets in the mouth.
Two experiments: 1) test about 15 different items (nail, coin, wire, foil, pencil, key, etc.) with a magnet and record “attracted” or “not attracted”. 2) Tie 20 cm of thread to a paper clip and tape the other end to the table; hold the magnet 2–3 cm above the clip, keeping the thread taut: the clip hangs in the air. Place paper between the magnet and clip, then increase the distance. Safety: handle sharp-pointed items carefully; do not trap fingers between strong magnets; do not use magnets near phones or cards.