Question

(Practical magnetism) As we will see in lab, the magnitude ?? of the magnetic field of a permanent magnet drops with distance d so that ?? ∝ 1/? 2 when d is less than the size of the magnet (where you “feel” only the closest pole) but ?? ∝ 1/? 3 farther away (where both poles “blur together” and the field is weaker). (a) Could a coin-sized magnet in your wallet or purse erase the magnetic strip on your credit card or hotel room key? Suppose the magnet is 2 mm thick, and measure d from its center. Consider both a fridge magnet (?? = 5 mT at the surface) and a neodymium coin magnet (?? = 1 T at the surface). To erase a magnetic strip requires 4000 G for a “high-coercivity” (“HiCo”) card such as a credit card, but only 300 G for a “low-coercivity” (“LoCo”) card such as a hotel room key. (b) If so, how far apart should you keep them? (c) Could wearing a “magnetic stone” (with field strength comparable to a fridge magnet) affect your health? To answer this, ask yourself whether people’s health is affected by the Earth’s magnetic field, of strength about 0.2 G? At what distance from the stone do you experience this same field strength?

Answer 1