(Practical magnetism) As we will see in lab, the magnitude ?? ofthe magnetic field of a permanent magnet drops with distance d sothat ?? ∠1/? 2 when d is less than the size of the magnet (whereyou “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 themagnetic strip on your credit card or hotel room key? Suppose themagnet is 2 mm thick, and measure d from its center. Consider botha fridge magnet (?? = 5 mT at the surface) and a neodymium coinmagnet (?? = 1 T at the surface). To erase a magnetic striprequires 4000 G for a “high-coercivity†(“HiCoâ€) card such as acredit card, but only 300 G for a “low-coercivity†(“LoCoâ€) cardsuch as a hotel room key. (b) If so, how far apart should you keepthem? (c) Could wearing a “magnetic stone†(with field strengthcomparable to a fridge magnet) affect your health? To answer this,ask yourself whether people’s health is affected by the Earth’smagnetic field, of strength about 0.2 G? At what distance from thestone do you experience this same field strength?
