Chapter 20, Problem 18P

& You make a seesaw by placing a 50-g magnet (whose poles**#x2019; faces are 2-cm-by-2-cm squares) at one end of a 50-cm-long ruler and a small 50-g metal object at the other end and balancing the whole setup by placing a pencil directly under the middle of the ruler. You know that the magnet is horizontally magnetized, but you don’t know which face is north and which face is south. When you place a horizontal wire close to the pole of the magnet and send a 14-A current through the wire in the direction shown in Figure P20.18, the end of the seesaw with the magnet goes up and the other end goes down. You can get the seesaw to go back to equilibrium (while keeping the current in the wire) by placing another 10-g metal object 6.0 cm from the axis of rotation, also shown in the figure. (a) Determine the polarity of and (b) the magnitude of the $\stackrel{\to }{B}$ field at the location of the wire. Indicate any assumptions that you made.