Describe the magnetic field produced by a current-carrying wire and relate its direction and strength to current and geometry.
E\&M.3.
A conducting bar of mass M, length L, and negligible resistance is connected to two long vertical conducting rails of negligible resistance. The two rails are connected by a resistor of resistance R at the top. The entire apparatus is located in a magnetic field of magnitude B directed into the page, as shown in the figure above. The bar is released from rest and slides without friction down the rails.
Is the magnitude of the net magnetic field above the bar at point C greater than, less than, or equal to the magnitude of the net magnetic field before the bar is released? Greater than Less than Equal to
Justify your answer.
While the bar is above point D, is the magnitude of the net magnetic field at point D greater than, less than, or equal to the magnitude of the net magnetic field before the bar is released? Greater than Less than Equal to
Justify your answer.
Express your answers to parts (c) and (d) in terms of M, L, R, B, and physical constants, as appropriate.
2 points
For selecting "Less than" For a correct justification Example: Because as the bar falls the flux at point C is increasing, the emf generated must create a magnetic field to oppose this change. Therefore, it will create a magnetic field to decrease the flux and thus decrease the magnetic field. No points are earned if the wrong answer is selected.
2 points
For selecting "Greater than" For a correct justification Example: The field at point C, which is above the bar, is less than the original magnetic field, and point D is on the other side of the bar. Therefore, the direction of the magnetic field from the bar at point D is the opposite of the direction at point C, so the net magnetic field at D when the bar is falling must be greater than the original magnetic field. No points are earned if the wrong answer is selected.