1. A 0.80-m aluminum bar is held with its length parallel to the east–west direction and dropped from a bridge. Just before the bar hits the river below, its speed is 22 m/s, and the emf induced across its length is 6.5 x 10-4 V. Assuming the horizontal component of the earth’s magnetic field at the location of the bar points directly north, (a) determine the magnitude of the horizontal component of the earth’s magnetic field, and (b) state whether the east end or the west end of the bar is positive.

 

Two views of this situation are shown below.  One looking north the other looking down.

The induced emf is given by

Solving for B

 

The magnetic force is still

 

Here we have for the cross product use looking down for directions

 

 

So the eastern edge of the bar is more positive!

 

Eastern side of Bar is more Positive!

 

 

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This page last updated on June 23, 2018