A 20-cm-long conducting rod with mass 25 g is suspended by wires of negligible m

Question

A 20-cm-long conducting rod with mass 25 g is suspended by wires of negligible mass, as in the figure (Figure 1). The rod is in a region containing a uniform magnetic field of 0.25 T pointing horizontally into the page, as shown. An external circuit supplies current between the support points A and B What is the minimum current necessary to move the bar to the upper position, so it's supported against gravity? Express your answer using two significant figures. In which direction should the current flow? from A to B from B to A

Explanation / Answer

(A) F_magntic + Fweight = 0

I L B - m g = 0

I x 0.20 x 0.25 = (25 x 10^-3) (9.8 )

I = 4.9 A

(B) magnetic force = IL X B

force -> upward

B -> into the page

so IL = to the right

hence I -> A to B

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