Sat February 11, 2012 By: Advait Vasavada

In figure AB is a potentiometer wire of length 10 m and resistance . With key K open, the balancing length is 5.5 m. However on closing key K the balancing length reduces to 5 m. The internal resistance of the cell is. answer is 0.1ohm but i think wouldn't the current be zero in both cases through the cell

Expert Reply
Mon February 13, 2012
Internal resistance, r= [(l1 -l2)/l2]R
r= [(5.5-5.0)/5]x1
r= 0.5/5 =0.1 ohm
when the balanced point is obtained on the potentiometer wire, no current flows in the galvanometer circuit because the fall of potential across the potentiometer wire between zero end and balance point position becomes just equal to the e.m.f of the cell, whereas the current in the potentiometer wire continues flowing.
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