Question

the efficiency of a cell when connected to a resistance R is 60%.What will be its efficiency if the external resistance is increased by six times.
first answer will be marked brainliest.

Answer 1

see the above image for solution.

Answer 2

efficiency of a cell (n) = (useful power)/ (total power produced) 

then, if 

r = internal resistance of the cell 
R = external resistance 
I = current produced by the cell 

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n = R / (r + R)

0.6 = R / (r + R) 

0.6r + 0.6R = R 

R = 1.5r

therefore, when R becomes 6 times of its original value then new efficiency n2

n^​2 =  6R /(6R + r)

n​^2 =  (6*1.5 r) / ((6*1.5 r )+ r)

n^​2 = 9 r / 10 r

n^​2 = 0.9      or   90%