Two cells with the same emf E and different internal resistance $r_{1} a n d r_{2}$ , are connected in series to an external resistance R. The value of R so that the pd across the ﬁrst cell is zero, is:

Q. A cell has internal resistance

r

. It sends current through two resistances

R_{1}

and

R_{2}

separately. then, the heat produced in resistance

R_{1}

and

R_{2}

will be in the ratio

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If a cell of constant electromotive force produces the same amount of heat during the same time in two independent resistance R1 and R2, when connected separately one after the other, across the cell, then the internal resistance of the cell is

The correct option is D √R1R2Current in the cirit, I=E(R+r) and heat produced is I2R.So, H1=E2R1(R1+r)2=H2=E2R2(R2+r)2⇒R1(R1+r)2=R2(R2+r)2On solving we shall get, r=√R1R2

If a cell of constant electromotive force produces the same amount of heat during the same time in two independent resistance $R_{1}$ and $R_{2}$ , when connected separately one after the other, across the cell, then the internal resistance of the cell is