Question

A cell of emf 2V delivers current equal to 20% of the maximum current, which it can produce through a resistance 4 $\Omega$. Its internal resistance is

• A. 0.1$\Omega$
• B. 1$\Omega$
• C. 0.5$\Omega$
• D. 1.5$\Omega$

Answer 1

Answer: (B)

1$\Omega$

Case 1 max account $\Rightarrow E=I_{max}\left(R\right)$
Case 2 20% of max current $\Rightarrow E=I_{max}x0.2(R+r)$
R - Internal resistance $r=-4\Omega$
$\Rightarrow R=0.2R+0.2r$
$0.8R=0.2\times 4$
$R=\frac{0.8}{0.8}=1\Omega$