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Two identical cells each of emf 1.5 V and internal resistance 0.5 $\Omega$ are connected in parallel (like terminals joined). The equivalent emf and equivalent internal resistance of the combination are
A3.0 V and 1.0 $\Omega$
B1.5 V and 0.25 $\Omega$
C1.5 V and 1.0 $\Omega$
D3.0 V and 0.25 $\Omega$
Answer & Solution
Correct answer: B. 1.5 V and 0.25 $\Omega$
1. For identical cells in parallel, $1/r_{eq} = 1/0.5 + 1/0.5 = 4$, so $r_{eq} = 0.25\ \Omega$.
2. Use $\varepsilon_{eq}/r_{eq} = \varepsilon_1/r_1 + \varepsilon_2/r_2 = 1.5/0.5 + 1.5/0.5 = 6$.
3. So $\varepsilon_{eq} = 6 \times 0.25 = 1.5\ \text{V}$.
4. The 3.0 V options wrongly add emfs (that is the series rule); C uses the series resistance.
_Source: NCERT Class 12 Physics Ch 3 "Current Electricity", p.16_
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