Two metal wires of identical dimensions are connected in series. If $$\sigma_1~\text{and}~\sigma_2$$ are the conductivities of the metal wires respectively, the effective conductivity of the combination is:

 1 $$\frac{2\sigma_1 \sigma_2}{\sigma_1+\sigma_2}$$ 2 $$\frac{\sigma_1 +\sigma_2}{2\sigma_1\sigma_2}$$ 3 $$\frac{\sigma_1 +\sigma_2}{\sigma_1\sigma_2}$$ 4 $$\frac{\sigma_1 \sigma_2}{\sigma_1+\sigma_2}$$

Subtopic: Â Derivation of Ohm's Law |
Â 63%
From NCERT
NEET - 2015
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A wire of resistance $$4~\Omega$$ is stretched to twice its original length. The resistance of a stretched wire would be:

 1 $$4~\Omega$$ 2 $$8~\Omega$$ 3 $$16~\Omega$$ 4 $$2~\Omega$$
Subtopic: Â Derivation of Ohm's Law |
Â 82%
From NCERT
AIPMT - 2013
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Two rods are joined end to end, as shown. Both have a cross-sectional area of $$0.01~\text{cm}^2$$. Each is $$1~\text{m}$$ long. One rod is of copper with a resistivity of $$1.7\times10^{-6}~\Omega\text{-cm}$$, the other is of iron with a resistivity of $$10^{-5}$$ $$\Omega\text-\text{cm}.$$ How much voltage is required to produce a current of $$1$$ A in the rods?

1. $$0.00145$$ V
2. $$0.0145$$ V
3. $$1.7\times10^{-6}$$ V
4. $$0.117$$ V
Subtopic: Â Derivation of Ohm's Law |
Â 63%
From NCERT
NEET - 2013