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Physics · 2026 · 2 marks
CBSE 2026 · Region 2 · Set 3 · Q20
(a) Why manganin is used in wire-wound standard resistors? Calculate the temperature at which the resistance of a wire increases by 25% of its…
(a)
Why manganin is used in wire-wound standard resistors? Calculate the temperature at which the resistance of a wire increases by $\displaystyle 25$% of its resistance at room temperature $\displaystyle 27.5$ °C? The temperature coefficient of resistance of the material of the wire is $\displaystyle 0.004{ }^{\circ} \mathrm{C}^{-1}$.
(b)
How does the resistivity of a semiconductor depend on temperature? Show it on a plot.
Marking-scheme solution
(a)
Manganin (an alloy) exhibits a very weak dependence of resistivity on temperature.
$\displaystyle R_{2}=R_{1}\left[1+\alpha\left(t_{2}-t_{1}\right)\right]$
$\displaystyle 1.25 R_{1}=R_{1}\left[1+0.004\left(t_{2}-27.5\right)\right]$
$\displaystyle t_{2}=90^{\circ} \mathrm{C}$
(b)
The resistivity of semiconductors decreases with increasing temperature.
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