Ohm’s law cannot be applied to the non-linear clients such as circuits containing electronic tubes or transistors and the circuits used to produce electric arc.
Example 1.1
The specific resistance of platinum at 0°C is 10.5 microohm cm. What should be the length of platinum wire of No. 32 S.W.G. (diameter = 0.0274 cm) to have a resistance of 4 ohms at 0°C?
Solution:
Resistance of a wire at 0°C is calculated as follows:
where R0 = 4 Ω; ρ0 = 10.5×10−6 Ω cm = 10.5×10−8 Ω m
∴
Example 1.2
A current of 0.75 A is passed through a coil of nichrome wire which has a cross sectional area of 0.01 cm2. If the resistivity of the nichrome is 108×10−6 ohm cm and the potential difference across the ends of the coil is 81 V. What is the length of the wire? What is the conductivity and conductance of the wire?
Solution:
Resistance,
Where
ρ = 108 × 10−6 Ω cm = 108 × 10−8 Ω cm
∴
Conductivity,
Conductance,
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