Electric Intensity

Question

A standard annealed copper wire has a resistance of 3 ohms per metre length at 20°C. What must be the electric intensity in the wire if the current is 20 A? What electric intensity is required to force the same current through the wire at 200°C?

The value of a20 for copper wire can be taken as 0.00392/°C

Answer 1

We know, E = V/L

Since, V = RI,

E = RI/L = (R/L)I

Here,

I = 20 A

R₂₀/L= 3 Ω/m

α = 0.00392/°C

ΔT = 200 °C - 20 °C = 180 °C

Now, R₂₀₀ = R₂₀ (1 + αΔT)

∴ R₂₀₀/L = (R₂₀/L) (1 + αΔT) = 5.1168 Ω/m

∴ E₂₀ = (R₂₀/L) I = 60 V/m

and E₂₀₀ = (R₂₀₀/L) I = 102.3 V/m