Page 44 - Basic Electrical Engineering

Example 1.1 A copper wire has resistance of 0.85 ohms at 20°C. What will
be its resistance at 40°C? Temperature coefficient of resistance of copper at
0°C is 0.004°C.

Solution:

We know,

Here,

We know, R = R [1 + α (t − t )]
1 2
1
2
1
In this case, R = R [1+ α (40 − 20)]
20
20
40
= 0.85 [1+ 0.0037 × 20]
= 0.9129 Ω

Example 1.2 The heating element of an electric heater made of nicrome
−6
wire has value of resistivity of 1 × 10 Ohm-m. The diameter of the wire is
0.2 mm. What length of this nicrome wire will make a resistance of 100

Ohms?

Solution:

We know,

or,

−6
given R = 100 Ω, ρ = 1 × 10 Ohm-m, d = 0.2 mm
2
−8
−3 2
area, a = πd = 3.14 × (0.2 × 10 ) = 12.56 × 10 m 2