Page 50 - Basic Electrical Engineering

We know the relation,

R = R [1 + α (t − t )]
1
1
2
1 2
∴ R 2000 = R [1 + α (2000 − 25)]
1
25
−3
881.6 = R [1+ 4.44 × 10 × 1975]
25
R = 90.25Ω
25
The current flowing through the 60 W lamp at the instant of switching will be
corresponding to its resistance at 25°C.

Example 1.8 A coil has a resistance of 18 Ω at 20°C and 20 Ω at 50°C. At

what temperature will its resistance be 21 Ohms?

Solution:

R = 18, R = 20, R = 21 at what t?
50
20
t
we know,
R = R + [1 + α (t − t )]
1
11 2
2
1
∴ R = R [1+ α (50 − 20)]
50
20
20
or,

20 = 18 [1+ α (30)]
20

or,