A resistor is to have a constant resistance of 30(ohm), independent of temperature. For this, an aluminum resistor with resistance R1 at 0(Celsius) is used in series with a carbon resistor with resistance R2 at 0(Celsius). Evaluate R1 and R2,

Answer 1

R1 + R2 = 30 Ohms.
The temperature coefficient of R1 and R2 must be equal and opposite.

th

Answer 2

Henry's reply sucks

Answer 3

Since you did not give a unit to the temperature coefficients, I suppose they are in unit of C^-1 It is required that: R01 + R02 = 30 ohms --------------------- (1) Suppose the temperature rises by T degrees C change of resistance of R01 = (R01) x (3.9x10^-3) x T change of resistance of R02 = (R02) x (-0.5x10^-3) x T For the total resistance to be kept at 30 ohms, we need to have, change of resistance of R01 + change of resistance of R02 = 0 i.e. [(R01) x (3.9x10^-3) x T] + [ (R02) x (-0.5x10^-3) x T] = 0 [(R01) x (3.9x10^-3) = (R02) x (0.5x10^-3) (R02) = (R01) x [3.9x10^-3/0.5x10^-3] = 7.8(R01) Substitute into equation (1) R01 + 7.8(R01) = 30 hence, R01 = 3.409 ohms and R02 = 7.8 x 3.409 ohms = 26.59 ohms
The above reply by Henry is USELESS