Question

When a resistor is connected to a 12 volt source, it draws a 185 mA current. what's the resistance? (I said 65 ohms). the same resistor connected to a 90 volt source draws a 1.25 amp current. what's the resistance? is it ohmic?

Answer 1

Resistance-1 = (voltage-1) / (current-1) =

                       (12 V)  /  (0.185 A)  =  64.9 ohms .

Resistance-2 = (voltage-2) / (current-2) =
 
                           (90 V )  /  (1.25 A)  =  72 ohms .

The resistance changed between situation-1 and situation-2 .
How did that happen ?

Power = (voltage) x (current)

Power-1 = (12)  x  (0.185)  =  2.22 watts

Power-2 = (90)  x  (1.25)  =  112.5 watts

The poor resistor dissipated 51 times as much power during
the second trial.  It got all heated up, and its resistance went
through the roof.

Carbon resistors behave nicely and reliably, until you try to
toast bread or light up your bedroom with them.