Answer to Question 7


An accurately calibrated ammeter has a resistance of 400 W and is being used to measure a current of 100 mA from a 12.0 V supply through a resistor.  Calculate the resistance of the resistor.  Hence discuss the uncertainty in the reading of the current.

The current through the resistor and the ammeter is the same.

Total resistance = 12.0 V 100 10-6 A = 120 000 W


Resistance of the resistor = 120 000 W - 400 W = 119600 W


This is 99.6 % of the expected value of the, so there is 0.4 % uncertainly, which is not a lot.  There would be more uncertainty (1 %) in the value of the resistor, even if it were well made.