Evan's Space

Wonders of Physics

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Finding unknown resistor R and setting up the electrical circuit

To find the unknown resistor R, the following apparatus are setup.

Unknown resistor

Refer to the video below for the setting up of the apparatus.

Why do you need a variable resistor (rheostat)?

  • Without the variable resistor, you will have only one set of current I and potential difference V readings. Using the formula R = V/I, you are able to find the unknown resistor. But this method is not so accurate.


  • Hence, to make it more accurate, we include a variable resistor to control the size of the current through the circuit. Thus having different readings of the potential difference V across the unknown resistor.
  • Instead of just one set of readings of I and V, we now have about 5 sets.
  • This allows us to plot a graph of V against I.
  • By finding the gradient of the best fit line, we are able to find the resistance more accurately. [gradient = V / I = R, hence the gradient of V-I graph represents resistance R]


For pure metallic conductor, like the fixed resistor R, it obeys the Ohm’s Law, hence it is an ohmic conductor.

From the graph, the current I flowing the conductor is directly proportional to potential difference V across the conductor, provided physical conditions like temperature remains constant. [the graph is a straight line with constant gradient, and passes through the origin]