Evan's Space

Wonders of Physics

Finding unknown resistor R and setting up the electrical circuit

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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]

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