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Ohm’s Law


To obtain current and voltage readings for a resistor and a light bulb and determine which one obeys Ohm’s Law.

HOW DOES IT WORK?                                                                       

Time allocation: 45 Min

Learners connect a resistor, ammeter and a light bulb in series with a battery of four cells. A voltmeter is connected in  parallel with the resistor and light bulb. The rheostat is set at its lowest resistance and the voltmeter and ammeter readings are taken.

The resistance of the rheostat is increased and another set of readings is obtained. The steps are repeated at least another three times. The results are used to draw a graph of potential difference vs current strength for the resistor and the light bulb. The shape of the graphs provide an indication of the relationship between the two variables.

The graph for the ohmic conductor is a straight line. Potential difference is directly proportional to current in the conductor and the gradient of the graph is an indication of the resistance:


The graph for the light bulb is a line with an increasing gradient. The ratio  is not constant because the increase in current is no longer directly proportional to the increase in potential difference.


  • A battery holder
  • A resistor (Ni-chrome wire)
  • A light build in a light bulb holder
  • Rheostat
  • An ammeter
  • A voltmeter
  • Connecting wires with crocodile clips
  • A switch
  • 4 x 1,5 V cells (AA)


  1. Connect the resistor in series with the other components as shown in the diagram:

        1. Set the rheostat at its lowest resistance.
        2. Close the switch and take the readings on the ammeter and voltmeter quickly.
        3. Open the switch.
        4. Increase the resistance of the rheostat.
        5. Take another set of readings.
        6. Repeat the steps to obtain at least four sets of readings.
        7. Tabulate the results in the table below.
        8. Replace the resistor with the light bulb holder with light bulb.

      1. Close the switch so that the bulb is illuminated. Take the reading of the ammeter and voltmeter at various time intervals 10’s, 20’s, 30’s and 40’s. Note the readings in the below table.