Testing ohms law on a light bulb and a resistor connected to a source of potential difference.

Result of the experiment

1. Light bulb. 

          Graph 1.1 voltage versus current graph for a light bulb.

Table 1.1 the current and resistance of a light bulb when         

  the voltage is varied.

B .Resistor

                      Graph 1.2 voltage versus current for a resistor

Table 1.2 the current and resistance of a resistor when         

  the voltage is varied.

The resistance shown in table 1.1 and table 1.2 (for both the light bulb and the resistor) was calculated using the following formula:-

                                   Resistance = voltage (V)        ……………………………. Equation 1

                                                           Current (A)                                                        

                                           

                                                                           

Discussion: - electric current is the amount of charge that moves through the cross sectional area of a conductor per unit interval of time. Electric current occurs due to the movement of electric charges (electrons) through the conductor. In a certain conductor (wire, bulb, resistor) the electric resistance is defined as the potential difference across its ends divided by the current flowing through it. According to ohm’s law at a constant temperature the current through a conductor is proportional to the potential difference across it. Thus materials obeying ohm’s law will have a constant electric resistance at a constant temperature. ohm’s law also states that if the potential difference V between the ends of the conductor is altered and for every value of v, its corresponding value of current  I  flowing in the conductor is measured, a graph between V and I obtained is always a straight line. This means that current and voltage are directly proportional.

In this experiment two types of conductors were used. The first one was a light bulb. In Graph 1.1 it can be seen that the graph formed was not straight line but more of a curve. This in turn shows that the voltage and the current are not proportional. Also seeing from table 1.1 the light bulb does not have constant resistance. So the light bulb does not obey ohm’s law. This is because a light bulb is a non ohmic conductor.

In addition the resistance in light bulb is dependent on temperature (the bulb heats up as more voltage is added) and it increases significantly as the temperature (voltage) increases which is different from the resistor. So from this it can be said light bulb is a type of circuit which has a resistance which is dependent on temperature and therefore ohm’s law does not apply to a circuit which has a resistance that is dependent on temperature. Light bulb is therefore a non ohmic conductor.

The second conductor tested was a resistor. From the voltage versus current graph it formed it can see that it is a straight line. In addition the resistor fulfills another aspect of ohm’s law which states that the product of current flowing through a conductor and the resistor R is always equal to the potential difference V across the two ends of the conductor as long as the temperature and the other physical conditions of the conductor do not change.(V=R . I). By checking the literature value of the resistor used the resistance was found to be 270 Ω. using this experiment the resistance was found within the range of 260 up to 267 Ω ±3. The slight difference can be due to systematic error.

Conclusion: - The light bulb did not obey ohm’s law while the resistor obeys ohm’s law. In addition it can also be concluded that resistance can be dependent on temperature as seen in the resistance of the light bulb.

Limitations:-  the limitation of this experiment may be that the voltmeter and amphometer may have their own resistance which may affect the measuring of values. Since the experiment was only done once for each conductor systematic and random errors may affect the results obtained.

Improvement:   doing the experiment several times to get accurate readings.