Why a bulb cannot be used to verify ohm's law?

Answer 1

A filament lamp does not obey Ohm's Law because, for tungsten, the ratio of voltage to current isn't a constant for changes in voltage. Metals such as tungsten are, therefore, termed 'non-linear' or 'non-ohmic'.

There seems to be a major misconception surrounding Ohm's Law. Ohm's Law simply states that 'the current flowing along a wire, at constant temperature, is directly proportional to the potential difference across its ends'. For Ohm's Law to apply, the current/voltage graph line resulting from varying the voltage MUST be a straight line. If it is curved, then Ohm's Law doesn't apply. In fact, most metals and electronic components, such as diodes, do not follow Ohm's Law.

So Ohm's Law is NOT a universal law and applies in very few circumstances.

The equation R = V/I applies whether Ohm's Law is followed or not, because the ratio of V/I will always indicate what the resistance happens to be for that particular ratio. But the fact that you can use this equation under any circumstance doesn't mean Ohm's Law is applicable.

Answer 2

There are no difficulties; it's very straightforward in fact. If you gradually increase the voltage across the lamp, you will find that the resulting current, if plotted on graph, will produce a curve, rather than a straight-line. This means that the current is not proportional to changes in voltage and, so, the lamp's filament is non-linear or non-ohmic, which means it does not obey Ohm's Law.

Answer 3

By definition, Ohm's Law only applies to conductors whose resistance remains constant over the full range of potential differences applied. In other words, there must be a straight-line relationship when you plot current against voltage.

As you increase the voltage across a filament lamp, however, the conductor's temperature rises substantially, causing its resistance to rise substantially, too. If you plotted a graph of current against voltage, the result would be a curve -confirming that the current isn't constantly proportional to the voltage applied and, therefore, the lamp is not obeying Ohm's Law.

The general equation R = V/I (derived from the definition of the ohm, and NOT from Ohm's Law) can be used to determine the (increasing) resistance of the filament for each incremental change in voltage. But, for Ohm's Law to apply, the ratio of current to voltage (V/I) must remain constant for variations in voltage which it clearly does not for tungsten.

Answer 4

There is no difficulty; your experiment will simply prove that the filament of the lamp doesn't obey Ohm's Law.

When you plot the results of current against voltage for a lamp's filament, obtained from your experiment, the result will be a curved line, indicating that the current is not proportional to voltage (due to a changing resistance). This shows that the filament doesn't obey Ohm's Law. To obey Ohm's Law, the result must be a straight-line graph.

Although the resistance of the lamp can be found at any point along the curve from the ratio of voltage to current (i.e. R = V/I) at that particular point, the lamp does not obey Ohm's Law. Ohm's Law only applies when the ratio of voltage to current remains constant throughout the experiment.

So no difficulty has arisen with your experiment, you have simply proved that Ohm's Law doesn't apply to the lamp filament. Believe your results!!

Read more: What_difficulty_will_arise_while_testing_whether_the_filament_of_light_bulb_obeys_ohm's_law

Answer 5

Ohms law is based on the condition that the temperature is to be constant. But in case of bulb temperature will not be constant.