Question

State Ohms law. How can it be verified experimentally? Does it hold good under all conditions?

Answer 1

Ohm's law:

1. Ohm's law states that “The voltage across a conductor is directly proportional to the current through it provided that all physical conditions and temperature remain same.” That is, $V\propto I$, where $V$ is potential and $I$ is current.
2. The above statement can be further written as $V=IR$, where $R$, the proportionality constant is resistance. The value of this constant depends upon nature, length, area of cross-section, and temperature.

Experimental verification of Ohm's law:

1. Set up the circuit as illustrated above to verify Ohm's law experimentally.
2. Connect the resistance in parallel with the voltmeter (an instrument to measure potential in a circuit) and connect the ammeter (an instrument to measure current in a circuit) in series in the circuit.
3. Initially, the key K is closed, and the rheostat (a device used to control current by varying resistance) is adjusted to get the minimum reading in ammeter A and voltmeter V.
4. The current in the circuit is increased gradually by moving the sliding terminal of the rheostat. During the process, the current flowing in the circuit and the corresponding value of potential difference across the resistance wire R are recorded.
5. Similarly, different values of current and voltage are obtained.
6. Plot a graph between the values of current and voltage. The graph obtained will be a straight line.
7. This shows that current and potential are directly proportional to each other. Thus, it verifies Ohm's law.

Conditions for Ohm's law:

1. Ohm's law is followed only if the temperature remains constant.
2. When the temperature increases, Ohm's law is violated. For example, the temperature of the light bulb filament increases due to current, so Ohm's law is not followed in this case.
3. Thus, for Ohm's law to be followed, temperature and other physical conditions must remain constant.