Question
State Ohm’s law? How can it be verified experimentally? Does it hold good under all conditions? Comment.
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Answer
According to Ohm's law the electric current flowing through a conductor is directly proportional to the potential difference applied across its ends provided the physical conditions such as temperature etc remains unchanged. Let V be the potential difference applied across the ends of conductor through which current Iflows, then according to Ohm's law.
$\text{V}\propto\text{I}\Rightarrow\ \text{V}=\text{IR}$
Ohm’s law can be verified experimentally by the activity given below:
- Firstly, Set up a circuit as shown in figure given below, consisting of a nichrome wire XY of length, say 0.5m, an ammeter, a voltmeter and four cells of 1.5V each. (Nichrome is an alloy of nickel,chromium, manganese, and iron metals.)
- First use only one cell as the source in the circuit. Note the reading in the ammeter I, for the current and reading of the voltmeter V for the potential difference across the nichrome wire XY in the circuit. Tabulate them in the Table given.
- Next connect two cells in the circuit and note the respective readings of the ammeter and voltmeter for the values of current through the nichrome wire and potential difference across the nichrome wire.
- Repeat the above steps using three cells and then four cells in the circuit separately.
- Calculate the ratio of V to I for each pair of potential difference V and current I.
| S.No. | Number of cells used in the circuit | Current through the nichrome wire, I (ampere) | Potential difference across the nichrome wire, V (Volt) | $\frac{\text{V}}{\text{I}}$ $\Big(\frac{\text{Volt}}{\text{Ampere}}\Big)$ |
| 1. | 1 | |||
| 2. | 2 | |||
| 3. | 3 | |||
| 4. | 4 |
- Plot a graph between V and I. The graph will be a straight line as shown below.
- This verifies the Ohm’s law.
Ohm's does not hold under all conditions as it is basically not a fundamental law which means it has exceptions. Following are the conditions when Ohm’s law does not hold:
- It is not obeyed when physical conditions of conductors like temperature keep on changing.
- It is not obeyed by a lamp filament, junction diode, thermistor etc.
- It is not obeyed in case of superconductors whose resistance is equal to zero.
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