Apparatus:
Battery (12 volts), tungsten lamp (12 volts, 25 watt), rheostat, voltmeter, ammeter, key, connecting wires and sand paper.
Theory
According to Ohm’s law, when potential difference across the resistances, the current also increases. This means that current and voltage are directly proportional to each other. But for tungsten filament Ohm’s Law does not hold due to increase of the resistance of the tungsten filament with rise of temperature.
Procedure:
1. Draw the circuit diagram and make the connection accordingly. The rheostat is used as potential divider. Ammeter is connected in series with tungsten bulb while voltmeter is connected in series with tungsten bulb while voltmeter is connected parallel to it.
2. Put the plug in the key k, the current stars to flow through the bulb. Note the readings of the voltmeter v and ammeter i.
3. Change the potential, difference across the bulb whit the sliding terminal of the rheostat and note at least 8 to 10 readings.
4. Draw a graph between v and i by taking v along x-axis and i along y-axis. This represents a curved line which shows that tungsten filament does not obey ohm’s law as its resistance changes due to increase in temperature.
Observation & calculation
Conclusion:
As the graph between V and I is not a straight hence tungsten filament bulb not obey the Ohm’s Law.
Precautions:
1. All the connection should be clean and tight.
2. High resistance rheostat should be used.
3. Voltage should be changed in small and regular steps.
Viva voce:
Q.1: State Ohm’s law.
Ans: The potential difference between the two ends of a conductor is directly proportional to the current flowing
Q.2: Does the tungsten filament of lighted lamp obey Ohm’s Law?
Ans: No, because the resistance of the heated filament increase with the rise of its temperature.
Q.3: Define electric current?
Ans: Charge passing through a conductor in unit time is called electric current.
Q.4: What is resistance? Give its units.
Ans: It is measure of opposition to the motion of free electrons due to their collisions with the atoms of the lattice. Its unit is Ohm (Ω).
Q.5: What is the relation between I and V for tungsten heated filament buld?
Ans: the relation between V and I for tungsten heated filament bulb is log I x kv
Q.6: Which of the following is a base unit in SI unit? (1) coulomb(2) ampere (3) Volt?
Ans: Ampere is a base SI unit.
