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AimTo determine the resistance per cm of a given wire by plotting a graph of potential difference versus current
Materials RequiredA given wire
Power supply
Connecting wires
Crocodile clips
Graph paper
TheoryOhm’s law states that the flow of electric current through a conductor is directly proportional to the potential difference across its ends, provided the physical state of the conductor (pressure, temperature, and dimensions) remains unchanged.
If the current flowing through the conductor is represented by I, and the potential difference across its ends is represented by V, then the relationship between the two is given by:
V = RI
Here, R is the constant of proportionality and is known as the electrical resistance of the conductor. The value of R depends on the material and dimensions of the conductor. The relationship between the resistance of a material and its length and area of cross-section is given by the formula:
R = (ρ * L) / A
Where ρ is the specific resistance or resistivity, which is a characteristic of the material of the wire.
ProcedureUsing sandpaper, remove any insulating coating on the connecting wire ends to ensure proper electrical contact.
As shown in the figure, connect the battery, resistance, rheostat, key, voltmeter, and ammeter in the circuit.
Check and adjust the voltmeter and ammeter pointers to coincide with the zero mark on the measuring scale using the provided screw and screwdriver.
Note down the range and the least count of the voltmeter and ammeter.
Insert the key K and slide the rheostat to the minimum current flow position. Take note of the milliammeter and voltmeter readings.
Remove the key K and allow the wire to cool down. Then, insert the key again and slightly increase the voltage by moving the rheostat. Record the voltmeter and milliammeter readings.
Repeat step 6 for four different adjustments of the rheostat, and document the readings in a tabular column.
Observation and ResultCalculations:
Using the recorded values, plot a graph of potential difference (voltage) versus current. The graph should be a straight line passing through the origin.
The slope of the graph represents the resistance per unit length of the wire. From the graph, we find the slope to be 2.5 V/A. Therefore, the resistance per unit length of the wire is:
Resistance per unit length = 2.5 Ω
Now, calculate the resistance per cm using the formula:
Resistance per cm = (Resistance per unit length) / (Length of wire used)
Assuming the length of wire used to be 50 cm, we get:
Resistance per cm = (2.5) / (50) Resistance per cm = 0.05 Ω/cm
Therefore, the resistance per cm of the given wire is 0.05 Ω/cm.