Aim
To study the dependence of potential difference (V) across a resistor on the current (I) passing through it and
determine its resistance and plot a graph between V and I.
Materials Required
A battery, an insulated copper wire, a key, an ammeter, a voltmeter, a rheostat, a resistor and a piece of sand paper.
Theory
Ohm’s Law: The potential difference, V across the ends of a given metallic wire in an electric circuit is
directly proportional to the current flowing through it, provided its temperature is the same. This is Ohm’s
law.
V∝I
.’. V = IR, (Here R = Constant for the given metallic wire)
Circuit Diagram:
In a circuit ammeter is always connected in series and voltmeter is connected in parallel across the points
between which potential difference is to be measured.
A straight line graph obtained between V and I verifies the Ohm’s law.
Procedure
1. Keep the devices as shown in the circuit diagram.
2. Connect them with the connecting wires and keep the key open.
3. Positive terminal of the battery is connected to the positive terminal of the ammeter.
4. Check the +ve and -ve terminals of voltmeter before connecting it in the circuit.
5. Once the circuit is connected, insert the key and check the rheostat, adjust its slider and see whether the
ammeter and voltmeter readings are shown.
6. By using the slider of rheostat take three different readings of current 1 and voltmeter V.
7. Record your observations in the observation table.
8. Calculate resistance of a given resistor by formula .
9. Plot a graph of voltmeter reading and current reading. On x axis take V and on y axis take I.
Observation Table
A. Least count of ammeter and voltmeter
S. No. Ammeter (A) Voltmeter (V)
1. Range
2. Least Count
B. For reading of ammeter and voltmeter
Current in Ampere (I) Potential difference in Volts (V)
(Ammeter Reading) (Voltmeter Reading)
S. No. R = V/I(Ω)
Observed Corrected Observed Corrected
1.
2.
3.
Graph between voltage and current
Conclusions
1. The value of R is found to be same and constant in all three readings.
2. The graph of V and I is a straight line. This shows that V∝I. This verifies Ohm’s law.
Precautions
1. The connecting wires should be thick copper wires and the insulation of their ends should be removed using
the sand paper.
2. Connections should be made as per the circuit. The ammeter should be connected in series with the resister
such that the current enters at the positive terminal and leaves at the negative terminal of the ammeter.
3. Voltmeter should always be connected in parallel to resistor.
4. Calculate the least count of voltmeter and ammeter correctly.
5. Current should be passed through the circuit for a short time while taking observations; otherwise current
would cause unnecessary heating in the circuit.
