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Aim: To study the dependence of potential difference (V) across a resistor on the current (I) passing through it and determine its resistance. Also plotting a graph between V and I.
Apparatus Required: Resistance wire, cell, key, ammeter, voltmeter, rheostat, connection wires.
Theory: According to Ohm’s law, if the physical conditions (particularly the temperature) of a conductor do not change, the potential difference (V) across its ends is directly proportional to the current (I) flowing in it, i.e.,
V ∝ I or V = IR
Where R is the resistance of the conductor (or resistance wire).
Method:
- Complete the electric circuit as shown in the figure by connecting the resistance wire, ammeter, rheostat and key in series with the cell. Connect the voltmeter in parallel across the ends of the resistance wire.
- Read and note the least count of ammeter and voltmeter.
- Insert the plug in the key and adjust the rheostat to obtain minimum reading in the ammeter (0.1 A). Note the readings of ammeter and voltmeter.
- Increase current gradually in steps of 0.1 A using rheostat and note readings each time to obtain at least five observations.
Observations:
Least count of Voltmeter = 0.1 V
Least count of Ammeter = 0.1 A
| S. No. | Ammeter Reading (I) in Ampere | Voltmeter Reading (V) in Volt | V / I = Resistance (R) in Ohm |
|---|---|---|---|
| 1 | 0.1 | 0.2 | 2.0 |
| 2 | 0.2 | 0.4 | 2.0 |
| 3 | 0.3 | 0.6 | 2.0 |
| 4 | 0.4 | 0.8 | 2.0 |
| 5 | 0.5 | 1.0 | 2.0 |
Mean value of resistance R = 2.0 ohm
Result:
- On increasing current in the resistance wire, the potential difference across it also increases.
- The ratio of potential difference to current is constant, i.e., potential difference is directly proportional to current. This verifies Ohm’s Law.
Graph: Plot potential difference (V) on Y-axis and current (I) on X-axis. The graph is a straight line. The slope of the straight line gives the resistance of the wire.
Slope = (1.0 − 0.2) / (0.5 − 0.1) = 0.8 / 0.4 = 2.0 ohm
Precautions:
- The ends of the connection wires must be cleaned using sandpaper before use.
- Ensure proper connections to avoid loose contacts.
- Do not allow the current to flow for a long time to prevent heating.
Precautions
- All the connections must be tight.
- Current should not be allowed to flow in the circuit for a long time i.e., the plug in the key must be inserted only while taking the reading and thereafter the plug must be withdrawn from the key. Otherwise due to flow of current in the resistance wire for a long time, its temperature will increase.
- The range of voltmeter used must be more than the e.m.f. of the cell used in the circuit.
- After making the electric connections, see that the cell is not short-circuited.
- The positive terminal of the cell must be connected to the +ve marked ends of the ammeter and voltmeter.
- The connection wires must be small and straight.
Viva-Voce
Q.1. What is Ohm’s law?
Ans. Keeping the temperature constant, the potential difference across the ends of a conductor is directly proportional to the current flowing in it i.e.,
V ∝ I or V = IR
where R is a constant called the resistance of conductor.
Q.2. What do you mean by the resistance of a conductor?
Ans. The obstruction offered by the conductor in the flow of current is called the resistance of the conductor.
Q.3. On what factors does the resistance of a conducting wire depend?
Ans. The resistance of a conducting wire depends on the following four factors:
- On the length of the wire (resistance of wire increases with its length).
- On the area of cross-section of wire (resistance decreases with increase in its thickness).
- On the material of wire (resistance is low of conductors such as silver).
- On the temperature of wire (resistance of metals increases with increase in temperature).
Q.4. What are the units of current, potential difference and resistance?
Ans. The unit of current is ampere, of potential difference is volt and of resistance is ohm, where
1 ohm = 1 volt / 1 ampere.
Q.5. What instruments are used to measure the potential difference across the ends of a conductor and the current flowing in a conductor?
Ans. The potential difference is measured by the voltmeter and current by ammeter.
Q.6. How are the voltmeter and ammeter connected in an electric circuit?
Ans. The ammeter is connected in series with the conductor, while the voltmeter is connected in parallel across the conductor.
Q.7. Why are the + and − signs marked on the ammeter and voltmeter?
Ans. The +ve sign means that the current must enter the instrument through this end and it should come out from the −ve marked end, otherwise the deflection of pointer will be on the left of the zero mark which we cannot measure.
Q.8. What is the use of rheostat in your experiment?
Ans. It is used to change the resistance in circuit and thus changes the current in the circuit.
Q.9. Why have you used a plug key in your experiment?
Ans. It is used to switch on and off the circuit. When the plug is in the key, the circuit is complete. On taking out the plug from the key, the circuit becomes incomplete. The plug is inserted only when we have to take readings of ammeter and voltmeter and then after taking the reading the plug is immediately withdrawn from the key. Otherwise due to continuous flow of current the wire will get heated up and its temperature will increase due to which its resistance will also increase.
Q.10. If a graph is plotted for potential difference against current, what type of curve do you get?
Ans. A straight line.
Q.11. What main precaution do you observe in this experiment?
Ans. The current is not allowed to flow in the conductor wire for a long duration, otherwise its temperature will increase.
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