CLASS 10 CBSE PHYSICS PRACTICAL

EXPERIMENT: Studying the Dependence of Potential Difference (V) Across a Resistor on the Current (I) Passing Through It and Determining Its Resistance

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AIM:

1. To study the dependence of potential difference (V) across a resistor on the current (I) passing through it.
2. To determine the resistance of the resistor.
3. To plot a graph between V and I.

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MATERIALS REQUIRED:

S. No.	Apparatus	Range/Specification	Quantity
1	Resistor	1Ω, 2Ω, 5Ω or 10Ω (known/unknown)	1
2	Ammeter	0-500mA or 0-1A	1
3	Voltmeter	0-3V or 0-5V	1
4	Battery/Battery eliminator	3V-6V	1
5	Rheostat	Variable resistance	1
6	Plug key	One way/Two way	1
7	Connecting wires	–	As required
8	Piece of sandpaper	–	1
9	Graph paper	–	1 sheet

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THEORY:

Ohm’s Law:
At constant temperature, the current (I) flowing through a conductor is directly proportional to the potential difference (V) across its ends.

Mathematically:

V ∝ I
V = IR

Where:

• V = Potential difference across the resistor (in Volts)
• I = Current passing through the resistor (in Amperes)
• R = Resistance of the resistor (in Ohms, Ω)

From Ohm’s Law:

R = V/I = constant

Characteristics:

• The graph between V and I is a straight line passing through the origin.
• The slope of the V-I graph gives the resistance (R).
• Slope = tan θ = V/I = R

Circuit Diagram:
The circuit consists of a resistor connected in series with an ammeter, battery, rheostat, and plug key. A voltmeter is connected in parallel across the resistor.

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CIRCUIT DIAGRAM:

        ┌─────[Rheostat]─────[A]─────[R]─────┐
        │                                      │
     [+]Battery[-]                            │
        │                                      │
        └──────────[Key]──────────────────────┘
                           │
                          [V]
                           │
                    (Across Resistor R)

Legend:

• Battery = Source of EMF
• Key = Plug key (to open/close circuit)
• Rheostat = Variable resistor (to change current)
• A = Ammeter (measures current, connected in series)
• R = Resistor (whose resistance is to be found)
• V = Voltmeter (measures potential difference, connected in parallel)

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PROCEDURE:

1. Circuit Setup:

• Clean the ends of connecting wires with sandpaper.
• Connect the circuit as shown in the circuit diagram.
• Connect the ammeter in series with the resistor.
• Connect the voltmeter in parallel across the resistor.
• Ensure positive terminals of ammeter and voltmeter are connected towards the positive terminal of the battery.

1. Checking:

• Check that all connections are tight and correct.
• Note the least count and zero error (if any) of ammeter and voltmeter.
• Keep the rheostat at maximum resistance position initially.

1. Taking Readings:

• Insert the plug key to close the circuit.
• Adjust the rheostat to get a small current flowing through the circuit.
• Note the ammeter reading (I) and corresponding voltmeter reading (V).
• Record the readings in the observation table.
• Increase the current gradually by adjusting the rheostat and take at least 5-6 readings.
• Remove the plug key after each observation to avoid heating.

1. Plotting Graph:

• Plot a graph between V (on Y-axis) and I (on X-axis).
• Draw the best-fit straight line.

1. Calculation:

• Calculate resistance for each observation using R = V/I.
• Find the mean resistance.
• Also calculate resistance from the slope of the graph.

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OBSERVATIONS:

Least Count of Ammeter = _ A (typically 0.01 A or 0.05 A)

Least Count of Voltmeter = _ V (typically 0.1 V or 0.05 V)

Zero Error in Ammeter = Nil / _ A

Zero Error in Voltmeter = Nil / _ V

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OBSERVATION TABLE:

S. No.	Current I (A)	Potential Difference V (V)	Resistance R = V/I (Ω)
1	0.10	0.5	5.0
2	0.15	0.8	5.3
3	0.20	1.0	5.0
4	0.25	1.3	5.2
5	0.30	1.5	5.0
6	0.35	1.8	5.1

Mean Resistance (R) = (R₁ + R₂ + R₃ + R₄ + R₅ + R₆) / 6

Mean Resistance (R) = (5.0 + 5.3 + 5.0 + 5.2 + 5.0 + 5.1) / 6 = 5.1 Ω

(Note: The above values are sample readings. Actual values will vary based on the resistor used.)

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CALCULATIONS:

Method 1: From Observations

Mean Resistance = Sum of all resistance values / Number of observations
R = (R₁ + R₂ + R₃ + ... + R₆) / 6
R = 5.1 Ω (example)

Method 2: From Graph

Slope of V-I graph = Resistance (R)

Choose two points on the best-fit line:
Point 1: (I₁, V₁) = (0.10, 0.5)
Point 2: (I₂, V₂) = (0.35, 1.8)

Slope = (V₂ - V₁) / (I₂ - I₁)
      = (1.8 - 0.5) / (0.35 - 0.10)
      = 1.3 / 0.25
      = 5.2 Ω

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GRAPH:

Title: Graph between Potential Difference (V) and Current (I)

Axes:

• X-axis: Current (I) in Amperes →
• Y-axis: Potential Difference (V) in Volts ↑

Scale:

• X-axis: 1 cm = 0.05 A
• Y-axis: 1 cm = 0.2 V

Nature of Graph:
The graph is a straight line passing through the origin, which verifies Ohm’s law.

V(Volts)
↑
2.0 |                               ×
    |                          ×
1.5 |                     ×
    |                ×
1.0 |           ×
    |      ×
0.5 | ×
    |_________________________________→ I(Amperes)
    0   0.1  0.2  0.3  0.4  0.5

Inference from Graph:

• The graph is a straight line passing through origin.
• This shows that V ∝ I (Ohm’s Law is verified).
• The resistance remains constant throughout.

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RESULT:

1. The potential difference (V) across the resistor is directly proportional to the current (I) passing through it, thereby verifying Ohm’s Law.
2. The resistance of the given resistor:

• From observations: R = 5.1 Ω
• From graph: R = 5.2 Ω
• Mean Resistance: R ≈ 5.2 Ω

1. The V-I graph is a straight line passing through the origin, confirming that the resistor obeys Ohm’s Law.

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PRECAUTIONS:

1. Connections:

• All connections should be neat, clean, and tight.
• Connect ammeter in series and voltmeter in parallel to the resistor.
• Connect positive terminals towards the positive of the battery.

1. Instruments:

• Use appropriate range ammeter and voltmeter to avoid damage.
• Check for zero error in ammeter and voltmeter before starting.
• Ensure the instruments are in working condition.

1. During Experiment:

• Keep the plug key closed only while taking readings to avoid heating.
• Adjust rheostat slowly and smoothly.
• Do not exceed the maximum current rating of the resistor.
• Take at least 5-6 observations for accuracy.

1. Readings:

• Note readings only when the ammeter and voltmeter needles are steady.
• Avoid parallax error while taking readings (eye should be perpendicular to the scale).
• Start with low current and gradually increase it.

1. Graph:

• Choose appropriate scale for plotting the graph.
• Mark points clearly with a sharp pencil.
• Draw the best-fit straight line (not a zigzag line joining all points).

1. Safety:

• Do not touch the circuit while it is ON.
• Switch off the circuit immediately if any component gets heated.
• Handle the apparatus carefully.

1. General:

• The current should not be passed through the circuit for a long time to avoid overheating.
• If the graph does not pass through origin, check for zero errors or loose connections.

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SOURCES OF ERROR:

1. Loose connections in the circuit
2. Zero error in ammeter or voltmeter not accounted for
3. Heating of resistor due to prolonged current flow
4. Incorrect range of instruments
5. Parallax error while reading meters
6. Rheostat adjustment not smooth

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VIVA QUESTIONS:

1. Q: State Ohm’s Law.

• A: At constant temperature, the current flowing through a conductor is directly proportional to the potential difference across its ends.

1. Q: What is the unit of resistance?

• A: Ohm (Ω)

1. Q: Why is ammeter connected in series?

• A: To measure the total current flowing through the circuit.

1. Q: Why is voltmeter connected in parallel?

• A: To measure the potential difference across the resistor.

1. Q: What is the nature of V-I graph for an ohmic resistor?

• A: A straight line passing through the origin.

1. Q: What does the slope of V-I graph represent?

• A: The resistance of the conductor.

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Signature of Teacher: ___