Lab Manual: Ohm’s Law | Lab Manuals for Class 10 PDF Download

Objective

Materials Required
A voltmeter and ammeter of suitable range, resistance wire or resistor, some connecting wires, a key, a dry cell (or battery eliminator), a rheostat and a piece of sand paper.

Theory/Principle

Procedure

• Before starting the experiment, note down the range, least count and zero error (if any) of the ammeter and voltmeter.
• Wipe the ends of the connecting wires with sand paper to remove any insulating layer.
• Make a set up of the circuit by connecting different components with the help of connecting wires as shown in Fig. 1(a).
  
• Connect the key, the rheostat, the ammeter and the resistor in series with the connecting wires to the dry cell, as shown in the circuit diagram [Fig. 1(b)], Ensure that positive (+) terminal of ammeter is joined to the positive (+) terminal of the cell. Make neat and tight connections.
• Connect the voltmeter in parallel to the resistor, as shown in circuit diagram. Ensure that the positive (+) terminal of the voltmeter is joined to the positive (+) terminal of the cell, and the negative (-) terminal of the voltmeter is joined to the negative (-) terminal of the cell.
• Insert the key in the plug to let the current establish in the circuit.
• Adjust the rheostat, so that a small current passes through the resistor, say 0.1 A, so the reading of ammeter will be 0.1 A.
• Read the corresponding value of potential difference from voltmeter.
• Repeat the experiment by adjusting the slider, for the values of current 0.3A, 0.4A, 0.5A etc. and record the corresponding values of potential difference in voltmeter.
• Lastly, plot a graph between V and I taking V along y-axis and I along x-axis.

Observation
Range of ammeter = ……… A
Least count of ammeter = ……….. A
Zero error of ammeter = ……….. A
Range of voltmeter = ……….. V
Least count of voltmeter = ……… V
Zero error of voltmeter = ………… V

Calculations and Graph
Mean value of resistance = (R₁ + R₂ + R₃ + R₄)/4 = …….. Ω

Now, by plotting the graph between V and I taking V along y-axis and I along x-axis.
We get a straight line and the slope of V versus I graph gives the resistance of the resistor (as shown in Fig. 2).
i.e. tanθ = V/I = R = Resistance
⇒ Slope = BC/AC = (V₂ – V₁)/(I₂ – I₁) = R

Result

• From the above calculation and observation, we have concluded that for a resistor, if temperature remains constant then current through the resistor is directly proportional to the potential difference applied across it.
• The value of resistance R of resistor remains the same for all values of current through it. The graph between V and I is a straight line and passes through the origin. This verifies Ohm’s law.

Precautions

• Wires should be thick and their insulation of ends should be removed properly.
• All the connections should be tight otherwise some external resistance may introduce in the circuit.
• Ammeter must be connected in series with the circuit.
• Voltmeter must be connected in parallel with the circuit.
• Zero mark of ammeter and voltmeter should be checked properly.
• There should be tight connections of connecting wires.
• Put the plug in key, only when reading is to be recorded. Take off plug immediately after it, to avoid unnecessary heating of wires.
• Positive terminals of voltmeter and ammeter should be connected to positive terminal of the cell and vice-versa.

Sources of Error

• There may be a reading error of ammeter and voltmeter.
• Area of cross-section of wire may not be uniform.
• It may be possible that current has flown for longer period of time through the circuit.
• There may be a loose connection in electrical circuit.