In 1827, German physicist Georg Simon Ohm published one of the most consequential discoveries in the history of electrical science: that for a metallic conductor at constant temperature, the current flowing through it is directly proportional to the potential difference applied across it. This relationship — so simple that it fits in three letters — became the cornerstone of circuit analysis, electronics, electrical engineering, and every piece of technology that runs on electricity. We call it Ohm’s Law.
Today, the Ohm’s Law experiment is a mandatory practical in both CBSE Class 10 (Science, Chapter 12: Electricity) and CBSE Class 12 (Physics Practical, Experiment on resistance). Students use it to verify that the ratio V/I remains constant for a metallic conductor, to plot the V-I graph (a straight line through the origin), and to determine the resistance of an unknown conductor from the graph’s slope.
This guide covers Ohm’s Law completely: its definition, the V=IR formula with all three forms, the standard Class 10 and Class 12 experimental setup, circuit diagram, step-by-step procedure, observation table, V-I graph, result, precautions, ohmic vs. non-ohmic conductors, and the limitations of Ohm’s Law. All apparatus described is manufactured and supplied by AJKANT Overseas from Ambala, India.
- 1. What is Ohm’s Law? Definition and Statement
- 2. Ohm’s Law Formula — All Three Forms
- 3. Apparatus Required
- 4. Circuit Diagram and Component Symbols
- 5. Experiment Procedure — Step-by-Step
- 6. Observation Table
- 7. V-I Graph and How to Find Resistance
- 8. Result and Conclusion
- 9. Ohmic vs. Non-Ohmic Conductors
- 10. Limitations of Ohm’s Law
- 11. Frequently Asked Questions (FAQ)
1. What is Ohm’s Law? Definition and Statement
Ohm’s Law states: “The electric current flowing through a metallic conductor is directly proportional to the potential difference across its ends, provided the temperature and other physical conditions of the conductor remain constant.”
Mathematically: V ∝ I, which gives V = IR, where R is the constant of proportionality called the electrical resistance of the conductor.
The SI unit of resistance is the Ohm (Ω), named after Georg Simon Ohm. One Ohm is defined as the resistance of a conductor through which a current of one Ampere flows when a potential difference of one Volt is applied across it: 1 Ω = 1 V/A.
2. Ohm’s Law Formula — All Three Forms
The single equation V = IR can be rearranged into three forms depending on which quantity you want to calculate:
| Form | Formula | Use When You Know | Example |
|---|---|---|---|
| Voltage | V = I × R | Current (I) and Resistance (R) | I = 2 A, R = 5 Ω → V = 10 V |
| Current | I = V / R | Voltage (V) and Resistance (R) | V = 12 V, R = 4 Ω → I = 3 A |
| Resistance | R = V / I | Voltage (V) and Current (I) | V = 6 V, I = 0.5 A → R = 12 Ω |
In the Ohm’s Law experiment, the form R = V/I is the most important: by measuring V and I for different rheostat settings and calculating V/I each time, students verify that this ratio remains constant — confirming that R is indeed independent of V and I for the given conductor at constant temperature.
3. Apparatus Required
4. Circuit Diagram and Component Symbols
Circuit Connection (Series-Parallel Arrangement)
The standard circuit for the Ohm’s Law experiment connects components as follows:
Series branch (current path): Battery (+) → Plug Key (K) → Ammeter (A) → Rheostat (Rh, series end) → Resistance wire (R, under test) → Battery (−)
Parallel branch (voltage measurement): Voltmeter (V) connected directly across the two terminals of the resistance wire (R) only — NOT across the rheostat.
This arrangement ensures that: (1) the ammeter reads the current through the resistance wire, (2) the voltmeter reads the voltage across the resistance wire only, and (3) changing the rheostat slider changes both V and I simultaneously while maintaining V/I = R = constant.
5. Experiment Procedure — Step-by-Step
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Assemble the CircuitConnect the battery eliminator, plug key, ammeter, rheostat (in series), and resistance wire in a series loop. Connect the voltmeter in parallel (directly across the resistance wire terminals only). Check all connections are tight. Ensure the ammeter and voltmeter pointers are at zero before switching on. Verify polarities: (+) of ammeter connects towards (+) of battery.
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Set Rheostat to Maximum ResistanceBefore closing the key, set the rheostat slider to the position of maximum resistance (minimum current). This protects the ammeter from overload when the circuit is first switched on and gives the smallest starting reading on both instruments.
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Close the Key and Take the First ReadingInsert the plug key to close the circuit. Immediately read the ammeter (I₁) and voltmeter (V₁). Record both values in the observation table. Open the key immediately after recording — do not leave the circuit closed between readings to prevent heating of the resistance wire.
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Adjust the Rheostat and Take 5 More ReadingsMove the rheostat slider to decrease its resistance by a known increment. Close the key, read I and V, record them, then open the key. Repeat for at least 5–6 different rheostat settings, progressively increasing the current (and therefore the voltage across R). Each reading gives one (V, I) data point for the graph.
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Calculate R = V/I for Each ReadingFor each row of readings (Vᵢ, Iᵢ), calculate the ratio Rᵢ = Vᵢ / Iᵢ. If Ohm’s Law holds, all values of Rᵢ should be approximately equal (within ±5% is acceptable). Take the mean value of all Rᵢ values as the experimental resistance of the conductor.
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Plot the V-I GraphOn graph paper, plot V (Y-axis, in Volts) vs. I (X-axis, in Amperes). All points should lie approximately on a straight line passing through the origin. Draw the best-fit straight line. The slope of this line = ΔV / ΔI = R (the resistance of the conductor).
6. Observation Table
Name of conductor under test: Nichrome resistance wire | Least count of ammeter: 0.02 A | Least count of voltmeter: 0.1 V
| S.No. | Ammeter Reading I (A) |
Voltmeter Reading V (V) |
Resistance R = V/I (Ω) |
|---|---|---|---|
| 1 | ____ | ____ | ____ |
| 2 | ____ | ____ | ____ |
| 3 | ____ | ____ | ____ |
| 4 | ____ | ____ | ____ |
| 5 | ____ | ____ | ____ |
| 6 | ____ | ____ | ____ |
| Mean Resistance R̄ = | ____ Ω | ||
7. V-I Graph and How to Find Resistance from the Graph
📊 The V-I Graph of an Ohmic Conductor
For a conductor that obeys Ohm’s Law (an ohmic conductor), the V-I graph is a straight line passing through the origin (0,0). This is because V = IR means V is directly proportional to I (with R as the constant), and a direct proportionality always gives a straight line through the origin.
How to find R from the graph:
- Draw the best-fit straight line through the plotted points and through the origin.
- Choose two well-separated points ON THE LINE (P₁ and P₂), not the original data points.
- Read their coordinates: P₁ = (I₁, V₁) and P₂ = (I₂, V₂).
- Calculate the slope: R = (V₂ − V₁) / (I₂ − I₁) = ΔV / ΔI
- This slope value equals the resistance of the conductor. Compare with the mean R calculated from the observation table.
8. Result and Conclusion
Conclusion to write in the practical record: “The V-I graph is a straight line passing through the origin, confirming that the current through the conductor is directly proportional to the voltage across it (at constant temperature). The conductor therefore obeys Ohm’s Law. The resistance of the conductor, calculated from the slope of the V-I graph, is ____ Ω.”
Precautions
- Do not leave the circuit closed: Open the key between readings to prevent the resistance wire from heating up. Heating increases resistance and violates the constant-temperature condition of Ohm’s Law.
- Ammeter in series, voltmeter in parallel: Never reverse these connections. A voltmeter connected in series will not pass enough current to deflect the ammeter; an ammeter connected in parallel may be damaged by excessive current.
- Avoid loose connections: Loose connections introduce contact resistance, giving erroneously high V/I values. All connections must be tight and clean.
- Check for zero error: Before closing the circuit, check that both ammeter and voltmeter read zero. Correct for zero error if present.
- Use correct ranges: Select ammeter and voltmeter ranges appropriate for the expected values. Reading near the top of the scale gives the best accuracy.
- Take readings at steady state: After adjusting the rheostat, wait a moment for the current to stabilise before recording readings.
9. Ohmic vs. Non-Ohmic Conductors
Not all conductors obey Ohm’s Law. Materials that maintain a constant resistance (constant V/I ratio) regardless of the applied voltage are called ohmic conductors. Materials whose resistance changes with voltage or current are called non-ohmic conductors.
- V-I graph: straight line through origin
- Resistance R = V/I is constant
- Examples: Nichrome wire, Constantan, Manganin, most metallic resistors
- Resistance increases with temperature (positive temperature coefficient) but is constant at any fixed temperature
- Used in: standard resistors, resistance boxes, rheostats, resistance thermometers
- Ohm’s Law applies directly
- V-I graph: curve (not a straight line)
- Resistance R = V/I changes with applied voltage
- Examples: Diodes, LEDs, transistors, thermistors, tungsten filament lamps
- For a diode: conducts easily in forward bias, blocks in reverse bias — highly asymmetric V-I curve
- For a tungsten lamp: resistance increases steeply as filament heats up
- Ohm’s Law does not apply
10. Limitations of Ohm’s Law
While Ohm’s Law is one of the most powerful and widely used relationships in electrical science, it has important limitations that students must understand:
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11. Frequently Asked Questions (FAQ)
Ohm’s Law states: “The electric current (I) flowing through a metallic conductor is directly proportional to the potential difference (V) across its ends, provided the temperature and physical conditions of the conductor remain constant.” Mathematically: V ∝ I, giving V = IR, where R is the resistance of the conductor (a constant at constant temperature). The SI unit of resistance is the Ohm (Ω), where 1 Ω = 1 V/A.
The V-I graph is a straight line through the origin because Ohm’s Law establishes a direct proportionality between V and I (V ∝ I), with R as the constant of proportionality. Any direct proportionality of the form y = kx, when graphed, produces a straight line passing through the origin (0,0). The slope of the V-I graph equals the resistance R of the conductor. If the graph passes through the origin with a constant slope, it confirms that R = V/I is constant and the conductor obeys Ohm’s Law. A curve or a line that doesn’t pass through the origin indicates non-ohmic behaviour.
Ammeter in series: The ammeter measures the current flowing through the circuit. To measure all the current passing through the resistance under test, it must be placed in the same series path as the component. The ammeter has very low internal resistance so it does not significantly reduce the current. Voltmeter in parallel: The voltmeter measures the potential difference (voltage) across the component. To measure the voltage across a specific component, it must be connected across (in parallel with) that component. The voltmeter has very high internal resistance so it draws negligible current, not disturbing the main circuit current appreciably.
Resistance is a property of a conductor — its opposition to the flow of electric current — defined as R = V/I. Resistance exists whether or not Ohm’s Law holds. Ohm’s Law is a specific statement about certain conductors: that their resistance R = V/I remains constant regardless of the applied voltage (at constant temperature). All ohmic conductors have resistance, but not all resistors are ohmic. A diode has resistance (R = V/I at any given voltage) but it is not ohmic because this ratio changes with voltage. So: resistance is a universal concept; Ohm’s Law is a special condition that some conductors satisfy.
The main limitations of Ohm’s Law are: (1) Temperature dependence: It holds only at constant temperature. High currents heat the conductor, changing its resistance. (2) Non-ohmic devices: Semiconductors (diodes, transistors, LEDs) do not obey Ohm’s Law — their resistance is voltage-dependent. (3) Unidirectional conductors: Electrolytes and p-n junctions conduct differently in opposite directions, so Ohm’s Law (which assumes symmetrical conduction) does not apply. (4) High-frequency AC: At high frequencies, skin effect, inductance, and capacitance become significant, and impedance Z replaces simple DC resistance R.
Source Ohm’s Law Experiment Kits from Ambala
AJKANT Overseas manufactures and supplies complete Ohm’s Law experiment kits — ammeters, voltmeters, rheostats, resistance boxes, battery eliminators, resistance coils, plug keys, and connecting wires — for CBSE Class 10 and Class 12 physics practicals. Factory-direct from Ambala, India. Bulk supply for schools, colleges, and government tenders.
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