Circuit Diagram to Verify Ohm's Law: Experiment Setup and Results
This is a free printable circuit diagram to verify ohms law: download the diagram as SVG or open it and print to paper or PDF.
Ohm's Law states that the current through a conductor is directly proportional to the voltage across it, provided temperature and other physical conditions remain constant: V = I × R. The circuit diagram to verify Ohm's law uses a series connection of a variable power supply, an ammeter, and a resistor, with a voltmeter connected in parallel across the resistor, allowing precise measurement of both V and I at each supply setting. By plotting V against I on a graph, students confirm the linear relationship and determine the resistance from the slope.
The standard circuit for verifying Ohm's law consists of the following elements connected in series: a DC power supply (either a variable bench supply or a battery with a rheostat), a key (switch) to control current flow, a known resistor (for example 10 Ω or 100 Ω), and an ammeter (A) in series to measure current. A voltmeter (V) is connected in parallel directly across the resistor — not the supply — to measure only the voltage drop across R, excluding contact resistance and wire resistance.
Why the voltmeter must be in parallel: The voltmeter has a very high internal resistance (typically 1 MΩ to 10 MΩ on modern digital meters) so that it draws negligible current through the parallel branch, ensuring the ammeter reading accurately represents the current through R. Connecting the voltmeter in series would block current flow and give incorrect readings.
Experiment procedure: With the circuit assembled, close the switch and set the supply to the lowest value. Record the ammeter reading (I₁) and voltmeter reading (V₁). Increase the supply voltage in equal steps (e.g., 1 V increments) and record each I and V pair. Take at least five readings. Calculate V/I for each pair — if Ohm's Law holds, V/I should equal the resistance R within experimental error for every reading.
Key formula: V = I × R, which rearranges to R = V/I and I = V/R. The slope of a V-vs-I graph gives R: slope = ΔV/ΔI = R. A steeper slope indicates higher resistance.
Expected results: The V-I graph is a straight line through (or close to) the origin, confirming the directly proportional relationship. The gradient equals R. Any deviation from linearity indicates that temperature is changing the resistance (common with tungsten filament lamps, which are non-Ohmic) or that contact resistance is significant.
Ohmic vs non-Ohmic conductors: Metals like copper, nichrome, and carbon resistors are Ohmic — their R is constant over a wide range of V and I. Semiconductor diodes, tungsten filament bulbs, and thermistors are non-Ohmic — their R changes with current. The Ohm's law verification experiment is typically performed with a fixed carbon or wire-wound resistor to demonstrate the ideal linear case.
Sources of error: The ammeter has a small internal resistance (r_A, typically 0.1–1 Ω) adding to the circuit resistance. The voltmeter draws a tiny current. Heating of the resistor changes its resistance slightly during measurement. These errors can be minimised by using a digital multimeter and allowing the circuit to stabilise between readings.
Class 10 context: In the CBSE Class 10 physics curriculum (Chapter 12: Electricity), students draw this exact circuit diagram and perform the experiment with a 1 Ω–100 Ω resistor and a 0–6 V variable supply. The expected conclusion is that the graph is a straight line, confirming V ∝ I.
Simulate this experiment virtually using the free online circuit editor at circuitdiagrammaker.com — place a voltage source, resistor, ammeter, and voltmeter, and vary the source voltage to observe the current change in real time.
How to wire circuit diagram to verify ohms law
- Gather components Obtain a variable DC supply (0–6 V), a resistor (e.g., 10 Ω/1 W), an ammeter (0–1 A range), a voltmeter (0–10 V range), a switch, and connecting wires.
- Connect the series circuit Connect the positive terminal of the supply through the switch, then through the ammeter, then through the resistor, and back to the negative terminal. Ensure polarity is correct (current flows from + to −).
- Connect the voltmeter in parallel Connect the voltmeter directly across the resistor terminals (not across the supply). Confirm the voltmeter + lead is on the side closer to the supply positive terminal.
- Close the switch and set minimum voltage Turn the supply to its lowest setting, close the switch, and record the first ammeter reading (I) and voltmeter reading (V).
- Increase voltage in steps Increase the supply voltage by approximately 1 V at a time. At each step, record I and V in a results table. Take at least five readings.
- Calculate V/I for each reading Divide each V by the corresponding I and compare to the stated resistance R. The values should be approximately equal, confirming Ohm's Law.
- Plot V vs I and find the slope Draw the V-I graph with V on the y-axis and I on the x-axis. The slope (ΔV/ΔI) gives the experimental value of R.
Specifications
| Ohm's Law formula | V = I × R |
|---|---|
| Rearranged for R | R = V / I (units: Ω = V / A) |
| Rearranged for I | I = V / R (units: A) |
| Ammeter connection | Series with the resistor (low internal resistance) |
| Voltmeter connection | Parallel across the resistor (high internal resistance) |
| Typical ammeter resistance | 0.1 Ω – 1 Ω |
| Typical voltmeter resistance | 1 MΩ – 10 MΩ (digital) |
| Graph result | V vs I is a straight line; slope = R |
| Typical test resistor (Class 10) | 10 Ω – 100 Ω, 1 W carbon or wire-wound |
| Supply voltage range | 0 V – 6 V DC variable |
| Number of readings (minimum) | 5 data points at different V settings |
| Ohmic conductor example | Nichrome wire, carbon resistor, copper wire |
| Non-Ohmic conductor example | Tungsten bulb, silicon diode, thermistor |
Safety warnings
- Never exceed the rated current of the ammeter or resistor; verify that the supply current at maximum voltage stays within the ammeter's full-scale range before closing the switch.
- Always turn OFF the supply and open the switch before modifying any connections to prevent accidental short circuits or electric shock.
Tools needed
- Variable DC power supply (0–6 V) or battery + rheostat
- Ammeter (0–1 A range, analog or digital multimeter in ammeter mode)
- Voltmeter (0–10 V range, or digital multimeter in voltmeter mode)
- Known resistor (10 Ω–100 Ω, 1 W, ±5% tolerance)
- Switch (SPST key or push-button)
- circuitdiagrammaker.com for drawing and simulating the circuit
Common mistakes
- Connecting the voltmeter in series instead of parallel — this blocks the circuit current and gives a near-zero ammeter reading.
- Connecting the ammeter in parallel across the resistor, which creates a near-short circuit and can damage the meter.
- Using a rheostat in parallel instead of in series as a variable resistance, which gives incorrect current control.
- Not allowing the resistor to cool between readings, causing heating effects that alter R and produce a non-linear V-I graph.
- Reversing the ammeter polarity, causing the needle to deflect backwards (below zero) and potentially damaging an analog meter.
Troubleshooting
- Ammeter reads zero despite voltage applied
- Cause: The circuit is open — possibly the switch is open, a wire is disconnected, or the resistor has failed open. Fix: Check continuity through each component with a multimeter in resistance mode with the supply OFF.
- V/I ratio is not constant across readings
- Cause: Resistor is heating up (its temperature coefficient is non-negligible), or loose connections are introducing variable contact resistance. Fix: Allow cooling time between readings, tighten all connections, and use a wire-wound resistor with low temperature coefficient.
- Graph does not pass through the origin
- Cause: A systematic offset in the ammeter or voltmeter zero reading, or a contact EMF at a junction. Fix: Zero both meters before starting, ensure the graph Y-intercept represents only instrument offset and correct accordingly.
Frequently asked questions
What is the circuit diagram to verify Ohm's law?
It shows a DC supply, switch, ammeter (in series), and resistor in a series loop, with a voltmeter connected in parallel across the resistor to measure the voltage drop while the ammeter measures current.
Why is the ammeter connected in series in the Ohm's law circuit?
The ammeter measures current flowing through the circuit; it must be in series so all the circuit current passes through it. Its low internal resistance minimises the disturbance to the measured current.
Why is the voltmeter connected in parallel in the Ohm's law experiment?
The voltmeter measures the potential difference across the resistor only. Its very high internal resistance ensures it draws negligible current, so it does not significantly alter the circuit conditions.
What does the V-I graph look like when Ohm's law is verified?
A straight line passing through (or very close to) the origin. The positive slope equals the resistance R of the conductor in ohms.
What is the formula for Ohm's law?
V = I × R, where V is the voltage in volts, I is the current in amperes, and R is the resistance in ohms.
What is a non-Ohmic conductor and how does it differ from an Ohmic one?
A non-Ohmic conductor's resistance changes with current or temperature (e.g., a tungsten bulb). Its V-I graph is a curve, not a straight line. Ohmic conductors like nichrome and carbon resistors maintain constant R.
What are the main sources of error in the Ohm's law experiment?
The ammeter's internal resistance adds to the circuit resistance, the voltmeter draws a tiny current, and resistor heating can slightly change the resistance during the experiment.