4.2.4 Resistance – Experiment & Calculations
What We Want to Find
We will use a voltmeter and an ammeter to measure the voltage across and the current through a resistor.
From these two numbers we can calculate the resistance with Ohm’s Law:
\$R = \dfrac{V}{I}\$
Analogy – Water in a Pipe
Think of electricity like water flowing through a pipe.
- Voltage (V) is like the water pressure.
- Current (I) is like the flow rate of water.
- Resistance (R) is like a narrow section of the pipe that slows the flow.
The higher the pressure, the more water flows, but a narrow pipe (high resistance) reduces the flow for a given pressure.
Equipment Needed
- Power supply (DC, adjustable voltage)
- Unknown resistor (the one we want to find)
- Voltmeter (high internal resistance)
- Ammeter (low internal resistance)
- Connecting wires
- Clamp or clip leads for safe connections
Circuit Setup (No Images – Use Your Own Diagram)
- Connect the power supply in series with the unknown resistor.
- Place the ammeter in series with the resistor so that all current flows through it.
- Connect the voltmeter in parallel across the resistor to measure the voltage drop.
- Make sure all connections are secure and that the ammeter is rated for the expected current.
⚡️ Safety note: Always start with the lowest voltage setting and increase gradually. Never touch live connections.
Step‑by‑Step Procedure
- Set the power supply to a convenient voltage (e.g., 5 V).
- Record the reading on the ammeter: \(I\) (in amperes).
- Record the reading on the voltmeter: \(V\) (in volts).
- Repeat the measurement at least three times with different supply voltages (e.g., 3 V, 5 V, 7 V) to check consistency.
- Use the formula \(R = V/I\) to calculate the resistance for each set of readings.
- Take the average of the calculated resistances for the final value.
Sample Data Table
| Voltage (V) | Current (A) | Calculated R (Ω) |
|---|
| 3.00 | 0.150 | 20.0 |
| 5.00 | 0.250 | 20.0 |
| 7.00 | 0.350 | 20.0 |
Calculations Explained
For each measurement:
\$R = \frac{V}{I}\$
Example: For 5 V and 0.250 A,
\$R = \frac{5.00}{0.250} = 20.0\ \Omega\$
Exam Tips Box
Tip: When the exam asks for “calculate the resistance”, show the full calculation: write the measured values, plug them into \(R = V/I\), and give the final answer with units. Always check that your units are correct – volts over amperes gives ohms (Ω).
Tip: If the data shows a slight variation, explain that this could be due to instrument tolerances or contact resistance. Mention that averaging the results gives a more reliable value.
Tip: In a multiple‑choice question, look for the answer that matches the calculated resistance within the given tolerance (often ±5 %).
Common Mistakes to Avoid
- Connecting the voltmeter in series – it should always be in parallel.
- Using a voltmeter with a low internal resistance – it will load the circuit and give a wrong voltage.
- Forgetting to zero the instruments before taking readings.
- Not accounting for the ammeter’s internal resistance, which can slightly increase the total resistance.
Quick Recap
1️⃣ Measure \(I\) with the ammeter in series.
2️⃣ Measure \(V\) with the voltmeter across the resistor.
3️⃣ Calculate \(R = V/I\).
4️⃣ Repeat for different voltages and average the results.
5️⃣ Check your work against the exam’s required precision.