How Does a Potentiometer Work? Construction, Formula, and Uses
A potentiometer is a three-terminal variable resistor commonly used to control electrical devices, measure voltages, and compare electromotive force (EMF) in laboratory setups. Unlike a regular fixed resistor, a potentiometer’s resistance can be manually varied, acting as a voltage divider to obtain different output voltages as needed in a circuit.
This makes potentiometers fundamental components in electronics—ranging from simple volume controls in audio systems to precision tools for measuring unknown voltage and EMF in Physics experiments.
Definition and Construction
A potentiometer consists of a long, uniform resistance wire with two end terminals connected across a source of constant voltage (called the driver cell). A third terminal, called the sliding contact or ‘wiper’, moves along the resistor to tap off different voltages. This simple yet effective structure enables continuous control over the voltage at the output terminal.
The key distinction from a rheostat is that while a rheostat only controls current by varying resistance in series, a potentiometer is primarily used as a voltage divider.
Working Principle of Potentiometer
The fundamental working principle of a potentiometer is based on the fact that the voltage drop (potential) across a uniform resistance wire is directly proportional to its length when a constant current flows through it.
If the wire of length L has a total voltage V across it, then the potential gradient (k) is:
For any length ‘l’ of the wire, the potential difference is given by:
During measurement, an external cell (unknown emf) is connected in a secondary circuit, and the sliding contact is moved until no current flows through the galvanometer (null point). At this balance point, the EMF of the cell matches the potential across the balancing length.
Key Formulas and Calculations
| Concept | Formula | Description |
|---|---|---|
| Potential Gradient (k) | k = V/L | Potential drop per unit length |
| EMF using balancing length | E = k × l0 | EMF equals potential at the null point length |
| Comparing two EMFs | E1 / E2 = l1 / l2 | Ratio of balancing lengths gives EMF ratio |
| Internal resistance of a cell | r = ((l1 - l2) / l2) × R | r: internal resistance, R: known external resistance |
Stepwise Problem-Solving Approach
- Set up the potentiometer: Connect the potentiometer wire across a stable voltage source (driver cell) using a rheostat to control current.
- Connect the unknown/emf source: Attach the cell or voltage to be measured in series with a sensitive galvanometer and a sliding contact (jockey) on the potentiometer wire.
- Find the null point: Slide the jockey along the wire until the galvanometer shows zero deflection, indicating no current and a balanced circuit.
- Measure length: Note the exact length (l0) from one end to the null point.
- Solve using formulas: Calculate the required quantity (EMF, ratio, internal resistance, etc.) using the relations in the table above.
Example: If the potentiometer wire is 100 cm long with a total voltage of 2V, the potential gradient is k = 2V / 100cm = 0.02 V/cm. If balance occurs at 60 cm, then E = k × l = 0.02 × 60 = 1.2 V.
Types of Potentiometers
| Type | Key Details | Applications |
|---|---|---|
| Rotary | Wiper moves in a circular arc; commonly used for volume control. | Radios, audio systems, charging systems. |
| Linear | Wiper slides in a straight line along the resistor. | Voltage measurement, laboratory uses, audio mixing panels. |
| Digital | Resistance adjusted by electronic switches; no mechanical wear. | Precision applications, calibration, digital volume controls. |
Applications of Potentiometer
- Comparing EMF of a cell with a standard, for high accuracy: Compare EMF of Two Cells Using Potentiometer
- Measuring internal resistance of a battery cell: Determine Internal Resistance with Potentiometer
- Obtaining adjustable voltages in circuits (e.g., volume control for audio equipment)
- Calibration of voltmeters, ammeters, and other measuring devices
- Used in transducers for position, displacement, and motion measurement
Potentiometer vs Rheostat
| Aspect | Potentiometer | Rheostat |
|---|---|---|
| Function | Variable voltage divider, measures voltage | Variable resistance in series, controls current |
| Terminals | Three | Two |
| Connection | Used across the full resistance; outputs variable voltage | Used in series with circuit; outputs variable resistance |
Sensitivity of Potentiometer
Sensitivity refers to the smallest voltage difference a potentiometer can measure. Sensitivity increases with the length of the potentiometer’s resistance and decreases with higher driver voltage. For maximum sensitivity, use a long resistance wire and low driving voltage, provided it still exceeds the unknown voltage being measured.
Limitations & Modern Enhancements
Mechanical potentiometers can be affected by wear, drift, and environmental changes. Digital potentiometers resolve many of these issues: they have no moving parts, are more reliable, and maintain stable performance in harsh conditions. However, they may not be suitable for high-power or high-frequency applications.
Explore Further and Practice
- Potentiometer: Principle and Applications
- Electricity Concepts and Examples
- Difference Between EMF and Voltage
- Practice numerical: A potentiometer is 120 cm long with a total voltage of 3 V. Where will the null point occur for a cell with EMF 1.5 V? (Potential gradient = 3/120 = 0.025 V/cm; So, l0 = 1.5/0.025 = 60 cm.)
Potentiometers are vital for both experimental Physics and practical electronics. Mastering their principle and usage builds a strong foundation for understanding voltage, EMF, and circuit measurements with accuracy. For more advanced concepts, practice problems, and detailed explanations on related topics, explore other sections on Vedantu’s Electricity resources.
FAQs on Potentiometer Working Principle: Class 12 Physics Guide
1. What is the working principle of a potentiometer?
The working principle of a potentiometer is that the potential drop across a uniform wire is directly proportional to its length, provided a constant current flows through the wire. This allows accurate measurement of voltage or EMF without drawing current from the source.
Key points:
• V ∝ l, where V = potential difference and l = length
• Used for comparing EMF of cells, measuring internal resistance, and calibrating instruments
• No current is drawn from the cell at balance (null point)
2. What is a potentiometer used for?
A potentiometer is used for:
• Accurately comparing the EMF of two cells
• Measuring the internal resistance of a cell
• Calibrating ammeters and voltmeters
• Measuring potential difference (voltage) across circuit branches
It is preferred because it provides precise measurements without drawing current from the source under balance.
3. How does a potentiometer measure EMF of a cell?
A potentiometer measures the EMF of a cell by finding the balancing length on a uniform wire where the cell’s EMF equals the potential drop. At this null point, no current flows through the galvanometer.
Steps:
1. Connect cell to potentiometer circuit.
2. Slide the contact till the galvanometer reads zero.
3. Calculate EMF using E = k × l0, where k = potential gradient and l0 = balancing length.
4. What is the difference between a potentiometer and a voltmeter?
Potentiometer vs Voltmeter:
• Potentiometer: Measures EMF directly without drawing current at balance; very accurate; used for precision measurements and comparisons.
• Voltmeter: Measures potential difference by connecting across two points; draws some current; less accurate than potentiometer.
Thus, potentiometers are preferred when high accuracy is needed.
5. Why is a potentiometer considered more accurate than a voltmeter?
A potentiometer is more accurate because no current is drawn from the source at balance, eliminating errors due to loading effect. Voltmeters, even high-resistance types, draw some current, which can affect the true potential difference across the measured points.
6. How do you calculate the potential gradient in a potentiometer?
The potential gradient (k) is calculated as the potential drop per unit length of the potentiometer wire.
Formula:
k = V / L
Where:
• V = total potential difference across the length L of the wire
• L = length of wire
This gradient helps to determine unknown EMF or potential differences using balancing length.
7. How is the internal resistance of a cell found using a potentiometer?
Internal resistance is measured by comparing balancing lengths with and without external resistance:
Steps:
1. Find balancing length l1 (open circuit, no load).
2. Attach known external resistance R, and find balancing length l2 (closed circuit).
3. Calculate internal resistance: r = (l1 - l2) / l2 × R.
8. What are the main types of potentiometers?
The main types of potentiometers are:
• Linear Potentiometer: Sliding contact moves in a straight line
• Rotary Potentiometer: Contact moves along a circular track (common in electronics, e.g. volume knobs)
• Digital Potentiometer: Uses electronic switching to change resistance digitally
All types operate on the same basic working principle.
9. What are some applications of potentiometers in real life?
Potentiometer applications include:
• Comparing EMF of cells in laboratory experiments
• Measuring internal resistance of batteries
• Calibrating voltmeters and ammeters
• Volume and tone control in audio devices (rotary type)
• Electronic circuits for adjusting voltage or signal levels
10. Why is the length of potentiometer wire made long?
The length of potentiometer wire is made long to increase the sensitivity and accuracy of measurements. A longer wire means a smaller potential gradient per unit length, allowing finer distinction of small voltage differences and helping identify the null point more precisely.
11. What happens if the current in the potentiometer wire is not constant?
If the current in the potentiometer wire is not constant, the potential gradient becomes non-uniform, leading to inaccurate measurements. This is why a stable current source and a rheostat are used to maintain constant current throughout the experiment.
12. Can a potentiometer compare two unknown EMFs without a standard cell?
Yes, a potentiometer can directly compare the EMFs of two cells by measuring the respective balancing lengths. The ratio of their EMFs equals the ratio of their balancing lengths: E1/E2 = l1/l2. No standard cell is strictly required for comparison.
