Potentiometer - principle and its applications to measure potential difference and for comparing EMF of two cells; measurement of internal resistance of a cell
Grasp the principle of the potentiometer and its uses for accurate measurement of potential difference, comparing EMFs, and determining internal resistance.
What is Potentiometer - principle and its applications to measure potential difference and for comparing EMF of two cells; measurement of internal resistance of a cell?
The potential drop per unit length of the potentiometer wire.
Key formula / rule: Potential Gradient
Key points
- Understand the working principle of a potentiometer.
- Explain how a potentiometer measures potential difference accurately.
- Describe the method for comparing the EMFs of two cells using a potentiometer.
- Explain the procedure to measure the internal resistance of a cell using a potentiometer.
Common exam trap
Using a non-uniform wire.
Definitions
- Term
Potential Gradient
- Meaning
The potential drop per unit length of the potentiometer wire.
- Term
Balance Length
- Meaning
The length of the potentiometer wire across which the potential drop is equal to the EMF of the cell being measured, resulting in zero deflection in the galvanometer.
- Term
Null Deflection Method
- Meaning
A method of measurement where the instrument (galvanometer) shows zero deflection, indicating that no current is flowing through it, thereby ensuring accurate measurement without disturbing the circuit.
Learning objectives
Understand the working principle of a potentiometer.
Explain how a potentiometer measures potential difference accurately.
Describe the method for comparing the EMFs of two cells using a potentiometer.
Explain the procedure to measure the internal resistance of a cell using a potentiometer.
Formulae
- Name
Potential Gradient
- Note
V is the potential drop across the potentiometer wire of length L, usually provided by the driver cell and rheostat.
- Expression
k = V / L
- Name
EMF of a cell using Potentiometer
- Note
E is the EMF of the cell, k is the potential gradient, and l is the balance length.
- Expression
E = k * l
- Name
Comparison of EMFs of two cells
- Note
E1 and E2 are the EMFs of the two cells, and l1 and l2 are their respective balance lengths.
- Expression
E1 / E2 = l1 / l2
- Name
Measurement of Internal Resistance of a cell
- Note
r is the internal resistance, R is the external resistance connected in parallel to the cell, l1 is the balance length when the cell's terminals are open, and l2 is the balance length when current flows through R.
- Expression
r = R * (l1 / l2 - 1)
Prerequisites
Ohm's Law
Series and Parallel Combinations of Resistors
Electromotive Force (EMF) and Internal Resistance
Galvanometer and its working principle
Common mistakes
Using a non-uniform wire.
Not ensuring constant current in the potentiometer wire.
Connecting the galvanometer incorrectly.
Assuming the potential drop across the wire is equal to the EMF of the driver cell without considering the rheostat.
Not checking if the EMF of the driver cell is sufficient.
Keywords
Potentiometer
Potential Gradient
Balance Length
Null Deflection
EMF Comparison
Internal Resistance
Driver Cell
Uniform Wire
Practice preview
A cell of EMF E and internal resistance r is connected across an external resistance R. When using a potentiometer, the balancing length for the open circuit (cell alone) is L1, and for the closed circuit (with R connect…
medium
A potentiometer wire of length L and resistance R is connected in series with a driving cell of EMF E0 and an external resistance R1. An unknown EMF E is balanced at a length l of the potentiometer wire. If the resistanc…
hard
Which of the following is a key advantage of using a potentiometer over a voltmeter for measuring potential difference?…
easy