Step-by-Step Guide: Potentiometer Setup and Observations
What is a Potentiometer and How to Measure Internal Resistance?
A potentiometer is a three-terminal device that is used to determine the potential differences in a circuit by manually varying the resistances. With the help of a potentiometer, one can also find the internal resistance of a cell and the EMF between two cells. There are different types of potentiometers available such as rotary potentiometer, linear potentiometer etc.
Internal Resistance in Potentiometer
In an electrical circuit, we use a device called a Potentiometer to measure the following things
To measure the internal resistance of the cell
To compare emf or the electromotive force between two cells
To measure the potential difference across a resistor
A potentiometer consists of a long wire of uniform cross-sectional area and a length of 10 metres provided that the wire should have high resistivity and a low-temperature coefficient.
In this article, we will perform an experiment to determine the internal resistance of a cell using the Potentiometer formula.
What is Internal Resistance?
If an electrical power source is a linear electric circuit, then according to Thévenin's theorem, it can be represented as an ideal voltage source in series with an impedance, and this impedance is called the internal resistance of the source.
How to Find Internal Resistance of a Cell Using a Potentiometer?
Now, let’s understand the theory on how to find the internal resistance of a cell using a potentiometer followed by the experiment on the determination of potential difference using a potentiometer.
Theory: We know that a potentiometer is used to determine the internal resistance of a cell. Besides this, we will have a determination of potential difference using a potentiometer.
The internal resistance of a cell using the potentiometer formula is given by:
r = (E/I - R)......(1)
And,
Determination of Potential Difference Using Potentiometer, the formula is:
E = I (R + r).....(2)
Here,
r = internal resistance of the cell in
E = electromotive force in Volts
I = current in amperes (amp)
R = Load resistance of the load in the circuit in
We know by Ohm’s law:
V = IR = E - Ir ….(3)
Equation (3) indicates that the value of V is less than E by an amount equal to the fall of potential inside the cell due to its internal resistance.
From equation (3), we have:
r/R = (E - V)/V
Therefore, the internal resistance of the cell is:
r = R (E - V)/V …..(4)
So, equation (4) is the Formula for Internal Resistance In Potentiometer.
To Determine the Internal Resistance of a Given Primary Cell Using a Potentiometer Experiment
The aim of our experiment is To Determine The Internal Resistance Of A Given Primary Cell Using A Potentiometer Experiment.
Materials required for the experiment:
A galvanometer
One battery
A potentiometer
A rheostat of low resistance
One fractional resistance box
One high resistance box
One ammeter
Two one-way number keys
Connecting wires
A jockey
Sandpaper
One Leclanche cell
One set square
A voltmeter
Below is the Circuit Diagram for the Potentiometer
(image will be uploaded soon)
Where the internal resistance of the cell can be determined by the following formula:
r = (L1 - L2)/(L2) * R
Here,
L1 = a balancing length without a shunt resistance
L2 = a balancing length with a shunt resistance
Procedure to Perform the Experiment
We must make sure that connections should be according to the diagram drawn above.
Use sandpaper to clean the ends of the connecting wires and make sure that the connections are tight.
Make sure that the plugs in the resistance box are tight.
We must note that the e.m.f of the cell and battery is more than that of the cell. If it is not then we would not be able to obtain the null point.
We can minimise rheostat resistance by taking maximum current from the battery.
To check if the connections in the circuit are correct, the galvanometer deflections must be in the opposite direction. This is done by inserting the key K1 and making the note of the ammeter reading.
To get the null point on the fourth wire, the rheostat should be adjusted/fixed without inserting the key K2.
Take the small range resistance between 1-5 from resistance box R connected in parallel with the cell.
Slide the jockey and get the null point.
Finally, record your observations.
Our Observations on Determining the Internal Resistance in Potentiometer
Below are the calculations on determining the internal resistance in the potentiometer:
The least count or the L.C. of the voltmeter =.....
Range of voltmeters =.....
EMF of the cell =.....
EMF of a battery =......
Table for the measured lengths:
Internal Resistance Calculations
For each set of observations find mean and L2.
Then calculate the value of r for each set.
Take the mean of obtained values of r.
Result Obtained
The value of the internal resistance is……..Ohms.
Precautions on Determination of Internal Resistance in Potentiometer
Precautions while performing the experiment:
The e.m.f of the cell must be less than that of the battery.
Keep an eye to make sure that the ammeter reading remains the same or constant at least for the first set of readings.
Ensure that the current is passed only while obtaining the null point.
Make sure that the rheostat should remain fixed.
During the experiment, the cell should remain undisturbed.
Make sure that the jockey should not be rubbed against the potentiometer wire.
Points to remember about potentiometer-
A three-terminal device with a variable resistance to control the flow of the current.
Used to determine the EMF, unknown voltage, and the internal resistance.
Generally abbreviated as POT.
The resistive wire used in a potentiometer is usually made up of metals such as manganese and constantan due to their properties by which there is not much variation in the resistance when the temperature changes.
A potentiometer is pocket-friendly, easy to use, and has high accuracy.
Different types of potentiometers are available such as rotary potentiometers, linear potentiometers, digital potentiometers, mechanical potentiometers, etc. Rotary potentiometers are widely recognized and used.
The sensitivity of a potentiometer is directly proportional to the length of the resistance. Therefore, if the length of the potentiometer resistance then the resistance per unit voltage also increases.
Conclusion
This is how a potentiometer is used to calculate the internal resistance of a primary cell. Understand how the experiment is conducted to realize the use of a potentiometer and get the best results when you do it practically.
FAQs on How to Determine the Internal Resistance of a Primary Cell Using Potentiometer
1. What is the fundamental principle of a potentiometer that allows it to determine a cell's internal resistance?
The fundamental principle of a potentiometer is that the potential drop (V) across any portion of a wire with uniform cross-sectional area and composition is directly proportional to its length (L), provided a constant current flows through it. This relationship (V ∝ L) allows for the precise comparison of an unknown EMF or potential difference with a known potential drop along the wire, without drawing any current from the source being measured at the balance point.
2. How do you determine the internal resistance of a primary cell using a potentiometer experiment?
The determination involves a two-step process:
- Step 1: First, find the balancing length (l₁) on the potentiometer wire that corresponds to the EMF (E) of the primary cell. This is done by connecting the cell to the galvanometer and finding the null point with the shunt resistance key open.
- Step 2: Next, a known resistance (R) is connected in parallel to the cell using a resistance box, and the new balancing length (l₂) is found. This length corresponds to the terminal potential difference (V) of the cell when current is being drawn from it. The internal resistance can then be calculated using these values.
3. What is the formula used to calculate the internal resistance of a cell from the potentiometer readings?
The formula to calculate the internal resistance (r) of a cell using a potentiometer is:
r = R * ((l₁ - l₂) / l₂)
Where:
- r is the internal resistance of the cell.
- R is the known resistance connected in parallel to the cell from the resistance box.
- l₁ is the balancing length when only the cell is in the circuit (measuring EMF).
- l₂ is the balancing length when the resistance R is also connected (measuring terminal voltage).
4. Why is a potentiometer preferred over a voltmeter for accurately determining a cell's internal resistance?
A potentiometer is preferred because it measures the potential difference using a null deflection method. At the balance point, it draws no current from the cell being tested, thus measuring the cell's true EMF. A voltmeter, in contrast, must draw a small current to operate, so it can only measure the terminal potential difference, which is affected by the cell's internal resistance. This makes the potentiometer a more accurate instrument for this specific measurement.
5. What is the key difference between a potentiometer and a rheostat?
The primary difference lies in their function and connections. A potentiometer is a three-terminal device used primarily to measure potential difference or act as a voltage divider. A rheostat is a two-terminal variable resistor used to control the current in a circuit. While a potentiometer can be wired to function as a rheostat, a rheostat cannot be used as a potentiometer.
6. What happens in the experiment if the EMF of the driver cell is less than the EMF of the cell being tested?
If the EMF of the driver cell in the main circuit is less than the EMF of the primary cell being tested, you will not be able to find a null point. The maximum potential drop that can be provided across the entire potentiometer wire will be less than the EMF of the test cell. Consequently, the galvanometer will show a deflection in only one direction, regardless of where the jockey is placed on the wire.
7. What is the importance of the 'null point' in this experiment?
The null point is the most crucial part of the experiment. It is the exact position on the potentiometer wire where the galvanometer shows zero current flow. This indicates that the potential difference across that length of the wire is precisely equal to the EMF (or terminal voltage) of the cell being measured. Finding this point of balance ensures that the measurement is made without drawing any current from the cell, which is essential for accuracy.
8. What are some essential precautions to take while performing this experiment in the lab for the 2025-26 session?
As per the CBSE guidelines for practicals, students should observe the following precautions:
- The EMF of the driver cell (in the primary circuit) must be greater than the EMF of the cell being tested.
- All connections should be tight and clean, with connecting wires made of thick copper.
- The jockey should be tapped gently on the wire and should not be dragged, as this can alter the wire's uniform cross-section and resistance.
- The positive terminals of both the driver cell and the test cell must be connected to the same end of the potentiometer wire.
