Physics Experiment: Internal Resistance of a Cell

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Questions and Answers

What is the role of the internal resistance of a cell in an electric circuit?

The internal resistance of a cell affects the flow of current within the cell and impacts the voltage available to the external circuit.

How does a potentiometer function to measure potential difference?

A potentiometer measures potential difference by utilizing a uniform wire and the principle of a potential divider, allowing for precise voltage measurements.

What materials are typically used in the construction of the potentiometer wire, and why?

Manganin or Constantan are commonly used due to their low thermal coefficients of resistance, which minimizes changes in resistance with temperature variations.

Describe the principle of a potential divider in the context of this experiment.

A potential divider consists of two resistors in series, where the voltage is divided between them based on their resistances, allowing for the calculation of voltage across each resistor. Signup and view all the answers

What is the significance of the 10 segments in the potentiometer wire?

The 10 segments allow for a more precise measurement of voltage by dividing the total length and resistance into measurable parts. Signup and view all the answers

What conditions must be met for the galvanometer’s pointer to deflect towards both sides?

The polarities of the electrodes of E and E1 connected at junction A must be the same. Signup and view all the answers

What happens when the terminal J is positioned near A?

The magnitude of the current IG will be negative, causing it to flow in the opposite direction to the presumed flow. Signup and view all the answers

How does increasing the distance l affect the current IG?

If l is sufficiently larger, the current IG may become positive, flowing in the presumed direction. Signup and view all the answers

What is the significance of the 'null point' in the context of the galvanometer?

The null point is where no current passes through the galvanometer, resulting in IG being zero. Signup and view all the answers

What is the maximum value of l, as stated in the content?

The maximum value of l can be L. Signup and view all the answers

Why must Rh be less than the cut-off value R for the galvanometer to function correctly?

This ensures that the galvanometer's pointer can deflect in both directions based on the adjustments made. Signup and view all the answers

What equation can be used to describe the relationship between l, E, and E1 for IG to be positive?

The equation is (E - E1) > 0, implying that E must be greater than E1. Signup and view all the answers

How does the position of terminal J influence the galvanometer’s current and deflection?

The position of J adjusts the values of l, affecting current IG which subsequently changes the deflection of the pointer. Signup and view all the answers

What is the role of the potentiometer's wire in the circuit described?

The potentiometer's wire serves as the conductor AB, facilitating the measurement of internal resistance. Signup and view all the answers

How is the null point on the potentiometer's wire determined?

The null point is found by adjusting the jockey until the galvanometer reads zero current. Signup and view all the answers

Explain the condition under which current passes through the potentiometer's wire.

Current does not pass through the wire when the key K is open. Signup and view all the answers

What is the purpose of resistance R in the internal resistance measurement?

Resistance R is used to create a voltage drop across the cell when measuring V2. Signup and view all the answers

What happens to the potentiometer wire's resistance when current flows through it?

The resistance of the wire may increase due to temperature rise from the current flow. Signup and view all the answers

How is the internal resistance r calculated using voltages V1 and V2?

The internal resistance r is calculated using the formula $r = \left( V_1 - V_2 \right) R / V_2$. Signup and view all the answers

What precaution should be taken when recording data during the measurement process?

The key K1 should be kept open while recording data to avoid current flow. Signup and view all the answers

Why is it important to measure the voltage difference across the cell with and without a resistor?

Measuring with and without a resistor helps to determine the effect of load on the cell voltage. Signup and view all the answers

How does Kirchhoff's current rule apply to the currents IG and IC at junction A?

According to Kirchhoff's current rule, the total current I entering the junction A is equal to the sum of the currents IG and IC, which gives the relationship I = IG + IC. Signup and view all the answers

What does the equation VC - VJ = IG Ro represent?

This equation represents the voltage difference across the resistance Ro due to the current IG flowing through it. Signup and view all the answers

What is the significance of the output voltage difference given by VA - VJ?

The output voltage difference VA - VJ represents the combined effect of the cell's emf E1 and the voltage drop across the resistance Ro. Signup and view all the answers

How can the polarity of E and E1 affect the value of IG?

The polarity of E and E1 can result in IG being either positive or negative depending on which electrodes are connected at junction A. Signup and view all the answers

Explain the condition under which IG would always be negative.

IG would always be negative if the negative electrode of E and the positive electrode of E1 are connected at junction A. Signup and view all the answers

What is the role of Ro in the circuit described?

Ro represents the combined resistance of the galvanometer and the internal resistance of the cell E1, affecting the overall circuit behavior. Signup and view all the answers

What is the internal resistance of a cell?

The internal resistance of a cell is the resistance to the flow of current within the cell itself, which causes a voltage drop when current flows. Signup and view all the answers

How can you determine the average value of l1 in the experiment?

Measure the length of the wire-segment between end-A and the null point three times, then compute the average of these values. Signup and view all the answers

From the equations discussed, how is IG expressed in terms of E, E1, and Ro?

IG is expressed as IG = (E(l - E1)) / (Ro(L + Rh/σ)), which showcases its dependence on the circuit parameters. Signup and view all the answers

What does a null point signify in this experiment?

The null point indicates a position along the wire where no current flows, demonstrating balance in the circuit. Signup and view all the answers

What happens to the flow direction of current IG if E is positive and E1 is negative?

If E is positive and E1 is negative, the current IG would be positive, indicating that the flow direction aligns with the assumed direction from A to C. Signup and view all the answers

What is the significance of varying R in the experiment?

Varying R allows the determination of the internal resistance of the cell at different current levels, providing insight into the cell's performance. Signup and view all the answers

What should be done before attending the lab class?

Students must read and understand the theory, complete the 'Questions on Theory,' and prepare to discuss their answers in class. Signup and view all the answers

Why is it important to draw diagrams by hand in the lab report?

Hand-drawn diagrams ensure authenticity and help the student engage more thoroughly with the material. Signup and view all the answers

What is the relationship between Vout, E, l, L, R, Rh, and σ?

Vout can be expressed as a function of the electromotive force (E), the lengths (l and L), total resistance (R), internal resistance (Rh), and resistance per unit length (σ). Signup and view all the answers

What consequences might arise from committing plagiarism in this experiment?

Plagiarism can lead to receiving zero marks for the whole experiment and significantly affect academic integrity. Signup and view all the answers

What is the significance of the length segment 'l1' in the equation for E1?

'l1' represents the length of the conductor segment between the null point and point A, which is critical for determining the voltage E1. Signup and view all the answers

How does the substitution of E2 instead of E1 affect the equation and corresponding lengths?

Substituting E2 necessitates adjusting the length to 'l2', representing the new distance from A to the null point under the new emf. Signup and view all the answers

Explain the relationship between E1, l1, E2, and l2 as derived from equations (7) and (8).

The relationship is expressed by the equation E1/l1 = E2/l2, indicating that the ratios of emf to lengths remain constant. Signup and view all the answers

What does the term 'internal resistance' refer to in the context of a real cell?

Internal resistance refers to the resistance encountered by current flowing through the cell due to its internal components. Signup and view all the answers

In the context of the potential divider, what role does the galvanometer play?

The galvanometer is used to detect the null point, which indicates when no current flows through it, allowing for precise voltage measurements. Signup and view all the answers

Describe what happens to the actual electromotive force when a resistor is connected in the circuit.

When a resistor is connected, the actual emf is divided between the external resistor and the internal resistance of the cell. Signup and view all the answers

How do the principles of the potentiometer assist in measuring internal resistance?

The potentiometer compares voltages by measuring lengths corresponding to different emfs while accounting for internal resistance. Signup and view all the answers

What physical setup is described for measuring the null point in the context of a real cell?

The setup includes connecting a real cell alongside a galvanometer within a potential divider to measure the distance to the null point. Signup and view all the answers

Study Notes

Experiment: Determination of Internal Resistance of a Cell

Theory:
- Cells have internal resistance.
- Current flows both externally through an external circuit and internally through the cell.
- Internal resistance is the opposition to current flow inside the cell.
- Potentiometer is a device used to measure potential difference and electromotive force (emf).
- The potentiometer's wire is uniform, low thermal resistance and typically made of Manganin or Constantan.
Potentiometer
- A device to measure potential difference and emf.
- A long, uniform wire with significant resistance.
- Usually has multiple segments (e.g., 10 segments of 1m).
Potential Divider
- Two resistors connected in series with a voltage source (emf).
- Voltage across each resistor is proportional to its resistance.
- Total voltage equals the sum of individual voltages across each resistor
Principle of Potential Divider
- Voltage across a resistor in a series circuit is proportional to its resistance -Kirchoff's law determines total current in a closed circuit
Procedure:
- Construct a circuit with a cell, resistor, potentiometer, and galvanometer.
- Vary the position of the potentiometer until the galvanometer shows zero current
- Measure the length of the segment on the potentiometer for various resistor values.
- Calculate the internal resistance.
Important Equations:
- V₁ = (R₁ / (R₁ + R₂)) * E
- V₂ = (R₂ / (R₁ + R₂)) * E
Measuring Internal Resistance: - Connect the cell in series with a known resistor (external resistance) - Adjust the potentiometer to find the null point (zero current in galvanometer) for both cases: - The cell connected directly (no external resistance) & - The cell with the external resistor
Internal resistance is calculated from the ratios of lengths
Apparatus:
- Potentiometer
- DC voltage source
- Cell
- Rheostat
- Two switches
- Galvanometer
- Resistance box
- Copper wire(s)
Procedure Summary:
- Construct the circuit exactly as shown in the diagrams (especially Figure 8).
- Adjust conditions to find a null point for both cases to measure lengths (l₁) and (l₂).
- Measure, record, and calculate the internal resistance.

Questions on Theory

(1) Define internal resistance.
(2) Describe how a potentiometer works to determine emf
(3) Derive an equation for internal resistance using a potential divider, considering no current in the galvanometer when zero deflection is obtained.
(4) State the properties of a good potentiometer for experimentation.

Additional Information

Ensure the potentiometer wire has minimal temperature coefficient, for accurate results
The precise apparatus and procedure mentioned in the text for the experiment should be followed strictly if detailed results are required.

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Description

This quiz assesses your understanding of the experiment on determining the internal resistance of a cell. It covers concepts related to cells, potentiometers, and potential dividers, including their functions and principles. Test your knowledge on how current flows through cells and the role of various components involved.