Finding the EMF & Internal Resistance of a Cell

Finding EMF & Internal Resistance of a Cell

Experimental Setup

• To determine the EMF and internal resistance of a cell, an experimental circuit is set up.
• The circuit includes a variable resistor, an ammeter, a fixed resistor, a battery (cell), and a voltmeter connected across the battery terminals and the fixed resistor.

Data Collection

• The variable resistor is adjusted, and readings of potential differences (voltages) across the cell's terminals and the current passing through the cell are recorded.
• This data is used to create a graph of terminal potential difference (V) against current (I).

Graph Analysis

• The resulting graph is a straight line that slopes diagonally down from the upper left to the lower right.
• Key points on the graph provide valuable information.

Open Circuit Potential (EMF)

• At the point where the line meets the terminal potential difference (V) axis (y-axis intercept), there is no current drawn from the cell. This point is called the open circuit potential.
• Since no current flows, there are no lost volts at this point, making it the EMF of the cell.

Short Circuit Current (I_short)

• At the point where the line meets the current (I) axis (x-axis intercept), the maximum current is drawn from the cell.
• Achieving this maximum current involves short-circuiting the cell, which should be avoided due to safety concerns.
• The maximum current is known as the short circuit current.

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Finding the EMF & Internal Resistance of a Cell

Calculating Internal Resistance (r)

• The gradient (slope) of the line on the graph is calculated; it has a negative value.
• The internal resistance of the cell is the same value as the negative slope but without the negative sign.
• For instance, if the slope of the line is -2.0/A, then the internal resistance is 2 Ω.

Alternative Method for Internal Resistance

• Internal resistance (r) can also be found by dividing the EMF (E) by the short circuit current (I_short):
  • r = E / I_short

In Summary

• To determine the EMF and internal resistance of a cell, an experimental circuit is used.
• Data collected from the circuit is used to create a graph of terminal potential difference against current.
• The point where the line meets the Y-axis represents the EMF.
• The point where the line meets the I-axis represents the short circuit current.
• Internal resistance is calculated from the graph's slope or by dividing EMF by short circuit current.