Experiment to find the emf (E) and the internal resistance ...



Experiment to find the emf (E) and the internal resistance (r) of a cell

Apparatus : 1.5V (approx) cell, Resistance box, Push switch, Digital Ammeter (0-1A).

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Method :

1. Check the scale on the digital ammeter by comparing to other ammeters.

2. Set R at 10 Ohms. Reduce in steps of 1 Ohm, recording resistance and current. Read the Ammeter as accurately as possible. Release switch (not a tap switch) after each reading, otherwise the cell will run down during the experiment.

3. Repeat the readings, increasing R back up to 10 Ohms. Obtain average values of I, the current for each value of the resistance.

4. Calculate 1/I.

5. Plot a graph of R against 1/I. Draw the best possible straight line through the points (they might be quite scattered) in Excel, put the equation of the line on the graph.

6. The gradient of this graph is the emf of the cell. The negative intercept on the y-axis is the internal resistance.

Theory:

The emf of a cell is the total p.d. it can produce around the circuit, including any potential wasted in driving current through the cell itself.

The internal resistance of a cell is simply the resistance from one terminal of the cell to the other.

since V = IR

then E = I (R+r)

so E/I = R + r

Hence: R = E(1/I) - r

Therefore R plotted against 1/I has gradient E and intercept -r .

Report:

This is to be written as a formal report. The theory section should be expanded to include the origin of internal resistance, a discussion of power delivered by a battery, the definition of emf and other relevant concepts. Include a circuit diagram. Of course include graphs, tables and the results obtained from the graph.

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