# In this experiment, we will measure the e.m.f. and the internal resistance of a dry cell.

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Introduction

Experiment 2A: Centre of gravity of a body(irregular shape only)

Objectives:

To determine the center of gravity of a body of different shapes

## Experimental Design

Apparatus:

Name of apparatus | Number | Remarks |

Dry cell | 2 | 1.5 V |

d.c. Voltmeter (0-5 V) | 1 | Product code: 83040152 |

d.c. Ammeter (0 –1 A) | 1 | |

Switch | 1 | A plug key |

Rheostat | 1 | Enhance the result more significantly |

Connecting wire | 6 |

Electronic Diagram

Description of design:

In this experiment, we will measure the e.m.f. and the internal resistance of a dry cell. In order to investigate the objective of the experiment, we should connect the apparatus as the above electric diagram. The voltmeter should be connected in parallel circuit while the ammeter should be connected in series circuit, otherwise, it may cause the inaccurate reading of the meters. Besides, we investigate the terminal potential difference V varies with the current I, hence we find out the internal resistance and the e.m.f by plotting the voltage – current graph. By vary the resistance of the rheostat R, the current I also varies. The terminal potential difference V across the dry cell is given by V = – Ir.

Theory:

Electromotive Force (e.m.f) of a dry cell is the amount of electrical potential energy gained by a coulomb of charge which passes through the dry cell. Simply, A Voltage where the charge is gaining energy is an electromotive force.

Middle

## Secondly, the rheostat should be set to its maximum value in the beginning of the experiment, so that the current is the lowest at first, then increase gradually. As high current produces heat, it would increase the resistance of the connecting wires and the internal resistance. The current through the wire will heat up the wire and lead to increase resistance of the apparatus. It may lead to the inaccurate and imprecise data obtained and hence the inaccurate calculated value of the e.m.f and internal resistance in the dry cell. Therefore, we should not leave the circuit connected longer than necessary to take the readings.

## In addition, we should ensure the ammeter and voltmeter are connected to the cell with suitable way (positive terminal to the direction of positive terminal and negative terminal to the direction of negative terminal). Hence the ways of connection of ammeter and voltmeter also should be in correct ways (voltmeter in parallel while ammeter in series). Otherwise, the pointer will deflect to the opposite direction. The ammeter and voltmeter may be damaged.

## Results & Calculations

The values of e.m.f and the internal resistance of the dry cell used in the experiment are calculated in both mathematical and graphical method:

For mathematical method,

Two dry cells are used:

V/V± 0.10 | 1.40 | 1.30 | 1.10 | 1.00 | 0.90 | 0.80 | 0.60 | 0.50 | 0.20 |

I/A± 0.01 | 0.16 | 0.18 | 0.20 | 0.22 | 0.24 | 0.26 | 0.28 | 0.30 | 0.32 |

V

Conclusion

Conclusion

By both mathematical and graphical methods, we can find out the e.m.f and the internal resistance of one dry cell.

For mathematical method, the e.m.f and internal resistance of the cell are 1.3 V ± 0.1 V and 0.95 Ω ± 0.87 Ω. respectively. For graphical method, the e.m.f and internal resistance of the cell are 1.29 V ± 0.04 V and 0.7 Ω ± 0.3 Ω respectively. Either the e.m.f from the mathematical method or the one from the graphical method are not equal to the value stated on the experiment menu, 1.5V. It may result from the consumption of chemical energy in the dry cell as the dry cell may be used for a period of time. In addition, they are closed to each other; it means that the results calculated are precise.

However, internal resistances calculated from both methods are not closed to each other and there are many experimental errors in the experiment. To obtain a more accurate and precise result, we should have more preparations and minimize the experimental errors as much as possible. For further investigation, we can find out the effect of heating on the internal resistance of the apparatus especially the dry cell.

References

Wikipedia (internal resistance)

http://en.wikipedia.org/wiki/internal_resistance

New Way Physics for advanced level Fields, Electricity and Electromagnetism P.125 – 126 for better understanding of internal resistance.

This student written piece of work is one of many that can be found in our AS and A Level Electrical & Thermal Physics section.

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