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# Investigate methods of finding and comparing the e.m.f and internal resistance of different cells and power supplies and factors that can affect these values.

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Introduction

Comparson of the EMF of Cells and Power supplies Investigate methods of finding and comparing the e.m.f and internal resistance of different cells and power supplies and factors that can affect these values. Hypothesis 'For conductors at constant temperature, the current is proportional to the voltage across it', Ohm's law. From this it can be said that an increase in resistance should decrease the current proportionally to the voltage. The internal resistance is the gradient from the variation of voltage against current. The power source with the greater voltage will have a greater e.m.f, because e.m.f is defined as the energy transferred to a charge, which is the same definition given to potential difference. energy transferred E.m.f = charge = current (external resistance + internal resistance) Larger cells or power supplies should have greater internal resistance. This is because internal resistance is the resistance created by the chemical reactions that occur within the cells or power supplies. The larger sources will have more chemicals within it. Therefore more chemical reaction should take place. Diagram Power source Ammeter Voltmeter Variable resistor (Resistor Box) Method 1. Set up the equipment as shown above, use a milliammeter. 2. ...read more.

Middle

Source Electromotive force/V Internal resistance/? Mitsubishi R20 1.5V 1.364 1.9 Sony R6 1.5V 1.463 3.5 Duracell R6 1.5V 1.482 4.4 Four way rectified output power supply 2V 3.355 5.0 Analysis of results According to the result, the four-way rectified output power supply gave the largest e.m.f and internal resistance, and the Mitsubishi R20 gave the smallest values. This proves that the source with the largest voltage gave the largest e.m.f and largest internal resistance. However, it does not prove that the internal resistance is due to chemical reaction and amount or not, because the Mitsubishi R20 had more chemical content than the Sony R6 or the Duracell R6. Therefore it disproves the theory that internal resistance is greater in sources with greater chemical content, although chemical reaction does affect internal resistance as it provides heat that is also internal resistance. Errors and improvements The cells and supply used should have had an initial voltage of the same value to make the test much fairer. This would mean that the cells would have similar starting voltage of about 1.5V rather than a varied range. ...read more.

Conclusion

Evaluation The results of the experiment could not only be inaccurate, but can also prove to be inconclusive, as there was a great deal of errors that could cause any number of faults in the experiment. The errors include: 1. Initial voltage not equal to 1.5V. 2. The time period for which a source has been turned on or off for. 3. The temperature of the source at different intervals in time. 4. The age of a source (also affects the output voltage). The faults that could occur are: 1. Imprecise measurements of the terminal p.d and the current. Inaccurate values will give imprecise results for the e.m.f and internal resistance. 2. High temperatures can cause a resistance that would decrease the current. However, these errors and faults would give slightly imprecise results, they would have little effect on the outcome of the experiment. This means that the experiment should still be considered a valid success. Overall the experiment is correct and does prove that the greater the supply voltage, the greater e.m.f and internal resistance. Yet, it neither proves nor disproves the case of greater chemical content giving greater internal resistance. Name: Mohammed Husnat 1 Candidate number: 9834 ...read more.

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