# I intend to study a circuit, in which I will determine the electro motive force (emf) and total internal resistance (tir) of a power supply.

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Introduction

Hannah Proctor 3PAW

Practical assessment – Power supply.

Aim:

I intend to study a circuit, in which I will determine the electro motive force (emf) and total internal resistance (tir) of a power supply.

Diagram:

E = emf, r = tir, R = potentiometer, V = voltmeter, A = ammeter.

Results of preliminary trials:

I did some preliminary testing to determine the range over which the variables will change, here are the results…

V / volts | I / amps |

1.180 | 10.00 |

1.350 | 9.00 |

1.490 | 8.00 |

1.660 | 7.00 |

1.820 | 6.00 |

… From my preliminary tests I found that a decrease of 1A results in an increase of 0.16V on average. Thus I have decided that a decrease of 0.5A for each reading should give me enough information to calculate the emf and tir if I were to take seven readings.

Method:

I will set up my equipment as shown and as I vary the current on the potentiometer, and read the voltage from the digital voltmeter (accurate to 100th of a volt) I have chosen to use this degree of accuracy because my preliminary tests show accuracy to 2 decimal place will be most appropriate. I will vary A in steps of 0.5A and thus I will start at 10A, 9.5A, 9A etc until 7A

Middle

To ensure the utmost safety while conducting my experiments I will not run excessive energy through the circuit, I will try not to let other students or teachers near my circuit to make sure it is entirely my responsibility. Also I won’t be passing enough volts or amps to short out the classroom or electrocute myself.

Proposed Analysis:

My research has told me that the graph I will plot using my results will show indirect proportionality between the voltage and the current.

Observations:

Here are the results obtained from my experiment together with the mean average.

Current / A | 1. Voltage / V | 2. Voltage / V | 3. Voltage / V | Average / V |

0.060 | 1.840 | 1.840 | 1.840 | 1.840 |

0.070 | 1.690 | 1.690 | 1.680 | 1.690 |

0.080 | 1.540 | 1.540 | 1.530 | 1.540 |

0.090 | 1.380 | 1.370 | 1.360 | 1.370 |

0.100 | 1.210 | 1.220 | 1.220 | 1.220 |

0.110 | 1.140 | 1.120 | 1.100 | 1.120 |

Conclusion

I have mentioned that I took the mean of three results, for example for 0.9A I got the following results 1.38V, 1.37V, and 1.36V. These average out at 1.37V. I took the mean of a set of results because if I only took one reading I may gather some very un-reliable information and thus be misled as to the actual answer.

I drew two extra lines on my graph they indicate the line of most squares and that of the least squares; these show the uncertainty in my results. It shows almost 0.5V worth of error.

Conclusion:

In conclusion to this I can say that the Total Internal Resistance is 15.625Ω. The Electro Motive Force is 2.65V. The maximum power that could be derived is 0.11W.

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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