# To prove ohms' law, and to study the relationship between current and potential difference (voltage).

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Introduction

12/3/2001Ohm’s LawManaf Khatib

Aim:To prove ohms’ law, and to study the relationship between current and potential difference (voltage).

Hypothesis: The current flowing through a metal wire is proportional to the potential difference across it (providing the temperature is constant).

Apparatus:

- Power Supply
- Variable resistor
- Ammeter
- Voltmeter
- Resistors
- Electric wires

## Diagrams:

R1: R2:

R1 + R2 (in series): R1 + R2 (in parallel):

Method:

- Set up the apparatus 4 times as shown in the diagrams above
- Set the power supply to 6v
- Set the variable resistor to maximum resistance
- Switch on the power and note the ammeter and voltmeter readings
- Reduce the resistance of the variable resistor
- Repeat steps 4, and 5 until at least five 8 sets of readings have been obtained
- Change

Middle

3.99

- Table of results for two resistors in series of 10Ω + 12 Ω = 22Ω

Current (A) | Voltage (v) |

0.14 | 3.10 |

0.15 | 3.27 |

0.16 | 3.43 |

0.17 | 3.65 |

0.18 | 3.92 |

0.19 | 4.20 |

0.20 | 4.35 |

- Table of results of two resistors of 10 Ω and 12 Ω in parallel

1/Rtot. = 1/R1 + 1/R2

= 5.5Ω

Current (A) | Voltage (v) |

0.24 | 1.30 |

0.26 | 1.38 |

0.28 | 1.50 |

0.30 | 1.59 |

0.32 | 1.71 |

0.36 | 1.80 |

0.42 | 2.19 |

0.73 | 3.85 |

- Refer to graph paper for the following Data:

The resistance can be measured by measuring the gradient of the slope, by using the following formula:

y2 - y1/x2 - x1

- Resistance in x: y2 - y1/x2 - x1 = 4.4 - 0 / 0.36 - 0

= 12.2Ω

- Resistance in y: y2 - y1/x2 - x1 = 4 – 0 / 0.39 – 0

= 10.3Ω

- Resistance in z:y2 - y1/x2 - x1 = 2.2 – 0 / 0.42 – 0

Conclusion

1/R = 1/R1 + 1/R2 or R = R1 X R2/ R1 + R2

- The current through the circuit is proportional to the potential difference (voltage) across it, providing the temperature remains constant.

### I V

- Resistance can be measured by:

- Dividing the potential difference across a wire, over the current through the wire.
- Measuring the gradient of the slope, using y2 - y1/x2 - x1

- A variable resistor or rheostat is used to vary the current in a circuit. As the sliding contact moves, it varies the length of the wire in the circuit.

Evaluation:

I can say that the experiment was accurate, as the resistances calculated from the graph where equal to the actual resistance across the circuits, but there where some errors that could have been avoided in order to obtain more accurate results, and here are some of the sources of errors, and improvements:

Sources of Error | Improvements |

The voltmeter, and ammeter’s readings where fluctuating, where I had to choose one of the readings | Try to take the average reading i.e. if the readings where fluctuating between three numbers, choose the medium number |

From one experiment to the other, the wires, and the rheostat’s temperatures increased, thus the resistance increased | Try to use separate wires and a rheostat in each experiment. |

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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