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# Using a search coil and CRO to investigate the magnetic fields generated by alternating currents (A.C.) through a straight wire and a slinky solenoid.

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Introduction

Name: Pyrrhus Choi Tsz Kiu                               Date: 01-03-2010
Class: 6S    Class no.: 8                                 Marks:     /10

C15. Investigation of Magnetic Fields by Search Coil (Brief Report)

In the report, all the number will be taken in exact or 3 sig. fig.

Objective: Using a search coil and CRO to investigate the magnetic fields generated by alternating currents (A.C.) through a straight wire and a slinky solenoid.

Part A. Magnetic field around straight wire
Measuring the magnetic field generated 2cm away from the centre of the straight wire by search coil.
Result Table of the Induced E.M.

Middle

From the above V-t graph, the curve is a straight line passes through origin. Hence, the potential difference across the capacitor is directly proportional to the time for the CRO trace to rise in steps of 1V and the slope ( ) which represented the charging rate, is constant.
Relationship between the charge stored in the capacitor and the potential different across it when the charging current is constant

The area of the graph (Vt) represented the charge stored in the capacitor. As the rate of change of potential, it implies the charge stored in the capacitor increase with potential, thus the charge stored in the capacitor directly proportional to the potential difference across the capacitor when the charging current is a constant.

Conclusion

2. Systematic Error
In the experiment, we need to keep the charging current constant as I, i.e.80μA by hand. However, the rate of the decrease of the current is not constant. When the potential difference across the capacitor becomes larger, the rate of the decrease of the current will increase too. It is difficult to adjust the current constant as I.

Reaction time
We mark the time taken of the CRO trace to rise in steps of 1V. If the reaction time of the timer is large, the measurement will be less accurate. And the average reaction time of human is 0.02s
Hence,
Maximum percentage error of the measurement of time: × 100% = 0.227%
Improvement
Both of them can be improved by practicing the task in the experiment.

-End of report-

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