C2 Investigating the Capacitance of a Capacitor

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Title : C2 Investigating the capacitance of a capacitor

I. Objective : * To investigate the factor which affect the capacitance of a parallel-plate capacitor using a reed switch.

II. Apparatus * Reed Switch * Signal generator * Capacitor Plates (1 pair) * Polythene spacers * Low voltage power supply * Voltmeter * Variable resistance * Light-beam galvanometer * CRO * Standard mass (eg. 100g ) * Connecting leads * Drawing board * Vernier calipers for measuring thickness of spacer
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III.
Theory

A capacitor is a device which can store charge. It consists of two sheets of conductor separated by a layer of insulator called dielectric. The capacitance of a capacitor is defined as the charge stored per unit potential difference applied to the capacitor :

In the experiment, a reed switch allows the capacitor to be charged up and discharged rapidly : The reed switch consists of three small metal strips (F,N and K) inside a glass capsule which is surrounded by a coil. F and K are made of steel but N is non-magnetizable material. In normal condition, K (called the reed) is in contact with N. When a current passes through the coil, F and K become oppositely magnetized so that K is attracted towards F and makes contact with it. When the current falls, K springs back into contact with N.

If the frequency at which the reed switch is operated is f, the charging and discharging process will be repeated f times per second, and the charge Q on the capacitor is delivered to the microammeter at the same rate. The capacitor is fully charged up and discharged every time, the total quantity of charge passes through the microammeter in a second is : Q=CVf This gives the size of theoretical current I. The

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