Capacitance

Uploaded by mary34d on Oct 24, 2007

Introduction
Chapter 3, “Capacitance,” contains laboratory experiments designed to explore the relationship between voltage and the amount of charge stored in an object. These experiments involve measuring electrical properties of capacitors in series, in parallel, while charging, discharging, and at varying widths between the surfaces. Hands-on experience and resulting data should provide insight into the nature of capacitance.

Theory
In order to charge an object, a certain amount of energy is required to transfer charge to that object. The energy per unit of charge is called voltage. Given a certain voltage, charge can be transferred to an object until the amount of energy that is required to add more charge exceeds the energy potential. A derived unit is useful for expressing the capacity of charge (in Coulombs) that can be transferred to an object per unit of voltage (in Volts). Therefore, a unit of capacitance called the Farad exists, and is defined as C = Q/V.

A capacitor comprised of two parallel surfaces will have a capacitance equal to

8.85 ρF/m, times the area of one of the plates, divided by the distance between them. When sharing the charge applied to one capacitor with a second capacitor, charge is conserved, therefore Vf * (C1 + C2) = Vi * C1. When discharging a capacitor through a resistor, V(t) = V0 * e-t/RC. When charging a capacitor through a resistor, V(t) = Vf – Vf * e-t/RC.

Experiments
3.5.2: Charging a Capacitor
This experiment required a 9V battery, a voltmeter, and voltage a follower that were assembled in this way: The battery and voltage follower ground contacts were connected to the volt meter ground, while the voltage follower output was connected to the positive terminal of the volt meter. To measure the voltage across the 0.033 μF capacitor, I connected one end of the capacitor to the positive lead from the voltage follower, and connected the other end to the ground.

To charge the capacitor, I touched the positive probe from the 9V battery to the ungrounded side of the capacitor. The voltmeter displayed 8.90 V after removing the battery probe. Therefore, the charge on the capacitor was 0.033 μF * 8.90 V = 0.294 μC.

3.6.1: Measuring Unknown Capacitance
This experiment was the same as the previous one, except that after the capacitor was charged, a second one was connected to it in parallel. When...