Lab Report 4 for Phy

P221 Lab #4 NAN HUA

Title: Capacitors

Introduction

We are all familiar with batteries as a source of electrical energy. We know that when a battery is connected to a fixed load (a light bulb, for example), charge flows between its terminals. Under normal operation, the battery provides a constant current through out its life. Furthermore, the voltage across its terminal will not vary appreciably - and when it does, it is an indication that the battery needs replacement. Capacitors are devices in which electric charges can be stored. In fact, any object in which electrons can be stripped and separated acts as a capacitor. Practical capacitors are made of two conducting surfaces separated by an insulating layer, called a dielectric. The symbol used to represent capacitors in schematics reflects their physical construction with electrolytic

capacitor (which must be hooked up with the correct polarization) represented by a curved plate for its negative terminal. There are many different types of capacitors: tubular, mica, variable, and electrolytic to name a few. Capacitors are used in timing circuits in many devices. The time that your dome lights inside your car stay on after you turn off your car’s ignition at night is one example of how a capacitor can be used to maintain the lighting long enough for you to remove the keys and collect your things before exiting. The value we use to characterize these kinds of circuits is given by the time constant defined as:

τ τ= RC (1)

where R is the circuit resistance (your dome light in this case) and C is the capacitance, in Farads (F). A Farad is the unit of measurement used when describing capacitors. It is ridiculously large. So large, in fact, that most capacitance measurementsuse microFarads (μF), nano (nF), and picoFarads (pF) as their unit of measure. The development of the equations describing the properties of capacitors in series and in...