2LB_Lab_6_Capacitors8 - Capacitors(Contains Pre Laboratory...

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Capacitors (Contains Pre Laboratory Assignment) Objective: To measure (i) the properties of a single capacitor. (ii) the properties of series and parallel capacitors. (iii) the properties of a capacitor containing a dielectric. Apparatus The apparatus used includes: Pasco variable capacitor apparatus Regulated voltage supply (0-30 VDC) Electrometer voltmeter with low capacitance cables 330 pF ( ± 2%) capacitor Computer with Data Studio software Capacito Plates r Figure 1: Experimental setup. Certain parts of this experiment are sensitive to stray electrostatic charge. Avoid wearing clothing that contains polyester. Instead, wear fabrics that contain cotton. Red Banana Plug with Capacitor Voltage source Electrometer
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The Physics Capacitor A capacitor is an electrical device that stores charge. The basic unit consists of two metal plates which are parallel to each other (see the circular plates in figure 1 above). One of the plates can be charged positive and the other can be charged negative by connecting to the appropriate terminals of a voltage source such as a battery. Note that each plate has only one kind of excess charge either positive or negative. In the case of the positive plate, the positive charges have to be squeezed together against the repulsive forces between the neighboring positive chages. This means that somebody or something did work against the repulsive force to bring the charges together. In the case of a battery, it is work comes from the conversion of the chemical energy in the battery. This work done is stored as potential energy of the capacitor. It can be recovered as kinetic energy of moving charges (current) when the capacitor is discharged. Thus, the capacitor by storing charges, actually stores energy. The RAM in your computer is essentially a capacitor, where a charged capacitor corresponds to the bit ‘1’ and the discharged capacitor to the the bit ‘0’. The standard picture of a capacitor is two parallel plate electrodes on which the charge are stored. Such a capacitor has a capacitance C = κε 0 A d (1) where A is the plate area, d the plate separation, κ is the dielectric constant of the material between the plates, and ε 0 = 8.85 picoFarad/meter (pF/m). This week we will make some basic measurements on a capacitor with variable d . The apparatus will also be used to measure κ for a sheet of plastic. Electrometer Capacitance The electrometer is special kind of voltmeter for measuring voltages without reducing the charge on the capacitor. It has a large impedance (primarily a large resistance) which will not seriously change the properties of the circuit under test. The high input resistance is necessary to minimize its impact on DC circuits. There is also an input capacitance that cannot be ignored in this lab exercise. The potential difference across an external capacitor will be measured using the electrometer.
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This note was uploaded on 01/11/2010 for the course PHYS 2b taught by Professor Clare during the Fall '07 term at UC Riverside.

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2LB_Lab_6_Capacitors8 - Capacitors(Contains Pre Laboratory...

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