ME 140L - Prelab 3


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PRELAB 3 RESISTOR, CAPACITOR AND INDUCTOR CONSTITUTIVE RELATIONS FILTERS 1. OBJECTIVES ± Review the constitutive laws of basic analog components such as resistors, capacitors and inductors. ± Understand how passive elements can be arranged to build different type of filters. 2. NOTATIONS C : Capacitance (Farads) I : Electrical current (amperes) Q : Electrical charge (Coulombs) R : Resistance (Ohms) V : Voltage (Volts) L : Inductance (Henries) T : Time (seconds) 3. BASIC ELEMENTS CONSTITUTIVE RELATIONS 3.1 RESISTOR Resistors are electrical circuit elements that dissipate energy. The voltage across a resistor is always proportional to the current. This relation is also known as the Ohm’s Law and is expressed with the following equation: IR V = This equation means that for a resistor, the maximum value of the voltage is the current times the maximum current through the resistor. The unit of the resistance is Ohms (Volts/Amperes) 3.2 CAPACITOR Capacitors are passive components that store energy in the form of an electrostatic field. The unit of capacitance is the farad (Coulomb/volt). Practical capacitor values are between 1 pF (1 x 10 -12 Farads) to 1 μ F (1 x 10 -6 Farads). The voltage across a capacitor depends on the amount of electric charge stored in the plates. The constitutive relation of a capacitor is expressed by the following equation: C q V = Recall that the charge is the integral of the current in a period of time
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= = Idt C C q V 1 Also dt dV C I = Physically, capacitors are two metal plates separated by a dielectric material; and that is the way capacitors are represented in an electrical circuit. When a voltage is applied between the plates, charge is stored in the capacitor. The amount of charge stored in the plates depends on the capacitance; the larger the capacitance, the higher the charge stored. When the voltage applied is suspended, the charge in the capacitor can be used to temporarily power a circuit. Capacitors are used in delay, timing, filtering or switching circuits. Capacitors can be either polarized or un- polarized. Capacitors with a small capacitance are usually un-polarized and can be connected in any way.
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This note was uploaded on 09/18/2011 for the course ME 140L taught by Professor Staff during the Fall '09 term at University of Texas.

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