Current, Voltage, and Power

Voltage is related to potential-energy difference. The voltage drop across any circuit element is directly proportional to the change in energy of a charge as it traverses the circuit element. Specifically, 1 volt = 1 joule/coulomb. The potential energy (with respect to some reference point) is equal to the voltage multiplied by the charge.

Current refers to the motion of charges. The current through a given surface (e.g. the cross-section of a wire) is defined as the net charge passing through that surface per unit time. The unit for current is the ampere: 1 ampere = 1 coulomb/second.

The product of voltage and current has units of joules/second, otherwise known as watts.

If the voltage drop across a circuit element equals the change in potential energy per unit charge, and the current equals the amount of charge moving through the element per unit time, then their product equals the power released within the device!
 
The power dissipated within any device is given by
P = IV (2.11)

For resistive elements (or when an effective resistance can be defined), Eq. 2.11 can be combined with Ohm’s law to give:
P = IV = I 2 R = V 2 /R  (2.12)

Resistors, diodes, transistors, relays, integrated-circuit chips, etc., are rated (in part) by their maximum allowed power. Exceeding these ratings can have  disastrous effects on your circuit, and may even cause a fire! To illustrate this point, our first exercise will deliberately lead to the destruction of a carbon-film resistor.

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Capacitors (Part 2)

Capacitors are the second most commonly used component in electronics. They
can be thought of as tiny rechargeable batteries—capacitors can be charged and
discharged. The amount of charge that a capacitor can hold is measured in Far-
ads or the letter F. However, 1F is too large for capacitors, so microfarads (μF)
and picofarads (pF) are used:



micro = 1/1,000,000 and pico = 1/1,000,000,000,000
So, 100,000pF = 0.1μF = 0.0000001F

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Device Electrical Characteristics

The resistor is a linear device and is characterized by a “straight-line equation”. It dissipates power as heat, its value in ohms can vary as to the tolerance rating (ohms ± % of rated value). The resistor cannot store energy.

An inductor or capacitor is an energy storage device; a capacitor’s current or an inductor’s voltage does not change instantaneously. Initial conditions can apply to both of these devices.

The ideal capacitor has zero conductance or infinite resistance and the ideal inductor has zero resistance or infinite conductance. Ideally, neither device dissipates heat (power). The total power consumed or delivered in an RLC is presented as a complex variable (phasor) with a real (dissipated power by resistors) and imaginary (reactive power ) component.

It should be mentioned that, a capacitor’s conductance (or an inductor’s resistance) only approaches zero and the rated component value (Farads for capacitors or Henries for inductors) may also vary. These variants in addition to EMI and environmental effects would require you to alter your design or analysis somewhat, depending on how critical they are to your design or model.

The Ideal voltage vs. current characteristics for the resistor, inductor and capacitor are shown below in Table 1.

source : PDHengineer.com
Course No E-6002
First Order RLC Circuits: Time Domain
Analysis

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