Electric Circuits Fundamentals

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A circuit consists of electrical elements connected together in a closed path so that an electric current may flow continuously.

Electric Circuits

-Introduction-

Current

Time rate of flow of charge past a given point

Charge

Quantity of electricity responsible for electric phenomena

i = dq/dt

Units: Ampere (A) : 1 coulomb per second

q

Expressed in coulombs

Conventionally represented as the flow of positive charges. Complete description of current reqiures a value (negative or positive) and a direction.

Direct current (dc) = current of constant magnitude

Voltage

The work (energy) required to move a unit positive charge from the - terminal to the + terminal.

v= dw/dq

Power and Energy

Power: time rate of supplying or receiving power

p= dw/dt

Units: Watts (w)

Unit: Volts (v)

pi= v * i

When current enters the + terminal and exits at the - terminal of voltage, voltage and current adhere to passive sign convention

Passive sign convention: power received (absorbed).

Not: power supplied.

Circuit elements

A linear element satisfies the properties of both super position and homogeneity

if v1 is caused by i1

and v2 is caused by i2

then i1+i2 results in v1+v2

Multiplying input by a constant = multiplying out put by a constant

Passive element = absorbs energy

Active element = supplies energy

Resistors

Ability to resist the flow of charge= resistivity, p

Resistance: property of an element that impedes the flow of current, represented by R

Ohms Law: v=Ri

R= Ohms= Ω = V/A

p = vi = v^2 / R = i^2 * R

Independent Sources

Source = voltage or current generator capable of supplying energy to a circuit

Independent = doesn't depend on other circuit variables

Voltmeter: measures voltage.

Ideal: measures voltage from red to black probe ( + to -) and has terminal current = 0

Ammeter: measures current.

Ideal: measures current through terminals and has 0 voltage.

= Open circuit

= Short circuit

Dependent Sources

Model the situation in which the voltage or current of one circuit element is proportional to the voltage or current of the second circuit element.

Represented by a diamond shape.

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