电路原理课件讲义英文版 Chapter_2.ppt

Chapter 2 Basic Laws
Introduction
Ohm’s Laws
Nodes, Branches, and Loops
Kirchhoff’s Laws
Series Resisters and Voltage Division
Parallel Resisters and Current Division
Wye-Delta Transformations
Summary
Introduction
To actually determine the values of these variables in a given circuit requires that we understand some fundamental laws that govern electric circuits.
Basic Laws:
Ohm’s Law
Kirchhoff’s Law
Ohm’s Laws
Resistance: the physical property (or ability) to resist current
Resistor: the circuit element used to model the current-resisting behavior of a material
(Simplest passive element)
Ohm defined the constant of proportionality for a resistor to be the resistance, R
Ohm’s Law:
Ohm’s Law states that the voltage v across a resistor is directly proportional to the current i flowing through the resistor
The resistance R of an element denotes its ability to resist the flow of electric current, measured in ohms( )
Two extreme possible case:
Short Circuit: A short circuit is a circuit element with resistance approaching zero.
Open Circuit: An open circuit is a circuit element with resistance approaching infinity
Passive sign convention:
If not conform with it:
Fixed: its resistance remains constant.
(wire-wound type; carbon film type)
Variable: variable resistors have adjustable resistance.
(composition type; slider pot)
Resistor:
Potentiometer
In general
Linear resistor: obey Ohm’s Law.
Nonlinear resistor: does not obey Ohm’s Law.
Conductance is the ability of an element to conduct electric current, measured in siemens (S).
Power representation of a resistor:
The power dissipated in a resistor is a nonlinear function of either current or voltage
Since R and G are positive quantities, the power dissipated in a resistor is always positive. Thus, a resistor always absorbs power from the circuit. This confirms the idea that a resistor is a passive element, incapable of generating energy.
Notice: