Saturday 22 December 2012

Ohm's Law and kirchoff's law


Ohms law:
v(t) = i(t) R  - or -  V = I R
p(t) = i2(t) R = v2(t)/[+ (absorbing)]
Kirchoffs law:

Kirchhoffs Current Law (KCL)

sum of all currents entering a node is zero

sum of currents entering node is equal to sum of currents leaving node


Kirchhoffs Voltage Law (KVL)
sum of voltages around any loop in a circuit is zero
Kirchoffs Current Law :

The sum of currents entering the node is zero:

Analogy: mass flow at pipe junction


KVL Polarity
A loop is any closed path through a circuit in which no node is encountered more than once
Voltage Polarity Convention
A voltage encountered + to - is positive
A voltage encountered - to + is negative


Electrical Analogies (Physical)






Thursday 20 December 2012

Basic Electrical Quantities


Basic quantities: current, voltage and power
Current: time rate of change of electric charge
  I = dq/dt 
1 Amp = 1 Coulomb/sec
Voltage:  electromotive force or potential, V 
1 Volt = 1 Joule/Coulomb = 1 N·m/coulomb
PowerP = I V 
1 Watt = 1 Volt·Amp = 1 Joule/sec

Active elements can generate energy
–Batteries
–Voltage and current sources
Passive elements cannot generate energy
–Resistors
–Capacitors and Inductors (but CAN store energy)
An independent source (voltage or current) may be DC (constant) or time-varying, but does not depend on other voltages or currents in the circuit.
The dependent source magnitude is a function of another voltage or current in the circuit.