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)/R  [+ (absorbing)]

Kirchoffs law:

•Kirchhoff’s 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

•Kirchhoff’s 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)

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

–Power:  P = 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.