Lecture 2

2.1 Sources2.2 Ohm’s Law

2.4 Kirchhoff’s Laws

Voltage and Current Sources

Independent Sources

Characteristics

• An independent source establishes a voltage or current in a circuit without relying on voltages and currents elsewhere in the circuit.

• The value of the voltage or current supplied is specified by the value of the independent source alone.

Exercise

Dependent Sources

Characteristics

• Dependent sources establish a voltage or current whose value depends on the value of a voltage or current else where in the circuit.

• You cannot specify the value of a dependent source unless you know the value of the voltage or current on which it depends.

Exercise

Exercise

What value of Vg is required for the wiring to be valid?

Ohm’s Law

Analogy

• The height of the water=voltage• The volume of flow through the hole per second=current• The smallness of the hole = resistance• Water wheel hit by the flow from the hole = power

Thought Experiment #1

Fixed hole dimension↔ Fixed RHeight of water↑ → flow rate↑.V↑ → I↑

Thought Experiment #2

Fixed height of water↔ Fixed VHole dimension↑ → flow rate↑R↓ → I↑

Derivation of Ohm’s Law• Three variables: V,I and R.• Different Possibilities:

– V=IR (1)– I=VR (2)– R=VI (3)

• Intuition – If R is constant, V↑ → I↑.

• (3) is not possible. (1) and (2) are possible.– If V is constant, R↓ → I↑.

• (2) and (3) are not possible

Ohm’s Law

• V=IR• Mnemonic: Victory Is Rare

Exercise

• Calculate the value va• Determine the power dissipated in the resistor

Exercise

Calculate id

Kirchhoff’s Laws

KCL

• Kirchhoff’s Current law (KCL)–The algebraic sum of all the currents at

any node in a circuit equals zero

Exercise

Sum the currents at each node

Multisim Demo

KVL

• Kirchhoff’s Voltage Law–The algebraic sum of all the voltage

around any closed path in a circuit equals zero

Exercise

Sum the voltages around each designated path in the circuit

Example 2.8

1.KCL at b2.KVL around cabc loop3.Solve for io and i1

Problem 2.18

1.KCL at the top node2.KVL around the right loop3.Find i1, i2, vo

Problem 2.21

Find i1 if i0 is 1A.