Kirchhoff’s Laws - Basic Electrical Engineering- EEE
Subject: Basic Electrical Engineering
Topic: Kirchhoff’s Laws
Subject Code: EEE-4201
Teacher's Name: Mohammad Mosarof Hossain
The sum of the currents entering a node equals the sum of the currents exiting a node.
Kirchhoff’s Current Law
Topic: Kirchhoff’s Laws
Subject Code: EEE-4201
Teacher's Name: Mohammad Mosarof Hossain
Kirchhoff’s Laws
KCL
(Kirchhoff’s Current Law)
The sum of the currents entering a node equals the sum of the currents exiting a node.
Kirchhoff’s Current Law
—Or
KCL for short
—Based upon conservation of charge – the algebraic sum of the charge within a system can not change.
KVL
(Kirchhoff’s Voltage Law)
The sum of the potential differences around a closed loop equals zero.
What KVL Really Means
Sum of the Voltage drops across resistors equals the Supply Voltage in a Loop.
Kirchhoff’s Voltage Law
—Based upon conservation of charge – the algebraic sum of the charge within a system can not change.
Where
N is the total number of branches connected to a node.
The sum of the potential differences around a closed loop equals zero.
What KVL Really Means
Sum of the Voltage drops across resistors equals the Supply Voltage in a Loop.
Kirchhoff’s Voltage Law
—Or
KVL for short
—Based upon conservation of energy – the algebraic sum of voltages dropped across components around a loop is zero.
Example 1
—Determine I, the current flowing out of the voltage source.
—Use KCL
—1.9 mA + 0.5 mA + I are entering the node.
—3 mA is leaving the node.
—Based upon conservation of energy – the algebraic sum of voltages dropped across components around a loop is zero.
Where
M is the total number of branches in the loop.
—Determine I, the current flowing out of the voltage source.
—Use KCL
—1.9 mA + 0.5 mA + I are entering the node.
—3 mA is leaving the node.
Example 2
—Suppose
the current through R2 was entering the node and the current through R3 was
leaving the node.
—Use KCL
—3 mA + 0.5 mA + I are entering the node.
—1.9 mA is leaving the node.
Example 3
—If
voltage drops are given instead of currents, you need to apply Ohm’s Law to
determine the current flowing through each of the resistors before you can find
the current flowing out of the voltage supply.
Example 3 (con’t)
—For
power dissipating components such as resistors, passive sign convention means
that current flows into the resistor at the terminal has the + sign on the
voltage drop and leaves out the terminal that has the – sign.
—Since R3 is a resistor, passive
convention means that the positive sign of the voltage drop will be assigned to
the end of R3 where current enters the resistor.
—As I1 is in series with R3, the direction
of current through R3 is determined by the direction of current flowing out of
the current source.
—Because I1 and R3 are in series, the
magnitude of the current flowing out of I1 must be equal to the magnitude of
the current flowing out of R3.
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