Handout 7

• Kirchhoff's Laws


  Describe current in a node and voltage around a loop
• Kirchhoff's Laws are the foundation of advanced circuit analysis
• Node


  A junction where 2 or more elements connect
• Lines on a schematic represent perfect zero-resistance conductors, there is no rule that says lines from multiple elements are required to meet in a single point junction
• Distributed node


  A node represented as a distributed connection rather than a single point
• Branch


  Connections between nodes, an element (resistor, capacitor, source, etc.)
• The number of branches in a circuit is equal to the number of elements
• Loop


  Any closed path going through circuit elements
• A loop can visit (pass through) a node only one time
• Currents into a node

  1. Currents flowing into the node find some way to flow out on another branch

  2. We don't expect flowing charge to pile up inside the node
• Kirchhoff's Current Law


  The sum of all currents flowing into a node equals the sum of currents flowing out of the node
• Solving a circuit using Ohm's Law


  1. Compute the current through the series circuit

  2. Compute the voltage across each resistor
• The sum of voltages across the resistors equals the source voltage
• Kirchhoff's Voltage Law


  The sum of voltages around a loop is zero
• Kirchhoff's Voltage Law holds true for any loop in a circuit, regardless of the starting point or direction
• Gustav Kirchhoff - German Physicist who is known for Kirchhoff's Laws
• Kirchhoff's Law for current and voltage lie at the heart of circuit analysis.
• Kirchhoff's Current Law - the sum of all currents flowing into a node equals the sum of currents flowing out of the node.
• Kirchhoff's Current Law

  
• mA - SI unit for Kirchhoff's Current Law
• Kirchhoff's Voltage Law - The sum of voltages around the loop is zero.
• n - counts the element voltages around the loop

  A) n

  
• Kirchhoff's Voltage Law - The sum of voltage rises equals the sum of voltage drops around the loop