Ph272- General Physics II (Electricity and Magnetsism)

Lecture 11-12: Ch. 26 - Resistor Circuits

Key Topics

Overview

• Resistors in series: increases resistance.
• Resistors in parallel: allows more current to go througth the circuit => lower resistance.
• Kirchoff's Rules: Use to analyse more complicated circuits.

Resistors in series

• Concept (qualitative) : The current accounters more resistance so the equivalent resistance is greater.
• Quantative:

The emf from the battery is shared by the resistor in series: EMF = V1 + V2 + V3 +...

Same current going through all the resistors, so V1 = IR1 and V2=IR2, etc

Combining these two eqautions give the equivalent resistance, Eq. 22-19.

 

Resistors in parallel

• Concept (qualitative) : The current can go through all branches of the circuit => more current and less resistance.
• Quantative: V1 = V2 = V3 .. ; I total = I1 + I2 + I3 + ...
• Combining theses two equations gives the equivalent resistance, Eq. 22-23.

For circuits with both series and parallel parts (but only one battery), the general rule is to combine the parallel resistance first.

Kirchoff's Rule

• A circuit consists of more than one battery requires the use of Kirchoff's Rules.
• A complicated circuit is made up of individual circuits and junction points
• Kirchoff's Rules (two of them) are based on two simple concepts:

(1) At a junction: current flowing in = current flowing out (principle: conservation of charge)

(2) The algebraic sum of all the voltage drop along a circuit = zero. (The sign of the voltage drop obeys a convention. Discuss this in class)

• Apply Kirchoff's Rules to a simple series circuit and a simple parallel circuit to see if you get the previous results.
• Apply Kirchoff's Rules to more complicated circuits: Do Problems 23-6, 23-7, P. 773-774.

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