DC Circuits AP Physics Chapter 18

DC Circuits 19.1 EMF and Terminal Voltage

The Electric Battery EMF – electromotive force – the potential difference between the terminals of a source when no current flows to an external circuit (  ) 19.1

The Electric Battery A battery will have an internal resistance (r) So there is a potential drop due to the current that travels through the cell So the actual potential across the terminals of a cell will be This is called the terminal voltage 19.1

DC Circuits 19.2 Resistors in Series and in Parallel

Resistors in Series and in Parallel When resistors are place in a single pathway They are said to be in series A schematic would look like this 19.2

Resistors in Series and in Parallel The current in a series circuit is the same throughout the circuit The potential across the source of EMF is equal to the sum of the potential drops across the resistors 19.2

Resistors in Series and in Parallel Since potential can be defined as We can rewrite the equation for potential as 19.2

Resistors in Series and in Parallel When resistors are place in a multiple pathways They are said to be in parallel A schematic would look like this 19.2

Resistors in Series and in Parallel The potential difference in a parallel circuit is the same throughout the circuit The current through the source of EMF is equal to the sum of the current through the resistors 19.2

Resistors in Series and in Parallel Since current can be defined as We can rewrite the equation for potential as 19.2

Resistors in Series and in Parallel Circuits that contain both series and parallel components need to be solved in pieces This circuit contains 20  resistors in series 25  resistors and load series to each other and parallel to the 40  resistor 19.2

DC Circuits 19.3 Kirchoff’s Rules

Kirchoff’s Rules Circuits that are a little more complex We must use Kirchoff’s rules Gustov Kirchoff They are applications of the laws of conservation of energy and conservation of charge 19.3

Kirchoff’s Rules Junction Rule – conservation of charge At any junction, the sum of the currents entering the junction must equal the sum of all the currents leaving the junction 19.3

Kirchoff’s Rules Loop Rule – the sum of the changes in potential around any closed pathway of a circuit must be zero For loop

Kirchoff’s Rules Steps 1.Label the current in each separate branch with a different subscript (the direction does not matter, if the direction is wrong, the answer will have a negative value) 2.Identify the unknowns and apply V=IR 3.Apply the junction rule (at a in our case) so that each current is in at least one equation 19.3 I1I1 I2I2 I3I3

Kirchoff’s Rules Steps 4.Choose a loop direction (clockwise or counterclockwise) 5.Apply the loop rule (again enough equations to include all the currents) a. For a resistor apply Ohm’s law – the value is negative if it goes in the direction of the current b. For a battery, the value is positive if the loop goes from – to + (nub to big end) 19.3 I1I1 I2I2 I3I3

Kirchoff’s Rules Steps We’ll do the two inside loops 6. Combine the equations and solve 19.3 I1I1 I2I2 I3I3

DC Circuits 19.5 Circuits Containing Capacitors in Series and in Parallel

Circuits Containing Capacitors 19.5 For a parallel set of capacitors – the total charge is the sum of the individual charges In all parallel circuits – the potential across each branch is the same as the total

Circuits Containing Capacitors 19.5 The equivalent capacitance is the value of one capacitor that could replace all those in the circuit with no change in charge or potential Since And We combine and get

Circuits Containing Capacitors 19.5 Series capacitors The magnitude of the charges is the same on each plate

Circuits Containing Capacitors 19.5 The total potential is the sum of the potential drops across each capacitor We then use that equation and the equation for capacitance We get

DC Circuits 19.6 RC Circuits-Resistors and Capacitors in Series

RC Circuits 19.5 Used windshield wipers timing of traffic lights camera flashes