Lecture Outline Chapter 18 College Physics, 7 th Edition Wilson / Buffa / Lou © 2010 Pearson Education, Inc.

Lecture Outline

Chapter 18

College Physics, 7th Edition

Wilson / Buffa / Lou

© 2010 Pearson Education, Inc.

18.1 Resistances in Series, Parallel, and Series–Parallel Combinations

• A few things about resistance before we start!!!!

• The resistance symbol is …..– We can use the resistance symbol to

represent ANY type of circuit element.

• Assume constant resistance unless otherwise stated.

18.1 Resistances in Series, Parallel, and Series–Parallel Combinations

Resistors in series all have the same current.

Individual voltages across the resistors add up to equal the voltage across the battery terminals.

© 2010 Pearson Education, Inc.

18.1 Resistances in Series, Parallel, and Series–Parallel Combinations

• Series means head to tail (so a + is connected to a -)

• Why must current stay the same in each through each resistor?

18.1 Resistances in Series, Parallel, and Series–Parallel Combinations

• Let’s simplify the first picture:

18.1 Resistances in Series, Parallel, and Series–Parallel Combinations

The resistance value of a single resistor that could replace all resistors, but maintain the same current is called: Equivalent Series Resistance

This formula is valid for any number of resistors in series.

Let’s do a simple example….

© 2010 Pearson Education, Inc.

18.1 Resistances in Series, Parallel, and Series–Parallel Combinations

• 2 major disadvantages of Series Connections: – If one bulb burns out…– Each bulb operates at less than the battery

voltage…

18.1 Resistances in Series, Parallel, and Series–Parallel Combinations

Resistors in parallel have common connections (+ and +)

Resistors in parallel all have the same voltage.

© 2010 Pearson Education, Inc.

Parallel circuits divide into different paths.

Current wants to take path of least resistance.

18.1 Resistances in Series, Parallel, and Series–Parallel Combinations

• Let’s simplify the first parallel picture:

18.1 Resistances in Series, Parallel, and Series–Parallel Combinations

The equivalent parallel resistance is the value of a single resistor that could replace all the resistors in the parallel configuration and maintain the same current.

The equivalent resistance in a parallel configuration is always less than the smallest resistance….for example…

© 2010 Pearson Education, Inc.

18.1 Resistances in Series, Parallel, and Series–Parallel Combinations

• What is the equivalent resistance of 3 resistors (1.0 ohm, 2.0 ohm, and 3.0 ohm) when connected (a) in series and (b) in parallel.

• (c) How much total current will be delivered by a 12 V battery in each of these arrangements?

• (d) How much current will be in each resistor and what is the voltage drop across each resistor in each arrangement?

18.1 Resistances in Series, Parallel, and Series–Parallel Combinations

• Christmas Tree Lights:– How they use to work– How they work now…

18.1 Resistances in Series, Parallel, and Series–Parallel Combinations

• Consider a string of Christmas tree lights composed of bulbs with jumper shunts, as in the picture on the last slide. If the filament of one bulb burns out and the shunt completes the circuit, will the other bulbs each

• (a) glow a little more brightly• (b) glow a little more dimly• (c) be unaffected

18.1 Resistances in Series, Parallel, and Series–Parallel Combinations

• Here is a procedure to analyze Series-Parallel Resistor Combinations– Determine which groups of resistors are in series

and which are in parallel and then reduce. (You may have to reduce multiple times)

– Find the current delivered to the reduced circuit using I = V/Rtotal

– Expand the reduced circuit back to the actual circuit to find currents and voltages for the resistors in each step.

18.1 Resistances in Series, Parallel, and Series–Parallel Combinations

For combination circuits, simplify piece by piece.

© 2010 Pearson Education, Inc.

18.1 Resistances in Series, Parallel, and Series–Parallel Combinations

18.1 Resistances in Series, Parallel, and Series–Parallel Combinations

18.1 Resistances in Series, Parallel, and Series–Parallel Combinations

18.1 Resistances in Series, Parallel, and Series–Parallel Combinations

18.1 Resistances in Series, Parallel, and Series–Parallel Combinations

• What are the voltages across and currents in each of the resistors R1 through R5 in the figure to the right?

• This is a REALLY great FRQ!

18.2 Multiloop Circuits and Kirchhoff’s Rules

Some circuits can’t always be reduced by pure series and parallel methods.

A dude named Kirchhoff made some rules…

© 2010 Pearson Education, Inc.

18.2 Multiloop Circuits and Kirchhoff’s Rules

Kirchhoff’s first rule is the junction rule:

*First a junction (or node) is a point where 3 or more wires are joined.

*A path connecting 2 junctions is called a branch.

The sum of all currents at any junction is 0. This means that the current coming into the junction must be equal to the current leaving the junction.

© 2010 Pearson Education, Inc.

18.2 Multiloop Circuits and Kirchhoff’s Rules

Kirchhoff’s second rule is the loop rule:

The sum of the potential differences around a closed loop is zero.

Sign conventions for traversing batteries and resistors are at left.

© 2010 Pearson Education, Inc.

18.2 Multiloop Circuits and Kirchhoff’s Rules

• Two resistors R1 and R2 are connected in parallel. This combination is in series with a third resistor, R3, which has the largest resistance of the three.

• (a) Which resistor will carry the most current: R1, R2, or R3? Explain.

Skipping

• Pages 637-641 we are skipping! You will not be tested on RC circuits.

• Also, from pages 641-643 is discussing galvanometers, but these work also on magnetic properties so we will discuss in Chapter 19.

• Finally, Household circuits and electrical safety will not be tested as well so we can skip Section 18.5.

18.4 Ammeters and Voltmeters

The deflection of a galvanometer is proportional to the current.

© 2010 Pearson Education, Inc.

18.4 Ammeters and Voltmeters

An ammeter measures current. In order to do this, it must be connected in series; so as not to change the existing current significantly, its resistance should be as small as possible.

© 2010 Pearson Education, Inc.

18.4 Ammeters and Voltmeters

A voltmeter measures voltage. In order to do this, it must be connected in parallel across the voltage to be measured; so as not to change the existing voltage significantly, its resistance should be as large as possible.

© 2010 Pearson Education, Inc.

18.4 Ammeters and VoltmetersMultirange meters have a selection of shunt and multiplier resistors, to optimize the measurement of currents and voltages of different magnitudes.

© 2010 Pearson Education, Inc.