Department of Physics and Applied PhysicsPHYS.1440 Lecture 13 A.Danylov

Lecture 13

Chapter 28

Resistors in Series and Parallel

Physics II

I wish my clicker could mute him

Equivalent resistance,.. blah, blah, blah

Course website:https://sites.uml.edu/andriy-danylov/teaching/physics-ii/

Department of Physics and Applied PhysicsPHYS.1440 Lecture 13 A.Danylov

Today we are going to discuss:

Chapter 28:

Section 28.1-6 Resistors 28.7 (Example 28.29)

Department of Physics and Applied PhysicsPHYS.1440 Lecture 13 A.Danylov

Circuit Elements

Slide 31-22

Department of Physics and Applied PhysicsPHYS.1440 Lecture 13 A.Danylov

Resistorsin series/parallel

Department of Physics and Applied PhysicsPHYS.1440 Lecture 13 A.Danylov

+ +

Resistors in ParallelConsider three resistors connected in parallel.

I

Real

circ

uit

Equi

vale

nt c

ircui

t

ΔV

Resistors in parallel have the same potential difference, ΔV

I + +

;

We have replaced 3 resistors with an “equivalent” resistor.

+  + 

Conservation of current

Req is inserted without changing the operation of the circuit, so I and ΔV are same as in the real circuit

Equivalent resistance of resistors in parallel.

=

I1

I2

I3

Ohm’s law;

ΔV

=

Department of Physics and Applied PhysicsPHYS.1440 Lecture 13 A.Danylov

+ +

Resistors in SeriesConsider three resistors connected in series.

Rea

l cir

cuit

Equ

ival

ent c

ircu

it

ΔV

+ +

Ohm’s law ∆

Req is inserted without changing the operation of the circuit, so I and ΔV are same as in the real circuit

Equivalent resistance of resistors in series.

ΔV

ΔV1 ΔV2 ΔV3

∆∆

+ +

Areheadlightwired: A)inparallel?

B)inseries?

ConcepTest Headlights

ConcepTest Resistors I

ThebatterycurrentIis

A)3A

B) 2A

C) 1A

D) 2/3A

E) ½A

=2/3 A

+

ConcepTest Resistors II

ThebatterycurrentIis

A)3A

B) 2A

C) 1A

D) 2/3A

E) ½A

=3 A

+ 

=4

Department of Physics and Applied PhysicsPHYS.1440 Lecture 13 A.Danylov

Exa

mpl

e:

Ana

lyzi

ng a

com

plex

cir

cuit

a)Find the equivalent resistance.b)Find the current through and the potential difference across each of the resistors in the circuit.

Department of Physics and Applied PhysicsPHYS.1440 Lecture 13 A.Danylov

Real batteries

Department of Physics and Applied PhysicsPHYS.1440 Lecture 13 A.Danylov

Give me a break! I do it as fast as I

can!

Real Batteries. Todriveacurrentinacircuitweneeda“chargepump”,adevicethatbydoingworkonthechargecarriersmaintainsapotentialdifference.Let’slookatagravitationalanalogofabattery:

Apersondoesworktomaintainasteadyflowofballsthrough“thecircuit”.However,thisguycannotmoveballsinstantaneously.Ittakestime.Sothereisanaturalhindrancetoacompletelyfreeflow.Todescribethishindrancewecanintroducetheinternalresistance,r.Itisinsideabatteryanditcannotbeseparatedfromthebattery.

Pot. difference of a battery without an internal resistance is called an electromotive force.(EMF, ε)

∆Terminal voltage

(Internal resistance)

Department of Physics and Applied PhysicsPHYS.1440 Lecture 13 A.Danylov

Thank you