L10 Ch27 Caps in Series Parallel

PHYS.1440Lecture10 A.DanylovDepartmentofPhysicsandAppliedPhysics

Lecture 10

Chapter 26

Capacitorsin Series and Parallel

Physics II

Is there life before coffee?Hmm! 8am Physics class triggers

philosophical questions

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


Today we are going to discuss:

Chapter 26:

Section 26.5

Oncethebatteryisdisconnected,Q hastoremain

constant,sincenochargecanfloweithertoor

fromthebattery.

Aparallel‐platecapacitorinitiallyhasapotentialdifferenceof400V andisthendisconnectedfromthechargingbattery.Iftheplatespacingisnowdoubled,whatisthenewvalueofthevoltage?

A)100V

B)200V

C)400V

D)800V

E)1600V

+Q –Q

ConcepTest Varying Capacitance II

Since,whenthespacingd isdoubled,

thecapacitanceC ishalved.Andsince ,thatmeansthevoltagemustdouble

400 VQ =CΔV≝


Combinations of Capacitors

In practice, two or more capacitors are sometimes joined together. The circuit diagrams below illustrate two basic combinations:

parallel capacitors and series capacitors.

The equivalent capacitance is the capacitance of the single capacitor that can replace a set of connected capacitors without changing the operation of the circuit


Capacitors in ParallelConsiderthreecapacitorsconnectedinparallel.

QReal

circ

uit

Equi

vale

nt c

ircui

t

ΔV

, ΔV

, ΔV

, ΔV

Q ΔV

Ceq

Capacitors in parallel have the same potential difference, ΔV

Qisatotalchargedrawnfromthebattery+ +

Since ≝;;;

Wehavereplaced3capacitorswitha“equivalent”capacitor.

+ +

+ +

Conservationofcharge

Ceq isinsertedwithoutchangingtheoperationofthecircuit,soQandΔVaresameasintherealcircuit

Equivalentcapacitanceofcapacitorsinparallel.

= =


+ +

Capacitors in SeriesConsiderthreecapacitorsconnectedinseries.

QReal

circ

uit

Equi

vale

nt c

ircui

t

+Q

Q ΔV

Ceq

Capacitors in series have the same charge, Q.

+ +

Since ≝

Ceq isinsertedwithoutchangingtheoperationofthecircuit,soQandΔVaresameasintherealcircuit

Equivalentcapacitanceofcapacitorsinseries.

C1 C2 C3

+ -

-Q +Q -Q +Q -Q

ΔV

ΔV1 ΔV2 ΔV3

Q


Example Equivalent capacitance


Lecture 11

Chapter 27

Current

Physics II

Is there life before coffee?Hmm! 8amPhysics class triggers

Philosophical questions

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


Today we are going to discuss:

Chapter 27:

Section 27.1 Section 27.3-5


A Model of Conduction

Uptothispointweweretalkingaboutelectrostaticequilibriumwhenaconductorwasatthesamepotentialandtherewasnocurrent.

Nowifweaddabattery,apotentialdifferencewillbeimposedandtheelectronswillstarttravellingcreatingacurrent

Inthiscase,anelectronbouncesbackandforthbetweencollisions,butitsaveragevelocityiszero.


Current (definition)

IfQisthetotalamountofchargethathasmovedpastapointinawire,wedefinethecurrentIinthewiretobetherateofchargeflow:

The SI unit for current is the coulomb per second, which is called the ampere.

1 ampere = 1 A = 1 C/s.

current is the rate at which charge flows

dQ


Direction of current (convention)

Byconvention,currentisdefinedasflowingofmotionpositiveparticlesfrom+to‐.

Electronsactuallyflowintheoppositedirection.

Current (by convention motion of positive particles)

Current

Currentflowsfromapositiveterminalofabatterytoanegativeone.

.

Everyminute,120Cofchargeflowthroughthiscrosssectionofthewire.

A)240A

B)120A

C)60A

D)2A

E)Someothervalue

ConcepTest Current

Thewire’scurrentis


The Current Density in a Wire

Thecurrentdensity Jinawireisthecurrentpersquaremeterofcrosssection:

ThecurrentdensityhasunitsofA/m2.

A


Thank you

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L10 Ch27 Caps in Series Parallel