电池电路元件挑战问题解决方案1mitge sol13.pdfVIP

BatteriesandCircuitElements

ChallengeProblemSolutions

Problem1:

(a)Ifchargesflowveryslowlythroughametal,whydoesitnotrequireseveralhoursfor

lighttocomeonwhenyouthrowaswitch?

(b)Whatadvantagedoes120-Voperationofferover240V?Whatarethedisadvantages?

(c)Whyisitpossibleforabirdtostandonahigh-voltagewirewithoutgetting

electrocuted?

(d)Ifyourcar’sheadlightsareonwhenyoustarttheignition,whydotheydimwhilethe

carisstarting?

(e)Supposeafallingfromabuildingonthewaydowngrabsahigh-voltagewire.

Ifthewiresupportshimashehangsfromit,willhebeelectrocuted?Ifthewirethen

breaks,shouldhecontinuetoholdontoofthewireashefalls?

(f)Aseriescircuitconsistsofthreeidenticallampsconnectedtoabatteryasshowninthe

figurebelow.WhentheswitchSisclosed,whathappenstothebrightnessofthelight

bulbplainyouranswer.

Problem1Solution:

(a)Eventhoughtheaveragevelocityofchargesisveryslow,whenyouconnecta

potentialdifferenceacrossacircuit(wirelightbulb),anelectricfieldisestablished

almostinstantly(propagatingnearthespeedoflight)everywhereinthecircuit,andthe

electronsbegintodrifteverywhereallatonce.

(b)Both120-Vand240-Vlinescandeliverinjuriousorlethalshocks,butthereisa

somewhatbettersafetyfactorwiththelowervoltage.Ontheotherhand,a240-Vdevice

requireslesscurrenttooperateatthesamepower,andhencegenerateslessheatingand

loss(I2R)inwiring.

(c)Thereasonisbecausethepotentialontheentirewireisnearlyuniform,andthe

potentialdifferencweenthebird’sfeetisapproxima