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Electrostatics - Solved Exercise - Basic Concepts of Physics, Exercises of Physics

Central University of South BiharPhysics

"Basic Concepts of Physics" course objective is to introduce the terms like: Newton’s Motion Laws, Momentum, Energy, Rotation, Gravity, Liquids, Gases, Plasmas, Heat, Vibrations and Waves, Sound, Electrostatics, Electric current, Magnetism, Electromagnetic Induction, Properties of Light, Color. In this exercise questions are about: Electrostatics, Electrical Force, Gravitational Force, Thunderstorm, Electric Potential Difference, Current, Conduction Electrons

Typology: Exercises

2012/2013

Uploaded on 08/13/2013

madangopal
madangopal 🇮🇳

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1)Considertwochargedparticlesacertaindistanceapart,e.g.twoelectrons.
a)Whatislikelytobelarger,thegravitationalortheelectricalforcebetweenthem?
b)Howwouldtheelectricforcebetweenthemchangeiftheyarebroughttohalftheiroriginaldistance
ofseparation?Whataboutthegravitationalforce?
a)Theelectricalforceisfarlargerthanthegravitationalforce,sincekqeqeismuchlargerthanGmeme,as
showninclass.
b)Theelectricforcevarieswithseparationastheinverse‐squarelaw,1/d2.Soifdwashalved,thend2is
quartered,and1/d2is4xasmuch,i.e.theelectricalforcebecomes4timesaslarge.Thegravitational
forceobeysthesamedistance‐dependence,soitwouldalsobe4timesaslarge.
2)Ifyouarecaughtoutdoorsinathunderstorm,whyshouldyounotstandunderatree?Canyouthink
ofareasonwhyyoushouldnotstandwithyourlegsfarapart?Orwhylyingdowncanbedangerous?
(Considerelectricpotentialdifference).
Thetreeislikelytobehitbecauseitprovidesapathoflessresistancebetweenthecloudoverheadand
theground.Thetreeandthegroundnearitarethenraisedtoahighpotentialrelativetotheground
fartheraway.Ifyoustandwithyourlegsfarapart,onelegonahigher‐potentialpartofthegroundthan
theother,orifyouliedownwithasignificantpotentialdifferencebetweenyourheadandyourfeet,
youmayfindyourselfaconductingpath.That,youwanttoavoid!
3)Willthecurrentinalightbulbconnectedtoa220‐Vsourcebegreaterorlessthanwhenthesame
bulbisconnectedtoa110‐Vsource?
Thecurrentwillbeless:Current=Voltage/Resistance,soiftheVoltageishalvedthen,therewillbehalf
asmuchcurrentthroughthesamebulb.
4)Ifelectronsflowveryslowlythroughacircuit,whydoesitnottakeanoticeablylongtimeforalamp
toglowwhenyouturnonadistantswitch?
Howquicklyalampglowsafteranelectricalswitchiscloseddoesnotdependonthedriftvelocityofthe
conductionelectrons,butdependsonthespeedatwhichtheelectricfieldpropagatesthroughthe
circuit—aboutthespeedoflight.
5)Considerapairofflashlightbulbsconnectedtoabattery.Willtheyglowbrighterconnectedinseries
orinparallel?Willthebatteryrundownfasteriftheyareconnectedinseriesorinparallel?Notethat
thebrightnessofabulbmeasuresthepower.
Thebulbswillglowbrighterwhenconnectedinparallel,forthevoltageofthebatteryisimpressed
acrosseachbulb:Power=VoltagexCurrent=(Voltage)2/Resistance.Whentwoidenticalbulbsare
connectedinseries,halfthevoltageofthebatteryisimpressedacrosseachbulb.Thebatterywillrun
downfasterwhenthebulbsareinparallelbecausethisdrawsmorepowerfromthebattery.
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Download Electrostatics - Solved Exercise - Basic Concepts of Physics and more Exercises Physics in PDF only on Docsity!

  1. Consider two charged particles a certain distance apart, e.g. two electrons. a) What is likely to be larger, the gravitational or the electrical force between them? b) How would the electric force between them change if they are brought to half their original distance of separation? What about the gravitational force? a) The electrical force is far larger than the gravitational force, since kqe qe is much larger than Gm (^) e m (^) e , as shown in class. b) The electric force varies with separation as the inverse‐square law, 1/d 2. So if d was halved, then d^2 is quartered, and 1/d^2 is 4x as much, i.e. the electrical force becomes 4 times as large. The gravitational force obeys the same distance‐dependence, so it would also be 4 times as large.
  2. If you are caught outdoors in a thunderstorm, why should you not stand under a tree? Can you think of a reason why you should not stand with your legs far apart? Or why lying down can be dangerous? (Consider electric potential difference). The tree is likely to be hit because it provides a path of less resistance between the cloud overhead and the ground. The tree and the ground near it are then raised to a high potential relative to the ground farther away. If you stand with your legs far apart, one leg on a higher‐potential part of the ground than the other, or if you lie down with a significant potential difference between your head and your feet, you may find yourself a conducting path. That, you want to avoid!
  3. Will the current in a light bulb connected to a 220 ‐V source be greater or less than when the same bulb is connected to a 110 ‐V source? The current will be less: Current = Voltage/Resistance, so if the Voltage is halved then, there will be half as much current through the same bulb.
  4. If electrons flow very slowly through a circuit, why does it not take a noticeably long time for a lamp to glow when you turn on a distant switch? How quickly a lamp glows after an electrical switch is closed does not depend on the drift velocity of the conduction electrons, but depends on the speed at which the electric field propagates through the circuit—about the speed of light.
  5. Consider a pair of flashlight bulbs connected to a battery. Will they glow brighter connected in series or in parallel? Will the battery run down faster if they are connected in series or in parallel? Note that the brightness of a bulb measures the power. The bulbs will glow brighter when connected in parallel, for the voltage of the battery is impressed across each bulb: Power = Voltage x Current = (Voltage)^2 /Resistance. When two identical bulbs are connected in series, half the voltage of the battery is impressed across each bulb. The battery will run down faster when the bulbs are in parallel because this draws more power from the battery.

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