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Forces and Fields. Electrostatics Lesson 3. Electrostatic Series. Increasing tendency to hold on to electrons. Increasing tendency to lose electrons. Whichever material is closer to the top holds electrons more tightly so will gain a negative charge. - PowerPoint PPT Presentation
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Forces and FieldsForces and FieldsElectrostatics Lesson 3
Electrostatic SeriesElectrostatic SeriesSulphur
BrassCopperEbonite
Paraffin waxSilk
LeadFur
woolglass
Increasing tendency to lose electrons
Increasing tendency to hold on to electrons
Whichever material is closer to the top holds electrons more tightly so will gain a negative charge.Whichever is closer to the bottom has a tendency to lose electrons so will gain a positive charge.
Two Static ExamplesTwo Static ExamplesRubbing ebonite with fur
produces a negative charge on the ebonite and a positive charge on the fur.
Rubbing glass with silk produces a negative charge on the silk and a positive charge on the glass because the glass gives up electrons more easily.
GroundingGroundingThe process of sharing charge with the earth. The earth can gain or donate electrons as needed to transfer a charge. The overall effect of this process is to make the object that comes into contact with the earth neutral.
The symbol for ground is
+ + + + + + + +
Earth
Conductive wire
electrons
Three Ways Charge Can Be Three Ways Charge Can Be TransferredTransferred
1. Friction (also called abduction)2. Conduction – touching a charged
object to an neutral object3. Induction – bringing an charged
object near a neutral object to cause temporary charge separation which polarizes the neutral object
Electrons in the paper are repelled to the most distant point leaving a localized positive charge near the negative comb.
Result – paper is attracted to the comb
When the comb is removed, electrons are redistributed in the paper.
Charge Migration
InductionInductionSteps to charge by induction:1.Bring a charged object close to (but
not touching) a neutral object.2.Ground the neutral object.3.Remove the ground.4.Remove the charged object.(The previously neutral object will
retain a charge because of the grounding step.)
Diagram of InductionDiagram of Induction
The ElectroscopeThe Electroscope
A simple device used to detect a charge on an object
It does not identify the quantity of charge, only whether a charge is present. It can be used to detect the sign of the charged object with some additional steps, but not directly.
Two TypesTwo Types1. Pith Ball ElectroscopeWhen pith balls are touched with a
charged object, the two balls acquire the same charge and therefore repel each other.
before After (repel)
2. Gold Leaf/Foil Electroscope◦When the metal knob is touched with a
charged object, the leaves/foil will repel each other.
Metal knob
Insulator
Glass container
Metal rod
Gold leaves (touching)
Metal knob
Insulator
Glass container
Metal rod
Gold leaves (repelling)
There are two ways to There are two ways to charge an electroscope.charge an electroscope.
Charging by CONDUCTIONTouch the ball with ebonite, (excess e’) and remove the
rod. Some e’ are transferred from the ebonite which makes the ball negative. The leaves are now both negative and will repel. The electroscope’s leaves will have the same charge as the object.
Charging by INDUCTIONConnect a wire to the electroscope. Bring the ebonite
rod close to but do not touch the electroscope. The negative charges in the electroscope are repelled, and move through the wire into the ground, (grounding). This causes the remaining positive charges in the leaves to repel each other, therefore spreading the leaves. Keep the ebonite in place and remove the grounding wire. The electroscope stays positively charged. It will have an opposite charge to the object.