Transcript Static Electricity 6 Electric Charge

Static Electricity 1
1020 C
Static Electricity
Chapter 10 part 1
Static Electricity 2
Introductory Question
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A woman rubs her feet on the carpet and gives
a shock to her identical twin. If the twin also
rubs her feet on the carpet before being
touched, the shock will be
A.
larger
smaller
the same size
B.
C.
Static Electricity 3
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Observations about
Static Electricity
Static electricity builds up on non-metallic things
Objects with static charge may cling or repel
Static electricity can lead to shocks
Static electricity can make your hair stand up
Static Electricity 4
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4 Questions about
Static Electricity
Why do some clothes cling while others repel?
Why do sticky clothes stick to uncharged walls?
Why do clingy clothes crackle as they separate?
Why do some things lose their charge quickly?
Static Electricity 5
Question 1
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Why do some clothes cling while others repel?
Static Electricity 6
Electric Charge (Part 1)
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Charges appear in two types: positive & negative
Like charges repel
 Opposite charges attract
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Two charges push (or pull) on one another
with forces that are exactly equal in magnitude
 but exactly opposite in direction.
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The forces increase as the separation decreases
Coulomb constant  charge1  charge 2
force =
(distance between charges) 2
Static Electricity 7
Electric Charge (Part 2)
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Electric charge
is a conserved quantity,
 is measured in coulombs,
 is quantized in multiples of the fundamental charge.
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One fundamental charge is 1.6  10-19 coulombs
Charge is an intrinsic property of matter
Electrons are negatively charged
 Protons are positively charged
 Each has one fundamental charge
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Static Electricity 8
Net Charge
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An object’s net charge
is the sum of its individual charges
 and tends to be zero or nearly zero.
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A neutral object has
zero net charge
 and contains as many + charges as – charges.
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Static Electricity 9
Charge Transfers
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Contact can transfer charge between objects
The object with the stronger affinity for electrons
 becomes negatively charged upon contact
 will the other object becomes positively charged.
 These oppositely charge objects attract one another.
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Rubbing the objects together ensures
excellent contact between their surfaces
 and consequently substantial charge transfer.
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Static Electricity 10
Introductory Question (revisited)

A woman rubs her feet on the carpet and gives
a shock to her identical twin. If the twin also
rubs her feet on the carpet before being
touched, the shock will be
A.
larger
smaller
the same size
B.
C.
Static Electricity 11
Question 2
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Why do clingy clothes stick to uncharged walls?
Static Electricity 12
Electric Polarization
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A neutral wall contains countless charges
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that respond to any nearby charged object.
When a negatively charged sock nears a wall,
the wall’s positive charges shift toward the sock,
 the wall’s negative charges shift away from it,
 and the wall becomes electrically polarized.
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The charged sock clings to the polarized wall
Static Electricity 13
Question 3
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Why do clingy clothes crackle as they separate?
Static Electricity 14
Voltage
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Charge has electrostatic potential energy (EPE)
Voltage measures the EPE per unit of charge
Raising the voltage of positive charge takes work
 Lowering the voltage of negative charge takes work
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Voltage is measured in joules/coulomb or volts
Static Electricity 15
Separating Opposite Charges
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Separating opposite charges takes work,
so the positive charges undergo a rise in voltage
 and the negative charges undergo a drop in voltage.
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Positive charge at high voltage can release EPE
by moving to lower voltage
 and it often does this by way of a discharge or spark!
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Negative charge behaves oppositely,
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releasing EPE by moving to higher voltage.
Static Electricity 16
Question 4
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Why do some things lose their charge quickly?
Static Electricity 17
Conductors and Insulators
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All objects contain positive and negative charges
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but most of those charges are immobile.
Insulators have no mobile electric charges
Conductors have mobile electric charges,
which are usually electrons (e.g., metals)
 but are occasionally ions (e.g., salt water).
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Conductors can lose net charges easily.
Static Electricity 18
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Summary about
Static Electricity
Even neutral objects contain countless charges
Charge can be transferred during contact
Clothes often develop net charges during drying
Oppositely charged clothes cling to one another
and spark as separation affects their voltages.
Conductivity tends to let objects neutralize.