Transcript 16-1 and 16-2 Electric Charge

Chapter 16
Electric Charge and
Electric Field
Units of Chapter 16
• Static Electricity; Electric Charge and Its
Conservation
• Electric Charge in the Atom
• Insulators and Conductors
• Induced Charge; the Electroscope
• Coulomb’s Law
• Solving Problems Involving Coulomb’s Law
and Vectors
• The Electric Field
Units of Chapter 16
• Field Lines
• Electric Fields and Conductors
Objectives: The students will be able to:
1. Demonstrate that charged objects exert forces, both attractive
and repulsive.
2. Explain that charging is the separation, not the creation, of
electric charge.
Objectives: The students will be able to:
1. State from memory the magnitude and sign of the charge on an
electron and proton and also state the mass of each particle.
2. Apply Coulomb's law to determine the magnitude of the
electrical force between point charges separated by a distance r
and state whether the force will be one of attraction or repulsion.
3. State from memory the law of conservation of charge.
4. Distinguish between an insulator, a conductor, and a semi
conductor and give examples of each.
5. Explain the concept of electric field and determine the resultant
electric field at a point some distance from two or more point
charges.
6. Determine the magnitude and direction of the electric force on a
charged particle placed in an electric field.
7. Sketch the electric field pattern in the region between charged
objects.
8. Use Gauss's law to determine the magnitude of the electric field
in problems where static electric charge is distributed on a surface
Electricity
• Hewitt’s intro to Electricity
Static electricity
Static electricity is caused when
certain materials are rubbed against
each other.
 Electrons can be rubbed off one
material and on to another.
 The material that has got extra
electrons is now negatively charged
 The material which has lost electrons
is positively charged.
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16.1 Static Electricity; Electric Charge
and Its Conservation
Objects can be charged by rubbing
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Static Electricity
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Static electricity
is the electric
charge at rest on
an object.
When something
is static, it is not
moving.
The charges of
static electricity do
not move away
from the object
that they are in.
So, the object
keeps its charge.
Ex. Clothes taken
out of a dryer
Static electricity
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It is this imbalance of positive and
negative charges that causes:
Balloons to stick to walls.
Your hair to stand on end when
brush your hair on a dry day.
And the electric shock you
sometimes get from the door
handle.
Exploring Electric Charge
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phET site
Electric Discharge
The loss of static electricity as charges
move off an object is called electric
discharge.
Sometimes,
electric
Sometimes
discharge
, electric
happens
discharge
quickly.
happens
slowly.
Ex. wearing
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Ex: static
on clothes
rubber-soled
shoes on
carpet,
lightning
Static Discharge
Human body can not feel
less than 2,000 volts of
static discharge
Static charge built up by
scuffing shoes on a carpet
can exceed 20,000 volts?
Gas Station Fires
• Carol said a static gas pump fire is blamed for
burning her daughter so badly she needed skin
grafts on her legs.
• Carol had put the gas pump nozzle on automatic
and re-entered her car to write a check. When her
then-12-year-old daughter, wearing a sweater and
jacket that may have created static electricity,
reached for the nozzle, flames suddenly ignited her
clothing.
Grounding
What is grounding?
An object is grounded when it is connected
to the earth through a connecting wire.
If a charged conductor is grounded, it
will become neutral.
Grounding
b How does grounding occur?
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When we touch a metal ball of
positive charge...
electrons flow from the
earth to the metal ball to
neutralize the metal ball.
Metal ball becomes neutral.
Grounding
How does grounding occur?
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– –
–
Similarly, if the metal ball is of
negative charge...
extra electrons flow
from the metal ball to
the earth and the ball
becomes neutral.
Why do gasoline tankers usually have metal
chains at the back?
When cars run, their tires and bodies are usually
friction For gasoline tankers, if the
charged by _______.
accumulated charge is large enough, _______can
be
sparks
explosion will occur if gasoline vapor is
produced and _________
ignited. Those metal chains conduct the charge on the
ground and avoid the danger.
bodies of tankers to the _______
16.1 Static Electricity;
Electric Charge and Its
Conservation
Charge comes in two
types, positive and
negative; like charges
repel and opposite
charges attract
16.1 Static Electricity; Electric Charge
and Its Conservation
Electric charge is conserved – the
arithmetic sum of the total charge cannot
change in any interaction.
Electric Charge
• All matter is made up of atoms
•
1.
2.
3.
Atoms contain
Protons (+)
Neutrons (0)
Electrons (-)
16.2 Electric Charge in the Atom
Atom:
Nucleus (small,
massive, positive
charge)
Electron cloud (large,
very low density,
negative charge)
16.2 Electric Charge in the Atom
Atom is electrically neutral.
Rubbing charges objects by moving electrons
from one to the other.
Where do charges come
from?
If electrons = protons neutral
If electrons > protons  gaining electrons, negative
charge
If electrons < protons  losing electrons, positive
charge
Electro-negativity
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+++
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+
Relative
electro-negativity
ranking for some
common materials
from electron donating
materials (+, glass) to -electron accepting
--materials (-, teflon)
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Glass
Human Hair
Nylon
Silk
Fur
Aluminum
Paper
Cotton
Copper
Rubber
PVC
Teflon
Where do charges come from?
Rubbing materials does NOT create
electric charges. It just transfers
electrons from one material to the other.
Where do charges come from?
When a balloon rubs a piece of wool...
+
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wool
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electrons are pulled from the
wool to the balloon.
The balloon has more electrons than
usual.
The balloon: – charged,
The wool: +charged
16.2 Electric Charge in the Atom
Polar molecule: neutral overall, but charge not
evenly distributed
Law of Electric Charges
• The law of electric charges states
that like charges repel, and
opposite charges attract.
• Protons are positively charged and
electrons are negatively charged,
so they are attracted to each other.
• Without this attraction, electrons
would not be held in atoms.
Law of Electric Charges
Elaboration
• Hand-out – Rule of Electric Charges
Homework for 16-1 and 16-2
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Questions on page 464
#s 2, 3, 4, 5
Closure
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Kahoot 16-1 and 16-2