Chapter 17 Electricity

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Chapter 17Electricity

• 17-1 Electric Charge and Force

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How can you produce static electricity

• Rub a glass rod with silk
• This will cause the rod to have a positive charge
• Move it close to a aluminum foil
• Rub a plastic rod with
• a wool cloth
• The rod acquires a negative charge

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Electric Charge

• 1 As you walk across a carpeted room during the
  winter what happens often when you go to touch a
  door knob?
• you get shocked
• 2. Are all the electrical charges concentrated in
  your hands?
• No
• 3. Electrical charges is a property of matter.
• 4. Objects can have either a Positive charge,
  negative charge or no charge at all.
• 5. If an object has an equal number of positive
  to negative charges it has no Net charge. Net
• 6. Energy cannot be Created nor destroyed this
  is one of the fundamental laws of nature.
• 7. Define electric charge
• An electrical property of matter that creates
  electric and magnetic forces and interactions

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Like charges repel, and opposites charges attract.

• 1.Which becomes electrically charged as you move
  a balloon across your hair, the balloon or the
  hair?
• both
• 2.When you move the balloon close to your hair
  what happens?
• the two are attracted to one another
• 3.Now if you rub two balloons together which
  becomes charged?
• both
•  4.What happens when you bring the two balloons
  close to one another?
• they repel one another
• 5.So this tells you that the electrical charges
  in your hair and the balloon are
• different
•  and the charges on the two balloons are the
• same.

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Electric charge depends on an imbalance of
protons and electrons.

• All matter is made up of
• atoms
• 2. Atoms are made of what three fundamental
  particles
• protons, electrons, and neutrons
•  
• 3. What are the electrical charges of the three
  particles
• Particle Charge of particle
• proton ,
• electrons , -
• neutron , 0

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• 4. If an object has a balanced number of
  positive and negative charges it has a
• . Net charge of 0
• 5.If an object has an imbalanced number of
  positive and negative charges it has a Net charge
  other than 0.
• 6.If an object has 500 protons and 495 electrons
  it has a net charge of 5
• 7. If an object has 500 protons and 505 electrons
  it has a net charge of -5
• 8.What is the unit we will use to describe
  electric charge
• coulomb (C)
• 9.The protons an electron both have the exact
  same amount of charge, 1.6x10-19 C.
• 10.The proton has 1.6x10-19 C and the electron
  has -1.6x10-19 C amount of charge to them.

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Add to your notes

• Recall that fundamental particles carry something
  called electric charge
• protons have exactly one unit of positive charge
• electrons have exactly one unit of negative
  charge
• Electromagnetic force is one of the basic
  interactions in nature
• like charges experience repulsive force
• opposite charges attracted to each other (like
  gravity)
• Electrical current is flow of charge (electrons)

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Add to your notes

• Neutral atoms are made of equal quantities of
  positive and negative charges
• Neutral carbon has 6 protons, 6 electrons, (
  neutrons)
• Electrons can be stripped off of atoms
• Electrons occupy the vulnerable outskirts of
  atoms
• Usually charge flows in such a way as to maintain
  neutrality
• Excess positive charge attracts excess negative
  charge
• Your body has 5?1028 positive charges and 5?1028
  negative charges, balanced within millions or
  billions

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Transfer of Electric Charge

• 1. Which electrons can be transferred from object
  to object?
• outermost

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Examples of Static Electricity
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Conductors allow charges to flow insulators do
not.

• 1.Materials which conduct electricity are
• conductors
• 2.Materials which prevent the conduction of
  electricity are
• insulators
• 3.give two examples of materials which conduct
  electricity _______________ , _______________
• Metals, salt water
• 4.Give two examples of materials which do not
  conduct electricity _____,_______
• cardboard, rubber, plastics

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Charges can move within uncharged objects

• 1. Is it possible to electrically charge an
  object without touching it?
• yes
• 2. The example in the book is
• Brining a charged rubber rod to a door knob and
  the electrons in the door knob are repelled from
  the electrons on the rubber rod
• 3. The charging of an object without actually
  touching it is called
• induction

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Induction

• Charge can also be coaxed to redistribute itself
  within an object

Charged rod approaches sphere
charge attracted to charge in rod
charge repelled by rod
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Induction

• Initially the paper is uncharged, but the comb
  polarizes the charges in the paper.

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Objects can be charged by contact.

• 1. What happens to the electrons as one
  electrically charged object touches another
  object which is not electrically charged?
• the electrons are transferred

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Objects can be charged by friction.

• 1. Depending on the materials when you rub two
  objects across one another what happens to the
  objects.
• One object gains electrons and becomes negatively
  charged while the other loses electrons and
  becomes positively charged
•  2. This often happens with your clothes in the
  dryer what do you often times call this charging
  of objects
• static electricity

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Static electricity

• Rubbing action redistributes charge (unbalanced)
• If enough charge builds up, we get discharge
• Air spark is actually due to breakdown of air
• neutral air molecules separate into ions
  (electrons are stripped away)
• current can then flow through the plasma-field
  air
• In essence, air becomes a wire for a short bit
• this happens at 3 million volts per meter
• 1 cm spark then at 30,000 volts
• typical finger-spark may involve a few billion
  electrons
• hold onto key to reduce pain of spark

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Lightning

• Lightning is an unbelievably huge discharge
• Clouds get charged through air friction
• 1 kilometer strike means 3 billion volts!
• Main path forms temporary wire along which
  charge equalizes
• often bounces a few times before equal
• Thunder is bang produced by the extreme pressure
  variations induced by the formation and collapse
  of the plasma conduit

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Lightning rods

• Perform two functions
• provide safe conduit for lightning away from
  house
• diffuse situation via coronal discharge

Charges are attracted to tip of rod,
and electric field is highly concentrated there.
Charges leak away, diffusing charge in what
is sometimes called St. Elmos Fire, or
coronal discharge
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Coronal Discharge

• Side-effect of the electric field
• Often youll hear a buzz around power lines -
  this is a coronal discharge
• Visible coronal occurs when we have the
  ionization of oxygen and nitrogen in the air
• You get a bluish haze around the electrical
  discharge

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A surface charge can be induced on insulators.

•  1. If insulators prevent the transfer of
  electricity how can it become charged?
• When a charged object comes near it the electrons
  move slightly from one side to another by
  repulsion of electrons this causes one side to
  be positive and the other side negative.
•  2. When one side of an object is positive and
  the other negative it is called _______________
• polarization

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add to your notes

• Can separate charges by rubbing
• feet on carpet
• atmosphere across ground
• silk on glass
• balloon on hair!
• Insulators keep charges where they are (no flow)
• Conductors distribute charge equally on surface
• charge is free to move about the cabin
• why do the charges collect on the surface?

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Electric Force

•  1. The attraction of a positively charged object
  to a negatively charged object or the repulsion
  of like charged objects are examples of
• electric force
• 2.Are electric forces only involved with specific
  objects
• no they are everywhere
•  3.The electric for is also one of the four
  forces which help to keep the _______________
  held together.
• Atom
•  
• The other three forces which help to keep the
  Atom held together are
• strong, weak, gravitational

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Electric Force

• Can think of electric force as establishing
  field telling particles which way to move and
  how fast

Electric field lines tell a positive charge
which way to move. For example, a positive
charge itself has field lines pointing away from
it, because this is how a positively-charged test
-particle would respond if placed in the
vicinity (repulsive force).

Run Away!

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Electric force depends on charge and distance.

•  1.The electric force between two objects is
  _______________ to the products of the charges
  on the objects.
• Proportional
• 2.If the net charge on one object doubles the
  electric force between the objects
  _______________
• doubles
• 3.The electric force is also _______________
  _______________ to the square of the distance
  between two objects.
• Inversely proportional
• 4. If the distance between two objects doubles
  the electric force between them is cut by
  _______________
• ¼
• 5. If the between to object increases by 8 times
  the electric force between them is cut by
  _______________
• 1/64

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Electric force acts through a field.

•  1.Do objects need to be in contact for there to
  be a change in electric charge? _______________
• no
• 2.Describe what an electric field is
• It is a field which exists in the space around a
  charged particle in which another charged object
  experiences an electric force
• 3.When drawing electric force lines we draw them
  _______________ a positive charged particle.
• Away
• 4.When drawing electric force lines we draw them
  _______________ a negatively charged particle.
• Towards

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Electric Fields
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Electric fields in circuits

• Point away from positive terminal, towards
  negative
• Channeled by conductor (wire)
• Electrons flow opposite field lines (neg. charge)

E
electrons direction of motion
E
Electric field direction
E
E
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Electron Beams Cathode Ray Tubes (CRTs)

• Televisions, Oscilloscopes, Monitors, etc. use an
  electron beam steered by electric fields to light
  up the (phosphorescent) screen at specified points

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