Electrostatics Notes

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Electrostatics Notes
Charge!
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The three fundamental facts about atoms

• Every atom is composed of a positively charged
  nucleus, surrounded by negatively charged
  electrons.
• The electrons of all atoms are identical. Each
  has the same quantity of charge and the same
  mass.
• Protons and neutrons compose the nucleus. Protons
  are about 1800 times more massive than electrons
  but carry an amount of charge equal to the
  negative charge of electrons. Neutrons have
  slightly more mass than the protons and have no
  charge.

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The normal configuration of the atom.

• Most atoms are neutral.
• Thus most atoms have an equal number of protons
  and electrons.
• Remember that the number of protons determines
  the element.

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What is an ion?

• When an atom gains an extra electron,
• it is negatively charged. It is then a negative
  ion.
• When an atom loses an electron,
• it is positively charged. It is then a positive
  ion.

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An Atomic Model

• Objects are made up of atoms
• Atoms have positively charged, massive protons
  and negatively charged, light electrons
• The electrons can move more easily than the
  protons
• We model objects as sets of positive charges
  fixed in place and negative charges that can
  shift in response to other charges
• Analogy Desks as protons, students as electrons

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What does it mean to say that an object is
neutral?

• A neutral object has no net charge.
• A neutral object has equal amounts of positive
  charge and negative charge.

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What does it mean to say that an object is
charged?

• A charged object has a net charge.
• A positively charged object has a greater
  quantity of positive charge than negative charge.
• A negatively charged object has a greater
  quantity of negative charge than positive charge.

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Electrons move, Protons dont!

• Protons are very massive. They have too much
  inertia. They are in the center of the atom.
• Electrons are outside the nucleus. It is easier
  to move particles that are on the perimeter.
• Generally, the electrons in the outermost orbits
  are the ones that transfer and move first.

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What do you have to do to make an object
positively charged?

• You need to take electrons away from the object.

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What do you have to do to make an object
negatively charged?

• You need to transfer electrons to the object.

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What does it mean for an object to be polarized?

• When an object is polarized, its charges have
  shifted so that one side of the object has a net
  negative charge and the other side of the object
  has a net positive charge.
• A polarized object might be neutral (no net
  charge) or have a net charge.

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Can charge be created or destroyed?

• No! Charge can not be created or destroyed.
• Conservation of Charge.

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Conductors vs. Insulators

• Loosely bound electrons.
• Allow the flow of electric charge.
• Examples include metals.

• Tightly bound electrons.
• Slow the flow of electric charge.
• Examples include rubber, plastic and Styrofoam.

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Coulombs Law

• Two charged objects exert a force on each other.
• The magnitude of the force is directly
  proportional to the product of the quantities of
  charge.
• The magnitude of the force is inversely
  proportional to the distance between the objects
  squared.

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More Coulombs Law (Well come back to this
later)
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Charged objects exert forces on each other.

• Like charges repel.
• Two positively charged objects repel each other.
• Two protons repel each other.
• Two negatively charged objects repel each other.
• Two electrons repel each other.
• Opposite charges attract.
• A positively charged object and a negatively
  charged object attract each other.
• A proton and an electron are attracted to each
  other.
• Newtons Third Law Equal and Opposite Forces.

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Interactions with Neutral Objects

• Remember that a neutral object is comprised of
  innumerable positively and negatively charged
  particles.
• A charged object (positive or negative) will be
  attracted to a neutral object.
• Well discuss exactly why later

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Consider two identical charged particles near
each other as shown.

• The force exerted by q1 on q2 points
• Left
• Right
• Up
• Down
• Nowhere there is no force.

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Two air pucks each carry a charged sphere.

• The sphere on the right carries three times as
  much charge as the sphere on the left.
• Which force diagram correctly shows the direction
  and magnitude of the electrostatic forces?

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What if one object is neutral?
Note that a neutral object and a charged object
will attract each other.

• A charged object will polarize a neutral (or
  weakly charged) object.
• Animation of Polarization
• Illustrations of Polarization

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Three pith balls hang from threads

• A pith ball is a small, light sphere coated with
  conductive material.
• The coating allows it to be easily charged or
  grounded, and its small mass allows it to respond
  easily to small forces.
• May we go on?

• Wait a minute!Whats a pith ball?

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Three pith balls each hang from an insulating
thread.

• PB 1 and PB 2 repel each other.
• PB 2 and PB 3 repel each other.
• Which of these statements is true about the
  charges on the PBs?

1. 1 and 3 have charges of opposite sign.
2. 1 and 3 have charges of the same sign, 2 has
   opposite charge.
3. All three have charges of the same sign.
4. One of the objects carries no charge.
5. We need more data to determine the signs of the
   charges.

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• And now for a similar, yet different, problem

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Three pith balls each hang from an insulating
thread.

• PB 1 and PB 2 attract each other.
• PB 2 and PB 3 repel each other.
• Which of these statements is true about the
  charges on the PBs?

1. 1 and 3 have charges of opposite sign.
2. 1 and 3 have charges of the same sign.
3. All three have charges of the same sign.
4. One of the objects carries no charge.
5. We need more data to determine the signs of the
   charges.

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Now to change the subject

• Youve been looking at the forces exerted on (or
  by) charged objects.
• Now, consider how objects become charged, or lose
  their charge.

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Three Charging Methods

• Triboelectricity
• Charging by Conduction
• Charging by Induction
• Animations of Charging and Grounding

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Conduction

• Example Touching charged styrofoam to
  electroscope
• Works best from conductor to conductor
• Requires the objects be in contact (or close
  enough for a spark)
• The total charge remains constant
• When two identical conductors come in contact,
  they share the total charge equally

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Induction

• Example Holding styrofoam close to electroscope,
  briefly touching scope with finger, then
  removing styrofoam
• A charged object charges a neutral conductor
  without contact
• The conductor does make contact with a neutral
  object
• The conductor ends up with a charge opposite that
  of the charged object brought near

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Triboelectric

• Examples Tape rubbing wool and styrofoam
• Two dissimilar materials brought into contact and
  separated will separate their charge
• The contact can involve friction or bonding
• Works best with insulators

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More on Triboelectricity

• Charge separation when two insulators are rubbed
  together.
• One of the insulators is more likely to grab
  electrons and one of the insulators is more
  likely to donate electrons.
• CHARGE IS NOT CREATED, electrons are transferred.

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Charging by Conduction

• Suppose you have two identical metal spheres,
  imaginatively named A and B. Each sphere is on an
  insulating stand.
• Originally, A has 10 units of charge, and B is
  neutral. If the two spheres are briefly touched
  together, then separated, what is the charge on
  each?
• A has 10 units, B has zero.
• B has 10 units, A has zero.
• A has 10 units, and B also has 10 units.
• A and B each have 5 units.
• A and B each have zero charge.

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Charging by Induction

• Notice that we are charging by induction,
  grounding the side of the sphere next to the
  positive object.
• What will the final charge on the sphere be?

1. Positive
2. Negative
3. Neutral
4. It depends

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Triboelectric Charging

• You rub a balloon against your hair, and the hair
  becomes positively charged. This means that
• Electrons moved from the balloon to your hair.
• Protons moved from the balloon to your hair.
• Protons moved from your hair to the balloon.
• Electrons moved from your hair to the balloon.
• The rubbing destroyed electrons in your hair,
  leaving it positive.

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Triboelectric Series

• A Triboelectric Sequence
• ELECTRON GRABBERS ELECTRON DONORS
• Rubber Amber Cotton Silk Cat fur
  Wool Glass Rabbit fur
• If you rub cotton with amber, which becomes
  positive?A. Amber B. Neither C. Cotton
• Which of the following can make glass
  negative?A. Amber B. Cat fur C. Rabbit fur

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Neutralizing/Grounding Objects

• When a charged object comes in contact with a
  very large, neutral conductor, the object becomes
  neutralized.
• Earth itself is a large, neutral conductor, so it
  neutralizes charged objects quite well.

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Review

• Charged objects exert forcesLike repels
  likeOpposites attractCharge and neutral attract
• How objects get charged or neutralizedConduction
  InductionTriboelectricGrounding

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The End

• Hooray!

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