The Big Picture

1
The Big Picture

• Whats the world made of what holds it together?

• The Standard Model
• All the known matter particles are composites of
  quarks and leptons, and they interact by
  exchanging force carrier particles
• Whats the world made of
• 6 quarks 6 anti-quarks
• protons and neutrons are made up of quarks
• 6 leptons 6 anti-leptons
• the electron is the most common lepton
• What holds it together
• force carrier particles
• like photons

2
The Big Picture Forces

• Common Forces
• Gravity
• Fma
• Centripetal force
• Friction
• Charged particles
• Magnetism
• Molecular attraction
• Nuclear Weak
• Nuclear Strong

3
How can I pick up small pieces of paper with a
comb?
Electrostatics
Your Prediction

• No problem, a comb will always pick up paper
  pieces
• Are you kidding, a comb cannot pick up paper
  pieces
• Run it through your hair first
• Close one eye, stand on your left leg, and say
  abracadabra when you do it

Your Observation
4
A conductive sphere has a net charge of zero and
is suspended by an electrically insulating
thread. A charged rod is brought in close to the
sphere without touching it.What happens to the
sphere?
Electrostatics
Your Prediction

• It is attracted to the rod
• It is repelled by the rod
• It may be attracted or repelled, depending on
  the charge on the rod (/-)
• It is unaffected by the rod

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Your Observation
5
A conductive sphere has a net charge of zero and
is suspended by an electrically insulating
thread. A charged rod is brought in close to the
sphere without touching it.The sphere is
attracted to the rod. Why?
Electrostatics
Your Prediction

• The sphere gets some electrons from the rod
• Negative and neutral are opposite charges
  andopposite charges attract
• The attraction is somehow magnetic
• None of the above

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Your Observation
6
A conductive sphere has a net charge of zero and
is suspended by an electrically insulating
thread. A charged rod is brought in close to the
sphere and touches the sphere.What happens to
the sphere?
Electrostatics
Your Prediction

• It is attracted to the rod
• It is repelled by the rod
• It may be attracted or repelled, depending on
  the charge on the rod (/-)
• It is unaffected by the rod

Your Observation
7
What happens when two identically charged objects
are brought close together?
Electrostatics
Your Prediction

• They repel
• They attract
• Nothing, there is no force

Your Observation
8
What happens when two differently charged objects
are brought close together?
Electrostatics
Your Prediction

• They repel
• They attract
• Nothing, there is no force

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Your Observation
9
Electric Forces
Draw the force vectors (length proportional to
magnitude) on each of these charged pith balls.
5 Charge
1 Charge






10
Electrostatics

• Summary
• What causes some objects to become charged?
• Rubbing
• How can we test if an object is charged?
• It attracts a small neutral object
• Can charge be transferred from one object to
  another?
• Yes
• How is the charge transferred?
• By contact
• How many different kinds of charge are there?
• At least two kinds
• What can we say about their forces?
• Opposite charges attract
• Like charges repel

11
How can we determine if an object is positively
charged?
Electrostatics
Your Prediction

• See if it attracts a neural object
• See if it is attracted to a negatively charged
  object
• Either of the above is sufficient
• None of the above are sufficient

Your Observation
12
Electrostatics
Metal
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Charged Rod
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Connecting Rod
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13
Consider two objects A and B. Object A has a net
charge, object B is uncharged. Based on this
information, can you tell whether or not either
object is a conductor or an insulator?
Electrostatics
Your Prediction

• Object A is a conductor and object B is an
  insulator
• Object A is a insulator and object B is an
  conductor
• Both objects are conductors
• Both objects are insulators
• Object A is a conductor and we cant tell what
  object B is
• Cant tell what object A is and object B is an
  insulator
• Cant tell what either object is

14
What happens when a neutral pith ball is brought
close to one end of a metal rod and a charged rod
is brought close to the other?
Electrostatics
Your Prediction

• It is attracted to the rod
• It is repelled by the rod
• Nothing, it will feel no force

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Metal Rod
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Unchargedpith ball
Charged Rod
Your Observation
15
Compared to the gravitational force, electric
forces are
Electrostatics
Your Prediction

• Much weaker than gravitational forces
• About the same as gravitational forces
• Much stronger than gravitational forces
• We cannot compare them

Your Observation
16
Two neutral, conductive spheres are in contact
with one another. A charged rod is brought close
to sphere A. The spheres are separated. What is
the charge on each sphere?
Electrostatics
Your Prediction

• Both remain neutral
• Both are positive
• Both are negative
• A is positive while B is negative
• A is negative while B is positive
• Cant tell

Metal
Metal
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B
A
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Your Observation
Charged Rod
17
Your instructor rubs a balloon against his hair.
Which statements are correct?
Electrostatics
Your Prediction

• The balloon becomes electrically charged
• Your instructors hair becomes electrically
  charged
• The balloon will be attracted to an electrically
  insulating wall
• The balloon will be attracted to an electrically
  conducting wall
• Your instructors hair will stand up and look
  funny (if he has hair)
• All of the above

Your Observation
18
Electrostatics
Electroscope
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Charged Rod
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19
A lightning rod works to protect buildings from
lightning strikes by
Lightning Rod
Your Prediction

• Creating a high electric field that attracts the
  lightning away from the building to the rod
• Creating a high electric field that enables
  charges to bleed off eliminating the need for a
  lightning strike
• Reducing the electric field near the rod, thus
  preventing a lightning strike
• We know they work but cannot explain how
• Lightning rods dont really do anything to
  protect buildings

Your Observation