Download Charging - University of Hawaii Physics and Astronomy

Physics 272
Electricity
Magnetism
Geometric Optics
Spring 2015
Prof. Philip von Doetinchem
[email protected]
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Electric charge
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Four different kind of forces exist:
Gravitation – Electromagnetic – Weak – Strong
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We will cover electromagnetism
Strength of interaction is determined by charge
(similar to mass for gravity)
Charges are accelerated by electric forces similar to
masses in gravitational fields
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Electric currents are streams of charged particles
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Charges exert electrostatic forces on each other
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Electric charge
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Important for chemistry, biology, technology
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Charges in motion: magnetism and nature of light
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Two positive charges or
two negative charges
repel each other. A
positive charge and a
negative charge attract
each other.
Source: http://en.wikipedia.org/wiki/Electric_charge
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Electric charge and the structure of matter
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Atoms are made of protons (positive charge), neutrons, and
electrons (negative charge)
Attractive electric force keeps atoms together
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Story more complicated: electrons
have to obey Pauli principle,
uncertainty relation
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Nucleus bound by strong
nuclear force
Protons and electrons have the
same absolute charge value, but
the proton is ~2000x heavier
Source: http://en.wikipedia.org/wiki/Atom
A atom is neutral if the number of protons and electrons is the
same
Ion: positive: electron was removed
negative: additional electrons
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Electric charge is conserved
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The algebraic sum of all the electric charges in
any closed system is constant.
If an object is charged it acquires or looses negative
charge → therefore it gains or loses mass
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Charge cannot be created or destroyed
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Universal conservation law
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The magnitude of charge of the electron or
proton is a natural unit of charge.
Charge can only be changed in integer numbers
(quantized)
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Charging
https://www.youtube.com/watch?v=KtvdaeYKCLE
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Charging
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Electrons have to be transferred from one object to
the other
A plexiglass rod rubbed with fur transfers electrons to
the fur: net positive electric charge
rubber rod rubbed with fur picks up a negative electric
charge
rods brought in contact with hanging tinsel
→ tinsel becomes charged and flares out
→ other rod will now attract the tinsel
→ touching the tinsel with your hand discharges it.
touching a charged rod to the balloon transfers
charge
→ balloon is repelled away from the rod
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Charging
Examples:
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Carpet fibers on a dry day
→ charge builds up on you
→ rapid charge transfer to
doorknob
Likes to lose electrons
your hand
glass
your hair
nylon
wool
fur
silk
paper
cotton
hard rubber
polyester
polyvinylchloride plastic
Likes to accept electrons
http://phet.colorado.edu/en/simulation/travoltage
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Conductors, insulators, induced charges
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Some materials are conductors of electricity,
some are insulators
Conductors allow charges to easily move through
them
Most metals: good conductors: outer electrons
of atoms become detached and move freely
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Insulator: electrons are bound and cannot move
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Semiconductors have intermediate properties
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Charging by induction
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Free electrons are
repelled and cannot
escape
Negative charge on one
site, positive charge on
the other:
induced charge
Forces reach
equilibrium
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Electric forces on uncharged objects
http://www.youtube.com/watch?v=TE2r0vjkXK0
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Charging by induction
https://phet.colorado.edu/en/simulation/balloons
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Electric forces on uncharged objects
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A charged body can exert forces on
uncharged objects
Charge-induced effect
Charged balloon causes slight shifting of
charge in the ceiling
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If the distance between two
charged objects is much larger
than their dimension
→ charges can be treated as
point charge
Strength of electric force is
proportional to 1/r2
Source: http://en.wikipedia.org/wiki/Charles-Augustin_de_Coulomb
Coulomb's law
Charles-Augustin de Coulomb
(1736-1806)
The magnitude of the electric force
between two point charges is directly
proportional to the product of the
charges and inversely proportional
to the square of the distance between them.
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Coulomb's law
https://www.youtube.com/watch?v=B5LVoU_a08c
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Coulomb's law
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Force magnitude is always positive
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Direction is always along the line of the two charges
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If both charges are positive: repulsive
If both charges are negative: repulsive
If have charges have opposite charge signs: attractive
Newton's third law is valid for charges:
actio est reactio
Form of force is similar to gravity, but two different types of forces
(gravity is never repulsive)
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Fundamental electric constants
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constant k depends on the system of units, in SI
(Système international d’unités) units:
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Electric current: charge per second: Ampere
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To make things easier for the following:
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Fundamental electric constants
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Natural unit of charge for proton and electron:
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Charge of 1 electron: -e
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Charge of 1 proton: +e
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Typical charges are 10-9C to 10-6C (1nC to 1C)
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Coulomb's law with vectors
absolute charge values
distance
between
charges
positive or negative
direction comes from
product of q1 and q2
unit vector along the
direct connection between
charges q1 and q2
→ do not confuse the
actual vector and the
unit vector of
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Coulomb's law
http://employees.oneonta.edu/viningwj/sims/coulombs_law_s.html
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Superposition of forces
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Total force is the vector sum of forces:
principle of superposition of forces:
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Electric field and electric forces
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How do electric charges know of each others
existence? What is an electric field?
A single charge causes an electric field in the
surrounding space
If you drop a second charge in this field the two
charges communicate through their fields
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Electric field and electric forces
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The fields are responsible for exerting the electric
force on the other charge
An electric field creates an electric force on a test
charge q0
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Electric field and electric forces
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The electric field concept is again analogous to the
gravitational field
Electrical field is useful because it does not depend
on the charge of the body on which the electric
force is exerted.
Calculation of electric field becomes more
complicated if the charged object is not point like.
Field strength and direction will depend on the
relative position to the object.
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