Transcript AP Electric Forces & Fields

Electric Forces & Fields
Chapter 18
The Origins of Electricity
• In the mid 18th century
Ben Franklin created the
idea on positive and
negative electric charge.
• It wasn’t until 150 years
later the electron was
discovered.
• Franklin described an
electric “fluid” that would
flow depending on
electric pressure.
Electric Charge
• In 1909 Robert Millikan discovered charge
was “quantized.”
• This means there is a smallest amount.
• Thing about it like this, In order to have a
car all the pieces must be there. If you try
to take the engine out, it’s no longer a car.
• Electric charge is much the same.
• The electron has a set charge, take some
away and it’s no longer an electron.
The Millikan Experiment
• Click here to recreate the Millikan oil-drop
experiment
Charged Particles
• Protons (+e): Mass = 1.673 x 10-27 kg,
Charge = 1.60 x 10-19 C
• Neutron: Mass = 1.675 x 10-27 kg, Charge
=0
• Electron (-e): Mass = 9.11 x 10-31 kg,
Charge = -1.60 x 10-19 C
e = 1.60 x 10-19 C
Neutral Objects
• If the number of electrons equals the
number of protons the object is said to be
electrically neutral.
• In general q (charge) = Ne, where N is an
integer.
• Since proton are much more difficult to
remove, most objects are charged by
removing or adding electrons.
Charged Objects
• When two dissimilar materials are rubbed
together electron usually go from one to
the other.
• Look on the triboelectric scale to see
which way they go, positive or negative
• Also, charge is conserved.
• The net charge of an isolated system is
constant
Like and Unlike charges
• Like charges repel
each other
• Unlike (opposite)
charges attract
Conductors & Insulators
• Materials that have lose valence electrons
are conductors
• Materials with tightly held valence
electrons are insulators
• Can you think of some?
Charging by Induction (Conductors)
Click on the picture to open an applet
Polarization (Insulators)
Coulomb’s Law
• F = 1/(4peo) q1q2 / r2
– F = Force (N)
– eo = 8.85 x 10-12 (electric permittivity of a
vacuum)
– q = charge (Coulombs)
– r = distance between charges
• 1/(4peo) = k
Point Charges
• When more than two charges are acting
on each other we sum the forces.
• Treat each pair independently, then add
the forces.
r2
r1
q1
q2
q3
Point Charges in 2D
• When more than two charges are acting on each
other in 2D, sum the forces for x and y
dimensions.
• Again, treat each pair independently, then add
the forces.
r2
q3
q1
q2
r1
Electric Field
• Just like mass create gravitational fields,
charges create electric fields
• With gravity the field strength is measure
as Newton per kilogram
• What do you think Electric fields are
measured in?
Newton's per Coulomb
Measuring the Electric Field
• If the unit is Newton's per Coulomb, what
is the equation?
E = F / qo
• Simple enough, right.
• E = Electric Field
• F = Force
• qo = charge producing field
Summing electric Fields
• It is the surrounding charges that create
an electric field at a given point in space.
• Look at Example 8
Parallel Plate Capacitors
• To store charge a
capacitor is used.
• The most common
type is a parallel plate
capacitor.
• Charge is spread
uniformly, not really,
through out the plate
• E = q /eoA = s / eo
• s = charge Density
Picturing the Electric Field
Click on the picture to open an applet
Electric Field Rules
• Fields start at positive and end at negative, or
start or end at infinity.
• This is by convention. The field is said to predict
the movement of a positive charge.
• The density of lines should represent the
strength of the field.
• A positive charge will have a velocity tangent to
a field line.
• Field lines do not actually exist since the are an
infinite number of paths a test charge can take.
Electric Fields Inside Conductors
• Excess electric charge moves
to the surface of a conductor
• At equilibrium the electric field
inside a conductor is zero
– This comes from the fact that free
electrons will not move inside the
conductor.
• So the electric field lines don’t
penetrate the conductor
• The electric field outside a
conductor is perpendicular to
the surface
Click here for more info
Triboelectric Scale
• Human hands (usually too
moist, though) (Very positive)
• Rabbit Fur
• Glass
• Human hair
• Nylon
• Wool
• Fur
• Lead
• Silk
• Aluminum
• Paper
• Cotton
• Steel (Neutral)
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Wood
Amber
Hard rubber
Nickel, Copper
Brass, Silver
Gold, Platinum
Polyester
Styrene (Styrofoam)
Saran Wrap
Polyurethane
Polyethylene (like Scotch
Tape)
Polypropylene
Vinyl (PVC)
Silicon
Teflon (Very negative )