Transcript Electric Field Strength

What Gives an Electric Charge?
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An imbalance of protons and electrons.
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Neutral objects have equal numbers of electrons and
protons.
Positively charged objects have more protons than
electrons.
Negatively charged objects have more electrons than
protons.
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When you rub a rubber rod with rabbit fur electrons are
transferred to the rubber rod and it becomes negatively
charged.
Conservations of charge
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When electrons are transferred from one substance to
another the overall charge is conserved.
The SI Unit for Charge is the
Coulomb
The charge of a proton is +1.6 x 10-19
Coulombs.
 The charge of a electron is -1.6 x 10-19
Coulombs.
 This is why an atom is neutral when there
are equal numbers of protons and
neutrons.
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Conductors
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Conductors have electrons that are more
loosely held and therefore allow charges
to flow more easily.
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Good conductors are:
 Metals
 Water
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and solutions with ions present.
Wires are usually composed of copper wire.
Insulators
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Insulators have more tightly held electrons and
are not good conductors of electricity.
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Good Insulators are:
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Rubber
Plastic
Air
Silk
Good electrical insulators are usually also good
thermal insulators.
Copper wires are usually encased in rubber to
prevent shock.
Millikan
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In 1909 Robert Millikan a physicist at the
University of Chicago ran an experiment that
revealed that charges occur in discrete
quantities.
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These quantities always occur in whole number
intervals and are said to be “quantized”.
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These quantities are known as electrons which have a
charge of -1.6 x 10-19 Coulombs.
Electrons cannot be divided into fractions. Any object that is
charges has a surplus or deficit of some whole number of
electrons.
# of electrons = total charge/charge of an electron.
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If something has a -1C of charge it contains 6.2 x 1018
electrons.
Millikan’s Oil Drop Experiemnt
Charging Objects
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Charging by Friction
Electrical charges can be transferred from
one object to another.
 Two substances can be rubbed together.
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 Rub
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a balloon with wool.
The wool will transfer electrons to the balloon and the
balloon will become negatively charged.
 Static
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Cling
Clothing in the dryer rub electrons off each other and
they become oppositely charged and stick together.
Conduction
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Objects can be charged by contact.
Since electrons can move from one place to
another they will move into another object to
even out the overall charge of the two objects.
 Conservation of charge.
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Induction
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Charges within a neutral conductor can move.
Objects can gain an induced charged when a
charges object is brought near a neutral object.
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Charges become polarized.
This induced charge is temporary if the objects never
touch.
Once the charged object is removed the charges
evenly spread out again.
Grounding
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Grounding an object provides a conduit in which
the excess electrons can leave the object.
Objects that are charged by induction can gain
a permanent charge by grounding.
Objects that are charged by induction and then
grounded end up with an opposite charge than
the conductor.
Objects charged by conduction end up with the
same charge as the original conductor.
Grounding
Grounding
Charging by Induction
Electric Forces
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Electric Force
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Force between charges
 Repulsion
between like charges and attraction
between opposite charges.
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Coulomb’s Law
 Fe
= k q1q2
d2
The electric force decreases with the square of the
distance between the charges
Look Familiar?
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Remember Fg = G m1m2
d2
Quantity of charge is like the mass of an object
Let’s compare G and k
G = 6.67 x 10-11 N m 2/kg
k = 8.99 x 109 N m 2/C2
Which force is stronger?
A pair of positively charged particles of 1 C each 1
meter apart would experience a repulsive force of
9.0 x 109N
The weight of a battleship!
Law of Superposition
The resultant force on any given charge is
the vector sum of all the forces acting on
that charge.
 Applied to three or more charges with a
certain distance to each other.
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Must consider whether or not the force is
attractive or repulsive.
Gravitational and Electric Fields
 Review of gravitation
 Gravitational field strength “g” gives the ratio of force
to mass.
 Fg = mbodymearth
Fg is the force of gravity (weight)
d2
d is the distance from the center of the earth
 We can think of mearth as creating the field and mbody
“experiencing the” force in response to the field.
 “g” is the field strength of the earth’s gravitational field
(9.8N/kg at the surface).
 We can calculate “g” by using the equation:
 g = G mearth
d
2
Electric Field Strength
 Electric field strength E gives the ratio of force to charge.
Fe = q 2E
like Fg = mg
Fe = electrostatic force
q2= charge experiencing the field
E = field strength
 E = kq1
gives the electric field strength at any given
d2
distance from the charge creating the field.
 q1 is the charge creating the field
 K is 8.99 x 109 N m/C2
 d is the distance from the center of the charge to any given point in the field.
 Si unit for field strength is N/C
Field Lines Associated with Electric
Fields
 Stronger charges have more concentrated field
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lines.
A positive charge creates a field that emanates
outward from the charge.
A negative charge creates a field that is directed
towards the charge from the charge.
Field lines are always perpendicular to the
charge or charged object.
Field lines never intersect. (this would indicate
two different values of force at the same point)