Download Electric Charge

Electricity and Magnetism
Electric Charges and Forces
Electric Charge
Coulomb’s Law
Capacitors
Objectives
Describe and calculate the forces between like
and unlike electric charges.
2. Identify the parts of the atom that carry electric
charge.
3. Apply the concept of an electric field to describe
how charges exert force on other charges.
4. Sketch the electric field around a positive or
negative point charge.
5. Describe how a conductor shields electric fields
from its interior.
6. Describe the voltage and current in a circuit with
a battery, switch, resistor, and capacitor.
7. Calculate the charge stored in a capacitor.
1.
Vocabulary Terms: homework
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charge
electrically neutral
static electricity
positive charge
negative charge
electric forces
charge by friction
electroscope
protons
neutrons
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electrons
gravitational field
charged
induction
Coulomb’s law
capacitor
parallel plate
capacitor
 microfarad
 coulomb
 electric field
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capacitance
charge
polarization
shielding test
charge
farad
field inverse
square law
discharged field
lines
Electrostatics
In Physics
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Take notes on key
points during the
film
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E:\Physics
2010\Unit 8
Electrostatic
Forces\Physics__A_
World_in_Motion__
Electrostatics.asf
Electric Charge
Key Question:
How do electric
charges interact?

All ordinary matter
contains both positive and
negative charge.

You do not usually notice
the charge because most
matter contains the exact
same number of positive
and negative charges.

An object is electrically
neutral when it has equal
amounts of both types of
charge.
21.1 Electric Charge
Objects can lose or gain
electric charges.
 The net charge is also
sometimes called excess
charge because a charged
object has an excess of either
positive or negative charges.
 A tiny imbalance in either
positive or negative charge on
an object is the cause of static
electricity. Charge
Electric

Electric charge is a
property of tiny particles
in atoms.
 The unit of electric
charge is the coulomb
(C).
 A quantity of charge
should always be
identified with a positive
or a negative sign.
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Electric Charge
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Electric forces are created between all electric
charges.
Because there are two kinds of charge (positive
and negative) the electrical force between charges
can attract or repel.
Electric forces

The forces between the two kinds of charge
can be observed with an electroscope.
Electric forces

Charge can be transferred by conduction.
Electric forces
 The direction of current was historically defined as the
direction that positive charges move.
 Both positive and negative charges can carry current.
In conductive liquids (salt
water) both positive and
negative charges carry
current.
 In solid metal conductors,
only the electrons can
move, so current is carried
by the flow of negative
electrons.

Electric current

Current is the movement of electric charge
through a substance.
Current
(amps)
I=q
t
Charge that flows
(coulombs)
Time (sec)
Electric current
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Two coulombs of charge pass through a wire
in five seconds.
Calculate the current in the wire.
Calculate current
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All materials contain
electrons.
The electrons are what
carry the current in a
conductor.
The electrons in insulators
are not free to move—they
are tightly bound inside
atoms.
Conductors and insulators
A semiconductor has a few free electrons and atoms
with bound electrons that act as insulators.
Conductors and insulators
When two neutral objects
are rubbed together,
charge is transferred from
one to the other and the
objects become oppositely
charged.
 This is called charging by
friction.
 Objects charged by this
method will attract
each
Conductors
and
insulators
other.
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Coulomb’s law relates the force between two
single charges separated by a distance.
Constant
9 x109 N.m2/C2
Force
(N)
F = K q1 q2
Coulomb's Law
Charges (C)
r2
Distance (m)
The force between
two charges gets
stronger as the
charges move closer
together.
 The force also gets
stronger if the
amount of charge
becomes
larger.
Coulomb's Law
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The force between two
charges is directed
along the line
connecting their
centers.
Electric forces always
occur in pairs
according to Newton’s
third law, like all
forces.
Coulomb's
Law
The force between
charges is directly
proportional to the
magnitude, or amount,
of each charge.
 Doubling one charge
doubles the force.
 Doubling both charges
quadruples the force.
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Coulomb's Law
The force between charges is
inversely proportional to the
square of the distance
between them.
 Doubling the distance reduces
the force by a factor of 22 =
(4), decreasing the force to
one-fourth its original value
(1/4).
 This relationship is called an
inverse square law because
force and distance follow an
inverse square relationship.
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Coulomb's Law
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Two balls are each given a static electric charge of
one ten-thousandth (0.0001) of a coulomb.

Calculate the force between the charges when they
are separated by one-tenth (0.1) of a meter.

Compare the force with the weight of an average
70 kg person.
Calculating force
The concept of a field is used to describe any
quantity that has a value for all points in space.
 You can think of the field as the way forces are
transmitted between objects.
 Charge creates an electric field that creates
forces on other charges.
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Fields and forces
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Mass creates a gravitational field that
exerts forces on other masses.
Fields and forces

Gravitational forces are far weaker than electric
forces.
Fields and forces
Drawing the electric field
On the Earth’s surface, the gravitational field creates
9.8 N of force on each kilogram of mass.
 With gravity, the strength of the field is in newtons
per kilogram (N/kg) because the field describes the
amount of force per kilogram of mass.
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Electric fields and electric force
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With the electric field, the strength is in newtons
per coulomb (N/C).
The electric field describes the amount of force
per coulomb of charge.
Electric fields and electric force
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An electric field can be
produced by maintaining a
voltage difference across any
insulating space, such as air
or a vacuum.
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Electric fields are used to
create beams of high-speed
electrons by accelerating
them.
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Electron beams are used in
x-ray machines, televisions,
computer displays, and many
other technologies.
Accelerators
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Electric fields are created all
around us by electric appliances,
lightning, and even static
electricity.

These stray electric fields can
interfere with the operation of
computers and other sensitive
electronics.

Many electrical devices and wires
that connect them are enclosed in
conducting metal shells to take
advantage of the shielding effect.
Electric shielding
Key Question:
How strong are electrical forces?
Coulomb’s Law
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A capacitor is a storage device for electric charge.
Capacitors
Capacitors can be connected in series or parallel
in circuits, just like resistors.
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A capacitor can be charged by connecting it to a
battery or any other source of current.
A capacitor can be discharged by connecting it to
any closed circuit that allows current to flow.
Capacitors
The current flowing into or out
of a particular capacitor
depends on four things:
1. The amount of charge already
in the capacitor.
2. The voltage applied to the
capacitor by the circuit.
3. Any circuit resistance that
limits the current flowing in the
circuit.
4. The capacitance of the
capacitor.
Capacitors
The simplest type of
capacitor is called a
parallel plate capacitor.
 It is made of two
conductive metal plates
that are close together,
with an insulating plate
in between to keep the
charges from coming
together.
 How
Wires conduct
charges
a capacitor
coming in and out of the
capacitor.
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works inside
The amount of charge a capacitor can store
depends on several factors:
1. The voltage applied to the capacitor.
2. The insulating ability of the material
between the positive and negative plates.
3. The area of the two plates (larger areas
can hold more charge).
4. The separation distance between the
plates.
How a capacitor works inside
The ability of a capacitor to store charge is called
capacitance (C).
Capacitance
(coulombs/volt)
Charge
(C)
q = CV
Voltage (volts)
Capacitance
Cameras
use capacitors to supply quick bursts of
energy to flash bulbs.
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Capacitance is measured in farads (F).
A one-farad capacitor can store one coulomb of
charge when the voltage across its plates is one
volt.
 One farad is a large amount of
capacitance, so the microfarad
(μF) is frequently used in place
of the farad.
Capacitance
A capacitor holds 0.02
coulombs of charge
when fully charged by
a 12-volt battery.
 Calculate its
capacitance and the
voltage that would be
required for it to hold
one coulomb of
Calculate
capacitance
charge.
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Capacitors
Key Question: homework
How does a capacitor work?
Application: How a Television
Works