Download Coulomb`s Law-ppt

Electricity and Magnetism
Electric Charges and Forces
Electric Charge
Coulomb’s Law
Objectives
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Describe and calculate the forces between like and unlike
electric charges.
Identify the parts of the atom that carry electric charge.
Apply the concept of an electric field to describe how
charges exert force on other charges.
Sketch the electric field around a positive or negative
point charge.
Describe how a conductor shields electric fields from its
interior.
Describe the voltage and current in a circuit with a
battery, switch, resistor, and capacitor.
Calculate the charge stored in a capacitor.
Vocabulary Terms
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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
Electric Charge
Key Question:
How do electric charges
interact?
Electric Charge
 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.
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.
Electric Charge
 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.
Electric forces
 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 current
 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)
Calculate current
 Two coulombs of charge pass through a wire in five
seconds.
 Calculate the current in the wire.
Conductors and insulators
 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 other.
Coulomb's Law
 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
Charges (C)
r2
Distance (m)
Coulomb's Law
 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
 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.
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 onefourth its original value (1/4).
 This relationship is called an
inverse square law because force
and distance follow an inverse
square relationship.
Calculating force
 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.
 1) You are asked to calculate the force and compare it to a person’s
weight.
 2) You are given the charges and separation, and the mass of the
person.
 3) Use Coulomb’s law, F= -Kq1q2/d2, for the electric force and F=mg for
the weight.
 4) Solve:
 F = (9×109 N•m2/C2)(0.0001C)(.0001C) ÷ (0.1 m)2 =
9,000 N
 The weight of a 70 kg person: F = mg = (70 kg)(9.8
N/kg) = 686 N
 The force between the charges is 13.1 times the weight
of an average person (9,000 ÷ 686).
Fields and forces
 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.
Fields and forces
 Mass creates a gravitational field that exerts forces on
other masses.
Fields and forces
 Gravitational forces are far weaker than electric forces.
Drawing the electric field
Electric fields and electric force
 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.
Electric fields and electric force
 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.
Accelerators
 An electric field can be
produced by maintaining a
voltage difference across any
insulating space, such as air or a
vacuum.
 Electric fields are used to create
beams of high-speed electrons
by accelerating them.
 Electron beams are used in x-ray
machines, televisions, computer
displays, and many other
technologies.
Electric shielding
 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.
Coulomb’s Law
Key Question:
How strong are electrical forces?
Capacitors
 A capacitor is a storage device for electric charge.
 Capacitors can be connected in series or parallel
in circuits, just like resistors.
Capacitors
 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:
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.
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How a capacitor works inside
 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.
 Wires conduct charges
coming in and out of the
capacitor.
How a capacitor 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.
Capacitance
The ability of a capacitor to store charge is called capacitance
(C).
Capacitance
(coulombs/volt)
Charge
(C)
q = CV
Voltage (volts)
Cameras use capacitors to supply quick bursts of
energy to flash bulbs.
Capacitance
 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.
Calculate 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 charge.
Capacitors
Key Question:
How does a capacitor work?
*Students read Section 21.3 BEFORE Investigation 21.3
Application: How a Television Works