CPO Science
Foundations of Physics
Unit 7, Chapter 21
Unit 7: Electricity and Magnetism
Chapter 21 Electric Charges and Forces
 21.1 Electric Charge
 21.2 Coulomb’s Law
 21.3 Capacitors
Chapter 21 Objectives
1. 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.
Chapter 21 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
21.1 Electric Charge
Key Question:
How do electric charges
interact?
*Students read Section 21.1
AFTER Investigation 21.1
21.1 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.
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.
21.1 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.
21.1 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.
21.1 Electric forces
 The forces between the two kinds of charge can
be observed with an electroscope.
21.1 Electric forces
 Charge can be transferred by conduction.
21.1 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.
21.1 Electric current
 Current is the movement of electric charge through
a substance.
Current
(amps)
I=q
t
Charge that flows
(coulombs)
Time (sec)
21.1 Calculate current
 Two coulombs of charge pass through a wire
in five seconds.
 Calculate the current in the wire.
21.1 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.
21.1 Conductors and insulators
A semiconductor has a few free electrons and atoms
with bound electrons that act as insulators.
21.1 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.
21.2 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)
21.2 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.
21.2 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.
21.1 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.
21.1 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.
21.2 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.
21.2 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.
21.2 Fields and forces
 Mass creates a gravitational field that exerts
forces on other masses.
21.2 Fields and forces
 Gravitational forces are far weaker than
electric forces.
21.2 Drawing the electric field
21.2 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.
21.2 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.
21.2 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 xray machines, televisions,
computer displays, and many
other technologies.
21.2 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.
21.2 Coulomb’s Law
Key Question:
How strong are electrical forces?
*Students read Section 21.2 BEFORE Investigation 21.2
21.3 Capacitors
 A capacitor is a storage device for electric charge.
 Capacitors can be connected in series or parallel
in circuits, just like resistors.
21.3 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.
21.3 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.
21.3 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.
21.3 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.
21.3 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.
21.3 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.
21.3 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.
21.3 Capacitors
Key Question:
How does a capacitor work?
*Students read Section 21.3 BEFORE Investigation 21.3
Application: How a Television Works