Transcript Chapter 14: Electrical Charges and Forces

Electricity
Unit 4: Electricity
Chapter 14: Electric Charges and Forces
 14.1
Electric Charge and Current
 14.2
Electric Current, Resistance, and
Voltage
 14.3
Capacitors
14.1 Investigation: Electric Charge
Key Question:
What is static electricity?
Objectives:
Explain that static electricity is caused by an imbalance of
charge.
 Build an electroscope and describe its function.


Infer that a relationship exists between charge and the
strength of electrical force.
Electric charge
 There
are two kinds of charge (positive and
negative) the electrical force between charges can
attract or repel.
Electric charge
 Electric
charge, like
mass, is also fundamental
property of matter.
 Inside
atoms found in
matter, attraction between
positive and negative
charges holds the atoms
together.
Coulomb
 The
unit of charge is the
coulomb (C).
 The
name was chosen in
honor of Charles Augustin
de Coulomb (1736-1806),
the French physicist who
performed the first
accurate measurements
of the force between
charges.
Neutral objects
 Virtually
all the matter around you has electric
charge because atoms are made of electrons and
protons (and neutrons).
 Because
ordinary matter has zero net (total)
charge, most matter acts as if there is no electric
charge at all.
Charged objects
 An
object is charged when its net charge is not
zero.
 If
you have ever felt a shock when you have
touched a doorknob or removed clothes from a
dryer, you have contacted a charged object.
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 forces
 Electric
forces are
incredibly strong.
 A millimeter
cube of
carbon the size of a
pencil point contains
about 77 coulombs of
positive and negative
charge.
Coulomb’s law
 The
force between two charges depends on the
charge and the distance.
 Coulomb’s
law explains the relationship between
the amount of each charge (q1 and q2), the distance
between their centers (r), and the electrical force
(FE).
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.
Coulomb's Law
 Coulomb’s
law is similar in form to Newton’s law
of universal gravitation.
Constant
9 x109 N.m2/C2
Force
(N)
F = K q1 q 2
Charges (C)
r2
Distance (m)
Electrostatics
 Electrostatics
is the
part of physics that
studies the forces
created by unmoving
charges.
 A photocopier
uses
electrostatic forces.
Using Coulomb’s law
Two steel marbles are each given a static
electric charge of 0.001 coulombs. Calculate
the size of the force on the marbles, in newtons, if
they are held 2 m apart.
Looking for: …force
Given: …charges (.001 C), distance (2 m)
Relationships: Use Coulomb’s law:
F = -Kq1q2 ÷ r2
4. Solution:
F = -(9 x 109 Nm2/C2) (0.001 C)(0.001 C) ÷ (2 m)2
= 2,250 N
1.
2.
3.
Electric forces
 The
forces between the two kinds of charge can
be observed with an electroscope.
Electric forces
 Once
an electroscope is charged it can be used to
test other charged objects.
Charging by friction
 Under
dry conditions, a
balloon rubbed on hair will
transfer electrons from hair
to the balloon.
 This
is called charging by
friction.
 Objects
charged by this
method will attract each
other.
Once the balloon is
charged, it can also stick
to a neutral wall through
“polarization”
Charging by induction
 Charging
by induction
is a method of using one
object to charge another
without changing the net
charge on the first
object.
Induction

To charge by induction, the
electroscope is first connected by a
wire to a large neutral object.

When the balloon comes near, the
charge on the balloon induces an
opposite charge to flow through the
wire onto the electroscope.

The wire is then removed quickly so
the charge on the electroscope
cannot flow back where it came
from.

The electroscope stays charged
after the balloon is removed.

The leaves spread apart because
the added charges repels each other
and spread out onto the
electroscope leaves.
Unit 4: Electricity
Chapter 14: Electric Charges and Forces
 14.1
Electric Charge and Current
 14.2
Electric Current, Resistance, and
Voltage
 14.3
Capacitors
14.2 Investigation: The Flow of Electric
Charge
Key Question:
How much charge moves
when current flows?
Objectives:
Discuss how the voltage across a capacitor varies with
time as it discharges.
 Use Ohm’s law to calculate the current in a circuit when
voltage and resistance are known.
 Create a current versus time graph and use it to calculate
the electrical charge flowing in a circuit.

Electric charge and current
 The direction of current was historically defined as
the direction that positive charges move.

Benjamin Franklin first used
the terms “positive” and
“negative” to describe charge.

According to Franklin’s theory,
a positive object’s extra fluid
naturally flowed toward a
negative object.
Conventional current
 Because
of Franklin’s work,
the direction of electric current
is defined as going from
positive to negative.
 Long
after Franklin’s work,
scientists discovered that
current in wires is the flow of
electrons which move in a
circuit from negative to
positive.
Electric charge and 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 charge and current
 In
a conducting metal like as
copper, the atoms of copper
bond together by sharing
electrons.
 Some
of the electrons can
move freely anywhere within
the copper.
Electrons in random motion
 If
a copper wire is not
connected to a battery, the
free electrons move
around at high speeds.
 They
have no net motion
because as many move
one way as the other way.
Electrons and drift velocity
 If
voltage is applied to a
copper wire, electrons
slowly drift while
randomly colliding with
atoms in the wire.
 This
drift velocity is
what creates electrical
current.
Conductors and insulators
A semiconductor has a few free electrons and
atoms with bound electrons that act as insulators.
Voltage and charge

Now that you know that current is really moving charge, how
do we understand voltage in terms of charges?

Voltage measures electrical potential energy per unit of
charge.

One volt is 1 joule per coulomb.
Unit 4: Electricity
Chapter 14: Electric Charges and Forces
 14.1
Electric Charge and Current
 14.2
Electric Current, Resistance, and
Voltage
 14.3
Capacitors
14.3 Investigation: Making an Electrophorus
Key Question:
How do electric charges
interact?
Objectives:

Triboelectrically charge different materials.

Use a triboelectric series to make predictions about
charged objects.

Make an electrophorus and explain how it works.
Capacitors
 A capacitor
is a storage device for electric charge.
 Capacitors can be connected in series or parallel
in circuits, just like resistors.
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.
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:
1.
2.
3.
4.
The amount of charge already in
the capacitor.
The voltage applied to the capacitor
by the circuit.
Any circuit resistance that limits the
current flowing in the circuit.
The capacitance of the capacitor.
Current and voltage change with time

As the capacitor charges, the current in the circuit
decreases.

As the voltage on the capacitor increases, the circuit’s
voltage difference decreases and so does its current flow.
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.
Variables in a capacitor
The amount of charge a
capacitor can store
depends on three factors:
The insulating ability of the
material between the positive
and negative plates.
2. The area of the two plates
(larger areas can hold more
charge).
3. The separation distance
between the plates.
1.
Discharging capacitors
 If
connected in a circuit with a low
resistance, a capacitor can discharge
very quickly, creating a large amount
of current.
 This
can be very useful in devices
that require a brief burst of a large
amount of current for their operation,
but can also be dangerous.
How is a
defibrillator
both useful
and
dangerous?
Lightning

Lightning originates in
towering, dark storm
clouds. Inside these
clouds, charges begin to
separate.

Scientists still don’t
completely understand
how this happens.