Transcript ELECTRICITY I

ELECTRICITY I
Statics
1
If you rub a piece of amber
With the fur of a rabbit
Electrostatics if the study
of electrical charges that
can be contained in one
place
It will attract bits of stuff
(paper, leaves, etc)
2
Charges are measured in units
called Coulombs.
• 1 C = 6.25 X 1018 electrons
• The charge
of
one
electron
is
-19
1.602 x 10 C. This magnitude
is called e-.
• e also can be used to represent
a proton charge.
• Charges are often expressed in
units of e. (Ex: 2e, 3e- )
3
Charge and Mass of Atomic
Particles
Particle
electron
proton
neutron
Charge (C)
-19
-1.60x10
-19
+1.60x10
0
Mass (kg)
-31
9.109 x 10
-27
1.673 x 10
-27
1.675 x 10
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The Law of Conservation of
Electric Charge…
• Charge can neither be created
nor destroyed, only transferred.
• Transferring charge creates
ions…which are charged
particles.
• Removing electrons creates a
positive charge.
• Adding electrons creates a
negative charge.
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Electric Charges
Like charges
repel
Unlike charges
attract
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Polarization
• Bringing a charged object near (but not
touching) a neutral object polarizes
(temporarily separates) the charge of the
neutral object.
Like charges in the neutral object are repelled
by the charged object.
Unlike charges in the neutral object are
attracted by the neutral object.
The neutral object returns to normal when the
charged object is removed
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Electric Dipoles
• An object that is electrically neutral
overall, but permanently polarized, is
called an electric dipole.
Example: H20 molecule
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Charging by Contact
• If a charged object is brought in contact
with a neutral object, charges will be
repelled from (or attracted to) the
charged object.
The neutral object will gain a charge of
the same sign as the charged object.
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Charging by Induction
•
Bring a charged object near (but not
touching) a neutral object.
Ground the neutral object.
Remove the ground.
Remove the charged object
The neutral object now has a charge
opposite to the charged object.
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Grounding
• Providing a path from a charged object
to the Earth is called grounding it.
Charges will be attracted from (or
repelled to) the Earth by the charged
object.
Since the Earth is so large, both the
charged object and the Earth are
neutralized.
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Electrons in Solids
• When atoms are arranged in a solid
they “share” electrons
– High electron mobility = conductor
– Low electron mobility = insulator
• In order to produce a net flow of
electrons, they must increase their
energy
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Superconductors
• Superconductors are materials that
lose all resistance to charge movement
at temperatures near absolute zero (0 K
or about -273oC).
Recently, “high temperature” (above 100
K) superconductors have been
discovered.
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The Periodic Table
14
Static charges in Nature
• A typical
thunder cloud
has both + and
– charges
• Lightning is
static
electricity
– Bolt travels
negative to
positive
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LIGHTNING
Though air is not a conductor, the
buildup of charges during a storm
forces the electricity to flow
through the air anyway, ripping
apart gas molecules along the way.
This “energized gas” state is called
plasma - which is a good
conductor.
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Question 1
• Explain from an atomic
standpoint why charge is
usually transferred by electrons.
• Protons are relatively fixed in
the nucleus of an atom, while
electrons can be transferred
from one atom to another.
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Question 2
• Calculate the net charge on a
substance consisting of a
combination of 7.0 x 10 13
protons and 4.0 x 10 13
electrons.
• 4.8 x 10 -6 C
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Question 3
• A negatively charged balloon
has 3.5 mC of charge. How many
excess electrons are on this
balloon?
• 2.2 x 10 13 electrons
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ELECTRICITY I
Coulomb’s Law
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Coulomb experimented with
charged spheres.
• He found that:
– electrostatic force varied inversely
with square of the distance between
the spheres.
– electrostatic force varied directly
with magnitude of the charge.
• He combined these discoveries to
make his law.
Fe = kqq’/d2
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Electrical Forces
• The electrical force between 2 charges
depends on:
The size of each charge
More charge means more force.
The distance between the charges
More distance means less force.
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Comparing gravitational force
to electric force
• Gravity
– Is a weak force
– Acts over long distances
– Is attractive only
• Electricity
– Is a strong force
– Acts over short distances
– Is attractive and repulsive
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What are q,d and k?
• q is the magnitude of
the charge.
– q’ can indicate a test charge
• d is the distance between the
charged particles.
• k is Coulomb’s constant of
proportionality.
– k = 8.99 x 109 N·m2/C2
(Compare this to G which is
6.67 x 10-11 N·m2 / kg2 !)
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Sample Problem:
• The electron and proton of a
hydrogen atom are separated,
on average, by a distance of
about 5.3x10-11 m.
• Find the magnitudes of the
electric force and the
gravitational force that each
particle exerts on the other.
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For the electric force, we use
Coulomb’s Law:
Fe = kqq’/d2
Fe =
(8.99 x 109 N·m2/C2) * (1.6 x 10-19 C) * (-1.6 x 10-19 C)
(5.3 x 10-11)2
=
-8.2 x 10-8 N
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For the gravitational force, we
use Newton’s Law of Gravitation:
F = Gm1m2/d2
Fg =
(6.67 x 10-11 Nm2/kg2) * (9.11 x 10-31 kg) * (1.67 x 10-27 kg)
(5.3 x 10-11 m)2
=
3.6 x 10-47 N
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Question 4
• How does the electric force
between two charges change
when the distance between
them is doubled?
• The force is quartered
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Question 5
• What is the mathematical
representation of Coulomb’s
Law?
• F = kqq’/d2
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Question 6
• What is the numerical value and
unit of Coulomb’s constant?
• 8.99 x 109 N m2 / C2
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ELECTRICITY I
Electric Fields
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The Electric Field
• An electric field caused by a point
charge exerts a force on any
other charge in its environment.
• A collection of all the forces
makes up an electric field.
• Faraday (1791-1867) defined the
electric field as the region of
space around a charged object.
• When another charged object
enters the field, electrical forces
arise.
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Electric Field lines
for a positive point
charge
Electric Field lines
for a negative point
charge
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Electric Dipoles
Field lines for two point charges of
equal magnitudes, but opposite
signs. This charge configuration
is called an electric dipole.
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Electric Field
A positive test charge
experiences a force
directed away from
the central charge
+
+
As the charge is moved closer to
the central charge the force
increases in magnitude
+
+
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Effects of Fields
• No effect on neutrals.
• Electric fields can accelerate charged
particles or repel them.
• Magnetic fields can turn a charged
particle in a circle
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Electric Field Intensity
is the ratio of the force on a test charge to
its charge.
F
E '
q
The units are Newtons per Coulomb (N/C)
• E is the electric field intensity
• F is the force
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• q’ is the test charge
ELECTRICITY I
Electric Potential
39
Electric Potential
• It takes a force to raise an
object in the Earth’s
gravitational field. When we do
raise an object in the field we
change its gravitational
potential energy. (mgh)
• When a charge is moved against
an electric field, its potential
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energy also changes
Electric Potential
• A positive charge
experiences a
downward force in the
direction of the E-field
• It takes work to move the
charge against the electric
field increasing its PE
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Electrical Potential (V)
• The electrical potential (V) is
defined as the electrical
potential energy per unit charge.
• Potential (V)=
electrical potential energy
charge
• Electric potential is a scalar
42
Electrical Potential and
Current
• A common analogy is to compare
potential to water pressure.
• The higher the potential, the
higher the pressure to transfer
charge.
• A battery maintains a continuous
potential difference.
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Electrical Potential (Voltage)
• The unit for electrical
potential is the Volt, named
after Alessandro Volta, an
Italian scientist who
developed the battery
• 1 Volt = 1 Joule / 1 Coulomb
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Potential Difference (Voltage)
The voltage difference between 2
points is equal to the work
done against the field to move a
positive charge from A to B
with no acceleration
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Uniform Electric Field
• Two parallel conducting plates a
distance, d, apart constitute a uniform
electric field.
• The uniform field emanates from the
positive plate to the negative plate.
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Voltage in a Uniform E-field
•
•
•
•
•
Voltage equals work per unit charge.
That is V = Nm/C
N/C is E (electric field intensity).
m is distance between the plates.
Therefore…
V  Ed
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Electron Volt (eV)
• When a particle with
a charge equal to that of an
electron moves through 1 volt in
an E-field, it changes energy by 1
eV.
• PE = qV
• 1 eV = 1.6 x 10-19 J
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ELECTRICITY I
Capacitance
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Capacitance
• Capacitance is the ability to
store charge.
• A charge storing device is called
a capacitor.
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Capacitance
• At a given potential (V), the
amount of charge (Q) that can
be stored by a body depends on
its physical characteristics.
• These physical characteristics
are lumped together under the
term capacitance (C).
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Capacitance (C)
• C = Q/V
• The unit of capacitance is the
Farad (F), named to honor
Michael Faraday
• 1 Farad = 1 Coulomb / 1 Volt
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Capacitance
• 1 Farad is a large amount of
capacitance.
• Usually a capacitor found in a
piece of electronic equipment
will be rate in microfarads (mF)
or picofarads (pF)
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