Transcript Electric Forces and Electric Fields

Electric Forces and
Electric Fields
Electrics
Section 1 Objectives:
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Understand the basic properties of electric
charge.
Differentiate between conductors and insulators.
Distinguish between charging by contact,
charging by induction, and charging by
polarization.
Properties of Electric Charge:
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Like charges repel, unlike charges attract.
Two types of electric charge: (named by Benjamin Franklin)
Positive
 Negative
These charges are opposite one another.
Protons and neutrons are relatively fixed in the nucleus of the atom, but
electrons are easily transferred from one atom to another.
Ions-atoms with a negative or positive charge.
Charge has a natural tendency to be transferred between unlike materials.
What would happen if you rubbed two materials together? The area of
contact would increase and the charge-transfer process would be
enhanced.
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Application:
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If you rub a balloon against your hair, some of the electrons of
your hair will transfer to the balloon. Your hair will become
more _________, and the balloon will become more
_________.
*Only a small portion of the total available charge is transferred
from one object to another.
Since the positive charge on your hair is equal in magnitude to
the negative charge on the balloon, electric charge is conserved
in the process.
Experiment:
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In 1909, Robert A. Millikan performed his oil-droplet
experiment. Because of this experiment, he found that when an
object is charged, its charge is always a multiple of a fundamental
unit of charge, e. Charge is said to be quantized.
Value of e: 1.602176 x 10-19C, where C (coulomb) is the SI unit
of electric charge.
Transfer of Electric Charge:
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Once materials such as Copper, zinc, and silver are charged in a
small region, the charge readily distributes itself over the surface
of the entire material (unlike the balloon/hair situation).
Substances are classified by their ability to transfer electric
charge.
Electrical conductors-materials in which electric charge moves
freely such as metals.
Electrical insulators-materials in which electric charges do not
move freely (e.g. glass, rubber, silk, plastic).
Conductors:
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Semiconductors-a class of materials with electrical properties
somewhere between those of insulators and conductors;
insulators in their pure state, however, carefully controlled
addition of specific atoms as impurities can dramatically increase
ability to conduct electric charge. Silicon and Germanium are
semiconductors utilized in many electronic devices.
Superconductors-class of materials that can conduct electric
current indefinitely without heating; have zero electrical
resistance at or below a certain temperature (e.g. certain metals
and compounds).
Conductors:
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Insulators and conductors can be charged by contact. E.g.
balloon/hair situation.
Conductors can be charged via induction.
Once a conductor is connected to Earth via a conducting wire or
copper pipe, it is said to be grounded.
Induction-the process of charging a conductor by bringing it
near another charged object and grounding the conductor.
Situation:
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A charged rubber rod brought near a metal sphere and the
charge on the sphere is redistributed. If it is grounded, some of
the electrons travel to the wire through the ground. As wire is
removed, the sphere has an excess of positive charge. Electrons
redistribute evenly on surface of sphere as rod is removed.
A surface charge can be induced on insulators by polarization.
When an object becomes polarized, it has no net chare but can
attract or repel objects due to a realignment charge. So, a plastic
comb can attract small pieces of paper with no net charge. As
with induction, in polarization one object induces a charge on
the surface of another object with no physical contact.
Section 2 Objectives:
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Calculate the electric force using Coulomb’s law.
Compare electric force with gravitational force.
Coulomb’s law:
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Charles Coulomb found that the electric force between two
charges is proportional to the product of the two charges. He
also found that the electric force is inversely proportional to the
square of the distance between the charges.
Felectric=kCq1/q2, kC= Coulomb’s constant
Force is a vector quantity.
Sample Problem A: The electron and proton of a hydrogen
atom are separated on average by a distance of about 5.3 x 1011m. Find the magnitude of the electric force and the
gravitational force that each particle exerts on the other.
Knowns:
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r=
kC=
me=
mp=
qe=
q p=
G=
Electric force and Fgrav:
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The electric force between two objects always acts along the line
that connects their centers of charge. Coulomb’s law applies to
only point charges or particles and to spherical distributions of
charge.
Electric force is a field force as is gravitational force. Coulomb
quantified electric force with a torsion balance. A torsion
balance consists of two small spheres fixed to the ends of a light
horizontal rod.
In his experiment…
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One sphere is given a charge and another charged object is
brought near the charged sphere.
The attractive or repulsive force between the two causes rotation
and twisting of the suspension by the rod.
Section Objectives
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Calculate electric field strength.
Draw and interpret electric field lines.
Identify the four properties associated with a
conductor in electrostatic equilibrium.
The Electric Field
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Field forces, unlike contact forces, are capable of acting through
space, producing an effect even when there is no physical
contact between the objects involved.
Electric field-a region where an electric force on a test charge can
be detected.
E, electric field=Felec/qo (a location)
The SI unit of E=newtons per coulomb (N/C)
What type of quantity is electric field?
Electric field strength depends on charge and distance.
Re-arrangement of Coulomb’s law
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If q is positive, the field due to this charge is directed outward
radially from q.
If q is neg., the field is directed towards q.
Electric field lines:
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What do physicists use to assist in visualizing electric field patterns? Lines
drawn and pointing in the direction of the electric field called electric field
lines. These lines really do not exist. They are generally drawn so that the
electric field vector, E, is tangent to the lines at each point. E is stronger
where the field lines are close together and weaker where they are far apart.
Rules for drawing electric field lines:
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1. The lines must begin on positive charges or at infinity and
must terminate on negative charges or at infinity.
2. The number of lines drawn leaving a positive charge or
approaching a negative charge is proportional to the magnitude
of the charge.
3. No two field lines from the same field can cross each other.
Conductors in electrostatic
equilibrium:
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Electrostatic equilibrium occurs when no net motion of charge is occurring
within a conductor.
Good electrical conductors (e.g. copper) contain charges (e’s) that are only
weakly bound to the atoms in the material and are free to move about within
the material.
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Characteristics of conductors in electrostatic equilibrium:
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The electric field is zero everywhere inside the conductor.
Any excess charge on an isolated conductor resides entirely on the conductor’s
outer surface.
The electric field just outside a charged conductor is perpendicular to the
conductor’s surface.
On an irregularly shaped conductor, charge tends to accumulate where the radius
of radius of curvature of the surface s smallest, that is at sharp points.
Electric Potential
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Electric potential energy-potential energy associated with a
charge due to its position in an electric field.
Mechanical energy is conserved in the absence of friction and
radiation.
Electric potential energy is a component of mechanical energy.
If a gravitational force, an elastic force, and an electric force are
all acting on an object, the ME can be written as follows: ME=
KE + PEgrav + PEeleastic + PEelectric
Whenever a charge moves, because of the electric field produced
by another charge or group of charges, work is done on that
charge.
Electric potential energy
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Electric potential energy can be associated with a charge in a
uniform field: ∆PEelec = -qEd
The negative sign indicates that the electrical PE will increase if
the charge is negative and decrease if the charge is positive.
Electric PE is similar to gravitational PE.
At any point in an electric field, as the magnitude of the charge
increases, the magnitude of the associated electrical PE increases.
Electric potential-the work that must be performed against
electric forces to move a charge from a reference point to the
point in question, divided by the charge: V=PEelec/q
The SI unit of potential difference and electric
potential is the volt, V.
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The volt equals one joule per coulomb.
The electric potential at a point is independent of the charge at
that point. Potential difference is a change in electric potential.
∆V = ∆PEelec/q
Daily application: batteries.
For a typical battery, there is a potential difference of 13.2-V
between the negative and positive terminals.
The potential difference in a uniform field varies with the
displacement from a reference point.
PEelec = -qEd, ∆V = ∆(-qEd)/q
Sample Problem A:
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A charge moves a distance of 2.0-cm in the direction of a
uniform electric field whose magnitude is 215-N/C. As the
charge moves, its electrical PE decreases by 6.9 x 10-19J. Find
the charge on the moving particle. What is the potential
difference between the two locations?
∆PEelec = ?
d=?
E=?
q=?
∆V=?
Solve the problem.
Capacitance
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Capacitor-a device that is used to store electrical potential
energy. Used in tuning the frequency of radio receivers,
eliminating sparking in automobile ignition systems, and storing
energy in electronics flash units.
A charged capacitor is useful because energy can be reclaimed
from the capacitor when needed for a specific application. A
typical design for a capacitor consists of two parallel metal plates
separated by a small distance.
Capacitance-the ability of a conductor to store energy in the
form of electrically separate charges. It is the ratio of charge to
potential difference.
Measuring Capacitance:
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The SI unit for capacitance is the farad, F, which is equivalent to a Coulomb
per volt (C/V).
Most capacitors have capacitancies ranging from microfarads to picofarads.
Capacitance= C =Q/∆V
What does capacitance depend on? The size and shape of the capacitor
Capacitance for a parallel-plate capacitor in a vacuum: C=εo A/d=permittivity
of a vacuum x area of one of plates/distance between the plates
ε-epsilon denotes permittivity of the medium. When followed by a zero, it
refers to a vacuum. The magnitudeis 8.85 x 10-12C2/Nxm2.
Q=εOA/d ∆V for spheres, ∆V=kC Q/R; Csphere=Q/∆V=R/kC
Capacitances:
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Earth has an extremely large capacitance because it is so large.
Earth can provide or accept a large amount of charge without its
electric potential changing too much. This is why Earth is used
as a reference point for measuring potential differences in
electric circuits. The material between a capacitor’s plates can
change its capacitance.
Discharging a capacitor releases its charge.
Most capacitors are filled with a dielectric, an insulating material
like air, rubber, glass, or waxed paper. The dielectric tend to
increase capacitance.
Energy and Capacitors:
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A charged capacitor stores electrical PE because it requires work
to move charges through a circuit to the opposite plates of a
capacitor.
Electrical potential energy stored in a charged capacitor:
PEelec=1/2 Q∆V
PEelec=1/2 C(∆V)2; PE=Q2/2C
Sample Problem B:
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A capacitor, connected to a 12-V battery, holds 36µC of charge
on each plate. What is the capacitance of the capacitor? How
much electrical PE is stored in the capacitor?
List Knowns:
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List Unknowns:
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Solve:
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Electrical Current:
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Current-the movement of electric charge through a medium.
What types of things are powered by electric currents? Lights,
radios, TV's, air conditioners, and refrigerators, automobiles.
Electrical current is a part of the human body.
Luigi Galvani: What connection did Galvani make between
physics and biology? Internet.
Electric current-the rate at which electric charges pass through a
given area.
Variables and Equations:
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∆Q=amount of charge passing through the area
∆t=time interval
I=current, ratio of amount of charge to the time
interval. I=∆Q/∆t
The SI unit of charge is the ampere, A. One ampere is
equivalent to one coulomb of charge passing through a
cross-sectional area in a time interval of one second
(1A=1C/s).
Sample Problem C:
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The current in a light bulb is 0.835-A. How long does
it take for a total charge of 1.67-C to pass through the
filament of the bulb?
I=0.835-A
Q=1.67-C
t=?
Current and Voltage:
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V=IR, I=V/R
Resistance-the opposition to the motion of charge
through a conductor; the opposition presented to
electric current by a material or device. R=∆V/I
The SI unit for resistance is the Ohm or 1V/A.
Ohm’s Law: Resistance is constant over a range of
potential difference. Georg Simon Ohm was 1st to
study electrical resistance in a systematic manner. This
law does not hold for all materials.
Resistance
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What does resistance depend on? Length, area, material, and
temperature.
What is used to control the amount of current in a conductor?
Resistors
Resistor-a simple electrical element that provides a specified
resistance.
What lowers the body’s resistance? Salt and water perspiration.
The human body has a high resistance to current when the skin
is dry.
Potentiometer-a special type of resistor with a fixed contact on
one end and an adjustable, sliding contact that allows the user to
tap off different potential differences.
Sample Problem D:
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The resistance of a steam iron is 19.0-Ω. What is the
current in the iron when it is connected across a
potential difference of 120-V?
V=120-V
R=19-Ω
Electric Power:
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Batteries and generators supply energy to charge carriers.
Batteries convert chemical energy to electrical PE.
Generators are preferred over batteries because batteries need
recharging or replacing. Generators convert ME into electrical
energy.
Generators are the source of electric current to a wall outlet in a
home and supply the electrical energy to operate appliances.
Current can be direct or alternating. Direct current is
unidirectional and negative charges move from low to higher
electrical potential. E.g. batteries. With alternating current, the
terminals of the source of potential difference are constantly
changing sign.
Power
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Electric power- the rate at which charge carriers do work.
P=W/t=∆PE/∆t, P=I∆V, ∆V=∆PE/q
P=I∆V=∆V/R(∆V)=(∆V)2/R, P=VI, P=I2R
Sample Problem E:
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An electric space heater is connected across a 120-V
outlet. The heater dissipates 1320-W of power in the
form of electromagnetic radiation and heat. Calculate
the resistance of the heater.
V=IR, I=P/V
Circuits and Circuit Elements:
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Schematic diagram-a diagram that depicts the
construction of an electrical apparatus.
Electric circuit-a set of electrical components
connected such that they provide one or more
complete paths for the movement of charges.
Any element or group of elements in a circuit
that dissipates energy is a load.
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Sample Problem A:
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A 9.0-V battery is connected to four light bulbs, find
the equivalent resistance for the circuit and the current
in the circuit.
List Knowns:
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List Unknown:
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Solve:
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