Download Electric Fields and Charges

Electrostatics,
Electric Fields
CHHS Physics
Mr. Puckett
Electrical Charges

ELECTRICAL CHARGES are natural forces
that exist on matter under certain conditions of
state.
 They are the result of an excess or deficiency
of electrons on atoms, making them ions.
Positive ions are Cations and Negative ions
are Anions. Protons are much harder to move
because of their mass and nuclear forces.
 These charges normally move around to
mingle with opposite charges and become
more stable. Most books show + charges in
RED and – charges in GREEN or BLUE
LAWS OF ELECTROSTATIC
CHARGE INTERACTIONS:

A. Like charges repel
each other just as similar
poles of two different
magnets will repel away
from each other.

B. Opposite charges
will attract each other.
This is the same as putting
a north and south poles of
two
different
magnets
together. They attract and
stick together.
Opposites Attract; Similar Repell
Charging by Friction and
Conduction
Electrostatics
ELECTROSTATICS is the study of
electrical charges that can be collected
and held in one place.
 The force exerted by charged objects is
greater than gravitational force.
 The electrical effect and the effect of
gravity are different because only the
effect of gravity is constant.

Balanced Atoms and Charges

The positive charge of the nucleus is
balanced by the negative charge of the
electrons, so the balanced atom has no net
charge.
 The removal of electrons by friction or
rubbing leaves a positive ion because the
positive charge of the nucleus is no longer
balanced by the negative electrons. If the
removed electrons become attached to
another atom, it changes the atom to a
negative ion and carries a net negative
charge.

Conservation of charge and energy are maintained
throughout this change because the electrons are not created or
destroyed, only put in different places.
Conduction and Induction:
You can cause charges to move around
by either touching them to another object
(conduction), or by bringing them near
another object (induction).
 The study of electrical charges is called
ELECTROSTATICS. The figure below is
charging
by conduction

With friction on the
Plastic wand
Charging by Induction
Charge Induction by Grounding
Charging action by Induction
Charging by Induction
Electroscope Diagram

This device detects
electrical charges.
 If a non-metallic
insulator touches the
mast, the excess
electrons will move to
the insulator and out of
the conductive mast
into the leaves. They
will spread due to
similar charge
repulsion.
Induction: Charging by
Proximity Charge Attraction

Charges are induced by coming close to
but NOT touching. It is caused by the
electrostatic laws of opposite attraction.
Charging by Polarization
Conduction:
Charging by Touching

Charges are
transferred by
touching.
Note that the
positive
charges
spread out
evenly
Conductors and Insulators

If they are in a medium that allows them to
move easily, then that would be a conductor.
 Metals are the normal conductors. Some
metals are better conductors than others.
Copper, gold and aluminum are good
conductors.
 If the charges are in a medium that does not
readily allow for charge movement then that
is an insulator. Glass and rubber are
normal insulators.
Charge Separations and
Placements
On a sphere
the charges
will go to the
surface and
spread out
evenly. On an
insulator the
charges will
stay where
they are
placed.
Distribution of Charges on a
Surface

Critical Angles vs. Smooth surfaces.
The shape of an object will be important
in determining the surface charge
concentrations. Charges will spread out
evenly over a smooth surface but will
concentrate at sharp angles. This
critical angle will promote charge
discharge.
Safe from Lightning in the Car?
Charge Distribution on
Different Sizes and Angles

Sharp angles increase the charge
density like decreasing size.
Charge and Conduction
Conventions
The following symbols and sign
conventions are used throughout the
unit.
 Charges are + and – signs and the
conductors are shown as charge flow
paths.

History of Electrical Fields

The concept of the electric field was introduced
by Michael Faraday.
 An electric field is created by a charged body in
the space that surrounds it, and results in a force
exerted on any other charges placed within the
field. The electric field acts between two charges
in a similar manner to the way that the
gravitational field acts between two masses, and
like it, extends towards infinity.
 It shows an inverse square relationship with
distance.
 However, there is an important difference.
Gravity always acts in attraction, drawing two
masses together, while the electric field can
result in either attraction or repulsion.
Electric Fields of Force
All electric charges produce a force field
around them that interact with other
force fields and some types of matter.
 These force fields produce Potential
Energy and produce effects that help us
predict interactions in the future.

The fields look analogous to
gravity and topo maps that
have high and low areas. The
high areas are + charges and
the lows are – charges.
Electrical vs. Gravitational Fields
As the picture above shows potential energy with the
top of the stadium seats would have more potential
energy and the track would have less energy. Think
of how rain water would flow from the + to the - .
That is how electrical fields work.
The rows of seats would be lines of equal potential
energy. Equipotential energy lines are common in
electrical and magnetic force fields.
Compare Gravity to Electrical
Charge Forces
Gravity Force vs. Electrostatics
Electrical Fields Visualized
Electrical Field Visualization (-)
Electrical Field Visualization (+)
Test Charges (+) are Used to Test
the Strength of Electrical Fields

A small positive
test charge “q” is
placed around
the electrical
field and the
forces are
measured that
affect it.
Electrical Field Force Lines

The force lines
go from + to –
charges.
Potential
Energy lines
(red) are
perpendicular
to the force
lines (purple).
Coulomb’s Law of
Electric Force

COULOMB'S LAW describes the force
between two charged objects. The electric
force varies inversely with the square of the
distance between the two charged objects.
The electric force varies directly with the
product of the charges on the objects.
Coulomb's law can be written as the equation

F = K q q'
d2 ,
where F is force, q and q' represent the
charges of the objects, d represents the distance between the
objects and K is a proportionality constant of charge conduction
through free space;. K is equal to 9 x 109 Nm2/C2.
The Coulomb Unit:
The SI UNIT OF CHARGE is the
Coulomb (C) defined as the charge of
6.25 X 1018 electrons.
 The magnitude of the charge of one
electron is called the elementary
charge. The charge on one electron is
-1.6 x 10 -19 C.
 The electric force is a vector quantity
and has both magnitude and direction.
A repulsive force has a positive sign
and an attractive force has a negative
sign.

Coulomb’s Example

What is the force of a hydrogen nucleus on its
electron. Orbital radius = 0.53 x 10-10m and
charge on the electron is 1.6 x 10-19 C.

F = K q q‘
d2
= (9.0x109 Nm2/C2)(-1.6x10-19C)(1.6x10-19C)
0.53 x 10-10m2
= - 8.2 x 10-8 N (the negative sign means
attractive forces because opposite charges
attract)
Electron Charge Calculations

What is the charge on 5000 electrons?
5000 e- X -1.6 x10-19 Coulombs/electron =
8x10-16 C or 8x10-10 μC
How many electrons are in 2 μC ?
2x10-6 C / 1.6x10-19 C / e- = 1.25x1013 e-
Calculate the Force on a Charge (+
or -) in an Electrical Field
A uniform electric field has a value of
8e5 N/C and a positive test charge of
2e-5C is placed in the field. What force
does the charge experience?
 F = qE
 (2e-5C)(8e5 N/C) = 16 N
repulsive ( attractive forces are negative

because opposites attract and similar
charges repel)
Forces on the Center Charge:
 -1 μC
-3 μC
+5 μC
 q1●-------------q2 ●----------------------- ● q3
|---- 2cm ---------------| ---------- 5 cm-------------------|
What is the Force on the center charge?
Use Coulombs law to compute the individual
electrical forces and then add them.
F21 = K q1 q2 / r2 = 68N and F 23 = 54 N so the
sum is 122 N to the right.
Testing Electric Fields
To test electric fields we place a small +
charge around the field and measure
the interaction. Ex: a positive test
charge of 1.85e-5C is placed in a field
and experiences a force of 14 N.
 What is the Electric field strength at that
point?
 E = F/q = 14N/1.85e-5C = 7.5e5 N/C

Electric Field of a Single Point
Charge.
A positive test charge of 2e-5C is placed
in an electrical field of 8e5 N/C.
 What force does the charge
experience? F = qE
 F = 2e-5C X 8e5 N/C = 16 N

Electrical Potential Energy = Volts
Calculate the electrical potential energy
of a 0.05C charge that is 5 m away.
 V = Kq/r = (9e9 )(0.05C) / 5 m =
9e7 Volts

Electrical Fields do WORK !!
Electrical fields have the ability to create
a force that moves things a given
distance. This is how you get electricity
out of the wall socket.
 Ex: The potential energy difference (
Voltage) is 6 V. How much work does
it take to move 2 coulombs of charge
across a filament?
 W = qV so 2C (6 J/C or V) = 12 J

Voltage and Work
Voltage (PEe) is equal to the amount of
work it takes to move a charge from the
negative side of an electrical field to the
positive side, divided by the amount of
charge you are moving.
 W = Fd = qEd
 V = PE/q = W/q = Ed

How Do We Use Electrical Fields
and Electrostatics in Daily Life?
Storing the Charges of
Electrostatics: CAPACITANCE





A capacitor is a device that is used to store
electrical charges and electrical potential
energy. It changes AC to DC current
A capacitor is “charged” when a potential
difference (voltage) is applied.
The capacitance, C, of an object is the
amount of charge, Q, it can store for a given
voltage. The UNIT is the FARAD, F.
Formula for capacitance: C = Q / ΔV
Formula for PE = ½ Q ΔV
Potential Energy in a Capacitor
A charged capacitor stores electrical PE
because it requires work to move charges
through a circuit to the opposite plates of a
capacitor.
 PE electric capacitor = ½ QV = ½ CV2 = Q2/2C

Capacitors
Semiconductors
Materials that have medium qualities of
conduction and insulation. These
properties can be changed by coatings or
materials processing methods (alloying).
They are mostly used in computer chips
and electronics.
Electric fields
allow these
chips to process
electrical
signals very
rapidly.

Superconductors

Superconductivity is an electrical resistance
of zero which occurs in certain materials
below a characteristic temperature.
 It was discovered by Heike Kamerlingh
Onnes in 1911. Superconductivity is a
quantum mechanical phenomenon.
The electrical resistivity of a
metallic conductor decreases
gradually as the temperature is
lowered. However, in ordinary
conductors such as copper and
silver, this decrease is limited by
impurities and other defects.
Superconductors in your Future
Van de Graaff Generator

A Van de Graaff generator is a charge
producing device that has a frictional
belt that separates electrons and allows
them to collect on the surface of the
dome. The voltage is very high but the
amperage is very low.
Van de Graaff Generator makes
hair stand up by static electricity
Electrical Fields
The Photocopier Charged Images
How is Lightening Made?
Grounding: Discharging Static


When static charges
are given a route to
discharge; it is called
Grounding; like when
lightning uses a barn
to “strike” and ground
out.
Ben Franklin first
came up with this idea
to stop the burning of
church steeples by
lightening in the early
colonial days of the
USA.
THE
END !!
Go forth and get a CHARGE out of Life!
 You have the capacitance to do
anything that you put your mind to.
 May the Volts be with you !
