Transcript Slide 1

Electrodynamics
Electrostatics
the study of electrical
the study of electrical
charges in motion
charges at rest
Two opposite types of charge exist, named
positive and negative by Benjamin Franklin.
Charge is a
property of
matter.
Charged particles exist in atoms.
Electrons are responsible
for negative charge;
protons for positive charge;
neutrons have no charge.
Small amounts of ordinary matter contain
incredible amounts of subatomic particles!
Conductor
link
Insulator
link
material that allows charges to move about easily
material through which charges will not easily move
Basic Law of Electrostatics
Like charges repel; unlike charges attract
Link
Click here to view a
simulation of the
behavior of pith balls
in the vicinity of
charged rods.
See a movie here.
Click here to read about charging objects
by contact (friction). See shoes on
carpet here.
View a simulation
of charging a
balloon by rubbing
it on your hair
and then sticking
it to a neutral
wall here.
Learn about Ben Franklin
and his work with electricity here.
rod and electroscope
positively and negatively
by conduction and induction
charging a
When charging by conduction,
the rod touches the electroscope.
The electroscope gets the same charge as the rod.
When charging by induction, the rod does not
touch the electroscope. The electroscope gets
the opposite charge of the rod.
Go here , here, here, and here
to view simulations of charging
an electroscope. Read more here.
COULOMB’S LAW
The force between two charged objects is
directly proportional to the product of their
charges and inversely proportional to their
separation distance squared.
link1, link2, link3, link4, link4
In equation form:
q
q
F = k 12 2
d
F is the force of attraction, measured in NEWTONS,
between charges q1 and q2
k is the Universal Electrostatic Constant, equal to
9.00 x 109 N m2/coul2
q1 and q2 are the attracting charges, measured in
Coulombs
d is the distance between the charges,
and is measured in METERS
The SI unit of charge
is the Coulomb,
named in honor of
Charles Augustin Coulomb.
1 C = charge on 6.25 x 1018 electrons (or protons)
1 e- = 1.60 x 10-19 Coul = elementary charge
Electric force is a vector and must be treated as such.
Electric Fields
An electric field exists in a region
if space if a charge placed in that
region experiences an electric force.
The magnitude of an electric field at any
given point is defined to be the ratio of
the force on a
charge at that
point to the
amount of charge.
E = F/Q
Electric field strength has units
of Newtons/Coulomb (N/C).
The direction of the electric field at any point is defined
to be the same direction as the direction of force on a
positive test charge placed in the region at that point.
Field lines point away from positive
and toward negative charges.
Click here to view a simulation
showing the magnitude and direction of the
electric force on a test charge when
placed near other charges.
Click here to view a simulation of a
charged particle moving through a region
occupied by other charges.
Electric Potential Difference
the change in
electric potential energy
per unit charge
V = W/Q
The SI unit of electric potential
difference is the VOLT, named in
honor of Alessandro Volta.
One VOLT
is the electric potential difference
between two points when one Joule of work
is done in moving one Coulomb of charge
between the points.
electric cell - a device that
converts one form of energy
to electrical energy
Chemical cells convert chemical
energy into electrical energy.
Chemical cells can be
“wet” or “dry”.
Solar cells convert light energy
into electrical energy.
A generator converts mechanical
energy into electrical energy.
battery - two or more cells
connected in series or in parallel
the flow of
charged particles;
can be positive or negative,
but usually negative (electrons)
through a conducting metal
Electric current is measured
in Amperes, in honor of
Andre Marie Ampere.
One Ampere is the flow of
one Coulomb of charge per second.
1 Amp = 1 Coulomb per second
= 1 C/s
I = Q/t
Ammeter
a device that measures current
Voltmeter
a device that measures
electric potential difference
power = work/time
.
= (work/charge) (charge/time)
.
= electric potential difference current
P (Watts) = V (Volts).I (Amps)
Analogies of simple circuits are these links:
Water circuit analogy link
Air flow link
Various link
Teaching with Analogies link1, link2
Resistance
determines the amount of current flow
= the ratio of potential difference to current
R=
V
The SI unit of
resistance is the
I Ohm, W, named
in honor of Georg Simon Ohm.
One Ohm of resistance is the resistance
such that one Volt of potential difference
is needed to obtain a current of one Amp.
The resistance of a circuit element depends on:
1. the length of the conductor
as length increases, resistance increases proportionally
2. the cross-sectional area of the conductor
as area increases, resistance decreases proportionally
3. the resistivity of the conductor
as resistivity increases, resistance increases proportionally
Resistivity
The resistivity, r, of a conductor is equal to the
resistance of a wire 1 cm long and having
a cross-sectional area of 1 cm2.
l
R =r
A
R = resistance, measured in Ohms
r = resistivity, usually in units of W.cm
l = length, measured in cm
A = cross-sectional area, measured in cm2
Investigate resistivity here
Ohm’s Law
The ratio of potential difference to current
is constant.
If R = V/I is a constant value
for a given resistor, then that
resistor is said to obey Ohm’s Law.
Click here and here to link to pages describing resistor
color codes.
Many circuit elements do not
obey Ohm’s Law. Resistors
that get hot, like light bulbs
and heating elements, do not
keep a constant resistance.
Resistance generally increases as
objects become hotter.
Click here and here to run
simulations of Ohm’s Law.
Series
Resistor Circuits
1. total resistance is the sum of the
separate resistors
RT = R1 + R2 + R3 + ...
2. current is the same through each resistor
IT = I1 = I2 = I3 = ...
3. total potential difference is the sum of each
VT = V1 + V2 + V3 + ...
In other words, in a series circuit,
resistance and voltage add,
but current stays the same.
R,
W
E = 12 V
R1
R3
R2
R1
8.0
R2
2.0
R3
5.0
RT =
VT =
IT =
PT =
V,
V
I,
A
P,
W
R,
W
E = 12 V
R1
R3
R2
V,
V
I,
A
P,
W
R1
8.0 6.4 0.80 5.1
R2
2.0 1.6 0.80 1.3
R3
5.0 4.0 0.80 3.2
RT = 15 Ω
VT = 12 V
IT = 0.80 A
PT = 9.6 W
Parallel
1. reciprocal of the total resistance is the
sum of the reciprocals of the separate
resistors
1/RT = 1/R1 + 1/R2 +1/R3 + ...
2. total current is the sum of the current
through each resistor
IT = I1 + I2 + I3 + ...
3. potential difference is the same across
each resistor
VT = V1 = V2 = V3 = ...
In other words, in a parallel circuit,
resistance adds as reciprocals,
voltage stays the same, and current splits.
R,
W
E = 12 V
R1
R1
12
R2
8.0
R3
12
R2
R3
RT =
VT =
IT =
PT =
V,
V
I,
A
P,
W
R,
W
E = 12 V
R1
V,
V
I,
A
R1
12 12
R2
8.0 12 1.5 18
R3
12 12
1.0 12
1.0 12
R2
R3
P,
W
RT = 3.42 Ω
VT = 12 V
IT = 3.50 A
PT = 42 W
Kirchhoff’s Rules
Loop Rule: The sum of the potential differences
around any closed circuit loop is zero.
Junction Rule: The sum of the currents
into any circuit junction is zero.
Go to link1, link2, link3, link4, link5, and link6 to view
pages and simulations examining
Kirchhoff’s Loop and Junction Rules.
The sites linked here and
here (click on “Circuit
Construction Kit”) allow
you to build and test your
own series, parallel, and
combination circuits.
For a complete
interactive tutorial
on electricity and
magnetism, go here.