Transcript Electricity

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
charging a rod and electroscope
positively and negatively
by conduction and induction
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.
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.
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
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
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
The site linked
here allows you
to build and test
your own series,
parallel, and/or
combination circuits.
For a complete interactive tutorial on
electricity and magnetism, go here.