Download Ohm`s Law

INTO THE GRID
A Study of the Power Grid
THE BIG PICTURE
• The power grid is a
very simple thing. It
consists of a set of
large power plants
(hydropower plants,
nuclear power plants,
etc.) all connected
together by wires.
One grid can be as
big as half of the
United States.
•
http://people.howstuffworks.com/black
out.htm
THE BIG PICTURE
• Power travels
from the power
plant to your
house through
an amazing
system called
the power
distribution
grid.
http://people.howstuffworks.com/blackout.htm
HOW DOES A BLACKOUT
OCCUR?
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POWER PLANT TRIPS OFF LINE
Lightning Strike, fire
When that plant disconnects from the grid, the other plants connected to it
have to spin up to meet the demand. If they are all near their maximum
capacity, then they cannot handle the extra load. To prevent themselves
from overloading and failing, they will disconnect from the grid as well. That
only makes the problem worse, and dozens of plants eventually disconnect.
That leaves millions of people without power.
TRANSMISSION LINE FAILURE
Lightning/Sagging
When that transmission line failed, all of its load shifted to neighboring
transmission lines. They then overloaded and failed, and the overload
cascaded through the grid.
In nearly every major blackout, the situation is the same. One piece of the
system fails, then the pieces near it cannot handle the increased load
caused by the failure, so they fail. The multiple failures make the problem
worse and worse and a large area ends up in the dark.
SMALLER PICTURE
• WHAT HAPPENS WHEN YOU
PLUG IN THE TOASTER????
CIRCUITS
• Behind the outlets in our home is a series of
complex circuits
• A circuit is simply a closed loop through which
charges can continuously move
http://www.glenbrook.k12.il.us/gbssci/phys/Class/circuits/u9l1b.html
CIRCUIT REQUIREMENTS
• There must be a closed conducting path which
extends from the positive terminal to the
negative terminal.
There must be an electric potential difference
across the two ends of the circuit. This is can be
established by the use of a battery or generator,
http://www.glenbrook.k12.il.us/gbssci/phys/Class/circuits/u9l1b.html
ELECTRIC POTENTIAL DIFFENERENCE
VOLTAGE
•
Gravitational potential energy is
dependent on mass and location.
The boulder on the right will move
from an area of high gravitational
potential energy to an area of low
gravitational potential energy
when dropped.
•
Electric potential energy is
dependent on
1) Electric charge - a property of
the object experiencing the
electrical field,
2) Distance from source - the
location within the electric field
Charges will move from high to
low electric potential
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http://www.glenbrook.k12.il.us/gbssci/phys/Class/circuits/u9l1b.html
Electric Potential Difference
Voltage
• Electric potential difference is the difference in
electric potential (V) between the final and the
initial location when work is done upon a charge
to change its potential energy.
• Electric Potential difference or voltage is
measured in a unit known as the volt
http://www.glenbrook.k12.il.us/gbssci/phys/Class/circuits/u9l1c.html
VOLTAGE
• In a battery the chemical energy
produced by the reaction of the
chemicals inside the battery is used
to do work on a positive test charge
to move it from the low potential
terminal to the high potential
terminal. Once at the high potential
terminal, a positive test charge will
then move through the external
circuit and does work upon the light
bulb or the motor or the heater coils,
transforming its electric potential
energy into useful forms for which
the circuit was designed. The
positive test charge returns to the
negative terminal at a low energy
and low potential, ready to repeat
the cycle (or should we say circuit)
all over again.
http://www.glenbrook.k12.il.us/gbssci/phys/Class/circuits/u9l1b.html
CURRENT
• Current is the rate at which charges flow
past a point in a circuit.
• Current is charge transfer per unit time
• CURRENT = CHARGE/TIME
The standard metric unit for current is the
ampere
1 ampere = 1 coulomb / 1 second
ALTERNATING AND DIRECT
CURRENT
• Alternating Current
(AC)
• the type of electricity
commonly used in homes
and businesses
throughout the world. the
current alternates in
direction, back-and-forth
between 50 and 60 times
per second. AC is created
by an AC electric
generator
DIRECT CURRENT (DC)
the continuous movement of
electrons from an area of
negative (-) charges to an
area of positive (+) charges
through a conducting
material such as a metal
wire..
Such a circuit consists of a
source of electrical energy
(such as a battery) and a
conducting wire running
from the positive end of the
source to the negative
terminal.
RESISTANCE
• Resistance is the hindrance to the flow of
charge
• Devices within a circuit( light bulb,
toaster,etc.) offer resistance to the current
• The standard metric unit for resistance is
the ohm, represented by the Greek letter
omega -
OHM’S LAW
• the electric potential difference between
two points on a circuit ( V) is equivalent to
the product of the current between those
two points (I) and the total resistance of all
electrical devices present between those
two points (R).
•
V=I•R
SO WHAT HAPPENS WHEN YOU
PLUG IN THE TOASTER?
• The outlet supplies the voltage necessary to
move the charge (AC current) In the US typical
household voltage is 120V. That voltage is
actually created in a power plant somewhere
else and then delivered to your house by the
power wires that are on poles or buried
underground.
• The wires of the toaster conduct the current
through the toaster
• The toaster itself offers resistance to the flow of
current (so too does the plug of the toaster)
POWER
• Electrical power is the rate at which electrical
energy is supplied to a circuit or consumed by a
load. It is the product of voltage and current.
• Power is measured in a unit known as the watt
• P = IV
• P = power in watts
• I = current in amperes
• V = voltage in volts
POWER
• USING OHM’S LAW
WE CAN DERIVE
TWO NEW
EQUATIONS FOR
POWER
P = IV
P=IV
P=I(IR) P=(V/R) V
P=I2•R P = V2 / R
ENERGY
• Electric Energy is a measure of the
amount of power that is consumed in a
certain amount of time
• Energy = Power x time
• Energy is measured in a quantity known
as kilowatt hours
• You are charged for the amount of kilowatt
hours you consume
Quantity
Symbol
Potential Difference
(a.k.a. voltage)
I=Q/t
I= V/R
I
Power
P
Resistance
R
Energy
V=I•R
V
Current
E or
Equation(s)
P= E/t
P=IV
P=I2R
P=V2/R
R=
PE
V/I
E=P•t
Standard
Metric Unit
Other Units
Volt (V)
J/C
Amperes (A)
Amps or C / s
or V /
Watt (W)
J/s
Ohm ( )
V/A
Joule (J)
V • C or
W