Transcript Potential energy

Energy: Basics
Definitions
Energy - the ability to do work
Work - the transfer of energy by applying a
force through a distance
But what is a “force”?
Position
Position - orientation and distance an object
is from some origin; measurement of
position requires a coordinate system
If the position does not change, the object is easily found
Displacement - change in position; if position is designated
with the vector r, then displacement is Dr
Velocity
Defn. - time rate of change of
displacement; is a vector
quantity; SI unit = m/s
Displacement
Dr
Average velocity =
=
Elapsed time
Dt
Instantaneous velocity = limit (average velocity)
Dt0
What is the average velocity of a dragster that takes 5.5 seconds
to go the 400 meters down the dragstrip?
Speed
Some books say that velocity is speed + direction. WRONG!
Average speed =
Distance traveled
Elapsed time
Displacement = Distance traveled
Displacement on racetrack is 0,
while distance travelled is not
Acceleration
Defn. - time rate of change of velocity;
is a vector quantity; SI unit is
m/s2
Dv
Average acceleration =
Dt
Accelerations can occur without
changing the magnitude of velocity;
Ex. Object going in circle at constant
rate
Newton’s First Law
Really, Galileo’s
“An object at rest, or in a state of constant motion,
will continue in that state unless acted upon by an
unbalanced force.”
Inverse of statement is very important: if an object is acceleration,
then a net force is operating on it, even if you cannot see the
reason for the force.
Is there a force operating in this picture,
and if so, from what direction?
Newton’s Second Law
F = ma
Relates kinematic variables to dynamic ones
Can measure accelerations  calculate forces
Note: SI unit is newtons, English is pounds
Incorrect to say that X pounds = Y kilograms
Not all forces are constant
What force is needed to accelerate a 1000 kg car to 5 m/s2?
Newton’s Third Law
“For every force, there is an equal and opposite
reaction force.”
Often misunderstood; actually means that one object acting
on a second object will have the second object act on it
Mule pulls on cart. Cart pulls back on
mule with equal and opposite force.
“Why pull?”, says mule, if force will
be negated.
Get Back To Work
Work - the transfer of energy by applying a
force through a distance
W=Fxd
DW = Fn x Dd
if F is constant
if F varies
Lifting box: F = mg
Distance lifted = h
W = mg x h = mgh
Simple Machines
Allow for the same amount of work
to be done, but with smaller forces
Trade-off of using a smaller force is
that the force is applied through a
longer distance
Box lifted straight up a height h, force supplied is F = mg
Force of gravity down inclined plane is F = mg sinq = mgh/L
Distance pushed up plane = L
Power
Power = DE = rate of energy usage
Dt
Can deliver the same amount of energy to a system using less
power, but it takes a longer amount of time
Our Western mindset usually screams for more power
Ex. SUV’s require more powerful engines; larger homes
require more powerful a.c.
How much power do you expend by climbing 3 flights of
stairs (10 m) in 10 seconds?
Potential energy
Energy stored within the force between two
objects separated by a distance; if objects
are allowed to move, force is applied through
distance = work done
TYPES OF POTENTIAL ENERGY:
Gravitational
Chemical
Nuclear
Example: Gravitational
potential energy
Potential energy due to gravity
EXAMPLES:
Water behind a dam
A rock at the top of a
steep hill
If the water or rock drops, gravity
operates over a distance, thereby doing work. This work converts
the potential energy to kinetic energy.
Kinetic energy
ENERGY OF MOTION
A moving object has momentum. If it hits
another object, it will transfer energy to it by
applying a force through a distance, i.e. work
Some of the bullet’s kinetic
energy is transferred to the
apple during the collision
Kinetic energy of falling water is
converted to motion of turbines
when water falls on them
How do you make electricity?
An important energy transfer example
Charge
Defn: Property of matter that determines how it will interact
with other matter via the electrostatic force; opposite charges
attract, like repel
All atoms are composed of subatomic
particles that contain charge; equal
amounts of + and - results in neutrality
Designation of positive and negative is
completely arbitrary
If like charges repel and unlike attract, what holds protons and
neutrons together?
Currents - Moving Charge
Oersted (1820) discovered that a current
creates a magnetic field
This means a current has a force on a
magnet
Newton’s Third Law means that a
magnet has a force on a moving charge
Direction of force is perpendicular to
velocity and magnetic field
Generator
Electric generator: rotate coils of wire perpendicular
to magnetic field
Magnet
Layers of wire coils
How do you make electricity?
Need energy source to do the work of turning the
magnet, causing a flow of current (electricity) in
the wire coils
Front view
Convert the potential energy
in something to kinetic
energy to turn magnet
Options
Water behind a dam
Chemical energy in
biomass or fossil fuels
How do you make electricity?
Most electricity formed by steam turbine electric
generators
Boiler
Turbine
Generator
Steam input
from boiler
Turbine
Shaft
Steam under pressure
presses on blades and turns
turbine and shaft
Turbines
Electrical Power
We know that P = DE/Dt from the definition of power
The amount of energy delivered by a current is equal to the
amount of charge delivered times the voltage,
DE = Dq V
But
I = Dq/Dt
 P=IV

P = (Dq V )/ Dt
Energy needs
in the modern
world
How do our current
uses of energy
compare with those
in the “old days”?
AGRICULTURE
THEN:
Chemical energy
in livestock (sugar, fat)
NOW:
Chemical energy
in gasoline
INDUSTRY
THEN:
Chemical energy
in humans (sugar, fat)
NOW:
Fossil fuels, electricity from
chemical energy in coal
LIGHT
THEN:
Chemical energy
in biomass
NOW:
Electricity from chemical
energy in coal
HEAT & COOKING
THEN:
Chemical energy
in biomass (wood)
NOW:
Fossil fuels, electricity from
chemical energy in coal
LANDSCAPING
THEN:
Chemical energy
in humans (sugar, fat)
NOW:
Chemical energy in
fossil fuels
TRANSPORTATION
THEN:
Chemical energy
in humans or animals
NOW:
Chemical energy in
fossil fuels
EDUCATION
THEN:
Chemical energy
in humans
NOW:
Electricity from chemical
energy in coal
MORAL:
We now use energy
from fossil fuels
instead of energy
from humans,
animals or biomass
U.S. Energy Consumption
Over the last 50 years, our
consumption of energy has
increased (except for after energy
crises)
Because of more efficient
devices, our consumption per
person has stayed about the
same over the last 30 years
Source: Dept. of Energy, http://eia.doe.gov/
One Case: Crude Oil
We get energy from many different sources. One of the more
important ones we will discuss is crude oil.
What are the
implications of
this graph? What
historical events
occurred during this
time that relate to
crude oil?
Source: Dept. of Energy, http://eia.doe.gov/
Import Countries
Since the mid-1970’s, we
have increased our
dependence of oil imports
on non-OPEC countries
Why?
We have increased our
reliance on oil from North
and South America
Why?