Energy: Basics

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Energy Basics
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Definitions
Energy - the ability to do work
Work - the transfer of energy by applying a
force through a distance
But what is a force?
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Position
Position - orientation and distance an objectis
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
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Velocity
Defn. - time rate of change ofdisplacement 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 secondsto go the 400 meters down the
dragstrip?
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Speed
Some books say that velocity is speed
direction. WRONG!
Distance traveled
Average speed
Elapsed time
Displacement Distance traveled
Displacement on racetrack is 0, while distance
travelled is not
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Acceleration
Defn. - time rate of change of velocityis a
vector quantity SI unit ism/s2
Dv
Average acceleration
Dt
Accelerations can occur without changing the
magnitude of velocity Ex. Object going in circle
at constantrate
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Newtons First Law
Really, Galileos
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?
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Newtons Second Law
F ma
Relates kinematic variables to dynamic ones
Can measure accelerations ? calculate forces
Note SI unit is newtons, English is
poundsIncorrect 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?
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Newtons Third Law
For every force, there is an equal and opposite
reaction force.
Often misunderstood actually means that one
object actingon a second object will have the
second object act on it
Mule pulls on cart. Cart pulls back onmule with
equal and opposite force.Why pull?, says mule,
if force willbe negated.
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Get Back To Work
Work - the transfer of energy by applying a
force through a distance
W F x d if F is constantDW
Fn x Dd if F varies
Lifting box F mgDistance lifted h W mg
x h mgh
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Simple Machines
Allow for the same amount of workto be done, but
with smaller forces
Trade-off of using a smaller force isthat 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/LDistance pushed up plane
L
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Power
DE
Power rate of energy usage
Dt
Can deliver the same amount of energy to a system
using lesspower, but it takes a longer amount of
time
Our Western mindset usually screams for more
powerEx. SUVs 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?
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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
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Example Gravitational potential energy
Potential energy due to gravity
If the water or rock drops, gravity operates
over a distance, thereby doing work. This work
converts the potential energy to kinetic energy.
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Kinetic energy
Some of the bullets 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
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How do you make electricity?
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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 subatomicparticles
that contain charge equal amounts of and -
results in neutrality
Designation of positive and negative
iscompletely arbitrary
If like charges repel and unlike attract, what
holds protons andneutrons together?
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Currents - Moving Charge
Oersted (1820) discovered that a currentcreates
a magnetic fieldThis means a current has a
force on a magnet
Newtons Third Law means that amagnet has a
force on a moving chargeDirection of force is
perpendicular tovelocity and magnetic field
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Generator
Electric generator rotate coils of wire
perpendicularto magnetic field
Magnet
Layers of wire coils
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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
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How do you make electricity?
Most electricity formed by steam turbine electric
generators
Turbine
Boiler
Generator
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Turbine
Steam input from boiler
Shaft
Steam under pressure presses on blades and turns
turbine and shaft
Turbines
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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 ? P
(Dq V )/ DtBut I Dq/Dt
? P I V
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Energy needs in the modern world
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How do our current uses of energy compare with
those in the old days?
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AGRICULTURE
THEN Chemical energy in livestock (sugar, fat)
NOW Chemical energy in gasoline
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INDUSTRY
THEN Chemical energy in humans (sugar, fat)
NOW Fossil fuels, electricity from chemical
energy in coal
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LIGHT
THEN Chemical energy in biomass
NOW Electricity from chemical energy in coal
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HEAT COOKING
THEN Chemical energy in biomass (wood)
NOW Fossil fuels, electricity from chemical
energy in coal
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LANDSCAPING
THEN Chemical energy in humans (sugar, fat)
NOW Chemical energy in fossil fuels
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TRANSPORTATION
THEN Chemical energy in humans or animals
NOW Chemical energy in fossil fuels
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EDUCATION
THEN Chemical energy in humans
NOW Electricity from chemical energy in coal
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MORAL
We now use energy from fossil fuels instead of
energy from humans, animals or biomass
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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/
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One Case Crude Oil
We get energy from many different sources. One
of the moreimportant ones we will discuss is
crude oil.
What are the implications of this graph?
Whathistorical eventsoccurred during thistime
that relate tocrude oil?
Source Dept. of Energy, http//eia.doe.gov/
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Import Countries
Since the mid-1970s, we have increased our
dependence of oil imports on non-OPEC countries
Why?
We have increased our reliance on oil from
Central and South America, as well as Canada and
Africa
Why?