Transcript What is Energy?

What is Energy?
• Physics Definition: The ability to do work
• Work: Force applied over a distance (W =f*d)
• Force: From Newton, force is the product of a
mass and its acceleration (F=ma) also known
as Newton’s second law.
• But this applies mostly to mechanics, the
study of the physics behind an objects motion
What is Energy?
• Thermodynamics: the study of the conversion
of heat energy into other forms of energy.
• In themodynamics, work is defined as the
quantity of energy transferred from one
system to another without a change in its
amount of order (called entropy)
Units of energy
• Joules:
– The work done by a force of one newton traveling through a
distance of one meter;
– The work required to move an electric charge of one coulomb
through an electrical potential difference of one volt; or one
coulomb volt, with the symbol C·V;
– The work done to produce power of one watt continuously for
one second; or one watt second (compare kilowatt hour), with
the symbol W·s. Thus a kilowatt hour is 3,600,000 joules or 3.6
megajoules;
– The kinetic energy of a 2 kg mass moving at a velocity of 1 m/s.
The kinetic energy is linear in the mass but quadratic in the
velocity, being given by E = ½mv²
Everyday examples of the Joule
• the energy required to lift a small apple one meter
straight up.
• the energy released when that same apple falls one
meter to the ground.
• the energy released as heat by a quiet person, every
hundredth of a second.
• the energy required to heat one gram of dry, cool air by
1 degree Celsius.
• one hundredth of the energy a person can receive by
drinking a drop of beer.
• the kinetic energy of an adult human moving a
distance of about a hand-span every second.
Power
• Power: the rate at which work is performed
– Or, the rate at which energy is transmitted
– Or the amount of energy expended per unit time
•
• Measured in Watts:
• Other units:
– HP or horse power
– BTUs
Horse power
• Arose as a result of the invention of the steam
engine. People needed a way to compare the
power of a steam engine to that of the horses
it was replacing.
• Confusing unit there are too many different
definitions!
BTU
• BTU: British Thermal Units - an energy unit
– the amount of heat required to raise the temperature
of one pound of liquid water by one degree from 60°
to 61°Fahrenheit at a constant pressure of one
atmosphere
• Used in the power, steam generation, heating and
air conditioning industries and the energy
content of fuels.
• However, BTU is often used as a unit of power,
where BTU/hour is often abbreviated BTU.
– So you need to watch the context!
Back to Watts…..
• A human climbing a flight of stairs is doing work
at a rate of about 200 watts.
• A typical household incandescent light bulb uses
electrical energy at a rate of 25 to 100 watts,
while compact fluorescent lights typically
consume 5 to 30 watts.
• A 100 Watt light bulb consumes energy at the
rate of 100 joules/second.
• After 1 hour, this light bulb uses 100 watt-hours
• 1 kilowatt (kw) is 1000 Watts
Examples
• In a certain room in your house, you use a 100 W
light bulb. This light is on for 5 hours every day.
How much energy does it use?
• 1 W = 1 J/s and there are 5h x 60min/hour x 60
sec/min = 18,000s in 5 hours so the total energy
used is 100 j/s *18000s = 1.8 x 10 6 J.
• Lets assume the same lighting level can be
achieved using a 30 W compact florescent bulb.
How much energy is used by the compact
florescent bulb?
Examples
• Total energy = 30 j/s x 18000 s = 5.4 x 105 j.
• So how much energy is saved every day using
the compact florescent bulb? Take the
difference between the energy used by the
two different light bulbs: 1.8 x 10 6 j - 5.4 x 105
j = 1.3 x106 j.
• Lets look at this in something you might be
able to relate to better than joules---dollars!
Example continued
• After 5 hours, our 100 W light bulb uses 500
Watt-hours, or 0.5 Kwh. The 30 W bulb will
use 150 Watt hours or 0.15 Kwh.
• Assume electricity costs 11 cents/Kwh
(average cost in the US in April 2008). So it
costs .5 KwH x 11 cents/Kwh = 5.5 cents every
day to run the 100 W light bulb and 0.15Kwh
x 11 cents = 1.65 cents every day to run the
compact florescent.
Example continued
• So in a year, the 100 W light bulb costs you 5.5
cents/day X 365 days/year = $20.00 and the
30 W bulb costs costs you 1.65 cents/day x
365 days/year = $5.50.
Types of Energy:
kinetic and
potential Energy
Kinetic energy energy of a moving
object KE=1/2mv2
Potential Energy –
Energy stored in a
system, for example
an object of mass m,
a distance h above
the surface of the
earth has a potential
energy given by
mgh. g is the
acceleration due to
gravity = 9.8 m/s2
More examples of
potential energy
Another example is
a spring,
compressed a
distance x from its
equilibrium point
has a potential
energy 1/2kx2,
where k is the
spring constant, a
property of the
spring.
Chemical Energy
Energy that is released
via chemical reactions.
Often times release is
through combustion
such as energy
generation via coal
Another example is a
battery
Heat Energy
Energy associated
with the random
motions of the
molecules in a
medium.
Measured by
temperature
• Temperature Scales:
• Fahrenheit – based on the
height of liquid (often
mercury or alcohol) in a
glass tube.
• Celsius – another scale
using height of liquid in a
tube
• Kelvin-absolute scale
– True measure of energy