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
Fahrenheit
temperature scale
• Freezing point of water set
at 32 and boiling point set
at 212, so there is 180
degrees between them and
each degree is 1/180 of the
difference between these
two points.
Celsius
temperature scale
• Freezing point of water set
at 0 and boiling point set at
100, so there is 100
degrees between them and
each degree is 1/100 of the
difference between these
two points.
Kelvin
temperature scale
• O k is absolute zero. All
molecular motion stops.
• Interval set so that 1 k = 1 c
• So to convert from c to k
k=c+273
Mass Energy
• E = mc2
• Energy and mass are
equivalent
• C = 3 x 108 m/s.
• A big number and its
squared! So even if m is
small, E is big.
• A small mass, converted to
energy, gives a lot of energy!
Example
Electromagnetic
energy
•Light displays properties of both
waves and particles.
•Light is an electromagnetic wave-a
wave created by alternating electric
and magnetic fields.
•“Light” is more than just visible light,
it covers wavelengths from radio thru
Gamma rays
•Light is also a “particle” called a
photon.
•Photons have energy given by E=hν or
E=hc/λ. H is constant, c is the speed of
light , ν is the frequency of light and λ
is the wavelength of the light.
Conservation of Energy
• The principle of
conservation of energy
states that energy
cannot be created or
destroyed. But it can be
converted form one
form to another
• This idea of energy
transformation is at the
heart of energy
generation.
Energy Sources renewable vs nonrenewable
• Renewable – can’t be
exhausted
• Solar
• Geo-thermal
• Tidal
• Wind
• Hydro
• Non-renewable-can be
exhausted
• Fossil fuels (oil, coal etc)
uranium
How much do we use?
• World energy
consumption
• US energy consumption
How much do we use?
How much do we use?
• Almost 95% of the energy we use comes from
non-renewable energy sources!
• One of these days we will run out, and then
what?
• What are some short and long term answers
to this question?