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Milk production at a diary farm was low so the farmer
wrote to the local university asking for help from academia.
A multidisciplinary team of professors was assembled and
two weeks of intensive on-site investigation took place.
The scholars then returned to the university, notebooks
crammed with data, where the task of writing the report
began. Shortly thereafter the farmer received the write-up,
and opened it to read on the first line:
“Consider a spherical cow …”
What is ‘the ability to do work’?
Energy
Other definitions:
- Measure of ability
- Capacity to produce heat
- E=mc2
Energy isn’t so easy to define.
BUT, the point to grasp is
To DO anything,
energy is required.
Definitions
 Work
 Force acting over a distance.
 A measure of accomplishment.
 Doing.
Units of energy:
Joule (J), btu, Calorie
 Power
 Rate of energy use – energy/time
 units: Watts (W), horsepower (hp)
 Efficiency
 (what you get out)/(what you put in)
 Potential energy – stored energy.
 Kinetic energy – energy associated with motion.
 There are an incredible number of overlapping
classifications of energy.
More definitions
 Thermal energy - kinetic energy of molecules
 We think of this as heat.
 Radiation (electromagnetic energy)
 This includes X-Rays, light, radio waves, cell phone
communications, microwaves, and more
 This stuff is very cool.
 Electrical energy – kinetic energy of charges
 Mechanical energy – energy associated with moving
parts
 Chemical potential energy – energy stored in
molecular bonds
 Nuclear energy – energy stored in the nucleus of
atoms
Energy ‘Laws’
 Conservation
 Energy can be created or
destroyed, it can change
forms but it always exists.
 1st Law of
Thermodynamics
 Conversion of thermal
energy to mechanical
energy
 Can’t be 100% efficient
 2nd Law of
Thermodynamics
Eout
Ein
Eearth
Ein + Eearth – Eout = constant
Mechanical
Thermal
Thermal
Mechanical
+ Thermal
Mechanical vs. Muscular Energy
 The incredible increase in economic and
technological advances of the ‘modern’ world
were brought about by humankind’s switch from
muscular to mechanical energy.
 How do they compare?
 Let’s look at an example that may give us all a
better grasp of energy.
 Example – my daily commute
 Niwot to CU
Muscular Energy
(in honor of bike week)
 If I bike, I am using muscular
energy.
 How much?
 260 kJ
 Where does the muscular energy
come from?
 Food (potential energy of food
molecules)
 Our bodies convert food energy
to muscular energy with ~20%
efficiency.
 SO, I need to input 1300kJ to get
to work on muscular energy.
Mechanical Energy
 If I drive to work, I am using
mechanical energy.
 How much?
 Let’s skip this step and directly
calculate the amount of energy in
the amount of gasoline that I
consume.
 I use ~1 L to get to work.
 So, I use 48,000 kJ of energy to
drive to work.
 I use 37x more energy driving
than biking.
 BUT, when driving, I get there
faster (3x) and I can carry more
stuff.
Another example:
How does my toaster do work?
 Coal is mined in Wyoming and shipped via rail to Cherokee






Station in Denver.
The chemical potential energy in coal is converted to thermal
energy and other less energetic molecules. (We call this
process combustion.)
The thermal energy is converted to mechanical energy in a
steam turbine.
The mechanical energy is converted into electrical energy in
a generator.
The electrical energy, which is easy to move, is sent to my
house on wires.
The electrical energy reaches my toaster through it’s cord
and is converted into thermal energy via a big resistor.
The thermal energy warms my toast.
Last points
 Net energy use:
 I have also heard this concept called ‘lifecycle energy’.
 There is ‘other’ energy involved with me driving or biking to
work.
 Energy required to build my bike.
 Energy required to build the paths that I bike on.
 Energy required grow and process the food that I need as fuel.
 Energy required to process and get water to me so I can ride my
bike.
 All the energy required needs to be included when
developing comparisons.
 In any energy conversion process, what happens to
the energy that is converted to the useful form?
 Combustion
 What happens to molecules with lower internal energy?
 What happens to the thermal energy not converted to mechanical
energy?
 Metabolism of food