Download Chap 6 - College of Science | Oregon State University

Energy Fundamentals
The Law of Conservation of Energy
Energy can be neither created nor destroyed but only changed in form…
(Don’t be confused with Energy Conservation that means using less energy or using energy more
efficiently…)
If energy can’t be destroyed, why is there any problem with “running out”? THE FORM!!!
Kinetic Energy – Energy due to motion of object.
- the obvious (moving things!)
- sound (vibration of medium)
- heat (thermal energy) (molecular/atomic vibrations)
Potential Energy – Energy due to position of object. Oftentimes referred to as stored energy.
- Gravitational
- Elastic
- Chemical
- Electrical/Magnetic
- Nuclear
Radiant Energy – Energy of electromagnetic radiation
- solar
- heat (infrared)
Mass Energy – Equivalence of mass and energy
- E = m c2
The “problem” (with energy) is almost always in our attempt to turn energy from one form into
another form that is more useful to us. For example, we turn the chemical potential energy of
gasoline into the kinetic energy of our car moving down the road. But, the chemical potential energy
of that gasoline is now gone (forever essentially!)
Currently, our most popular/useful sources of energy are all non-renewable.
oil + natural gas + coal = source of 85% of our energy
oil + natural gas + coal + uranium = 92% of our energy
What IS Energy?
The ability to do work.
What IS Work?
Work
For work to be done on an object, the following two conditions MUST be met:
- A force must be applied to the object.
- The object must move in the same direction as some component of the applied force.
- Work is a type/form of energy.
- Work is a scalar quantity. (Energy is a scalar quantity.)
- Work is the product of (the component of) the force in the direction of the motion and
the distance the object moves because of that force:
W=Fd
(In SI units) Work has units of N m, or, kg m2 / s2, or joules (J). 1 N m = 1 J
Energy (work) can also be measured in calories, Calories, eV, BTU’s, ergs, ft lb, kW hrs, etc…
Common usage: Everyone knows what work means…
Physics usage: If the two above-mentioned conditions are not met, no work is being done!
Basic Energy (Work) Units
1 joule (J) = 1 newton meter (N m)
British Thermal Unit (Btu)
Energy required to raise the temperature of 1 pound of water by 1o F = 1055 J = 252 calories = 778 ft lb.
calorie (cal)
Energy required to raise the temperature of 1 gram of water by 1o C = 4.186 J
kilocalorie (kcal or Calorie) = 1000 calories
Energy required to raise the temperature of 1 kilogram of water 1o C = 4186 J = 4 Btu
Used for measuring energy content of food.
foot pound (ft lb)
A force of 1 pound moving through a distance of 1 foot = 1.36 J = 0.33 calories
electron volt (eV)
Energy of an electron moving through a potential difference of 1 volt. 1 eV = 1.6 x 10-19 J
6 x 1018 eV = 1 J
1 Quad = 1015 Btu
1 therm = 100,000 Btu
1 kilowatt-hour (kWh) = 3.6 x 106 J = 3413 Btu
1 megaton of TNT = 4.2 x 1015 J
Gravitational Potential Energy
Energy an object has due to its position in a gravitational field.
PE = m g h
where m is mass, and g is g, and h is the height of the object above some specific reference level.
Note that weight = mass*g so this is the same as weight time height.
PE = wt*h
Note also that the SI units will be kg (m / s2) m which is …
Kinetic Energy
Energy associated with (or due to) motion.
KE = ½ m v2
where m is mass, and v is speed.
Note the SI units will be kg m2 / s2, which is = joule.
Work and Conservation of Energy
The work done on an object by a net force is equal to the change in the KE of the object.
Work in direction of motion = Increase in KE
Work in opposite direction of motion = Decrease in KE = Increase in Thermal Energy
The Law of Conservation of Energy tells us that energy can be neither created nor destroyed but it
can be changed in form. Energy transformations happen all around us all the time.
- chemical (sugar) to KE (muscle moving)
- potential (object up high) to KE (as it falls)
- KE (motion of object) to heat (friction)
- chemical (fuel) to light and heat (fire)
- electrical (electricity) to light and heat (a light bulb)
In all these cases, the total energy just keeps getting shuffled around but is never created nor
destroyed!
Conservation of Energy (continued)
Note 1: Some transformations are difficult or impossible even. For example, we can’t easily (if at all)
transform the heat from friction that stops a sliding object back into useful KE of that object!
(Note 2: There’s a cool idea that maybe we can transform the energy required to stop a car into the
energy of a spinning flywheel on the car that could then be used to get the car moving again!)
Note 3: Note 1 means that USEFUL energy decreases! (Or is “lost”.) In fact, it isn’t really lost; it’s
just turned into a form that isn’t useful to us, like heat of friction.
For example, when we burn gasoline in a car, some of the chemical energy is transformed into the
useful KE of the car, but most (80%!) is transformed into heat that is useless to us (when it comes to
moving our car!) That energy is “lost”. AND, the gasoline, a non-renewable energy source, is also
gone for good!
Conservation of Energy is a topic that comes up in nearly all sciences. Its importance can’t be
overestimated!
Some references define “physics” as the study of energy and its transformations.
In physics (and other sciences, even biology) Conservation of Energy lets us solve some problems
that are otherwise very difficult. In some cases, problems may be solvable that are impossible using
Newton’s 2nd Law and his equations of motion.
Power
- Power is the (time) rate at which work is done.
- Power is work done per unit time.
- High output power is associated with being able to do a large amount of work (output a lot of
energy) in a short amount of time.
P=W/t
where W is work done and t is the elapsed time.
- Units will be energy per time units. In SI, that will be joules/second (J/s).
This unit has the special name of watts (W). 1 J/s = 1 W
(Note that a watt also = N m/s or kg (m2/s3).
- Engine power is commonly rated in horsepower.
1 horsepower = 550 ft lb/s = 746 W
Basic Power Units
1 watt (W) = 1 joule/second (J/s) = 3.41 Btu/hr
1 kilowatt (kW) = 103 watt (W)
1 horsepower (hp) = 746 watts = 2545 Btu/hr = 550 ft lb/s
A Very Short Energy History
Until 1850s – WOOD! (solar energy in disguise!)
supplied 90% of energy.
By 1910, COAL (solar energy in disguise!) supplied
75% of our energy.
Advent of automobile increased the use of oil (solar
energy in disguise!) to 40% today.
Today, 95% from 1) oil, 2) coal, 3) nuclear, 4) hydro
1940 – Hydropower (solar energy in disguise!) furnished 40% of U.S. electricity.
Today, 2400 dams in U.S. provide only 9% (and dropping).
Sources of Energy
95% of total energy consumed today comes from one of four sources:
- Petroleum (and natural gas) at 60%
- Coal at 25%
- Nuclear at 7%
- Hydropower at 3%
- These percentages change with time and with location.
- Wood provided 90% of all energy until mid 1800’s.
- After that, coal provided 75% of all energy until early 1900’s.
- These percentages will be shifting again as fossil fuels diminish.
- Solar and geothermal are less than 2%.
- Petroleum, natural gas, and coal are all fossil fuels.
- Fossil fuels are non-renewable resources. They are slowly being diminished; when they are gone,
they are gone!
- Nuclear (fission) is also a non-renewable resource. There is only a fixed amount of uranium and
plutonium on the planet.
- Nuclear, so far, means only fission reactors. Fusion reactors don’t yet exist (except in the
experimental stage.) (Fusion is what powers the sun and other stars.)
- Fusion power would be both cleaner and safer, but it’s not here yet!
- All sources have their advantages and disadvantages. (Such as?!…)
- Hydropower is really just another form of solar energy. (Why so?)
- Alternative Sources
- solar cells
- “power tower”
- passive solar
- active solar
- wind energy (just another form of solar energy! (Why so?!))
- biomass
- ocean thermal energy conversion
- others?!