What is energy?
Jan 06, 2016
What is energy?
Chain of energy
Newton’s Laws
1. An object in motion will stay in motion (or rest at rest) unless acted on by an outside force. (Inertia)
2. Force = mass x acceleration (Units: Newtons = kg x m/s2)
3. For every action there is an equal and opposite reaction.
Work
Energy = ability to do work
Work = force exerted over a distance
W (or “energy”) = F x d
1 Joule = 1 Newton x 1 meter
4.18 Joule = 1.00 calorie
Power
Rate at which work is done
Power = Work / time
Watts = Joules / seconds
SO… Energy can also be expressed this way:
Energy = Power x time
Joules = Watts x seconds
Consider:
What does it mean to give electrical appliances a rating in watts?
What is a kilowatt? What is a killowatt-hour?
Why do we describe food in terms or calories?
Forms of Energy
Kinetic energy = Energy of moving objects
KE = ½ mass x velocity2
KE = ½ mv2
Joules = ½ kg x (m/s)2
Potential energy = Energy stored due to position or composition
PE = mass x acceleration due to gravity x height
PE = mgh
Joules = kg x m/s2 x m
Consider:
How do each of the following represent KE?
Heat radiating from a car
The sound of my voice
Muscles contracting
Light traveling to your eye
Consider:
How do each of the following represent potential energy?
A book on the edge of a table
A slice of pizza
Two magnets near each other
An electrical outlet
A stretched rubber band
What is heat?
Kinetic energy caused by atoms moving and colliding
Flows from warm body to cold body
Measured as temperature
Temperature Scales
° F = (1.8 x ° C) + 32
° C = (° F – 32) / 1.8
Both use freezing and boiling points of water as standards
Kelvin = ° C + 273
0 K = absolute zero
Specific Heat
Ability of a material to absorb/lose heat
Q = mcΔt
Heat gained/lost = mass x specific heat x change in temp
If two substances have equal mass and equal change in temp., what does the amount of heat gained depend on?
Heat transfer
Conduction – transfer due to atoms colliding or vibrating
Convection – transfer due to bulk movement of a fluid (gas or liquid)
Radiation – transfer due to electromagnetic radiations (uses electrical/magnetic fields instead of matter)
file:///Users/johnb/Desktop/lsps07_int_heattransfer/lsps07_int_heattransfer.html
How can these types of transfer work together?
1st Law of Thermodynamics:Conservation of Energy
Energy cannot be created or destroyed but can be transformed
PE = KE
mgh = ½ mv2
How can our examples of PE be converted into KE? Is the reverse also true?
Are energy conversions 100% efficient?
2nd Law of Thermodynamics: Entropy
Statements:
1. Heat will not flow spontaneously from a cold to a hot body.
2. You cannot construct an engine that does nothing but convert heat to useful work.
3. Every isolated system becomes more disordered with time. (Entropy)
http://www.stolaf.edu/people/giannini/flashanimat/transport/diffusion.swf
Applications
What is a gradient? How do they work? What kinds of gradients exist?
Can any system transfer energy with 100% efficiency? What does this mean in living systems?
How do we maintain order in systems (especially living systems)?
Where does energy go? Is it truly “lost”?
Back to the Sun – Nuclear Fusion
High pressure and high temperature force atoms together
Mass converted to energy
Energy radiates into space
How do atoms relate to energy?