Energy vs. Power Sustainable Energy Jan 22, 2007 Lecture by Joel Bettner

The Basics: Force Force –Accelerating a mass What is keeping us on the earth? –Gravity accelerates us –We have mass –Gravity applies a force on our mass g m F = m x g

The Basics: Work Work –Force over a distance (i.e. a force causing something to move) Mass distance Force W = F x d

Energy –The ability to do work Potential Energy –How much potential energy does the mass have? –M x g x d = PE Kinetic Energy –How much energy does the mass have? –.5 x m x v 2 Total Energy Conserved –T.E. = P.E. + K.E. Mass distance v

Energy –The ability to do work Other forms of energy –Heat –Chemical –Nuclear

Power Power is energy per time –Watch each horse –Which horse uses/produces more energy? –Which horse produced greater power? –James Watt (1782) Coal mine horses lifting buckets of coal d M d #1 d M d #2

Energy vs. Power Ready to feel the power? –Break up into your groups –Grab one chair and go into the hall –WALK down the hall pushing a group member in the chair –Next, RUN down the hall pushing your group member

Energy vs. Power Did you use more energy while walking or running? Did you have a higher power production while walking or running?

Energy vs. Power Think about your car –What is in your gasoline, energy or power? What kind of energy is it? –What does the chemical energy in your gasoline produce Power! Hence the “Horse Power” rating of a vehicle. Kinetic Energy Heat

Units How do we measure force, energy, and power? Here are some common ways these are measured. –Force English Units  Pounds (lb) Metric Units  Newtons (N) –Energy English Units  Pound-Feet (lb-ft) Metric Units  Newton-Meters (N-m)  Joule (J) –Power English Units  Horse Power (HP)  1 HP = 550 ft-lb/sec Metric Units  Watt (W)  1 W = J/sec

Questions and Discussion Does anyone have any questions? Can anybody think of common places/applications where the distinction between energy and power is visually evident? –I think of batteries Battery in my car is MUCH larger than two 9-volt batteries. However, my car battery is 12.6 Volts. What do you think that means? If discharged at the same power, which do you think would last longer? Why?