Energy vs. Power

Sustainable EnergyJan 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• 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 v2

• Total Energy Conserved– T.E. = P.E. + K.E.

Mass

distance

v

Energy

• 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

• http://cusustainableenergy.pbwiki.com/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?