mfe chapter 5 notes

Energy: Forms and Changes

• The Nature of Energy• Kinetic and Potential Energy• Energy Conversions• Conservation of Energy• Physics and Energy

The Nature of Energy

• Energy can be defined as the ability to do work.

• Energy is often measured in Joules (J).• 1 Joule=1Kg-m2/sec2= 1 Nm• Energy comes in many forms. The

main forms are: Mechanical, Heat, Chemical, Electromagnetic, and Nuclear.

The Nature of Energy (continued)

• Mechanical energy is energy of motion.• Examples include a moving car and

sound.• Heat energy is energy of internal

motion (movement of atoms).• Friction is one cause of heat energy.• Chemical energy is stored in the bonds

that keep molecules together.

The Nature of Energy (continued)

• Electromagnetic energy is caused by moving charges.

• Examples include electricity and light.

• Nuclear energy is stored in the nucleus of an atom.

• All stars (including our sun) run on nuclear energy.

Kinetic and Potential Energy

• All of the types of energy just described fit into one of two categories of energy: Kinetic or Potential.

• Kinetic energy is the energy of motion.

• K.E.=(mass x velocity2)/2• Kg x (m/sec)2=Kg-m2/sec2=Joule

Kinetic and Potential Energy (continued)

• Potential energy is stored energy and is sometimes called energy of position.

• For example, raising a bowling ball 1 meter above the ground gives it potential energy relative to the ground.

• Gravitational Potential Energy (GPE)= Weight x Height.

• Newtons x meters=Nm=Joule

Energy Conversions

• Energy can be converted from one form to another.

• This process can happen over and over again.

• For example, I can convert the mechanical energy of rubbing my hands together into heat energy.

Energy Conversions (continued)

• One of the most common energy conversions is Kinetic-Potential.

• For example, by holding a 20 Newton bowling ball 1 meter above the ground, it has 20 Joules of gravitational potential energy.

• By releasing the bowling ball, its potential energy is converted into kinetic energy as it gains velocity.

Energy Conversions (continued)

• As the bowling ball speeds up towards the ground, its gravitational potential energy is reduced, and its kinetic energy is increased until the instant before it hits the ground.

• At that point, the bowling ball has converted all of its GPE into kinetic energy, so that it now has 20 Joules of kinetic energy.

Conservation of Energy

• The previous example shows how energy is always conserved.

• In fact the Law of Conservation of Energy says that energy can neither be created nor destroyed by ordinary means.

• Albert Einstein showed how energy can be converted into mass and mass into energy using his famous “E=mc2”

Conservation of Energy (continued)

• The Law is consequently referred to as “The Law of Conservation of Mass-Energy.”

Physics and Energy

• Understanding energy is critical to understanding the physical sciences.

• Everything we have studied in MFE connects in some way to energy.

• For example, the faster you move, the more momentum you have, the more kinetic energy you have, the higher your power, which means you consume energy faster.