Energy Conversions & Conservation

Bellringer  Where does the energy that makes a roller coaster car move come from? Where does the energy go?

Energy Conversion  An energy conversion is a change from one form of energy to another  Any form of energy can change into any other form of energy  Usually one form of energy changes into more than one other form

Potential & Kinetic Energy  As the skateboarder travels up and down the half-pipe, his energy changes back and forth between kinetic energy and potential energy

Elastic Potential Energy  Stretching a rubber band stores elastic potential energy in the rubber band  When you let the rubber band go, it goes back to its original shape, releasing its stored-up potential energy as it does so

Conversions: Chemical Energy  Chemical energy is stored in the food you eat  Your body uses this chemical energy to function  Example:  Converted into kinetic energy when active  Converted into thermal energy to maintain body temperature

Energy Conversion in Plants  The chemical energy in the food you eat comes from the sun’s energy  Plants use photosynthesis to convert light energy into chemical energy

Energy Conversions in a Hair Dryer

Conversions: Electrical Energy

Where Does the Energy Go?  Roller coasters have a mechanism that pulls the cars up to the top of the first hill  But the cars are on their own for the rest of the ride  As the cars go up and down the hills on the track, their potential energy is converted into kinetic energy and back again  But the cars never return to the same height at which they started  Energy does not get lost along the way; it is just converted into other forms of energy

Friction  Friction is a force that oppose motion between two surfaces that are touching  For a roller coaster car to move, energy must be used to overcome the friction between the car’s wheels and the track  As a result, not all of the car’s potential energy changes into kinetic energy and not all of the car’s kinetic energy changes back into potential energy

Closed Systems  A closed system is a group of objects that transfer energy only to each other  Example:  a closed system that involves a roller coaster consists of the track, the cars, and the air around them  On a roller coaster, some mechanical energy (the sum of kinetic and potential energy) is always converted into thermal energy because of friction  Sound energy also comes from the energy conversions in a roller coaster  If you add together the cars’ kinetic energy at the bottom of the first hill, the thermal energy due to overcoming friction, and the sound energy made, you end up with the same total amount of energy as the original amount of potential energy  Energy is conserved and not lost

The Law of Conservation of Energy  Energy cannot be created or destroyed, it can only change forms  The total amount of energy in a closed system is always the same

No Conversions without Thermal Energy  Any time one form of energy is converted into another form, some of the original energy always gets converted into thermal energy  The thermal energy due to friction that results from energy conversions is not usually useful energy