CHAPTER 15: ENERGY

You use energy when you: hit a softball. lift your book bag. compress a spring. Living organisms need energy for growth and movement. Energy is involved when: a bird flies. the wind blows. rain falls from the sky. electricity flows in a wire.

What is energy that it can be involved in so many different activities? Energy can be defined as the ability to do work. Work is the transfer of energy by a force moving an object some distance. The unit of energy is the Joule (J). If an object or organism does work (exerts a force over a distance to move an object) the object or organism uses energy.

Because of the direct connection between energy and work, energy is measured in the same unit as work: joules (J). In addition to using energy to do work, objects gain energy because work is being done on them.

The Law of Conservation of Energy and Matter states that energy and matter cannot be created or destroyed, but can be changed in form.

In 1905, Albert Einstein said that mass and energy can be converted into each other. He showed that if matter is destroyed, energy is created, and if energy is destroyed mass is created. E = mc 2 is the most well known formula in the world.

Chemical reactions do not change matter into energy or vice-versa, but nuclear reactions can change matter into energy and energy into matter.

Objects can gain energy when work is done on them. Objects can gain energy when they are heated. Objects can gain energy when they are raised higher. Objects can gain energy when they are stretched or compressed.

Objects can lose energy when they do work. Objects can lose energy when they are cooled. Objects can lose energy when they are lowered. Objects can lose energy when they relax back to their at-rest shape.

When energy changes form, some is always changed into waste heat. If enough escapes into the environment, we call it thermal pollution.

The food we eat is chemical energy, which is changed in our bodies to mechanical energy when we move…. or even sleep.

Energy can be changed from one form to another. Changes in the form of energy are called energy conversions OR transformations. All forms of energy can be converted into other forms. The sun’s energy (EM) through solar cells can be converted directly into electricity (electrical energy). Green plants convert the sun’s energy (EM) into starches and sugars (chemical energy)

Conversions Between Mechanical KE and PE Assume a closed system: mechanical energy is not added to system and FRICTION CAN BE IGNORED Total mechanical energy of the system remains CONSTANT KE and PE can be completely converted into one another Can be applied to any mechanical process Law of Conservation of Energy – energy cannot be created nor destroyed only converted Conservation of Mechanical Energy: (KE + PE) Beginning = (KE + PE) End

Energy Conversions However, any time an energy conversion takes place, some of the energy is transferred from the system in the form of HEAT Regardless of the form of energy, energy is conserved – LAW OF CONSERVATION OF ENERGY

The most common energy conversion is the conversion between potential and kinetic energy. Energy can be in either of two states: Potential – stored energy Kinetic – energy in motion

Roller coasters work because of the energy that is built into the system. Initially, the cars are pulled mechanically up the tallest hill, giving them a great deal of potential energy. From that point, the conversion between potential and kinetic energy powers the cars throughout the entire ride. 6.sci.phys.maf.rollercoaster/

A pendulum shows potential energy changing into kinetic energy and back to potential energy as it swings. maximum potential energy maximum kinetic energy

In an automobile engine, fuel is burned to convert chemical energy into heat energy. The heat energy is then changed into mechanical energy.

In an electric motor, electric energy is converted to mechanical energy. In a mp3 player the chemical energy in the battery is converted into electromagnetic energy for the display & mechanical energy of the music. The mechanical energy of a waterfall is converted to electrical energy in a generator.