Chapter 15 Energy

Objectives:

•Explain how an object’s kinetic energy depends on its mass and its speed.

•Demonstrate that near the Earth’s surface an object’s gravitational potential energy depends upon its weight and height above a reference surface.

•Trace the transformations of energy within a system and recognize that energy is conserved.

Nature of Energy

  • Energy is all around you!
  • You can hear energy as sound.
  • You can see energy as light.
  • And you can feel it as wind.

Nature of Energy

  • You use energy when you:
  • hit a softball.
  • lift your book bag.
  • compress a spring.

Nature of Energy

  • Living organisms need energy for growth and movement. They get that energy from food

I. Energy and Work

  1. Work is a transfer of energy.
  2. Both work and energy are measured in Joules (J).
  3. Can take many forms

1. The sun

2. Plants convert sun’s energy into food that people eat.

3. People convert food energy into muscle movement.

II. Kinetic Energy

  1. The word kinetic comes from the Greek word kinetos, meaning “moving.”
  2. The kinetic energy of any moving object depends upon its mass and speed.
  3. Kinetic energy = ½ mv2

1. m = object’s mass

2. v = speed

III. Gravitational Potential Energy

  1. An object’s gravitational potential energy depends on its mass, its height, and the acceleration due to gravity.
  2. Potential Energy (PE) = mgh

1. m = mass

2. g = gravity

3. h = height

  1. Relative position of height

1. Potential energy should be stated in terms of relative position.

ex. If you are standing on the ground your potential energy is 0 relative to the ground.

D. Gravitational Potential Energy

If you stand on a 3-meter diving board, you have 3 times the G.P.E, than you had on a 1-meter diving board.

IV. Elastic Potential Energy

  1. When you stretch a rubber band there is potential energy.
  2. When you stretch a spring there is potential energy.

Kinetic vs. Potential Energy

Notice how Potential and Kinetic are related.

PE = mass x gravity x height

V. Kinetic-Potential Energy Conversions

  1. As a basketball player throws the ball into the air, various energy conversions take place.

V. Forms of Energy

  1. Mechanical Energy

1. The sum of an object’s potential and kinetic energy.

Movement Energy

2. Ex. Speeding trains, bouncing balls, sprinting athletes

B. Thermal Energy

1. Total potential and kinetic energy of all the microscopic particles in an object make up its thermal energy.

C. Chemical Energy

•Energy stored in chemical bonds.

D. Electrical Energy

•Energy associated with electrical charges.

examples: batteries, lightning bolts

E. Electromagnetic Energy

  • Energy from the sun

F. Nuclear Energy

•Energy stored in atomic nuclei.

examples: nuclear fission

VI. Energy Conversion and Conservation

A. Energy can be converted from one form to another.

Example: striking a match…How?

Your muscles use mechanical energy to strike the match. Friction converts some of the match’s kinetic energy into thermal energy. The thermal energy triggers a chemical reaction. After the match ignites thermal energy is produced.

VII. Energy conversions

  1. All forms of energy can be converted into other forms.
  2. The sun’s energy through solar cells can be converted directly into electricity.
  3. Green plants convert the sun’s energy (electromagnetic) into starches and sugars (chemical energy).

VIII. Other energy conversions

  1. In an electric motor, electromagnetic energy is converted to mechanical energy.
  2. In a battery, chemical energy is converted into electromagnetic energy.
  3. The mechanical energy of a waterfall is converted to electrical energy in a generator.

IX. Energy Conversions

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

X. Conservation of Energy

•Energy cannot be created or destroyed.

a. One of the most important concepts in science.

Question: As you peddle your bike there is energy to keep the bike moving, where does the energy go as you slow down.

Answer: Friction slows the bike down and converts kinetic energy to thermal energy.

1. The gravitational potential energy of an object is converted to the kinetic energy of motion as the object falls.

XI. Energy Resources

  1. Non – renewable
  2. Exist in limited quantities and cannot be replaced
  3. examples: oil, natural gas, coal, and uranium.
  1. Renewable Energy Resources
  2. Hydroelectric – energy obtained from flowing water
  1. Solar energy – sunlight that is converted into usable energy
  1. Geothermal Energy – heat beneath the earth’s surface (used near volcanoes)

XII. Conserving Energy

  1. More efficient appliances.
  2. Recycling
  3. Hybrid Cars

XIII. Law of Conservation of Energy

  1. In 1905, Albert Einstein said that mass and energy can be converted into each other.
  2. He showed that if matter is converted, energy is created, and if energy is converted mass is created.
  3. E = MC2