Chapter 4Energy

What you will learn:

• Definition of energy, different forms of energy.• How to calculate kinetic energy.• How to calculate gravitational potential energy.

Section 1: The Nature of Energy

What is energy?

• Every change that occurs-large or small- involves energy.

• When something is able to change its environment or itself, it has energy.

• Energy is the ability to do work or cause change.

Different Forms of Energy

• Energy has several different forms-mechanical, heat, chemical, electromagnetic (or radiant), nuclear.

• The SI unit for energy is the Joule.

Different Forms of Energy

• Mechanical: energy that is associated with moving parts.

• Heat: caused by the motion of particles. This causes changes in phase and temperature.

• Chemical: energy required to hold atoms together. This is released when bonds are broken.

Different Forms of Energy

• Electromagnetic: energy associated with moving charges. Includes electricity, visible light, x-rays, radio waves, microwaves, and ultraviolet.

• Nuclear: released in the form of heat and light when the nucleus of an atom undergoes fission (splitting) or fusion (joining together). The sun’s energy is produced from nuclear fusion.

Kinetic Energy

• Kinetic energy is the energy of motion.• The formula for kinetic energy is half of the mass times

the velocity squared:

Example

• If Julie is running with a speed of 4 m/s and her mass is 60 kg. How much kinetic energy does Julie have?

Example

• How much mass does a turtle have if he is moving with 1.2 m/s and has 84 J of kinetic energy?

Potential Energy

• Even motionless objects can have energy. This is stored energy.

• Stored energy is referred to as potential energy, it depends on the position of the object.

• There are several different types of potential energy, for example: elastic potential energy, chemical potential energy, and gravitational potential energy.

Elastic Potential Energy

• Common examples of elastic potential energy are a stretched rubber band and a spring.

• Elastic potential energy is energy stored by something that can stretch or compress.

Chemical Potential Energy

• Energy stored in chemical bonds is chemical potential energy.

• Energy is stored in the bonds that hold the carbon and hydrogen atoms together and is released when gas from your car is burned.

• In this chemical reaction, chemical potential energy is released.

Gravitational Potential Energy

• Anything that can fall has stored energy.• Gravitational potential energy (GPE) is energy stored by

objects due to their position above Earth’s surface.• Gravitational potential energy can be calculated from this

equation:

• g=9.8 m/s/s remember this is due to Earth’s gravity.

Example

• If Charles throws a bucket that has a mass of 10 kg onto a truck bed that is 2.4 m tall, how much gravitational potential energy does the bucket have?

Example

• If a boulder is on top of a hill that is 10.3 m high and it has 356 J of potential energy, how much does the boulder weigh? What is the mass of the boulder?

Summary Question 1

• Energy is the ability to cause _____________.

A. change

B. heat

C. motion

D. work

Summary Question 1

• Energy is the ability to cause _____________.

A. change

B. heat

C. motion

D. work

Energy is the ability to cause change and has several different forms.

Summary Question 2

• What are the five forms of energy?

Summary Question 2

• What are the five forms of energy?

Mechanical, heat, chemical, electromagnetic (radiant), nuclear

Summary Question 3

• The kinetic energy of an object depends on ___________.

A. the object’s mass and speed

B. the object’s mass

C. the object’s speed

D. the acceleration of the object

Summary Question 3

• The kinetic energy of an object depends on ___________.

A. the object’s mass and speed

B. the object’s mass

C. the object’s speed

D. the acceleration of the object

Kinetic energy depends on both mass and speed of the moving object.

What you will learn:

• How different objects convert one type of energy into the form of another.

• What the law of conservation of energy is.• How to calculate total mechanical energy in a closed system.

Section 2: Energy Conversions

Energy Conversions

• Energy conversions are changes in the forms of energy. • KE to PE – examples are running up a hill or throwing a

ball• PE to KE – examples are an object falling or a pendulum.• All other forms of energy can be converted into other

forms; electromagnetic, chemical, mechanical, heat, and nuclear.

Energy Conversions

• A battery converts chemical energy into electrical energy.• A light bulb converts electrical energy into heat & light

energy.• An internal combustion engine converts heat energy into

mechanical energy.• A solar cell converts light energy into electrical energy.• A body converts chemical energy into heat & mechanical

energy.

Law of Conservation of Energy

• The law of conservation of energy states that energy can be neither created nor destroyed by ordinary means. It can however be changed from one form into another.

• In other terms the total energy of any closed system is constant.

• Mechanical Energy (ME) is the sum of the kinetic and potential energy of the system.

• ME=KE+PE

https://www.youtube.com/watch?v=mhIOylZMg6Q

Summary Question 4

• Mechanical energy is the total amount of ___________ in a system.

A. kinetic energy

B. momentum

C. potential energy

D. potential and kinetic

Summary Question 4

• Mechanical energy is the total amount of ___________ in a system.

A. kinetic energy

B. momentum

C. potential energy

D. potential and kinetic

Mechanical energy is the energy due to position and motion of all objects in a system.

Summary Question 5

• State the law of conservation of energy.

Summary Question 5

• State the law of conservation of energy.

The law of conservation of energy states that energy cannot be created or destroyed.

Summary Question 6

• Friction converts ______ energy into ______ energy.

A. electrical, thermal

B. mechanical, thermal

C. thermal, electrical

D. thermal, mechanical

Summary Question 6

• Friction converts ______ energy into ______ energy.

A. electrical, thermal

B. mechanical, thermal

C. thermal, electrical

D. thermal, mechanical

Friction converts mechanical energy into thermal energy.