Pearson Prentice Hall Physical Science: Concepts in Action

Chapter 15Energy

15.1 Energy and Its Forms• Objectives:• 1. Describe and compare how energy

and work are related• 2. Explain what factors kinetic energy

of an object depends on• 3. Discuss how gravitational potential

energy is determined• 4. Summarize the major forms of

energy

How Energy & Work are Related + Kinetic Energy

• Def: energy is the ability to do work• Work is a transfer of energy • Def: kinetic energy is the energy of motion• The kinetic energy of any moving object depends

on its mass and speed• The formula is: KE = ½ mv2 where m = mass and v is

the velocity (which must be squared)• the units for m are kg & v = (m/s)2 or m2/s2

• the units for PE are kg*m2/s2 which is also called joules, J

Potential Energy• Def: potential energy is stored energy as a

result of position or shape• PE is energy with the potential to do work• Two forms of PE are gravitational PE and

elastic PE• Def: gravitational PE is PE that depends

upon an object’s height• Gravitational PE increases when an object is

at a higher height

• An object’s gravitational PE depends on its mass, height & acceleration due to gravity

• The formula for gravitational PE = mgh where m= mass (kg), g= 9.8 m/s2 (the free fall acceleration of gravity) & h = height in meters, m

• When you multiply all the units together you get kg*9.8m/s2*m or kg*m2/s2 which is J

• Def: elastic PE is the PE of an object that is stretched or compressed

• An object is elastic if it springs back after being stretched

Practice Problems

• Calculate the KE of a 1500kg car moving at 29m/s.

• A bowling ball traveling at 2.0m/s has 16J of KE. What is the mass of the bowling ball in kg?

Practice Problems

• Calculate the PE of a car with a mass of 1200kg at the top of a 42m hill.

• Calculate the PE of a 55g egg held out of a 2nd story window, 6m off the ground.

Forms of Energy• The major forms of energy are mechanical

energy, thermal energy, chemical energy, electrical energy, electromagnetic energy and nuclear energy• Def: mechanical energy is the energy

associated with the motion and position of everyday objects• Def: thermal energy is the total PE and KE

of all the microscopic particles in an object

• Def: chemical energy is the energy stored in chemical bonds• Def: electrical energy is the energy

associated with electric charges• Def: electromagnetic energy is a form

of energy that travels through space in the form of waves• Def: nuclear energy is the energy

stored in atomic nuclei

15.2 Energy Conversion & Conservation

• Objectives:• 1. Describe how energy can be converted

from one form to another• 2. Explain the law of conservation of energy• 3. Discuss the energy conversion that takes

place as an object falls toward Earth• 4. Discuss how energy and mass are related

Energy Can be Converted from One Form to Another

• Def: energy conversion is the process of changing energy from one form to another

• Sometimes energy is converted to other forms in a series of steps

• Ex: striking a match uses chemical energy from your muscles, then friction between match and box converts KE to thermal energy, thermal energy triggers a chemical reaction releasing more chemical energy

• Often energy converts directly from one form to another

• a wind up toy, for example, is PE to KE

Conservation of Energy + Energy Conversions and Gravity

• The Law of Conservation of Energy states that energy cannot be created or destroyed

• The gravitational PE of an object is converted to the KE of motion as an object falls

• Pendulums constantly convert PE to KE and KE to PE as the pendulum swings

• At the bottom of the swing, the pendulum has maximum KE and zero PE

• On either side the pendulum will have a combination of PE + KE

• Q: Where is the PE the greatest and KE zero?

Energy and Mass• Mechanical energy = KE + PE• Mechanical energy is also conserved• (KE + PE)beginning = (KE + PE) end• Einstein has an equation: E = mc2 where E is energy

(J), m is mass (kg) & c2 is the speed of light squared (3 x 108 m/s)2

• This equation says that energy and mass are equivalent and can be converted into each other

• It also means that a tiny amount of matter can produce enormous amounts of energy

• Mass and energy together are always conserved

15.3 Energy Resources

• Objectives:• 1. Give examples of the major

nonrenewable and renewable energy sources• 2. Explain how energy

resources can be conserved

Nonrenewable and Renewable• Nonrenewable energy resources

include oil, natural gas, coal and uranium• Oil, natural gas and coal are fossil fuels• Fossil fuels were formed underground

from once living organisms• Most nonrenewable resources are

considered inexpensive, available and are known to cause pollution

• Renewable energy resources include hydroelectric, solar, geothermal, wind, and biomass

• Def: hydroelectric energy is energy obtained from flowing water

• Hydroelectric energy production usually involves the building of a dam

• Hydroelectric is available, used today and generally nonpolluting

• Def: solar energy is sunlight converted to usable energy

• Solar energy is expensive and its use is limited until technology improves

• Def: geothermal energy is thermal energy beneath the earth’s surface

• It is nonpolluting and available in this area due to naturally occurring hot springs

• most places are not near a volcano or hot springs• Def: biomass energy is the energy stored in living

things• Biomass can be converted directly to thermal

energy• Agricultural waste such as turning corn into

ethanol for auto fuel is an example• This technology is moderately expensive

• Wind energy requires a lot of land and a place that has a lot of wind

• It is expensive and not practical at this time although research continues

• Hydrogen fuel cells are being used in some places to generate electricity by reacting hydrogen with oxygen

• The main source of hydrogen is water• This technology is expensive and considered

to be a research and development project for future energy sources

Conservation of Resources• Energy resources can be conserved by

reducing energy needs and by increasing the efficiency of energy use• Def: energy conservation is finding

ways to use less energy or use energy more efficiently• Q: Can you think of some ways we can

conserve energy resources?