Energy and Heat Transfer Teacher Notes

							                                            Energy Notes
What is Energy?
Energy makes change. Energy moves cars along the road and boats over the water. It bakes a cake in the
oven and keeps ice frozen in the freezer. It plays our favorite songs on the radio and lights our homes.
Energy makes our bodies grow and allows our minds to think. Scientists define energy as the ability to do
work. People have learned how to change energy from one form to another so that we can do work more
easily and live more comfortably.
Forms of Energy
Energy is found in different forms, such as light, heat, sound and motion. There are many forms of energy,
but they can all be put into two categories: kinetic and potential.

             KINETIC ENERGY                                         POTENTIAL ENERGY
   Kinetic energy is motion—of waves, electrons,             Potential energy is stored energy and the
     atoms, molecules, substances, and objects.              energy of position (gravitational energy).

Electrical energy is the movement of electrical
charges. Everything is made of tiny particles called
atoms. Atoms are made of even smaller particles           Chemical energy is energy stored in the bonds of
called electrons, protons, and neutrons. Applying a       atoms and molecules. It is the energy that holds
force can make some of the electrons move.                these particles together. Biomass, petroleum,
Electrical charges moving through a wire is called        natural gas, and propane are examples of stored
electricity. Lightning is another example of electrical   chemical energy. Ex. H-O-H
energy.                                                   Elastic (Mechanical) energy is energy stored in
Radiant energy is electromagnetic energy that             objects by the application of a force. Compressed
travels in transverse waves. Radiant energy includes      springs and stretched rubber bands are examples of
visible light, x-rays, gamma rays and radio waves.        stored mechanical energy.
Light is one type of radiant energy. Solar (sun) energy   Nuclear energy is energy stored in the nucleus of an
is an example of radiant energy.                          atom––the energy that holds the nucleus together.
Thermal energy, or heat, is the internal energy in        The energy can be released when the nuclei are
substances––the vibration and movement of the             combined or split apart. Nuclear power plants split
atoms and molecules within substances. Geo-               the nuclei of uranium atoms in a process called
thermal energy is an example of thermal energy.           fission. The sun combines the nuclei of hydrogen
                                                          atoms in a process called fusion. Scientists are
Motion energy is the movement of objects and              working on creating fusion energy on earth, so that
substances from one place to another. Objects and         someday there might be fusion power plants.
substances move when a force is applied according
to Newton’s Laws of Motion. Wind is an example of         Gravitational energy is the energy of position or
motion energy.                                            place. A rock resting at the top of a hill contains
                                                          gravitational potential energy. Hydropower, such as
Sound is the movement of energy through                   water in a reservoir behind a dam, is an example of
substances in longitudinal (compression/rarefaction)      gravitational potential energy.
waves. Sound is produced when a force causes an
object or substance to vibrate––the energy is
transferred through the substance in a wave.
Law of Conservation of Energy
Conservation of energy is NOT saving energy. The law of conservation of
energy says that energy is neither created nor destroyed. When we use
energy, it doesn’t disappear. We change it from one form of energy into
another.
A car engine burns gasoline, converting the chemical energy in gasoline into
mechanical energy. Solar cells change radiant energy into electrical energy.
Energy changes form, but the total amount of energy in the universe stays
the same.




Energy Efficiency
Energy efficiency is the amount of useful energy you get from a system. A perfect, energy-efficient
machine would change all the energy put in it into useful work—an impossible dream. Converting one form
of energy into another form always involves a loss of usable energy.
In fact, most energy transformations are NOT very efficient. The human body is a good example.
Your body is like a machine, and the fuel for your machine is food. Food gives you the energy to move,
breathe, and think. But your body isn’t very efficient at converting food into useful work. Your body is less
than five percent efficient most of the time. The rest of the energy is lost as heat. You can really feel that
heat when you exercise!


What Does Energy Mean to Us?
All organisms must have energy to survive. Photosynthesis is the chemical process that provides that
energy! During photosynthesis, the sun’s energy is trapped in the chemical bonds of glucose. Plants
(producers) provide us with ENERGY!
                                            PHOTOSYNTHESIS
               Energy Conversion: RADIANT ENERGY is CONVERTED TO CHEMICAL ENERGY!

                 6CO2       +     6H2O       +    ENERGY                 C6H12O6 +           6O2
             carbon dioxide       water          from sunlight             glucose            oxygen


                                            CELLULAR RESPIRATION

               C6H12O6        +     6O2              6CO2        +     6H2O + ENERGY
                 glucose           oxygen          carbon dioxide         water         ATP
                    CHEMICAL ENERGY IS CONVERTED INTO ATP –bioCHEMICAL ENERGY!




                                                RESPIRATION
During cellular RESPIRATION the energy is released from it’s chemical bonds for use by the cells. The
usable energy form is called ADENSOSINE TRIPHOSPHATE (ATP).

Conversions efficiencies are always much less than 100%. At each link in a food chain, a substantial portion
of the sun's energy - originally trapped by a photosynthesizing autotroph - is dispersed back into the
environment (ultimately as heat). Our bodies convert only about 35% of the energy that it takes in. This
may seem inefficient, but our cars are even less efficient. Cars convert about 25% of its energy intake into
motion.

Sources of Energy
We use many different energy sources to do work for us. Energy sources are classified into two groups—
renewable and nonrenewable. Renewable and nonrenewable energy can be converted into secondary
energy sources like electricity and hydrogen.
In the United States, most of our energy comes
from nonrenewable energy sources. Coal,
petroleum, natural gas, propane, and uranium
are nonrenewable energy sources. They are used
to make electricity, to heat our homes, to move
our cars, and to manufacture all kinds of
products.
These energy sources are called nonrenewable
because their supplies are limited. Petroleum, for
example, was formed millions of years ago from
the remains of ancient sea plants and animals.
We can’t make more petroleum in a short time.
Renewable energy sources include biomass, geothermal energy, hydropower, solar energy, and wind
energy. They are called renewable energy sources because they are replenished in a short time. Day after
day, the sun shines, the wind blows, and the rivers flow. We use renewable energy sources mainly to make
electricity.
Energy Calculations
The S.I. unit for energy is Joules, J.

Kinetic Energy = 1/2 mass X velocity2               KE = mv2/ 2




Example Problem:
A 15 kg bicycle carrying a 50 kg boy is traveling at a speed of 5 m/s. What is the kinetic energy of the
bicycle (including the boy)?



Gravitational Potential Energy = mass X acceleration due to gravity X height
                                                                                     PE = mgh




Example Problem:
A 0.06 kg tennis ball starts to fall from a height of 2.9 m. How much gravitational potential energy does the
ball have at that height?


Example Problem:
Thomas is holding a tennis ball outside a second floor window (3.5 m from the ground) and Dustin is
holding one outside a third floor window (6.25 m from the ground). How much more gravitational potential
energy does Dustin’s tennis ball have? (Each tennis ball has a mass of 0.06 kg.)


Mechanical Energy = potential energy + kinetic energy
                                                              ME = PE + KE
Thermal Energy
All matter is made of tiny particles—atoms and molecules. In all materials—solids, liquids, and gases—
these particles are in constant motion. Like all objects that are moving, these moving particles have kinetic
energy. The faster these particles move, the more kinetic energy they have.

The temperature of an object is related to the average kinetic energy of the atoms or molecules. The faster
these particles are moving, the more kinetic energy they have, and the higher the temperature of the
object is. The SI unit for temperature is Kelvin (K).

The sum of the kinetic and potential energy of all the molecules in an object is the thermal energy of the
object.

***Heat is thermal energy that flows from something at a higher temperature to something at a lower
   temperature. Heat is a form of energy, so it is measured in joules—the same units that energy is
   measured in. Heat always flows from warmer to cooler materials.

The amount of heat that is needed to raise the temperature of 1 kg of
some material by 1°C or 1 K is called the specific heat of the material.
Specific heat is measured in joules per kilogram Kelvin [J/(kg K)].

Q = thermal energy
m = mass
                                            Q = mc∆T
c = specific heat of substance
∆T = change in temperature
SPECIFIC HEAT CALCULATIONS
1. If 150 kilograms of water is heated from 30oC to 40oC, the number of joules of heat energy absorbed is…




2. If a 2.0 kg sample of water at 5.0oC absorbs 26 J of heat energy, the temperature of the sample will be…




HEAT TRANSFERS BY:
Conduction—the movement of thermal energy from one object to
another when they are in direct contact (touching). Molecules can
transfer energy to a neighboring one within an object.

Examples:
    Bare feet touching the cement on a hot summer day
    Warming up with a hot cup of coffee
    Car seat warmers
Can you think of any other examples???



Convection—the movement of thermal
energy from one area to another in a
liquid or gas. Heat transfers by currents.

Examples:
    Wind Currents
    Hot Air Balloon
Can you think of any other examples?



Radiation—when warm or hot matter emits electromagnetic radiation – especially infrared radiation – that
is then absorbed by an object at a distance. The absorption heats the second object. This energy transfer
does NOT require matter.

Examples:
     A microwave oven
     The Sun
     UV light
Can you think of any other examples?
Controlling Heat Flow
Almost all living things have special features that help them control the flow of heat. For example, the
Antarctic fur seal’s thick coat and the emperor penguin’s thick layer of blubber help keep them from losing
heat. This helps them survive in a climate in which the temperature is often below freezing. In the desert,
however, the scaly skin of the desert spiny lizard has just the opposite effect. It reflects the Sun’s rays and
keeps the animal from becoming too hot. An animal’s color also can play a role in keeping it warm or cool.
The black feathers on the penguin’s back, for example, allow it to absorb as much radiant energy as
possible. How do you think humans control body heat flow?

Insulators—a material that does NOT allow heat to flow through it
easily. Materials such as wood, plastic, fiberglass, and air are good
insulators. Gases are usually much better insulators than solids or
liquids.

Conductors—a material that allows heat to flow through it easily.
Although conduction can occur in solids, liquids, and gases, solids
usually conduct heat much more effectively. Metals such as silver,
copper, and aluminum are among the best heat conductors.


A material that is a good conductor of heat, such as a metal, is a poor
insulator. Likewise, good insulators are poor conductors of heat. Why do
you think most cooking pots are made of metal but the handles usually are
not?


Heat Movers—a device that removes thermal energy from one location
and transfers it to another location at a different temperature such as a
refrigerator and air conditioner.

Change in Energy
      Endergonic Reaction—energy is absorbed; requires more energy to break bonds than is given off
       when new bonds are formed
       Endothermic Reaction—absorb heat from surroundings
       Ex: Commercial cold packs usually consist of two compounds - urea and
       ammonium chloride in separate containers within a plastic bag. When the bag is
       bent and the inside containers are broken, the two compounds mix together and
       begin to react. Because the reaction is endothermic, it absorbs heat from the
       surrounding environment and the bag gets cold.
      Exergonic Reaction—energy is released; less energy is required to break the original
       bonds than is released when new bonds form
       Exothermic Reaction—giving off heat or light release energy to the surroundings
       (usually in the form of heat)
       Ex: sodium and chlorine react so violently that flames can be seen as the exothermic
       reaction gives off heat.