Download Energy - Mr. Rowley - Physical Science 20

Unit 04 - Heat
The Concept of Energy
Although you may know what
energy is, it may be difficult to
define it.
 Energy:
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the ability to do work or produce
heat.
What forms can energy come
in?

Light energy, nuclear energy,
and electrical energy
 Biomass, petroleum, natural
gas, and coal are examples of
stored chemical energy
○

Chemical energy is converted
to thermal energy (heat) when
we break those bonds
In this unit, we will focus on
heat energy.
Potential & Kinetic Energy

Kinetic Energy
 Energy of motion
 i.e.:
○ a rock rolling down a hill
has kinetic energy
○ Atoms and molecules
have kinetic energy as
they are always in motion

Potential Energy
 Energy that is stored in
something, and has the
potential to do work, is
called potential energy.
 i.e.:
○ a rock sitting at the top of
a hill has potential energy
Potential Energy – Chemical
Bond Energy
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This energy exists
because of the
attractive forces
between molecules
and atoms.
It plays a large part in
chemical reactions.
Therefore, the bonds
that hold molecules
together are stored
energy and may do
work (or give off heat)
if they are broken.
The Law of Conservation of
Energy
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The Law of Conservation of Energy states that
energy cannot be created or destroyed, but only
change forms.
This law is also known as the First Law of
Thermodynamics.
i.e.:
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When octane (C3H8, the main component of gasoline) is
burned in your car engine, chemical bond energy (potential
energy) is converted into mechanical energy (pistons
moving in the car engine; kinetic energy) and heat.
When we eat, our bodies convert the chemical energy of
the food into movement of our muscles; again heat is also
a product of this conversion.
When we turn on a light switch, electrical energy is
converted into light energy and, you guessed it, heat
energy.
When talking about chemical reactions, all the energy
in the reaction must come from somewhere. That is, it
is not spontaneously created.
The only place that energy is stored is in the bonds
between atoms.
This potential bond energy will usually be converted to
kinetic energy (heat).
Kinetic Molecular Theory
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Previous science courses may have
taught us that molecules in a
substance, no matter its state, are in
constant motion.
Molecules move slowest in solids,
faster in liquids, and fastest in gasses.
The energy a particle has because of
its motion is called kinetic energy.
The energy associated with the
movement of particles is called heat
and thermal energy.
What happens to the motion of
particles as they are the temperature
of a substance is increased?
http://www.chem.iastate.edu/group/Gr
eenbowe/sections/projectfolder/flashfil
es/thermochem/eqilibrium-v1.html
Since the particles begin to move
faster, then they have more kinetic
energy. Therefore, the heat an object
gives off is directly related to the
motion of its particles.
Heat – A TRANSFER of Energy

Heat is the transfer of energy. Note
that heat always flows from hot to
cold, never from cold to hot.

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
For example, imagine you had an ice
cube in your hand. The heat from your
hand gets transferred to the ice cube.
That is, fast moving particles hit the slow
moving particles in the ice cube.
This interaction slows the particles down
in your hand (making it cold) and speeds
up the particles in the ice (making it
warm).
Thus, thermal energy is transferred from
your hand into the ice cube (coldness
does not travel from the ice cube to your
hand).
If you were holding a cold piece of
metal (instead of ice) your hand will
eventually warm the metal to the same
temperature as your body.

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At this point your hand and the metal are
in thermal equilibrium.
http://www.chem.iastate.edu/group/G
reenbowe/sections/projectfolder/flash
files/thermochem/thermoEquiv.html
Heat

Heat:
 Internal energy transferred
from one thing to another due
to a temperature difference
 Internal energy in transit
 Heat is a form of energy. It
is the total amount of kinetic
energy in a sample of
matter.
 Heat also flows from a
warmer object to a cooler
object.
 Therefore, the feeling of
getting colder is not
“coldness” entering your
body, but heat (energy)
leaving your body.
 The S.I. unit for heat is the
Joule (J).
Forms of Heat Transfer
3
types;
 Conduction
 Convection
 Radiation
Radiation
There are no particles between the
Sun and the Earth so it MUST travel
by radiation
?
How does heat energy get
from the Sun to the Earth?
RADIATION
Radiation
The transfer of heat in rays, from a hot object, without
needing a medium to pass through
 Electro Magnetic radiation infrared frequencies (not
visible)
 It travels in all directions from a hot object
 The hotter an object is, the more heat it will radiate out
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Conduction
Transfer of heat is through a SOLID by
being passed from one particle to the next
Requires contact
Particles at the warm end move faster and
this then causes the next particles to move
faster and so on.
e.g: poker in fire or spoon in tea
In this way heat in an object travels from:
the HOT end
the cold end
Conduction

When you heat a metal strip at one end, the heat travels
to the other end.

As you heat the metal, the particles vibrate, these
vibrations make the adjacent particles vibrate, and so on
and so on, the vibrations are passed along the metal and
so is the heat. We call this? Conduction
Convection
The transfer of heat by the
physical movement of more
energetic particles from one
location to another.
 It is the way in which particles in a
GAS or LIQUID move upwards,
carrying heat with them

Think about when you boil water, the bubbles move
upwards
 Or think of a heater in the room, the heat rises around
the room

Convection
Cools at the
surface
Cooler
water
sinks
Convection
current
Hot water
rises
Three forms of Heat Transfer
Heat - Calories: Another way to
measure heat energy

A thermochemical “calorie” is
defined as:

the amount of heat required to
raise the temperature of one
gram of water by one degree
Celsius (1°C).
Units for calories are written
as “cal”
 To convert between joules
and calories, use the
following conversion factors:
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1 J = 0.2390 cal and/or
1 cal = 4.184 J
Examples:
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Convert 8181 joules to calories
Convert 2019 calories to joules
Convert 1125 kJ to calories
Convert 4307 kcal to joules
Heat - Food for Thought…

Be careful – dietary calories can be
deceiving:
 A dietary calorie (as written on food labels),
written with a big C ( not “cal”) is actually a
kilocalorie, or 1000 calories.

What this means is when you look at a
label and it says “100 calories per
serving!” it really means:
 100 C x 1000 cal = 100 000 cal per serving
1C
Temperature

Temperature:
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is a measure of the average kinetic
energy of a sample of matter. It is the
intensity of heat energy.
There are two ways we will measure
temperature:
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1. Celsius scale based on the freezing
and boiling points of water.
2. Kelvin scale based on energy.
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At this temperature, a particle has no
energy, and thus, all movement is
ceased.
To converge between degrees Celsius
and degrees Kelvin, we will use:
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K = ˚C + 273 where:
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The Kelvin scale has no negative numbers.
In fact, the lowest temperature that can ever
be reached is absolute zero (0 K).
K = degrees in Kelvin
˚C = degrees in Celsius
Examples:
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Convert 25oC to Kelvin
Convert -35oC to Kelvin
Convert 187 Kelvin to Celsius
Heat vs. Temperature

In terms of heat, or energy, an iceberg
would have more than a boiling cup of
coffee.
 Although the temperature of the boiling water is
much higher than the iceberg, the iceberg has
more particles. Thus, it will have a higher total
energy.
A good way to think of temperature is the
intensity of heat.
 The coffee will have a more intense heat
than the iceberg.
