Transcript Thermal Energy and Heat + Conservation of Energy

Lesson 3
THERMAL ENERGY AND HEAT
+ CONSERVATION OF ENERGY
THERMAL ENERGY AND HEAT

Thermal energy and heat play significant roles
in our lives from the furnaces that heat our
homes to winds generated by the uneven
heating of the Earth’s surface. Even most of
the food that we consume is converted into
thermal energy.
THERMAL ENERGY
The total kinetic energy and potential energy of
the atoms or molecules of a substance.
 Depends on: mass, temperature, nature and
state of matter

HEAT

A measure of the energy transferred from a
warm body to a cooler body because of a
difference in temperature.
TEMPERATURE

A measure of the average kinetic energy of the
atoms or molecules of a substance.
EXAMPLE 1

- Consider two samples of water: 100 g at 50
°C and 500 g at 50° C
50° C
50° C
50° C
EXAMPLE 1

The two samples have the same temperature,
but the bigger sample contains more thermal
energy because there’s more of it. If the
samples were mixed, no heat transfer would
occur because they are the same temperature
50° C
50° C
50° C
EXAMPLE 2 
Consider two other samples of water: 500 g at
50 °C and 500 g at 90° C
50° C
90° C
70° C
EXAMPLE 2 
The masses are the same but the warmer sample has
more thermal energy because the water particles have
more motion, that is, the average kinetic energy of the
molecules is greater at a higher temperature. When
the two samples are mixed, heat would transfer from
the 90 °C sample to the 50° C.
70° C
50° C
90° C
METHODS OF HEAT TRANSFER

There are three methods that heat can transfer
from a warmer body to a cooler body:
Radiation, Conduction, and Convection.
Radiation - the emission of energy as
electromagnetic waves. When radiant energy
encounters particles of matter, it may be
reflected or absorbed. Absorbed energy
increases the motion of particles.
 An increase in kinetic energy increases the
temperature of the matter.


Any substance at a higher temperature than its
surroundings will emit radiant energy, usually
as infrared radiation. The warmed matter then
transfers some of its thermal energy to
substances at lower temperatures or re-emits it
as IR.
CONDUCTION
- The transfer of thermal energy through direct
contact between the particles of a substance,
without moving the particles to a new location.
 Thermal energy transfer by conduction usually
takes place in solids.


During conduction, particles with more kinetic
energy transfer some of their energy to
neighbouring particles with lower kinetic energy
increases the kinetic energy of the
neighbouring particles. Metals are the best
heat conductors as their electrons can vibrate
more freely than those of other substances.
CONVECTION
- The transfer of thermal energy through the
movement of particles from one location to
another. Thermal energy transfer by convection
usually occurs in gases and liquids.
 During convection, the movement of the particles
forms a current, which is a flow, from one place to
another in one direction. Liquid water has a high
heat capacity which means that it takes a lot of
energy to increase the temperature of a mass of
water.


Since Earth’s surface is over 70 percent water,
water has a large effect on Earth’s climate.
Therefore, regions closer to large bodies of
water tend to experience more moderate
weather conditions than regions farther from
them.
HTTP://WWW.YOUTUBE.COM/WATCH?V=VZE_IP
EDUJC
CONSERVATION OF ENERGY

When energy is transformed from one form to
another form the energy is said to be
conserved. None of the energy disappears and
no new energy suddenly appears.
THE LAW OF CONSERVATION OF ENERGY

When energy changes from one form to
another, no energy is created or destroyed.
This law applies to all energy transformations.
In ideal situations, just as in ideal machines, no
energy is lost to friction.
 However, some energy is usually needed to
overcome friction. This results in the production
of thermal energy and, sometimes sound
energy.


Energy-transformation equations are more
complete when they include thermal energy
and in some cases sound energy as well.

Consider the operation of a pile driver. The overall
goal of the energy transformation is the work done
on the pile.
The energy-transformation equation
is listed below.
 Echem  W1 + EChem  Eg  EK +
Etherm  W2 + Etherm + Esound

Echem = chemical potential energy
released in the burning of fuel in the
engine
 W1 = Work done by the force on the
hammer to raise it to Max Eg






Etherm = Thermal energy resulting from
heat losses in the engine and from
friction forces.
Eg = Max gravitational potential
energy of the hammer at its highest
position
Ek = Max kinetic energy of the
hammer right before it hits the pile
W2 = Work done by the hammer as it
pushes downward on the pile
Esound = Sound energy resulting from
the operating the engine, and the
collision of the hammer and the pile.
QUESTIONS
Explain the difference between thermal energy,
and the temperature of a metal coin.
 A parent places a baby bottle containing 150 mL
of milk at 7 ° C into a pot containing 550 mL of
water at 85 ° C.

Qualitatively compare the average kinetic energy of the
milk and the water.
 Qualitatively compare the thermal energy of the milk
and the water.
 Is there a stage at which the heat will stop transferring
from the water to the milk? Explain


Explain your answer to each question.
Would it be better to place an electric heater near the
floor or near the ceiling?
 Would it be better to place an air conditioner vent near
the floor or ceiling?
 Why are fans placed near the ceilings in tall rooms?


A ball is dropped vertically from a height of 1.5 m
and bounces back to a height of 1.2 m. Does this
violate the law of conservation of energy? Explain.
A 0.20 kg ball is held at rest 2.2 m above the
ground, and then it is dropped. Apply the law of
conservation of energy to determine the ball’s
speed after it has fallen.
 A) 1.1 m and
 B) 2.2 m. Ignore air resistance.
 C) As the ball falls, what type of energy does its
gravitational potential energy change into?
