Transcript Thermal Energy and Heat

THERMAL ENERGY AND
HEAT
SPH4C/SPH3U
THERMAL ENERGY
• James Prescott Joule (1818-1889) spent much of his
honeymoon studying waterfalls.
• He noticed that the water at the bottom
of a waterfall had a higher temperature
than at the top.
• How might this happen?
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.
CALCULATING HEAT
• The amount of heat released or gained during a
temperature change can be found from,
𝑄 = 𝑚𝑐Δ𝑇
• Where,
– 𝑄 is the heat in Joules
– 𝑚 is the mass of substance
– 𝑐 is the specific heat capacity J/kgoC
– Δ𝑇 is the change in temperature (𝑇𝑓 − 𝑇𝑖 )
SPECIFIC HEAT CAPACITY
• Scientists define specific heat capacity of a
substance as the amount of heat needed to raise
the temperature of 1 kg of that substance by 1 0C.
• These are generally known values for most
substances.
EXAMPLE
A beaker of 250 g of water is heated over a bunsen
burner from room temperature (20 0C) to boiling
point (100 0C). The heat capacity of water is 4180
J/kg0C. How much heat does the water gain?
TEMPERATURE
• A measure of the average kinetic energy of the
atoms or molecules of a substance.
HOW HEAT SPREADS FROM ONE REGION
TO ANOTHER
• All things are made up of molecules
• When objects are heated, they absorb thermal
energy.
• This means that the molecules are absorbing the
thermal energy.
• With more energy, the molecules are able to move
faster.
• When the molecules move faster, the temperature
of the object increases.
• Temperature increase means the object gets hotter.
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.
50° C
90° C
70° C
EXAMPLE (GRADE 11 PHYSICS)
A beaker containing 250 g of water at 250C is poured
into another beaker that initially contains 350 g of
water at 850C. What is the final temperature of the
mixed water?
METHODS OF HEAT TRANSFER
THREE METHODS OF HEAT TRANSFER
• Conduction
– Process by which the collision of atoms and electrons
transfers heat through a material or between two
materials in contact.
• Convection
– Process of transferring heat by a circulating path of fluid
particles.
• Radiation
– Process in which energy is transferred by means of
electromagnetic waves.
CONDUCTION
•
Collision of atoms and electrons transfers heat
•
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.
•
Occurs mainly in solids
•
Two types of conduction
–
Molecular vibration
–
Free electron diffusion
Note: Conduction is not the main form of heat transfer in
liquids and gases because their molecules are
spaced further apart.
MOLECULAR VIBRATION
• When heat is supplied to one end, the molecules at
the hot end start to vibrate more vigorously.
• In the process, they ‘bump’ into their neighboring
molecules. In doing so, some energy is transferred
to the neighbour.
• The neighbour molecule gains energy and starts to
vibrate more vigorously. The cycle continues.
FREE ELECTRON DIFFUSION
• This form of conduction takes place only in metals.
As only metals have free electrons.
• The electrons are freed from the molecule when
heated and they travel towards the cold end.
• At the cold end they collide into a molecule
therefore passing all their energy to the molecule.
METHODS OF CONDUCTION
Molecular vibration
Free electron diffusion
Occurs in all solids
Occurs in metals only
Slow process
Fast process
This explains why metals heat up faster:
1. Metals have 2 mechanisms of conduction occuring at
the same time.
2. In metals, free electron diffusion is the main
mechanism, which is faster.
CONDUCTORS AND INSULATORS
• Materials that can conduct heat easily
and readily (eg. Metals) are known as
conductors.
• Materials that do not conduct heat
easily (eg. Water, air, plastic) are known
as insulators.
CONVECTION
• Transferring heat by a circulating path of fluid
particles.
• Occurs in liquids and gases
• Does not occur in solids because the molecules are
not free to move around
EXAMPLE
Taking the example of heating water
• Water at the bottom is heated first
• Heated water expands
• When water expands density decreases
• Heated water of lower density starts to rise
• Cooler water of higher density rushes in from sides
to take its place
• The cooler water gets heated and the cycle repeats.
• Convection currents are set up.
CONVECTION
• 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.
RADIATION
• Energy is transferred by means of electromagnetic
waves.
• Radiation does not require a medium to transfer
heat. (can occur in a vacuum)
• Sun releases electromagnetic waves (heat is
contained in the waves as infra-red)
• Hotter objects radiates more heat.
RADIATION
• 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.
EMITTERS AND ABSORBERS
• The Sun gives out the heat.
–It is known as an emitter / radiator
• The Earth takes in the heat.
–It is known as an absorber.
CONSERVATION OF ENERGY
Law of Conservation of Energy
When energy changes from one form to
another, no energy is created or destroyed.
In ideal situations, no
energy is lost to
friction.
In real situations, some
energy is needed to
overcome friction.
This results in the
production of waste
thermal energy and,
sometimes, sound
energy.
ENERGY TRANSFORMATION
CONSERVATION OF ENERGY (GR. 11)
• The amount of heat gained by a cold substance is
equal to the amount of heat lost by a hot
substance.
𝑄𝑐𝑜𝑙𝑑 = −𝑄ℎ𝑜𝑡
𝑚𝑐 𝑇𝑓 − 𝑇𝑖 = −𝑚𝑐 𝑇𝑓 − 𝑇𝑖
EXAMPLE (GRADE 11 PHYSICS)
A beaker containing 250 g of water at 250C is poured
into another beaker that initially contains 350 g of
water at 850C. What is the final temperature of the
mixed water?
LATENT HEAT AND CHANGES OF
STATE
HEATING/COOLING CURVE
• We learned that when you add heat to a substance, the
temperature increases. When you remove heat, the
temperature decreases.
• BUT, an interesting thing occurs when an object is
undergoing a change of state. During a change of
state, the temperature remains constant. The heat
being added or removed is going into breaking or
creating the bonds between the particles in the
different states. If you measured the temperature of a
solid substance to the point where it melts to a liquid,
then continued heating the liquid until it boiled and
turned entirely to a gas, you would get the following
graph of temperature versus time.
HEATING/COOLING CURVE
LATENT HEAT
• Latent Heat of Fusion
– The amount of thermal energy absorbed when a substance
melts or released when it freezes.
𝑄𝑓 = 𝑚𝐿𝑓
– Where 𝐿𝑓 is the specific latent heat of fusion.
• Latent Heat of Vaporization
– The amount of thermal energy absorbed when a substance
evaporates or released when it condenses.
𝑄𝑣 = 𝑚𝐿𝑣
– Where 𝐿𝑣 is the specific latent heat of fusion.
EXAMPLE
A 300 g block of ice at -25 0C is heated until it
eventually becomes 300 g of water vapour at 110 0C.
How much total heat does this take?