Download Section 1 – Thermal Energy

PSCI II – Chapter 6– Thermal Energy (pages 156 - 189)
Section 1 – Thermal Energy
I.
Temperature
º We all know that hot means high temperature and cold means low
temperature.
º But how are they related.
Matter in Motion
º All matter is made up of tiny particles called atoms.
º All materials (solid, liquid, and gas) are in constant motion.
º Just like other objects in motion atoms have kinetic energy.
º Cool objects are moving slower so they have less kinetic energy than
warmer ones.
Temperature
º Defn. – related to the average kinetic energy of the atoms and molecules.
º The faster the atoms and molecules move the more kinetic energy they
have.
º SI units are in Kelvin (K) – remember a change of one degree Kelvin is the
same as one degree Celsius.
II.
Thermal Energy
º Defn. – the sum of the kinetic and potential energy of all the molecules in an
object.
Thermal Energy and Temperature
º When temperature of an object increases the average kinetic energy
increases.
Thermal Energy and Mass
º The more mass means more kinetic energy simply because there are more
molecules to move.
º This is only if the temperature doesn’t change.
III.
Heat
º Defn. – thermal energy that flows from something at higher temperature to
something at lower temperature.
º Heat is a form of energy so the units are in Joules.
PSCI II – Chapter 6 – Page 1 of 6
IV.
Specific Heat
º As a substance absorbs heat its temperature changes depending on the
nature of the substance.
º Also depends on how much heat is added.
º Specific Heat – the amount of heat that is needed to raise the temperature
of 1kg of a substance by 1 degree or 1K.
º Measured in joules per kilogram Kelvin [J/kgK].
Water as a Coolant
º Look at the table on page 161.
º Water has the highest specific heat and can absorb large quantities of heat
with out a large temperature change.
º Since it takes longer for something to heat up it also takes longer to cool
down.
V.
Calculating Changes in Thermal Energy
º Remember that the change in thermal energy is related to mass.
º Change in thermal energy = mass x specific energy x change in
temperature.
º Change in temperature – Tfinal – Tinitial
º Q is the change in thermal energy
º m is the mass
º C is the specific heat
º Q = m x (Tfinal – Tinitial) x C
º Usually I’ll give you C.
º Example: The temperature of a 32 g silver spoon increases from 20C to
60C. If silver has a specific heat of 235 J/kgK what is the change in the
thermal energy of the spoon.
m  32 g  0.032kg
T final  60C
Tinitail  20C
C  235 J
kgK
  301J
Q  0.032kg 60C  20C  235 J
kgK 

PSCI II – Chapter 6 – Page 2 of 6
Thermal Energy and Temperature Change
º When heat flows into an object the thermal energy usually increases (final
temperature is greater) and change is positive.
º When heat flows out of an object the thermal energy usually decreases
(final temperature is less) and change is negative.
Measuring Specific Heat
º Use a device called a calorimeter.
º Basically the device can measure the temperature changes after
something’s been heated.
Section 2 – Transferring Thermal Energy
I.
Conduction
º Defn. – transfer of thermal energy through matter by the direct contact of
particles.
º Think of a snowball melting in your hand.
Transfer by Collisions
º In your snowball the slower moving particles of your snowball come into
contact with the faster moving particles of your hand.
º As the particles collide energy is transferred.
º One transfers energy to its neighbor and so on.
Heat Conductors
º Conductors can happen in solids, liquids and gases.
º Solids are the best because the molecules are closer together.
º Metal solids are even better.
º Silver, copper, and aluminum are among the best.
º Wood, plastic, and glass are among the poorest.
II.
Convection
º Defn. – transfer of energy in a fluid by the movement of heated particles.
º Heated particles move from one location to another.
º They also expand but mass does not change.
º Usually causes particles to move up because of less density.
º Think of a lava lamp.
Heat Transfers by Currents
º The rising and sinking are called convection currents.
º Convection currents transfer heat from warmer to cooler parts of the fluid.
PSCI II – Chapter 6 – Page 3 of 6
Desert and Rain Forests
º The atmosphere has convection currents.
º These are what cause the weather.
III.
Radiation
º Defn. – the transfer of energy by electromagnetic waves.
º These waves can travel through space (called radiant energy).
º One example is the sun.
º Another one is a fire – if you stand next to a fire you feel heat.
Radiant Energy and Matter
º When radiant energy is transmitted some is absorbed and some reflected.
º This depends on the type of material.
º When radiant energy is absorbed thermal energy is increased.
Radiation in Solids, Liquids, and Gases
º Most important in gases since particles are more mobile.
IV.
Controlling Heat Flow
º We do lots of things to protect our bodies from losing or gaining to much
heat.
º Almost all-living things have things that protect them from extreme
temperatures.
V.
Insulators
º Defn. – a material that doesn’t allow heat to flow through it easily.
º An object that is a good conductor is a poor insulator.
º A good insulator is a poor conductor.
Section 3 – Using Heat
I.
Heating System
º Almost all buildings in the US have some kind of heating system.
º All heating systems require a heat source.
º Oldest is burning wood or charcoal.
º Heat is then transferred to surrounding area.
Forced-Air Systems
º Most common heating system.
º Fuel is burned in the furnace and a system of fans blows warm air through
vents in the building.
PSCI II – Chapter 6 – Page 4 of 6
Radiator Systems
º Before forced air systems.
º Closed metal container with hot water or steam where heat is transferred
through air by convection.
º Radiator systems heat water in a central core where water is pushed as
water cools it flows back to central core.
Electric Heating Systems
º Heating coils in ceiling and floors and heat through conduction.
º Convection carries warm air to where it needs to be.
º Less fuel is used but it still uses fuel.
II.
Solar Heating
º Think of green houses.
º Solar energy – energy from the sun.
º Free and seemingly limitless.
Passive Solar Heating
º Solar energy heats a room inside a building but no mechanical device is
used to transfer energy.
Active Solar Heating
º Solar collector – solar heating system device that absorbs radiant energy
from the sun.
º Solar collectors use radiant energy to heat water in the collectors and a
pump pushes warmth through the house.
III.
Using Heat to do Work
º Heat engine – an engine that converts thermal energy to mechanical
energy.
Internal Combustion Engines
º Defn. – fuel is burned inside the engine in chambers or cylinders.
º Four cycles or strokes.
º Intake stroke – valve opens and piston moves down and a mix of air and
gas go in.
º Compression stroke – valve closes, piston moves up compressing mixture.
º Power stroke – spark ignites fuel, gases expand pushing piston down.
º Exhaust stroke – piston moves up pushing exhaust out.
Why are Engines Hot?
º During the power stroke lots of heat is produced.
º Cooling systems are in cars to keep the systems cool.
PSCI II – Chapter 6 – Page 5 of 6
IV.
Heat Movers
º Defn. – a device that removes thermal energy from one location and
transfers it to another location at a different temperature.
Refrigerators
º Contains a coolant pumped through pipes.
º Evaporates at low temperature.
º When it changes to a gas it cools and as a result cools the air around it.
º Then goes and gets condensed.
Air Conditioners and Heat Pumps
º Like a refrigerator but for homes.
º A heat pump is a two way heat mover.
º Acts as an air conditioner in warm weather and as a heater in cool weather.
The Human Coolant
º You use sweat to cool your body.
º As sweat evaporates it carries heat away.
Why do Humid Days Feel Hotter?
º More water vapor is in the air so sweat doesn’t evaporate as much.
PSCI II – Chapter 6 – Page 6 of 6