Download Chapter 6 Thermal Energy

Chapter 6 Thermal Energy
Section 1
Page 158-162
Temperature and Heat
• What do we observe about hot and cold?
– Can we see hot?
– We can compare hot and cold by touch.
• Hot and cold are subjective
– For me, 72% is cold, for my wife it is hot
– Swimming
• If a hot object and a cold object are placed
side by side in an enclosed container, they
come to the same temperature.
Particles in Constant Motion
Modern Theory
Kinetic Theory of Heat Page 476
• Matter is made up of tiny particles, atoms and
molecules
• These particles are in constant random motion
– Move in all directions until they strike something
• Kinetic energy = ½ mv2
– More mass of the particle, more energy, more heat
– Greater speed, more energy, more heat
• Potential energy
– Particles attract each other so when separated they
have potential energy.
Heat/Thermal Energy
• Thermal Energy is the total kinetic and
potential energy of the particles making up an
object.
• Heat is thermal energy that flows from an
object at a higher temperature to an object at
a lower temperature.
• Heat always flows from higher temperature to
lower temperature.
Temperature
• The temperature of an object is a measure of the
average kinetic energy of the particles in an object.
– Higher temperature, more kinetic energy
– Lower temperature, less kinetic energy
• Temperature is measured by comparing
characteristics of a substance that changes with
temperature.
– The volume of most substances change with
temperature
Heat
• Heat is thermal energy that flows from an
object at a higher temperature to an object at
a lower temperature.
• Heat always flows from higher temperature to
lower temperature.
• Since energy is conserved, heat that flows
from one object must be gained by other
objects.
Heat Quantity
• At noon on the beach, how does the
temperature of the air, the water and the
sand compare?
• The more heat that is added to an object, the
higher the temperature rise.
• The amount of heat required to raise the
temperature of an object depends on the
material.
• The amount of heat (Joules) required to raise
the temperature of 1 kg of material by 1oC is
called the specific heat. (Page 161)
Quantity of Heat
•
•
•
•
•
•
•
•
•
Q = quantity of heat, measure in units of Joules
m= mass, measured in kilograms (kg)
Tf = final temperature, units of oC
Ti = initial temperature, units of oC
C = specific heat (c- used in most textbooks)
Q = m (Tf - Ti )C
∆T = (Tf - Ti )
or
Q= mc ∆T
Specific Heats (P161)
Substance
Aluminum
Gold
Concrete
Copper
Diamond
Ethyl Alcohol
Glass
Iron
Water
Specific Heat (J/kg Co)
899
129
880
386
518
2400
837
447
4186
Definitions
• Temperature – (measured with a thermometer) – a
measure of the average kinetic energy of the particles
that make up an object.
• Thermal Energy – the sum of the kinetic and potential
energy of the particles in an object.
• Heat – thermal energy that is transferred.
• Specific heat – the amount of heat that is needed to
raise the temperature of 1 kg of a substance by 1 oC
• Q = m (Tf - Ti )C or Q= mc ∆T
• The heat lost by one object must be gained by some
other object.
Transfer of Thermal Energy
• Conduction – thermal energy is transferred by
the collision of particles with high kinetic
energy with particles of lower kinetic energy.
– Metals are good conductors
• Convection – the transfer of thermal energy in
a fluid (liquid or gas) by the movement of
warmer and cooler fluid from place to place
• Radiation- the transfer of thermal energy by
electromagnetic waves (light, infra-red,
ultraviolet, microwaves)
Controlling Thermal
Energy Transfer
• Reflective surfaces reflect radiation and do
not let the radiation pass.
• Vacuum prevents both convection and
conduction.
• Air spaces prevents conduction.
• Examples
– Attic insulation
– Thermos bottle
– Clothing
Heating Systems
• Source of Heat
– Burning Fuel (propane, fuel oil, coal, wood)
– Electrical Resistance heating
– Solar Heat – absorbing heat from sun light
• Transfer of Heat
–
–
–
–
Radiant Heat – old fashioned fireplace or stove
Force Air – Fan blows air through ducts and vents p172
Circulating Hot Water or Steam – pump and radiators
Heat pumps – as a gas expands it absorbs heat as it
contracts it gives off heat
Solar Energy
• Passive solar – designing buildings to absorb
and store heat directly from sunlight.
– Orientation
– South facing windows
– Materials the store heat
• Active solar – systems that use sunlight to
heat fluid (usually water) these fluids are then
pumped to areas needing heat.
Thermodynamics
• Thermodynamics – movement of heat
• Study of the relationships between work,
thermal energy and heat.
• First Law of Thermodynamics
– In ordinary chemical and physical reactions,
energy cannot be created nor destroyed.
– The increase in thermal energy of a system must
equal the work done on the system and the heat
transferred to the system
Laws of Thermodynamics
• Second Law of Thermodynamics
– Energy always tends to go from a more usable
form to a less usable form
– It is impossible for heat to flow from a cooler
object to a warmer object unless work is done on
the cooler object.
Converting Work to Heat (p175, 177)
• Pushing the bicycle pump handle compresses
air.
• Pushing the pump handle also generates heat.
– Friction of the pump
– Compressing the air
• Friction from the parts of an automobile
produce friction which generates heat.
• Heat is usually a wasted byproduct of work
– Second Law of Thermodynamics
Refrigerator p178
• Second Law of thermodynamics (Page 175)
• It is impossible for heat to flow from a cool object
to a warm object unless work is done on the cool
object.
• A refrigerator takes heat from the cold inside of the
refrigerator and transfers it into the hot room.
• Impossible to move heat from a cold object to a
warmer object unless we do work on the colder
object.
Refrigerators and Heat Pumps
• Special materials – freon
• Freon changes to a gas at a low temperature if
the pressure is high.
• Freon absorbs heat as it is changed into a gas.
• The heat is absorbed from the surrounding air.
• A pump (compressor) then compresses the
Freon and it turns back to a liquid, but in the
process gives up heat.
• These two processes take place at low
temperatures.
Human Heating and Cooling
Human Coolant
• Evaporation of a liquid to its gaseous form
(vapor) requires energy.
• Evaporation of sweat on the body takes heat
energy from the body and uses this to convert
the sweat to water vapor.
• Heat is conducted to the surface of the skin by
blood vessels opening up to allow more blood
flow.
• In cold weather, the blood vessels close down to
prevent heat loss.
• We shiver to generate more heat.