Heat and Temperature
Physical Science
Chapter 14
Physical Science chapter 14 2
Temperature
Hot and cold can be used to describe temperature.
Heat is related to temperature, but they are not the same thing.
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Tiny moving particles
All matter is made up of tiny particles that are in constant motion.
The particles have kinetic energy.The faster they move, the more they have.Temperature is a measure of the average
kinetic energy of the particles in a sample of matter.
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Temperature
Which particles are moving faster, those in a hot cup of coffee or those in a bowl of ice cream?
The coffee – higher temperature means more kinetic energy, which means that the particles are moving faster
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Thermometers
Glass – use expansion of liquidsLiquids expand when heated and move up the
tube
In Thermostats – use expansion of metalsMetals expand at different rates when heatedMakes coil wind or unwind and moves pointer
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Temperature ScalesFahrenheit –
Water freezes at 32 °F and boils at 212 °FUsed in US for weather
Celsius – Water freezes at 0 °C and boils at 100 °CUsed in US for science and for nearly everything in most
of the world
Kelvin – Water freezes at 273.15 K and boils at 373.15 KDegree symbol not usedEach degree is the same size as a Celsius degreeUsed for science
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Converting between temperature scales
1.832.0 FC TT
32.0
1.8F
C
TT
273K CT T
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Examples
Practice problem 2 on page 477: complete the table.
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Heat
The energy that flows from something with a higher temperature to something with a lower temperature.Always flows from warmer to cooler
Heat is measured in joules
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Feelings associated with temperature differenceObject at lower temperature
Takes energy away from your skin when you touch it
Feels cold
Object at higher temperatureAdds energy to your skin when you touch itFeels warm
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Heat vs. work
Heat is energy transferred between objects at different temperatures.
Work is energy transferred when a force acts over a distance.
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Discuss
Describe the relationship between temperature and energy
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Conduction
The transfer of energy by direct contact of particles.
When particles collide, the faster moving one gives some of its energy to the slower moving one.
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Conduction
Can transfer energy through a given material or from one material to another.Example: holding a metal spoon with one end
in boiling water.
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Conduction
Can take place in solids, liquids, or gases.Solids usually conduct heat better
particles are closer together
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Conductors and Insulators
Good heat conductors – conduct heat easilyMetals
Insulators - poor heat conductors – don’t conduct heat easilyplasticwoodglassfiberglass
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Convection
The transfer of energy by the movement of matter
The particles move from one place to another, carrying the energy with them.
When a fluid is heated, the particles move faster. Since they can move, they do – and they spread out.
Fluids expand when heated.
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Heating water
When the water at the bottom gets hot, it expands, and becomes less dense.
The cooler, more dense water above it sinks and pushes the warm water up.
As the water rises, it becomes cooler and more dense, and moves towards the bottom again.
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Convection currents
This movement creates convection currents that transfer energy from warmer to cooler parts of the fluid.
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Radiation
The transfer of energy in the form of invisible rays.
Does not require matter to be present.
Radiant energy – energy that travels by radiation
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Discuss
Compare and contrast conduction, convection, and radiation.
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Specific Heat
Also called heat capacityDifferent materials require different
amounts of energy to produce the same temperature change.
The specific heat (c) of a material is the amount of energy it takes to raise the temperature of 1 kg of the material 1 kelvin.
Units are J/kg∙K
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Specific heat
Water has a high specific heat, so it takes a lot of energy to raise its temperature.
That’s why the temperature of a lake or unheated swimming pool is always cooler than the temperature of the air around it.
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Using Specific heat
We can’t measure change in thermal energy directly.
However, we can measure the change in temperature and use the specific heat to calculate the change in thermal energy.
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Change in Thermal Energy
change specific change inenergy mass
heat temp.
energy cm T
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Delta
The Greek letter D (delta) means “change in”DT means change in temperatureAlways take final temperature minus initial
temperature.When DT is positive, the object has increased in
temperature and taken in heat.When DT is negative, the object has decreased
in temperature and given off heat.
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Example
Calculate the thermal energy change when 230 g of water warms from 12 °C to 90 °C.
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You try
A 3.1 kg block of aluminum cools from 35°C to 20 °C. What is the change in its thermal energy?
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You try
How much energy must a refrigerator absorb from 225 g of water to decrease the temperature of the water from 35 °C to 5 °C?
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Adding Heat to an object
Raises temperatureORChanges state (solid-liquid-gas)
Not both at the same time
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Forced air heating systems
Fuel is burned to heat air.A fan forces the warm air into a room.Convection currents carry the warm air
throughout the room.Cool air returns to the furnace to heat
again.
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Hot water heating systems
A fuel is burned to heat water.The hot water travels through pipes to the
radiator.The cooled water returns to the furnace to
heat again.
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Steam heating systems
A fuel is burned to boil water.The steam travels through pipes to the
radiatorThe cooled water returns to the furnace to
heat again.
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Active solar heatingCollectors on the roof or the south side of
the building.Energy is absorbed by liquid in pipes in
collectors.Heated liquid runs through house to heat
exchanger.Cooled liquid is pumped back to collectors to
heat again
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Refrigerators
Heat naturally flows from warmer to colder.
Refrigerators move the warm air from inside to the even warmer air outside.Work must be done for this to happen
This work is powered by electricity.
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Refrigerators
A liquid is pumped through the refrigerator coils.
As it evaporates, it absorbs heat from inside the fridge.
It is then compressed, causing it to lose heat to the room.
Work is done in pumping and compressing the liquid.
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Air conditioners
Work like refrigerators, only they are designed to cool larger areas.
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First law of Thermodynamics
The total energy used in any process is conserved.
Energy might be transferred as work, heat, or both.
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Second law of Thermodynamics
Heat always moves from an object at a higher temperature to an object at a lower temperature.Unless work is done, like in a refrigerator
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Entropy
The amount of disorderAny system will naturally move towards
higher entropy (and lower energy)
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Usable energy always decreases
The total amount of energy stays the same whenever energy is transferred or transformed.
Often, it is transformed into unusable forms – like heat coming from friction.
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Heat engines
Devices that convert chemical energy into mechanical energy by combustion (rapid burning)
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Internal combustion engine
Fuel burns inside the engine, in the cylinders.Each cylinder has two valves that open and
close.A piston is moved up and down in the cylinders.The piston moves the crankshaft, which moves
the car’s wheels.The wheels exert a force on the road. The equal
and opposite force of the road on the tires accelerates the car forward
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Four-stroke cycle
See figure 4 on page 493Each movement of the piston up or down
is called a stroke
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Intake stroke
For a fuel injected engine, air enters the cylinder through the open intake valve.
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Compression stroke
The intake valve closes.The piston moves up, compressing the air
into a smaller space.Fine droplets of fuel are injected into the
compressed air.
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Power stroke
When the air-fuel mixture is very compressed, a spark plug produces a hot spark that ignites the mixture.
As it burns, the hot gases produced expand, forcing the piston back down.
Energy is transferred from the piston to the crankshaft, powering the motion of the car.
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Exhaust stroke
The piston moves up again, compressing the leftover waste products.
The exhaust valve opens, releasing the exhaust
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Carburetor
In engines with a carburetor instead of a fuel injector,
the gasoline is mixed with air in the carburetor
and the fuel-air mixture enters the cylinder on the intake stroke
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Diesel engines
No spark plugsThe fuel air mixture is compressed so
much that it ignites without a spark.