Heat and Temperature Physical Science Chapter 14

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.

Physical Science chapter 14 3

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.

Physical Science chapter 14 4

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

Physical Science chapter 14 5

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

Physical Science chapter 14 6

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

Physical Science chapter 14 7

Converting between temperature scales

1.832.0 FC TT

32.0

1.8F

C

TT

273K CT T

Physical Science chapter 14 8

Examples

Practice problem 2 on page 477: complete the table.

Physical Science chapter 14 9

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

Physical Science chapter 14 10

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

Physical Science chapter 14 11

Heat vs. work

Heat is energy transferred between objects at different temperatures.

Work is energy transferred when a force acts over a distance.

Physical Science chapter 14 12

Discuss

Describe the relationship between temperature and energy

13

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

Physical Science chapter 14 21

Discuss

Compare and contrast conduction, convection, and radiation.

Physical Science chapter 14 22

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

Physical Science chapter 14 23

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.

Physical Science chapter 14 24

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.

Physical Science chapter 14 25

Change in Thermal Energy

change specific change inenergy mass

heat temp.

energy cm T

Physical Science chapter 14 26

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.

Physical Science chapter 14 27

Example

Calculate the thermal energy change when 230 g of water warms from 12 °C to 90 °C.

Physical Science chapter 14 28

You try

A 3.1 kg block of aluminum cools from 35°C to 20 °C. What is the change in its thermal energy?

Physical Science chapter 14 29

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?

Physical Science chapter 14 30

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.

34

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.

Physical Science chapter 14 38

First law of Thermodynamics

The total energy used in any process is conserved.

Energy might be transferred as work, heat, or both.

Physical Science chapter 14 39

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

Physical Science chapter 14 40

Entropy

The amount of disorderAny system will naturally move towards

higher entropy (and lower energy)

Physical Science chapter 14 41

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.