May-14General Science Chapter 141 Heat and Temperature Chapter 14.

04/10/23 General Science Chapter 14 1

Heat and Temperature

Chapter 14


Temperature

What do you know about temperature?Hot and cold can be used to describe

temperature.Heat is related to temperature, but they

are not the same thing.


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


Tiny moving particles

All matter is made up of tiny particles that are in constant motion. (Kinetic Theory)

The particles have kinetic energy.The faster they move, the more kinetic

energy they have.Temperature is a measure of the average

kinetic energy of the particles in a sample of matter.


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

Temperature

We use thermometers to measure changes in temperatures.

Absolute Zero: Lowest temperature that an object can have (-273.15°C)

Common units for temperature areKelvin SI unitCelsiusFahrenheit


Temperature conversions


K = C + 273 C = K - 273


Heat and Phase Changes

Heat is the transfer of energy from a warmer temperature to a cooler temperature.

In some cases this temperature change will cause the substance to go through a phase change


Phase Changes

Solid to a liquid ---- MeltingLiquid to a gas ---- EvaporationGas to a liquid ---- CondensationLiquid to a solid ---- FreezingWhen sufficient heat is added or removed

from a substance (which slows or increases the motion of the particles) a phase change will occur


Phase Changes

The temperature at which the phase occurs is called the melting point or freezing point, etc.Melting point and Freezing point occur at the

same temperatureIt just depends on whether the substance is

gaining heat or losing heatTherefore evaporation point/boiling point and

condensation point occur at the same temperature


Heating curve for water


Heating Curve

Important information for the heating curveWhat is the independent variable?What is the dependent variable?Can you identify what phase a substance is in

when looking at the heating curve?Can you identify where phase changes occur

on the heating curve?


Discussion #1

Why do ice cubes melt when you put them in your drink?

What is absolute zero and what is the value of absolute zero?

What does temperature measure?What are the 3 temperature scales?


Discuss #1

What happens to molecules during a phase change?

Explain the parts of the heating curve.Explain what is happening on the constant

sloped lines on the heating curve.What is a phase change and what are 3

examples?


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.


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.

Can take place in solids, liquids, or gases.Solids usually conduct heat better

particles are closer together


Conductors

Good heat conductors – materials that transfer heat easily.Metals

Poor heat conductors – don’t conduct heat easilyplasticwoodglassfiberglass


Reducing heat flow

Good heat insulators reduce the flow of heat.

They are bad heat conductorsGases such as airWoodPlasticGlassfiberglass


Air pockets

Many insulating materials make use of small pockets of air inside them. The pockets are too small to allow convection currents to form, so they are good insulators.Styrofoam coolersDown blanketsInsulation for your house


R-value

Resistance to heat flowHigher values mean it is a better insulator.You should use higher R-value materials

in roofs and ceilings because more heat escapes upward by convection currents.


Double-paned windows

Heat is lost through glass windows.By putting a thin layer of air between two

panes of glass gives a window a higher R-value.

High R-values keep heat inside in the winter and outside in the summer.


Thermoses

Also called vacuum bottles.Contain a double glass wall with a

vacuum in the middle to prevent heat transfer.


Fluid

Any material that can flow Liquid or gas

Air is a common example


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.


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.


Convection currents

This movement creates convection currents that transfer energy from warmer to cooler parts of the fluid.


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


Energy from the sun

Radiant energyit travels through mostly empty space to reach

us.

Shiny materials reflect radiant energy, while dull materials absorb it.

Dark-colored materials absorb more radiant energy than light-colored materials.


Discussion #2

List the 3 types of heat transferWhat is a convection current?What is the difference between an

insulator and a conductor?List 3 insulators and 3 conductors.What does an R-value tell us?What is the advantage of a double pane

window?


Discuss #2

What is conduction?What is radiation?What is convection?Why do birds fluff their feathers and

mammals fluff their fur to keep warm?


Specific Heat

Different materials require different amounts of energy to produce the same temperature change.

The specific heat (Cp) of a material is the amount of energy it takes to raise the temperature of 1 kg of the material 1 degree Celsius.

Units are J/kg∙°CAlso called heat capacity


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.


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.


Change in Thermal Energy

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change in

thermal energy= mass( ) ×

specific

heat

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⎠ ⎟×

change in

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Q = m ×Cp × ΔT


Delta

The Greek letter (delta) means “change in”T means change in temperatureAlways take final temperature minus initial

temperature.When T is positive, the object has increased in

temperature and taken in heat.When T is negative, the object has decreased

in temperature and given off heat.


Example

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

Q = m∙Cp∙T

Q = (0.230 kg)(4184J/kg∙°C)(90 °C – 12 °C)Q = (0.230 kg)(4184 J/kg∙°C )(78 °C)Q = 75061 J


You try

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

Q = (3.1 kg)(-15 °C)(920 J/kg∙°C)Q= - 42780 J


Discussion #3

What do each of the symbols in the equation Q = m∙Cp∙T stand for?

What substance has the larger specific heat? Water or Aluminum

Would 750 grams of Iron (Cp = 500) or 750 grams of Zinc (Cp = 800) require more energy to warm up? Explain

Would your answer above change if the substances were cooling down? Why?


Stoves or fireplaces

Simplest heating systemsFuel is burned in the area to be heatedHeat transferred to surrounding air by

conduction, convection, and radiation.


Radiator

Device with a large surface areaHeats surrounding air by conductionConvection currents carry warm air

throughout the room.


Radiator (Hot water or Steam)

A fuel is burned to heat water.The hot water travels through pipes to the

radiator.Or, the water is boiled and the steam travels

through the pipes

The cooled water or steam returns to the furnace to heat again.


Forced air

Fuel is burned to heat air.A blower 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.


Radiant Heat (electric or water)

Heating coils in the floor or ceiling are heated by electricity or with water.

Nearby air is heated by conduction.Materials in the room are heated by

radiation.


Radiant Energy Examples

Pictures from Zach Wiltse


Heat Pump

Heat energy is collected with a network of coiled pipes on the outside of the home.

Heat from the ground evaporates a liquid inside the coils.

The vapor moves to a condenser which increases the pressure to increase the temperature of the vapor further.


Heat Pump

The heated vapor warms the surrounding air.

The heated air is moved through the building with a blower, similar to a forced air system.

As the vapor cools, it condenses to a liquid and is returned to network of pipes on the outside of the home to repeat the process


Geothermal picture


Heat Pump


Solar Heating

Uses energy from the sun.Two kinds of solar heating

passiveactive


Passive solar heating

Windows allow solar energy in.It is absorbed as thermal energy by

materials in the room.Later, it is released to the room.


Active solar heating

Collectors on the roof or the south side of the building. (solar collectors)

Energy is absorbed by liquid in pipes in collectors.

Heated liquid runs through house to radiators.

Cooled liquid returns to collectors


Discussion #4

List the 5 main types of central heating systems.

What are the 2 types of solar heating?What is the difference between the 2

types of solar heating?


Discuss #4

What types of heating systems heat by conduction?

What types of heating systems heat by convection?

What types of heating systems heat by radiation?


Thermal Expansion

An increase in volume of a substance due to a change in the substances temperature

As materials heat up the particles begin to move faster, this makes them spread apart slightly in all directions.Freezing water, thermostat, an inner tube on a

bicycle tire


Thermal Expansion

Expansion jointsGaps that allow materials to expand and

contract with temperature changesMetal joints on bridgesSpaces with “tar” in sidewalks


Thermal Expansion Examples

Thermodynamics

1st Law of Thermodynamics: The net change in energy equals the energy transferred as work and heat

2nd Law of Thermodynamics: energy transferred as heat always moves from objects of high temperature to objects of low temperatures. The total disorder of a system will increase.


Entropy

Measure of the amount of disorder in a system.

Symbolized by the letter S.Higher temperature = more entropyLower temperature = less entropyGas > Liquid > Solid



Heat movers

If heat always flows from warmer to colder, how do refrigerators move the warm air from inside to the even warmer air outside?

Work must be doneThis work is powered by electricity.


Heat movers

Devices that remove thermal energy from one location and transfer it to another location at a different temperature.


Cooling systems (Fridge or AC)

Main purpose of a cooling system is to remove heat energy by use of refrigerantsThe refrigerant will gain heat energy from the

surrounding environment.The refrigerant begins in the liquid form causing

it to evaporate as it gains heat energy.The refrigerant then travels to a compressor to

remove the heat and condense back to a liquid and begin the process again.


Refrigerants

Cooling systems use refrigerants to help remove the heat energy. Chemicals used in cooling systems that usually

evaporate quickly at low temperatures.Air, water, ammonia and carbon dioxide are

common natural refrigerantsFreon is the most commonly used refrigerant,

because it is nontoxic, odorless, and non-corrosive.Freon is harmful to the environment


Other heat movers

Air conditioners – work like refrigerators, only they are designed to cool larger areas.

Heat pumps – work like air conditioners when it is warm out. When it is cold, they take thermal energy from the cold air outside and transfer it inside the house.


Your bodies cooling system

People normally sweat when they become warm.

Our body does this so the sweat evaporates by using the heat from our bodies.

The heat used from our body to evaporate the sweat makes our bodies feel cooler due to the transfer of heat.


Discussion #5

Before there were refrigerators, people had iceboxes.

Ice was placed on the top shelf of an insulated box, and food was placed on the shelves below it.

How did this work?


Discuss #5

What is a heat mover?List some examples of heat movers


Heating curve for Iron

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May-14General Science Chapter 141 Heat and Temperature Chapter 14.

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