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Why does heat transfer happen?Heat is a type of energy called thermal energy.

1. conduction

2. convection

3. radiation

Heat can be transferred (moved) by three main processes:

Heat energy only flows when there is a temperature difference from a warmer area to a cooler area.

Why does heat transfer happen?During heat transfer, thermal energy always moves in the same direction:

HOT COLD

Why do objects get hotter or colder?Temperature is a measure of how hot an object is. Why does an ice lolly melt on a warm tongue?

How might climate change cause the polar ice caps to melt?

So… about Thermal Energy

What’s up with Temperature vs Heat?

Temperature is related to the average kinetic energy of the particles in a

substance.

You know the SI unit for temp is Kelvin

K = C + 273 (10C = 283K)

C = K – 273 (10K = -263C)

Thermal Energy is the total of all the kinetic and potential energy of all the particles in a substance.

Thermal energy relationships

As temperature increases, so does thermal energy (because the kinetic energy of the particles increased).

If the temperature stays the same, the thermal energy in a more massive substance is higher (because it is a total measure of energy).

Heat

The flow of thermal energy from one object to another.

Heat always flows from warmer to cooler objects. Ice gets

warmer while hand gets

cooler

Cup gets cooler while hand gets

warmer

How are the particles arranged in a solid, a liquid and a gas?What is conduction?

solid liquid gasParticles that are very close together can transfer heat energy as they vibrate. This type of heat transfer is called conduction.

What type of solids are the best conductors?

Conduction is the method of heat transfer in solids but not liquids and gases. Why?

Metals are good conductors of heat. The outer electrons of metal atoms are not attached to any particular atom. They are free to move between the atoms.

When a metal is heated, the free electrons gain kinetic energy.

How do metals conduct heat?

heat

This means that the free electrons move faster and transfer the energy through the metal.

This makes heat transfer in metals very efficient.

Insulators do not have free electrons and so they do not conduct heat as well as metals.

What happens a fluid is heated?Liquids and gases can both flow and behave in similar ways, so they are called fluids.

heatless dense

fluidThe heated fluid particles gain energy, so they move about more and spread out. The same number of particles nowtake up more space, so the fluid has become less dense

What happens to the particles in a fluid when it is heated?

What is convection?

Warmer regions of a fluid are less dense than cooler regions of the same fluid.

called a convection current.

The warmer regions will rise because they are less dense.

The cooler regions will sink as they are more dense.

This is how heat transfer takes place in fluids and is called convection.

The steady flow between the warm and cool sections of a fluid, such as air or water, is

Why is the freezer compartment at the top of a fridge?

The freezer compartment is at the top of a fridge because cool air sinks.

This warmer air rises and so a convection current is set up inside the fridge, which helps to keep the fridge cool.

Why is convection important in fridges?

The freezer cools the air at the top and this cold air cools the food on the way down.

It is warmer at the bottom of the fridge.

infraredwaves

The Earth is warmed by heat energy from the Sun.

There are no particles between the Sun and the Earth, so the heat cannot travel by conduction or by convection.

?

The heat travels to Earth by infrared waves. These are similar to light waves and are able to travel through empty space.

How does heat travel through space? How does this heat energy travel from the Sun to the Earth?

Heat can move by travelling as infrared waves.

This means that infrared waves act like light waves:

They can travel through a vacuum.

They travel at the same speed as light – 300,000,000 m/s.

They can be reflected and absorbed.

What are infrared waves?

Infrared waves heat objects that absorb them and are alsoknown as thermal radiation.

These are electromagnetic waves, like light waves, but with a longer wavelength.

Emitting thermal radiationAll objects emit (give out) some thermal radiation.

Matt black surfaces are the best emitters of radiation.

white silvermattblack

best emitter worst emitter

Certain surfaces are better at emitting thermal radiation than others.

Shiny surfaces are the worst emitters of radiation.

Which type of kettle would cool down faster: a black kettle or a shiny metallic kettle?

Absorbing thermal radiationInfrared waves heat objects that absorb (take in) them.

best emitter worst emitter

best absorber worst absorber

white silvermattblack

Matt black surfaces are the best absorbers of radiation.

Certain surfaces are better at absorbing thermal radiation than others. Good emitters are also good absorbers.

Shiny surfaces are the worst emitters because they reflect most of the radiation away.

Why are solar panels that are used for heating watercovered in a black outer layer?

Things heat up or cool down at different rates.

Land heats up and cools down faster than water, and aren’t we lucky for

that!?

Specific heat is the amount of heat required to raise the temperature of 1 kg (but in Chem we use g) of a material by one degree (C or K, they’re the same size).

C water = 4184 J / kg C (“holds” its heat)

C sand = 664 J / kg C (less E to change)

This is why land heats up quickly during the day and cools quickly at night and why water takes longer.

Why does water have such a high specific heat?

Water molecules form strong bonds with each other water molecule; (including H-bonds!)so it takes more heat energy to break the bonds. Metals have weak bonds

(remember the “sea of e-) and do not need as much energy to break them.

water metal

How does energy affect materials?Do different materials need the same amount of energy to increase their temperature by the same amount?

To increase the temperature of 1 kg of water by 1°C, requires 4200 J.

To increase the temperature of 1 kg of copper by 1°C, requires 390 J.

How does energy affect materials?Water and copper require different amounts of energy because they have different values for a property called specific heat capacity.

It is the amount of energy required to increase the temperature of 1 kg of a material by 1°C.

So, the specific heat capacity for water is 4200 J/kg°C and for copper is 390 J/kg°C.

• Specific Heat Capacity can be thought of as a measure of how much heat energy is needed to warm the substance up.

• You will possibly have noticed that it is easier to warm up a saucepan full of oil than it is to warm up one full of water.

http://www.cookware-manufacturer.com/photo/418fa6490f24202f2cc5b5feee0fdde3/Aluminum-Saucepan.jpg

• Approximate values in J / kg °K of the Specific Heat Capacities of some substances are:Air 1000 Lead 125Aluminium 900 Mercury 14Asbestos 840 Nylon 1700 Brass 400 Paraffin 2100 Brick 750 Platinum 135 Concrete 3300 Polythene 2200 Cork 2000 Polystyrene 1300 Glass 600 Rubber 1600 Gold 130 Silver 235 Ice 2100 Steel 450 Iron 500 Water 4200

What is specific heat capacity?The specific heat capacity of a material is the amount of energy required to raise 1 kg of the material by 1 °C.

Energy is measured in joules (J).

It can be used to work out how much energy is needed to raise the temperature of a material by a certain amount:

energy specific heatcapacity

temperaturechange= mass x x

Mass is measured in kilograms (kg).

Temperature change is measured in °C.

Specific heat capacity is measured in J/kg°C.

The amount of heat energy (q) gained or lost by a substance = mass of substance (m) X specific heat capacity (C) X change in temperature (ΔT)

q = mCΔT

The equation:

Specific heat capacity exampleUsing the specific heat capacity of water (4200 J/kg°C), how much energy is needed to increase the temperature of 600 g of water by 80°C in a kettle?

energy = 0.6 kg x 4200 J/kg0C x 80 0C

= 201 600 J

energy specific heatcapacity

temperaturechange= mass x x

How much energy would be needed to heat 450 grams of copper metal from a temperature of 25.0ºC to a temperature of 75.0ºC? (The specific heat of copper at 25.0ºC is 0.385 J/g ºC.)

Explanation:The change in temperature (ΔT) is:

75ºC - 25ºC = 50ºCGiven mass, two temperatures, and a specific

heat capacity, you have enough values to plug into the specific heat equation

q = m C ΔT .and plugging in your values you get q = (450 g) x (0.385 J/g ºC) x (50.0ºC)

= 8663J

Specific Heat Capacity

If 25.0 g of Al cool from 310 oC to 37 oC, how many joules of heat energy are lost by the Al?

(c= 0.897 J/g•K)

heat gain/lose = q = (c)(mass)(∆T)

where ∆T = Tfinal - Tinitial

q = (0.897 J/g•K)(25.0 g)(37 - 310)Kq = - 6122 J

Notice that the negative sign on q signals heat “lost by” or transferred

OUT of Al.

How much heat is required to

melt 500. g of ice and heat the

water to steam at 100 oC?

Cice = 333 J/g oCCwater = 4.2 J/g oCCsteam = 2260 J/g

oC

How much heat is required to melt 500. g of ice and

heat the water to steam at 100 oC?

1. To melt ice q = (500. g)(333 J/g) = 166500 J

or 1.67 x 105 J

How much heat is required to melt 500. g of ice and heat the water to

steam at 100 oC?

2. To raise water from 0 oC to 100 oC q = (500. g)(4.2 J/g•K)(100 - 0)K

= 210000 J or 2.1 x 105 J

How much heat is required to melt 500. g of ice and heat the water to

steam at 100 oC?

3. To evaporate water at 100 oC q = (500. g)(2260 J/g)

= 1130000 J or 1.13 x 106 J

How much heat must be absorbed by 375 grams of water to raise itstemperature by 25° C?

m = 375 gc = 4.2 J/g·KΔT = 25° C = 25 Kq = mcΔTq = 375 g x 4.2 J/g·K x 25 K

= 39375 J or 3.9 x 104 J

What mass of water can be heated from 25.0° C to 50.0° C by the addition of2825 J?

m = ?c = 4.2 J/g·KΔT = 50.0° C – 25.0° C = 25.0 Kq = mcΔTm = q/cΔTm = 2825 J/(4.2 J/g·K x 25.0 K) = 26.90g or 27g or .0269kg