D. Whitehall

1

HEAT

HISTORY

18th Century

In the 18th century it was assumed that there was an invisible substance called caloric.

When objects got it was assumed that they gained caloric, therefore hot objects should be

heavier than cold objects, but there was no evidence to prove this, hence this theory was

untrue.

1798 ‘Count Rumford’

Count Rumford observed the boring of a gun barrel; he noticed a lot of heat was generated,

while a small quantity of brass chips was removed from the barrel. He thought it was

unlikely that all the heat was stored in the small chips. Further investigations showed that:

(i) The longer the boring took place, the greater the amount of heat was produced.

(ii) If the gun boring was done in a tank of water, the water becomes heated even

though no flame was used.

From his observations he concluded that:

(i) Heat was created when mechanical work was done against friction (caused by

the action of metal on metal).

(ii) If heat can be created it could not a material substance.

1842 ‘J. P. Joules’

He conducted a series of experiments that heat was not a material substance. He also

converted different types of energy into heat energy. He then measured the amount of

energy created and produce, and found that they where in a constant ratio he was the first to

conclude that heat was a form of energy.

Today

Today we believe that when an object gains heat, its molecules gain kinetic energy, and

move and vibrate faster. We also believe that heat cannot be created or destroyed, but

changed from one form to another.

D. Whitehall

2

CONDUCTION, CONVECTION AND RADIATION Heat can be transferred by three methods:

(i) Conduction

(ii) Convection (iii) Radiation

Conduction

This is the flow of heat through a material, without the movement or flow of the material

itself. The experiment below demonstrates that there are materials which are good

conductors of heat (poor conductors are called insulators). We also realize all metals are

good conductors compared to others.

How Heat is transferred in Conduction

All materials transfer heat from molecule to molecule. As the material gains heat the

molecules that are closer to the heat source gain kinetic energy. They vibrate more and

bump into their neighbor and pass on some of their kinetic energy. The molecules become

excited and vibrate vigorously as well. The passing of the energy from molecule to

molecule does no involve the movement of molecules themselves. Metals however contain

free electrons, which move independently throughout the metal. When a metal is heated,

these free moving electrons move faster and diffuse themselves into the cooler parts of the

metal. They transfer kinetic energy to the metal molecules by colliding with them; hence,

the process of the transferring of energy is more quickly.

Convection

These are the flow of a liquid or gas caused by the change in density, in which the whole

medium moves and carries heat energy with it. If we observe the experiment on page , we

will observe purple streaks rising in the water above the crystals, and being carried to the

far side of the beaker. This flow of water is called convection current.

What Causes Convection Currents?

The water at the bottom of the tank, close to heat source expands. As is expands it becomes

less dense than the cooler surrounding water, so it rises. It moves away from the heat

source and loses some of its heat energy to the surrounding and begins to cool. As it

becomes denser, it sinks back to the bottom of the tank.

Radiation

This is the transfer of heat energy by means of electromagnetic waves. Radiation can take

place without a material medium.

D. Whitehall

3

Read and make notes:

PFC Page 161 (include diagrams)

Convection currents in the air.

Land and sea breezes.

PFC Page 163 (include diagrams)

The greenhouse effect

The vacuum flask

PFC Page 390 (include diagrams)

Solar panel

PFC Page 161 -162

Radiant heat (shiny polish surfaces vs dark matted

surfaces)

D. Whitehall

4

EXPANSION OF SOLIDS, LIQUIDS & GASES

Objects increase in size or expand, when they are heated, and contract when they are

cooled. They are three types of expansion and they are:

1.) Linear Expansion: objects increase in length when heated.

2.) Superficial Expansion: objects increase in area when heated.

3.) Cubical Expansion: objects increase in volume.

Why Solids Expand and Contract

When solid are heated their molecules gain extra energy and vibrate violently, and need

more room for movement. The molecules try to push their neighbors away against their

mutual attraction. The increase in distance between the molecules causes expansion in all

directions. If the solid has no room to expand, its molecules will produce a force of

expansion. When a hot solid tries to cool down, its molecules try to return to their original

positions. If the molecules are held too far apart and are not allowed to shrink, they pull on

their neighbours and produce a tension or a force of contraction.

Bimetallic Strip

A bimetallic strip is made of two different metals, e.g. brass and iron, welded or riveted

together. When cold the double strip is straight, fig. (a).

As it is heated the brass expands more than the iron and so the brass forms the outside of a

curve, and the iron on the inside, fig. (b).

Bimetallic strips are used in thermo stats and many other mechanical switching circuits.

D. Whitehall

5

Expansion of Liquids

When we first heat the round bottom flask, the liquid level drops, because glass is a poor

conductor by heat, so the glass flask expands and increase the inside volume. The liquid

which has not started to expand as yet drops to yet to fill the extra volume in the flask.

Once the liquid become heated it expands rapidly and spills over the top. This demonstrates

that cubical (volume) expansion of liquid is very large. The expansion of liquids which we

see is called apparent expansion; the real expansion is greater than what we observed,

because of expansion of liquid’s container which takes up some of the liquid’s expansion.

Expansion of Water

Most liquids contract as they cool and further contact when they reach their freezing point.

Water, however, contracts as it cools from 100°C - 4°C, and expands between 4°C - 0°C.

When a pond freezes over; the denser water (4°C) remains at the bottom of the pond. The

less dense water 3°C - 0°C floats in layers above the denser water. The water on the surface

is frozen, but floats because it is less dense than water below it (this is because it increases

in volume). The density layers stop convection currents from spreading the heat. Since ice

is a poor conductor of heat, the top layer of ice on the pond acts as an insulator blanket and

reduces further heat loss. Aquatic animals and plants use this phenomenon to live in ponds

during the winter.

D. Whitehall

6

Heat and Temperature

Heat flows from a body of high temperature to one of lower temperature. The thermometer

is used to measure temperature.

There are two scales:

(i) Celsius Scale (°C)

(ii) Absolute/Kelvin Scale (K)

Celsius Scale

On this scale there is a lower fixed point which is called the ice point (temperature of

melting pure ice, 0°C) and upper fixed point, called the steam point (temperature of steam

just above boiling water, 100°C)

Kelvin Scale

This scale is used for temperatures which are colder than the freezing point of ice and

higher than the boiling point of water. The lowest possible temperature is called absolute 0

which also known as 0K which is -273°C.

Relationship between Celsius and Kelvin

Converting Temperature

Kelvin temperature = Celsius temperature + 273

T/K = °C + 273

Examples:

°C

K

- 273

0

0

273

50

323

100

373

D. Whitehall

7

Temperature Change

Temperature change of 1°C = Temperature change of 1K

Examples:

(a) Initial temperature: 50 °C 323 K

Final temperature: 110 °C 383 K

Temperature change (∆T): 60 °C 60 K

[ Temperature change (∆T) = Final temperature – Initial temperature]

(b) Initial temperature: 80 °C 353 K

Final temperature: 10 °C 283 K

Temperature change (∆T): -70 °C -70 K

[the negative sign (-) means that the object is lossing heat energy or is cooling]

Read and make notes:

PFC Page 170 (include diagrams)

Different types of thermometers

D. Whitehall

8

Heat Capacity and Specific Heat Capacity

Heat Capacity (C)

This is the heat energy needed of an object to raise its temperature by one Kelvin (or one

degree).

The heat capacity of an object depends on:

(i) the type of material the object is made of.

(ii) the mass of the object.

The formula for heat capacity (C) is:

Heat Capacity = Heat Energy

Temperature Rise

C = E

∆T

Units: J/°C or J/K

[N.B. - The heat capacity refers to the whole object]

Specific Heat Capacity (c)

The specific heat capacity of a substance is the heat energy needed to raise the temperature

of 1kg of a substance by 1K (or one degree).

The formula for specific heat capacity (c) is:

Specific Heat Capacity = Heat Energy

Temperature rise × Mass

c = E

M × ∆T

Units: J/(kg°C) or J/(kgK)

D. Whitehall

9

We can arrange the formula to get:

heat energy = mass × specific heat capacity × temperature change

E = mc∆T

[This formula is used to calculate the heat energy required to heat up a substance]

The Relationship between Heat Capacity and Specific Heat Capacity

The heat capacity is when you are talking about the entire / whole object. The specific heat

capacity refers to 1 kg of the object. There is a relation which exists between the heat

capacity and the specific heat capacity of an object.

heat capacity = specific heat capacity × mass

C = mc

Table showing specific heat capacity of some materials

Substance

Specific Heat Capacity

J/(kg°C) or J/(kgK)

Water 4200

Aluminum (alloy) 880

Copper 380

Ice 2100

Nylon 1700

Glass 670

Lead 126

Marble 880

D. Whitehall

10

Examples:

(i) How many joules of heat are required to raise the temperature of 550 g of water

from 12oC to 18oC? (remember the specific heat of water is 4200 J/kg oC)

(ii) 8750 J of heat are applied to a piece of aluminum, causing a 56 oC increase in

its temperature. The specific heat of aluminum is 902.5 J/kg oC. What is the

mass of the aluminum?

(iii) A 250 g sample of water with an initial temperature of 98.8 oC loses 7500 joules

of heat. What is the final temperature of the water?

(Remember, final temp = initial temp - change in temp and that specific heat

capacity of water 4200 J/kg oC)

D. Whitehall

11

SOLID

Change of State

LIQUID

Change of State

GAS

Latent Heat of Vaporization

Latent Heat of Fusion

Latent Heat and Specific Latent Heat

Latent heat is “hidden heat”. That changes the state of an object without causing a

temperature change. For example: Latent heat changes ice at 0 °C to water at 0 °C.

State Of Matter

Latent Heat of Fusion (L)

The latent heat of fusion of a solid is the heat required to change a solid to a liquid without

a temperature change.

latent heat of fusion = heat energy needed to melt all of it.

L = E

Units: Joules (J)

D. Whitehall

12

Specific Latent Heat of Fusion ( ɭ )

The specific latent heat of fusion of a solid is the heat required to change 1kg of it, from a

solid to a liquid without any temperature change.

specific latent heat of fusion = heat energy

mass

ɭ = E

m

Units: J/kg

We can rearrange this formula, to obtain a formula for heat energy:

heat energy = mass × specific latent heat

E = m ɭ

Latent Heat of Vaporization (L)

The latent heat of vaporization of a liquid is the heat required to change a liquid to a gas,

without a temperature change.

Specific Latent Heat of Vaporization ( ɭ )

The specific latent heat of vaporization of a liquid is the heat required to change 1kg of it,

from a liquid to a gas without any temperature change.

Heat Formulas

We now have two formulas to use to determine the heat energy:

(i) E = mc∆T (this heat energy causes a change of temperature)

(ii) E = m ɭ (this heat energy causes a change of state, but no

temperature change)

D. Whitehall

13

Examples:

(i) An ice lolly has a mass of 100g, if the specific latent heat of fusion of ice is

340 000J / kg; calculate the amount of heat needed to melt the lolly.

(ii) Calculate the heat energy required to convert 4 kg of ice at -25°C, to stem, at

100°C, given that specific heat capacity of water is 4 200J/(kg°C), the specific

heat capacity of ice is 2 100J/(kg°C), the specific latent heat of fusion of ice is

340 000J/kg, and the specific latent heat of vaporization of water is

2 300 000J/kg.

D. Whitehall

14

Determining the Specific Heat Capacities of Metals and Liquids by Experimentation

There are two types of experiments we can use to determine the specific heat capacity of a

metal or a liquid.

Electrical Method

We set up the experiment as shown in the diagram above. We then determine the mass of

the material. We use a thermometer and measure the initial temperature of the material.

Next we supply a known amount of energy to the material and we measure the temperature

rise in the material.

We use a heater of known power supply and use the heater for approximately five (5)

minutes. We can use the formula below to determine how much energy was sent to the

material.

heat energy supplied = power of heater × time

E = Pt

We the find the temperature change of the material by using

temperature change (∆T) = initial temperature – final temperature

∆T = T final – T initial

Final we use the formula below to calculate the specific heat capacity of the material

specific heat capacity = heat energy supplied

temperature change ×mass

c = Pt

m ∆T

D. Whitehall

15

Method of Mixtures

This is the most common practical used to find the specific heat capacities of solids and

liquids. We usually add a hot solid (or a hot liquid) of known temperature to a cold liquid

and determine the final temperature.

We assume that all the heat from the hot substance goes to the cooler one if we can reduce

heat loss by using insulation.

We then use the formula below to determine the specific heat capacity of the substance

heat loss by solid = heat gained by liquid

m solid × c solid × (T solid – T final ) = m liquid × c liquid × (T final – T liquid )

Where:

m solid = mass of the solid

c solid = specific heat capacity of solid

m liquid = mass of liquid

c liquid = specific heat capacity of liquid

T solid = initial temperature of solid

T liquid = initial temperature of liquid

T final = final temperature of mixture

D. Whitehall

16

Example:

Find the specific heat capacity (c) of aluminum by the following procedure below:

(i) heat a 0.5 kg mass of aluminum in boiling water (100 °C).

(ii) put 1kg of water at 20°C in an insulated container.

(iii) quickly transfer the hot aluminum into the water.

(iv) stir and record the final temperature of the mixture, which is 28°C

(given that the specific heat capacity of water is 4 200J/(kg°C) ).