Reference: Nelson Physics VCE Units 1&2 Chapters 6 - 9 Page 137.

The Earth's main source of thermal energy is the Sun. There are many other sources of thermal energy some of these are: burning of oil, gas, coal, wood, etc; nuclear reactors, friction, volcanoes, etc.

Temperature (T) tells us the amount of hotness or coolness of an object. We use temperature for a number of things:

1. For cooking

2. For preserving food

3. To tell if someone is sick

4. To help us decide what to wear

5. To help us plan our activities

Temperature (T) is measured in degrees and there are a number of temperature scales. Fahrenheit, Celsius and Kelvin are three common ones.

Degrees Celsius (°C) and degrees Fahrenheit (° F) are related by the following formula

° F = ° C  + 32

Degrees Celsius (°C) and degrees Kelvin (K) are related by the following formula

(approx. °C + 273) and one Kelvin degree equals one Celsius degree.

Kelvin is significant because it is the temperature scale used by scientists in their work.0 K is the lowest possible temperature that can exist.

Heat is a measure of the amount of energy contained in the particles of an object. Heat and temperature are related but should not be confused. A small object such as a pin can have a very high temperature if heated but does not have much heat (energy) because it will cool down quickly. On the other hand a warm drink will have a relatively low temperature but a large amount of heat (energy) since it will take a lot longer to cool down.

Heat is defined as the amount of energy transferred from an object at a higher temperature to an object of lower temperature and is measured in joule (J).

Prac #1.11: Heat, Energy, Temperatures and Mixtures

Problem Set #1: Text Page 153 Questions 1 – 6, 15 – 18, 23 - 27

Heat is transferred in three ways: convection, conduction and radiation.

Demo: Convection in water (Condy's in beaker)

Transfer of heat by the circulation of gas or liquid is called convection. The hot parts of the liquid or gas will rise and the cool parts will fall creating a circulation of the liquid or gas an thus circulation of the heat.

Demo: Fast and slow conductors.

Heat can be transferred across a coin, glass or can to the other side of the object. This process is called conduction-heat was transferred through a solid (without movement of material) from the hot parts to the cold parts.

Different solids conduct heat at different rates in general, metals are fast conductors of heat, whereas substances like cork, wood, glass and plastics are slow conductors of heat. Many liquids and gasses, in particular air, are also slow conductors.

Demo: A Third Type of Heat Transfer (Radiation from a can.)

There is a third type of heat transfer, it is called radiation. Radiation does not rely on the movement of a liquid or gas nor does it rely on travelling through a material. Heat transfer by radiation occurs in much the same way as light travels. Radiation is the method by which heat is transferred from the Sun to you.

How could heat reach you through a window pane. What happens to the glass?

Problem Set #2: Text Page 193 Questions 1, 5, 6, 8, 10

Text Page 209 Questions 1 – 16, 21 – 25

Prac #1.12: Heat Capacity

The heat content of an object depends on three things:

1. it's temperature (T) measured in Kelvin
2. it's mass
3. the substance from which it is made.

We have seen that different substances conduct heat at different rates. Different substances also have differing abilities to hold heat. eg. water holds heat better than steel. Of course this will also depend on the amount of substance which we are testing, so scientists refer to a substances specific heat capacity (c) which is the amount of energy needed to raise the temperature of 1.00 Kg of a particular substance by 1.00 K. The unit of measurement is joule/kilogram/degree Kelvin, which is written as J Kg^-1 K^-1.

Example. It takes 4.2 x 10³ J of energy to raise the temperature of 1.00 Kg of water by 1.00 K. So the specific heat capacity (c) of water is 4.2 x 10³ J Kg^-1 K^-1.

To calculate the amount of energy, Q, needed to change the temperature of an object by D T we use the following formula

Q = m c D T

where:

Q = amount of heat energy

m = mass of the substance

c = specific heat capacity

D T = change in temperature

Note: Delta D is used to symbolise "change in"

Example.

Find the amount of heat energy needed to raise the temperature of 3.0 Kg of iron from 10 °C to 18°C, given that the specific heat capacity for iron is 450 J Kg^-1 K^-1.

Q = ? m = 3.0 c = 450 D T = 18 - 10 = 8.0

Q = 3.0 ´ 450 ´ 8.0

= 10800

= 1.1 ´ 10⁴ J Note: 2 significant figures in answer

Example.

A chef pours 200 g of cold water with a temperature of 15 ° C into a hot aluminium saucepan with a mass of 250 g and a temperature of 120 ° C. What will be the common temperature of the water and the saucepan when the thermal equilibrium is reached?

The amount of energy gained by the water (Q_w) is the same as the energy lost by the aluminium saucepan (Q_s)

Q_w = Q_s

1. Power

If a heat source is used to heat other objects, we can also find the power rating of the source. Power is defined as the rate of energy usage.

Power = Energy per unit time

or

The unit of power is the watt (W) and equals one joule per second (Js^-1).

1 kilowatt (kW) = 1000 watt

Example.

The hot plate of an electric stove heats 1.5 kg of water from 10° C to 80° C in 70s. Find the power rating of the source.

W

P = 6.3 kW

Problem Set #3: Text Page 174 Questions 1 - 29

The three states of matter are Solid, Liquid and Gas. These three states are different in the way that the particles are arranged and are held together.

The properties of these three states are as follows:

SOLID: A solid cannot move by itself.

A solid will keep it's shape unless broken.

The volume of a solid stays the same no matter what it is kept in.

In a solid, strong forces hold the particles together.

LIQUID: A liquid can flow.

A liquid take up the shape of it's container.

The volume of a liquid stays the same.

In a liquid, the forces that hold the particles together are weaker and the particles can move a bit.

GAS: A gas can flow, it will spread out as far as it can.

A gas can change shape.

The volume of a gas can change.

In a gas, the forces of attraction are very weak and the particles are free to move.

A substance can change it's state when heat is added or removed. When heat is added the following occurs.

Melting Vaporization

Sublimation

We can also take out heat (or cool down the system). If we do this the following occurs.

Solidification Condensation

Melting Point: This is the temperature at which a solid melts and becomes a liquid.

Boiling Point: This is the temperature at which a liquid boils and becomes a gas.

Prac #1.13: Change of State

When we heat water it's temperature increases until it boils, at about 100 °C. If we continue to put in more heat the water will change into a gas, steam. To change the state from a liquid to a gas requires more energy, more than it does to raise the temperature of the water. This extra heat required to change the state is called Latent Heat. Of course this will also depend on the amount of substance which we are testing, the substance and the two states, so scientists refer to a substances specific latent heat (l) which is the amount of energy needed to change the state of 1.00 Kg of a particular substance. The unit of measurement is joule/kilogram, which is written as J Kg^-1.

Example. It takes 3.4 x 10⁵ J of energy to change 1.00 Kg of water from a solid to a liquid and It takes 2.3 x 10⁶ J of energy to change 1.00 Kg of water from a liquid to a gas.

To calculate the amount of energy, Q, needed to change the state of a substance we use the following formula

Q = m l

where:

Q = amount of heat energy required or given off

m = mass of the substance

l = specific latent heat

This extra energy is required because changing the state of a substance requires a greater rearrangement of the structure of the substance.

Examples

1. Calculate the amount of heat energy necessary to change 1.5 Kg of ice at 0 °C to water at 0 °C. Given the latent heat of fusion for water is 3.4 ´ 10⁵ J Kg^-1.

Q = ? m = 1.5 l = 3.4 x 10⁵

Q = 1.5 ´ 3.4 ´ 10⁵

= 510000

= 5.1 ´ 10 ⁵ J

2. Calculate the amount of heat energy needed to convert 2.5 Kg of ice at -20 °C to water at 20 °C.

This involves three processes first raising the temperature of the ice to 0 ° C, then changing the state via fusion and finally raising the temperature of the water to 20 ° C.

First heating (using Q = m c D T)

m = 2.5 c = 2.1 x 10³ D T = 20

Q = 2.5 ´ 2.1 ´ 10³ ´ 20

= 110000 J

Changing the state (using Q = m l)

m = 2.5 l = 3.4 ´ 10⁵

Q = 2.5 ´ 3.4 ´ 10⁵

= 850000 J

Second heating (using Q = m c D T)

m = 2.5 c = 4.2 ´ 10³ D T = 20

Q = 2.5 ´ 4.2 ´ 10³ ´ 20

= 210000 J

Total energy required

Energy = 110000 + 850000 + 210000

= 1200000

= 1.2 ´ 10 ⁶ J

Each of the molecules in a liquid will be moving at different speeds. This is the reason that we see steam rising from the surface of water before it reaches boiling point, some of the molecules are moving fast enough to escape from the liquid. When a single molecule escapes from the liquid this causes a drop in both the speed of the other molecules in the liquid and the temperature of the liquid. This explains why evaporation is a cooling process.

Prac #1.14: Rate of Cooling

Cooling is the process where heat is lost from a system. The rate of cooling follows Newton's law of cooling which states that the rate of heat loss is proportional to the surface area of the object and the temperature difference.

There are two common ways where melting and boiling points can be changed. The first is to add impurities to the substance this will lower the melting and boiling points, anti freeze is put in car radiators for this reason. The second is the pressure. A high pressure will increase the boiling point and decrease the melting point, again car radiators are presurised for this reason. On the other hand a decrease in pressure will reduce the boiling point of a liquid, on Mt. Everest water boils at about 70 °C.

Insulation is used to reduce undesired heat transfer into the home during summer and out of the home during winter. Although the word insulator describes materials that are poor conductors of heat, it is used to describe materials that limit heat transfer by conduction, convection and radiation.

Home insulation products fall into two categories; bulk insulation and reflective insulation

Bulk insulation contains pockets of still air. The low thermal conductivity of air ensures that the rate of heat transfer by conduction is kept low. As long as the air is effectively trapped in pockets, heat transfer by convection is restricted also.

Reflective insulation consists of foil which reduces the transfer of heat by radiation by reflecting radiant energy. In order to work effectively, reflective insulation needs to face a still air space which is at least 25 cm wide.

Problem Set #4: Text Page 177 Questions 30 – 52