Teacher background information

This elaboration provides a context for students to consolidate their understanding of thermal energy transfer mechanisms. It provides opportunities for students to investigate particle and wave models of heat transfer in the context of the effective clothing technologies developed by Aboriginal peoples living in cool and temperate climatic regions of Australia. It also provides opportunities to investigate and compare the thermal conductivity of various materials, as well as factors that affect the rate at which energy is transferred as heat between bodies.

Australia is often characterised as a hot and dry continent and the stereotypical image of Aboriginal peoples is often that of desert people with minimal clothing. Similarly, it is easy to think that Torres Strait Islander peoples, living so close to the equator, perhaps may not have had the need to develop effective bedding and clothing. Australia has a large variety of climatic regions and First Nations’ peoples have thrived successfully in all its climates, including the cool temperate and oceanic climates of southern Australia, Tasmania and the Alpine regions.

In order to control heat loss, Aboriginal peoples developed technologies that enabled them to live and thrive in these cooler south-eastern parts of Australia. The most lauded of these technologies are cloaks made from the hides of animals such possums, kangaroos and wallabies.

For warmth, the traditional skin cloaks were typically worn with the fur lining next to the body, trapping a layer of air. During rainy weather, the cloaks could be worn with the fur on the outside. The oily, water-repellent hairs provided a waterproof protection for the wearer. The cloaks were also rubbed with fat to further improve their insulating properties. The cloaks were large and could also be used to sleep on at night, acting as both mattress and blanket.

Possum skin cloaks are of great cultural significance to some Aboriginal peoples. Often, the inner surface of each cloak was incised and painted with ochre to depict themes from nature or stories of identity, kinship, family group and Country. Prior to 1830 almost every person had his or her own possum skin cloak. They were worn from a young age, with additional skins added to the cloak as the individual grew. Possum fur cloaks were also greatly admired and sought after by European colonists for their functionality as well as for their aesthetic value. However, in the mid-1800s, due to the loss of access to their traditional lands and its resources, many Aboriginal people began using government-issued wool blankets that were less effective in keeping the wearer warm and dry and offered little protection in winter. During this time, many Aboriginal people became ill and died from common European colds and influenza viruses.

The heat-conserving properties of cloaks made from animal furs involve three principal modes of heat transfer: thermal conduction, convection and radiation. 

An object contains thermal (heat) energy because of the movement of the particles in the object. The faster the particles move, the greater the amount of thermal energy the object possesses.

Temperature is a measure, on a defined scale, of the average kinetic energy of molecular motion in an object. It is a measure of an object’s ability to transfer thermal energy to another object.

Temperature and heat are closely related concepts, but they are not the same. Heat is a form of energy; temperature is a measure of it. For example, 200 mL of water at 30 °C contains more thermal energy than 100 mL of water at the same temperature.

Thermal energy will flow from a high temperature substance to a lower temperature substance until they reach thermal equilibrium. This transfer can occur by particles transferring their kinetic energy to nearby particles (conduction) or by the bulk movement of particles in fluids (convection). Transfer of heat energy can also occur as electromagnetic radiation.

The primary purpose of clothing is to regulate these processes. Traditional possum fur cloaks not only act as an insulator by providing a barrier of trapped air that slows thermal conduction, it has also been shown that fur hairs play an important role in reflecting infrared radiation, thus leading to a significant enhancement of a hide’s insulating power.

Students could choose to investigate the situations for which the equation for the rate of heat transfer, in watts, through flat surfaces is applicable:

\(rate\;=\;\frac{kA(T_1\;-\;T_2)}d\)

Where  k = thermal conductivity in W m^-1 °C^-1

A = surface area in m²

T₁ = the temperature on one side of the wall in °C

T₂ = the temperature on the other side of the wall in °C

d = the thickness of the wall in m.

By studying energy transfer through different mediums in the context of traditional clothing, students can gain an appreciation of the technologies and culturally important methods used by Aboriginal peoples to survive in cold wet climates. There are also opportunities for students to consolidate their understanding about heat and temperature, to become more familiar with the models we use to explain thermal energy transfer, and to deepen their understanding of the differences between thermal conductors and thermal insulators.