Heat Transfer

Heat can be transferred by conduction, convection and radiation. It is only transferred from hotter things to cooler things.

Heat is transferred in order to equalise the temperatures of the object and its environment

Examples:

• A cup of coffee will cool down because it is giving out heat energy into the surroundings
• A cold drinks can (taken out of the fridge) will warm up because it is taking in heat energy from the surroundings

Conduction

In a hot solid, particles vibrate more. They collide with the particles next to them and set them vibrating. The kinetic energy is transferred from particle to particle. Metals are the best conductors. Solids are better than liquids. Gases are very poor conductors. They are insulators.

The video below shows how heat transfer occurs through conduction

Convection

In a hot fluid (gas or liquid) the particles have more kinetic energy so they move more. They spread out and the fluid becomes less dense. The hot fluid rises above the denser cold fluid forming a convection current.

The video below explains how heat transfer occurs by convection

Radiation

All objects emit and absorb infrared radiation. The higher the temperature the more they emit.

When objects absorb this energy their temperature increases.

Radiation will travel through a vacuum – it does not need a medium (material) to pass through:

• Dark and matt surfaces are good absorbers and emitters of infrared radiation.
• Light and shiny surfaces are poor absorbers and emitters of infrared radiation.
• Light and shiny surfaces are good reflectors of infrared radiation.

The video below explains about how heat transfer occurs by Radiation

Insulation

When we insulate our homes we reduce the heat lost, we use less fuel and it costs less. Still air is a good insulator, so materials with air trapped in them are often used:

• In cavity walls the air gap between the walls stops conduction.
• In cavity wall insulation the cavity is filled with foam or mineral wool.
• Loft insulation using layers of fibreglass or mineral wool
• Reflective foil on walls reflects infrared radiation.
• Draught-proofing stops hot air leaving and cold air entering the house.

All these improvements cost money to buy and install, but they save money on fuel costs. You can work out the payback timewhich is the time it takes before the money spent on improvements is balanced by the fuel savings, and you begin to save money:

Payback time (in years) = cost of insulation ÷ cost of fuel saved each year

If the price of the fuel increases, the payback time will be less.

Conductors and insulators

Equally as interesting as the effects of heat transfer on a system are the methods by which this occurs. Whenever there is a temperature difference, heat transfer occurs. Heat transfer may occur rapidly, such as through a cooking pan, or slowly, such as through the walls of a picnic ice chest. We can control rates of heat transfer by choosing materials (such as thick wool clothing for the winter), controlling air movement (such as the use of weather stripping around doors), or by choice of color (such as a white roof to reflect summer sunlight). So many processes involve heat transfer, so that it is hard to imagine a situation where no heat transfer occurs. Yet every process involving heat transfer takes place by only three methods:

1. Conduction is heat transfer through stationary matter by physical contact. (The matter is stationary on a macroscopic scale—we know there is thermal motion of the atoms and molecules at any temperature above absolute zero.) Heat transferred between the electric burner of a stove and the bottom of a pan is transferred by conduction.
2. Convection is the heat transfer by the macroscopic movement of a fluid. This type of transfer takes place in a forced-air furnace and in weather systems, for example.
3. Heat transfer by radiation occurs when microwaves, infrared radiation, visible light, or another form of electromagnetic radiation is emitted or absorbed. An obvious example is the warming of the Earth by the Sun. A less obvious example is thermal radiation from the human body.

We examine these methods in some detail in the three following modules. Each method has unique and interesting characteristics, but all three do have one thing in common: they transfer heat solely because of a temperature difference.