Thermal Radiation for the ESAT

Thermal Radiation as Infrared Electromagnetic Waves

Thermal radiation, commonly referred to as infrared (IR) radiation, is a specific type of wave found within the electromagnetic spectrum. As with all electromagnetic waves, it travels at the speed of light and does not require a physical medium to propagate. This distinguishing characteristic allows thermal radiation to transfer energy through a vacuum, which is fundamentally different from conduction and convection.

This process is the mechanism by which energy from the Sun reaches the Earth through the vacuum of space.

Radiation enables the direct transfer of heat from one object to another without the need to heat or pass through an intervening medium. A common example is a radiant heater, which warms nearby objects directly via infrared radiation without necessarily heating the air between them first. While the human eye cannot see infrared radiation, it is easily detected by the sensation of warmth it creates when it interacts with the skin.

Worked Example: Identifying Thermal Radiation Properties

Which of the following statements apply to thermal radiation?

1. It can travel through a vacuum.
2. It is a type of ionising radiation.
3. It travels slower than light.

Solution: Thermal radiation is a form of electromagnetic radiation. All electromagnetic waves can travel through a vacuum and move at the speed of light. Thermal radiation is not produced by the decay of unstable nuclei, meaning it is not ionising. Therefore, only statement 1 is correct.

Absorption and Emission of Radiation

Emission of Thermal Radiation

Every object or substance with a temperature higher than absolute zero (the theoretical minimum temperature) emits thermal radiation. The rate at which an object emits this radiation, measured as power or energy per second, is directly related to its temperature: the higher the temperature, the higher the rate of emission.

During emission, the thermal energy of the object is converted into the energy of the radiation, provided the object is not undergoing a change of state. Consequently, if an object emits radiation without receiving energy from another source, its internal thermal energy decreases, leading to a drop in temperature.

Absorption of Thermal Radiation

When thermal radiation strikes an object, there are three potential outcomes: the radiation may be absorbed, reflected (bouncing off the surface), or transmitted (passing through the object).

When radiation is absorbed, its energy is converted back into the thermal energy of the object. If absorption occurs without other energy transfers, the object's temperature will increase.

Combined Effect of Emission and Absorption

Objects are constantly emitting and absorbing thermal radiation simultaneously. The net change in temperature is determined by the balance between these two rates.

1. Higher temperature than surroundings: The object emits radiation at a higher rate than it absorbs it. This results in a net loss of thermal energy and a decrease in temperature.

1. Lower temperature than surroundings: The object absorbs radiation at a higher rate than it emits it. This leads to a net gain in thermal energy and an increase in temperature.

It is worth noting that air is a weak absorber of thermal radiation. This property allows solar radiation to pass through the Earth's atmosphere and reach the surface effectively.

Worked Example: Net Energy Transfer Calculation

Thermal radiation is incident on a window at a rate of $1000$ W. The window reflects this radiation at a rate of $100$ W and transmits it at a rate of $750$ W. Simultaneously, the window emits its own thermal radiation at a rate of $200$ W. Calculate the net rate of change of the window's thermal energy.

Step 1: Calculate the rate of absorption. Of the $1000$ W incident, we subtract the reflected and transmitted portions: $1000 - 100 - 750 = 150$ W. The window absorbs $150$ W.

Step 2: Calculate the net rate. The window gains $150$ W through absorption but loses $200$ W through emission. Net change = $150 - 200 = -50$ W. The window experiences a net loss of energy at a rate of $50$ W.

Factors Affecting Rates of Absorption and Emission

While temperature is a primary driver of emission, the physical characteristics of an object also dictate the rates of energy transfer. Generally, a surface that is a good absorber is also a good emitter, while a poor absorber is a poor emitter but a good reflector.

Factor affecting rates	Lower rates of absorption and emission	Higher rates of absorption and emission
Surface Texture	Shiny surfaces	Matt (dull) surfaces
Surface Area	Smaller surface area	Larger surface area

Practical Applications

Survival Blankets: These are used to prevent hypothermia. The blanket's surface is shiny, making it a poor absorber and emitter but a highly effective reflector. It reflects the person's own body heat back toward them and emits very little thermal radiation to the cold surroundings.

Solar Water Heaters: These systems use pipes painted matt black to transport water. Because matt surfaces are excellent absorbers, they maximise the heat taken in from the Sun. While they are also good emitters, the rate of absorption from solar radiation is significantly higher than the rate of emission, resulting in a net energy gain for the water.

Thermal Radiation and the Earth

The Earth's temperature is primarily determined by the long term balance between the rates at which the surface and atmosphere absorb solar radiation and emit thermal radiation back into space. Global warming, or the greenhouse effect, occurs when the rate of energy transfer to the Earth and its atmosphere exceeds the rate at which energy is radiated away.