# Cold, Colder, Coldest

#### Abstract

Ice water glows strongly on an infrared thermal camera compared to nearby dry ice and liquid nitrogen.

Yes

#### Principles Illustrated

Introduction

All surfaces at temperatures greater than absolute zero glow. This glow is often called blackbody radiation and it is proportional to the 4th power of the absolute temperature of the surface (and the emissivity of the surface). So yes, even a glass of ice water glows. It just glows less than a cup of hot tea.

In the photos above we have two Styrofoam calorimeter cups in a shallow styrofoam box filled with dry ice (-80°C or 193 K). One cup contains ice water (0°C or 273 K) and the other contains liquid nitrogen (-196°C or 77 K). Notice the ice water looks quite warm on the infrared thermal camera compared to

the dry ice and liquid nitrogen. But not as warm as the rim of the styrofoam cup which is closer to room temperature.

More detailed discussion

The power radiated per square metre of surface is given by the Stefan-Boltzmann Law

In this equation,

P is the radiated power (watts)

T is the temperature (kelvin),

is the Stefan-Boltzmann constant,

and ε is the emissivity (blackness) of the surface, a dimensionless number.

A true blackbody is perfectly black. In other words, it is a perfect absorber of radiation, and thus emissivity ε = 1. All of the visible light, all of the ultraviolet, infrared, microwave, and other electromagnetic radiation that strikes the surface is absorbed. A perfect absorber is also a perfect emitter. A blackbody surface glows more than any other surface at the same temperature. Surfaces that are not blackbodies have emissivity ε < 1

The glow that one sees from an object on an infrared thermal camera depends on the temperature of the object’s surface and also the emissivity of the surface. This makes quantitative work with infrared thermal cameras more challenging. They can be used to measure temperature from a distance but the widely varying emissivities of common surfaces complicate matters. One might try to repeat this demonstration with, for example, copper blocks cooled in ice water, dry ice, and liquid nitrogen. That might give a more quantitative result, but the emissivity of copper will depend strongly on how oxidized it is and also on its temperature. In short, thermal imaging cameras are great tools for qualitative work and can be used for quantitative work as well but not easily.

Extension

#### Instructions

See safety notes below. Teachers in the Wellington area can borrow our IR camera or we can bring this demonstration to your school. IRL has an IR camera that can be borrowed by schools throughout the country with some notice.

#### Safety

Be sure to use fairly thick styrofoam that can withstand cooling. And, as always, handle dry ice and liquid nitrogen using standard safety precautions. Both are potentially dangerous. If you are not trained in the handling of these materials ask a university physics or chemistry department for advice.

Individual teachers are responsible for safety in their own classes. Even familiar demonstrations should be practised and safety-checked by individual teachers before they are used in a classroom.

#### Related Resources

• Thermal Camera
• Infrared Webcam

#### Teaching Resources

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#### Credits

This teaching resource was developed with support from