Question

Why do we characterize the heat conduction ability of insulators in terms of their apparent thermal conductivity instead of their ordinary thermal conductivity?

Short answer

Answer: Insulators are characterized by their apparent thermal conductivity because it takes into account the combined effects of conduction, radiation, and convection, while ordinary thermal conductivity only measures the conduction of heat. As insulating materials often have low ordinary thermal conductivity values, other heat transfer mechanisms, such as radiation and convection, become more significant. Apparent thermal conductivity gives a more accurate representation of the insulators' overall heat conduction ability in practical applications.

Step-by-step solution

Define ordinary thermal conductivity

Ordinary thermal conductivity is the property of a material that quantifies its ability to conduct heat. It is typically denoted by the symbol 'k' and is expressed in watts per meter-kelvin (W/mK). This value is constant for a given material and can be used to determine the rate of heat transfer through the material under steady-state conditions.

Define apparent thermal conductivity

Apparent thermal conductivity, on the other hand, is a property that takes into account both the conduction of heat through the material as well as other heat transfer mechanisms such as radiation and convection. It is also expressed in watts per meter-kelvin (W/mK) but represents an effective value that is not solely dependent on the material's intrinsic conduction ability.

Differences between ordinary and apparent thermal conductivity

Ordinary thermal conductivity measures only the conduction of heat through a material and assumes that other heat transfer mechanisms are negligible. However, this may not always be true, especially for insulating materials, since they often have low ordinary thermal conductivity values and other heat transfer mechanisms become more significant. Apparent thermal conductivity takes these additional factors into account and gives a more accurate representation of the overall heat conduction ability of the material.

Suitability of apparent thermal conductivity for insulators

Insulators are materials that typically have low ordinary thermal conductivity values. As in insulating materials, the effects of radiation and convection could be more significant than conduction itself. Thus, using ordinary thermal conductivity to describe an insulator's ability to conduct heat may not give an accurate representation of its performance in practical applications. On the other hand, apparent thermal conductivity takes into account the combined effects of conduction, radiation, and convection, resulting in a more accurate value for insulators. This is why we characterize insulators' heat conduction ability using apparent thermal conductivity instead of their ordinary thermal conductivity.