Animals in cold climates often depend on two layers of insulation: a layer of body fat [of thermal conductivity 0.200 W/(m⋅K) ] surrounded by a layer of air trapped inside fur or down. We can model a black bear (Ursus americanus) as a sphere 1.40 m in diameter having a layer of fat 4.00 cm thick. (Actually, the thickness varies with the season, but we are interested in hibernation when the fat layer is thickest.) In studies of bear hibernation, it was found that the outer surface layer of the fur is at 2.90 ∘C during hibernation, while the inner surface of the fat layer is at 30.9 ∘C. Assume the surface area of each layer is constant and given by the surface area of the spherical model constructed for the black bear.
What should the temperature at the fat-inner fur boundary be so that the bear loses heat at a rate of 50.8?
How thick should the air layer (contained within the fur) be so that the bear loses heat at a rate of 50.8?

a) 29.25 ∘C b) 7.66 cm

Explanation:

a) We need to find the temperature at the fat-inner fur boundary so bear loses heat at rate of 50.8

Rate of heat is related to temperature as:

Rate= Thermal conductivity of fat * Area * Change in temperature / Fat layer length

So,

50.8= 0.2 * * ( 30.9 + 273 - T ) / 0.040

T= 302.25 K

T= 29.25 ∘C

b)

We have

Thermal conductivity of air= 0.024 W/(m⋅K)

Rate= Thermal conductivity of air * Area * Change in temperature / Air layer length

50.8= 0.024  * * ( 29.25- 2.9 ) / L

L= 0.076 m

L= 7.66 cm

electromagnetic shielding is the practice of reducing the electromagnetic field in a space by blocking the field with barriers made of conductive or magnetic materials. shielding is typically applied to enclosures to isolate electrical devices from their surroundings, and to cables to isolate wires from the environment through which the cable runs. electromagnetic shielding that blocks radio frequency electromagnetic radiation is also known as rf shielding.

the shielding can reduce the coupling of radio waves, electromagnetic fields and electrostatic fields. a conductive enclosure used to block electrostatic fields is also known as a faraday cage. the amount of reduction depends very much upon the material used, its thickness, the size of the shielded volume and the frequency of the fields of interest and the size, shape and orientation of apertures in a shield to an incident electromagnetic

from wikipedia