In a toaster, current is run through a piece of wire to produce heat.
1. If you hold your hand above the toaster it feels hotter than if you hold you hand beside the toaster. Why?
Answer: Hot air rises.
Why: When you place your hand above the toaster you are placing it in the path of the convection current. Since the toaster does not have a fan to blow air around this convection current occurs naturally due to the buoyancy of hot air in room temperature air.
2. If you look inside the toaster when it is on you can see the hot wire glowing red. Compare the temperature of the wire to the temperature of the filament in a typical incandescent light bulb.
Answer: The wire is at a lower temperature than the filament of most incandescent bulbs.
Why: The spectrum of light emitted by a hot object depends on its temperature. A cooler object will emit reddish light while a hotter object will emit yellowish light.
3. Why aren't toasters made with filaments that glow yellow?
Answer: If the filament is too hot it will start to evaporate.
Why: Since the purpose of a toaster is to cook food, not to light a room there is no need to run the toaster so hot that the wire starts to evaporate. By making the wire long it can produce plenty of thermal energy to cook with, but not be so hot that it evaporates quickly.
4. When the toaster turns off, the wire quickly stops glowing red. Why does radiation cause the wire to cool off very quickly?
Answer/Why: Radiated power is proportional to the fourth power of the objects temperature.
5. The outside surface of many toasters is shiny and mirror like. How does this help the toaster to be more efficient?
Answer: This reduces the emissivity of the surface so that less thermal energy is radiated to the objects around the toaster.
Why: Although it is often difficult to determine a surface's emisssivity simply by looking at it, objects that are highly reflective tend to have lower emissivity than objects that appear black.
Many animals have developed special ways of insulating themselves.
6. Deer have hollow hair. How does this help insulate them?
Answer: The deer's hollow hair traps air, which is a poor thermal conductor.
Why: Many synthetic fibers used in sleeping bags and winter jackets are also made of hollow strands.
7. Ducks have oily feathers. Not only does this help them float, but it also helps insulate them. If a duck had water trapped in the spaces between feathers it would cool off faster than if it had air trapped in these spaces. What difference between water and air would cause this?
Answer: The specific heat capacity of water is greater than air's. In other words water conducts heat better than air.
Why: The duck's feathers trap fluid and keep it from transferring heat through convection. However, heat can still be transferred via conduction.
8. A seal has a thick layer of fat under its skin. Why does a thick layer of fat help the seal to retain heat better than a thin layer?
Answer: The conduction of heat through a thick layer of fat is slower than through a thin layer.
Why: Heat transfer via conductivity depends not only on the thermal conductivity of the material, but also on the thickness of that material. It also depends on the temperature difference and the surface area.