Final answer:
Low watt density heating elements are typically long and coiled, a shape that allows the heat to be spread over a large area, reducing the watt density and preventing hotspots.
Step-by-step explanation:
Low watt density heating elements are designed in shapes that allow them to distribute heat over a larger surface area, reducing the watt density (watts per unit area).
These elements can be made in various shapes, such as flat, ribbon, or coil, but they are generally long and coiled in shape, like those used in radiative heaters with Nichrome wire. A long coiled design increases the surface area, allowing the element to emit the same amount of power (energy per time) without becoming excessively hot in one area, hence reducing the likelihood of burning out or damaging the material being heated.
Considering the problem related to the concave mirror reflecting infrared radiation, it isn't directly related to the shape of heating elements. However, understanding heating elements' shapes concerns heat transfer and distribution—concepts also relevant for understanding how a mirror can focus infrared energy from a heater's coils. Heat transfer efficiency and safety considerations lead to the optimization of the shape of these elements.