Hello, I was hoping to get help with this question. For part a) we are supposed

Question

Hello, I was hoping to get help with this question.

For part a) we are supposed to talk about it in terms of heat modes conduction, convection and radiation

For part b) Im guessing it has something to do with pressure and evaporation temperature but I am not quite sure how that would affect efficiency.

2. An incandescent light bulb is an electric light with a tungsten wire filament heated to a high temperature by passing an electric current through it until it glows with visible light. The hot filament is protected from oxidation with a glass bulb that is evacuated (vacuum). Incandescent bulbs are known to be rather inefficient forms of lighting, typically converting less than 5% of the energy they consume into visible light. Higher efficiencies can be achieved by increasing the filament temperature, but this is typically associated with reduced lifespan of the filament a) Explain how heat is transferred (which modes) from the hot tungsten filament surface of the incandescent light bulb shown below to the ambient air in the room. (Neglect heat transfer through the threaded portion of the light balb). (12 marks) b) Some light bulbs are filled with an inert gas rather than being evacuated. How would you expect this to affect the filament temperature and consequently the light bulb efficiency, and why? (8 marks) Tungsten Filament Evacuated Space (vacuum in bulb) Glass Bulb Ambient Air

Explanation / Answer

(a) First heat is transferred from the heat source to the tungsten filament by conduction phenomenon. Then there’s convection, which drives a flow inside the bulb transferring the heat from the filament throughout the bulb via the movement of inert gas if any(in case of vaccum convection transfer is not there). Finally, there is the radiation portion of the problem, and in this case that includes surface-to-surface and surface-to-ambient radiation. The radiation heat transfer is directly proportional to fourth power of temperature and is the most dominant of all ways.

(b) You can't fill a bulb with air or hot tungsten wire will combust in presence of oxygen. Maintaining vacuum either is sometimes dangerous as atmospheric pressure can break the glass, so usually filled with inert gas like Argon etc which does not chemically react with the filament.

In the first light bulbs, all the air was sucked out of the bulb to create a near vacuum -- an area with no matter in it. Since there wasn't any gaseous matter present (or hardly any), the material could not combust.

The problem with this approach was the evaporation of the tungsten atoms. At such extreme temperatures, the occasional tungsten atom vibrates enough to detach from the atoms around it and flies into the air. In a vacuum bulb, free tungsten atoms shoot out in a straight line and collect on the inside of the glass. As more and more atoms evaporate, the filament starts to disintegrate, and the glass starts to get darker. This reduces the life of the bulb considerably.

In a modern light bulb, inert gases, typically argon, greatly reduce this loss of tungsten. When a tungsten atom evaporates, chances are it will collide with an argon atom and bounce right back toward the filament, where it will rejoin the solid structure. Since inert gases normally don't react with other elements, there is no chance of the elements combining in a combustion reaction.

The role of the gas is to prevent evaporation of the filament, without introducing significant heat losses. For these properties, chemical inertness and high atomic or molecular weight is desirable. The presence of gas molecules knocks the liberated tungsten atoms back to the filament, reducing its evaporation and allowing it to be operated at higher temperature without reducing its life

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