Doing household chores is hardly anyone’s favourite way of spending their free time. Still, it is not without its silver linings, such as occasionally finding extra change in your laundry or stumbling upon a mystery of physics! One of these mysteries that many may have pondered while spiffing up their home is why their ceiling fan gradually gets coated with a thick layer of dust even though they are virtually on 24/7.
From what we know about dust and our experience in dusting off all kinds of surfaces in the home, it can be puzzling to see electric fans accumulate so much dust despite always being on the move. After all, the moving fan blades should send dust particles that land on them back into the air, right?
To understand why this is unfortunately not the case, we must first learn more about how dust works and the fundamentals of fluid dynamics and electromagnetic forces.
Dust: What It Is Made of and How It Forms
As gross as it may sound, the dust found in the everyday home is largely made up of dead hair and human skin cells that get shed off when our body regenerates thousands of skin cells day in and day out. This all adds up to countless tiny dust particles floating around your home’s atmosphere. And while other substances comprise a small percentage of dust in your home, most of it directly comes from our bodies!
Electrical Charges and Dust
Plastic is the most common material used in all types of electric fans, and this material for the fan blades and the electric charge of dust particles are the two important factors we must first look into.
As a refresher, most, if not all, of the dust particles drifting in your home have some sort of charge and cause them to cling to one another, hence the curious formation of dust bunnies under your bed. Meanwhile, as fan blades slice the air at great speeds, they rub against air molecules and cause a charge to build up on their leading edge. This charge is called static electricity, and as you might recall from listening to your physics tutor in Singapore, it occurs when two or more bodies or surfaces come into contact and separate again, resulting in the transfer of electrons from one atom to another.
As dust particles act on the leading edge of the blades, they get attracted to their surface due to the built-up charge and ultimately cling there. The edge develops and retains the most charge due to frictional forces, so it pulls in the most dust compared to other areas of the fan blade. Combine this with dust’s tendency to stick to each other, and you will eventually accumulate dust on your fan blades.
Note that this explanation generally only applies to plastic electric fans and thus answers just part of this question. Fret not, however, for there is another explanation regarding the dust-loving nature of electric fans, no matter what material they are made of.
Dusty Fans and Fluid Dynamics
Static electricity does not build up on fans with metal blades, unlike in plastic ceiling fans, yet it still picks up dust all the same. To explain why this occurs without needing an electrical charge, we must turn to fluid dynamics or the study of how gases and liquids move.
As the blades of a metal ceiling fan move in circles, the speed of the air at its precise surface is, surprisingly, zero. This is typically the point where dust deposits stick to the fan’s surface since there is minimal air movement. Dust particles and anything else that is as small in size get pressed against the surface and stay there, specifically on the blade’s leading edge. A thin layer of idle air above the surface then protects the accumulated dust deposit.
The airspeed and movement only increase as you move further away from the surface of the blades, hence why there is little to no dust in those sections. This means that the higher above the blade you go, the higher the friction and air movement, which explains why it is not common for piles of dust to stick to metal blades.
But over time, as more dust piles up, it creates a “new surface” where a great deal of dust can further accumulate. The fan’s speed has no effect on this fundamental principle of fluid dynamics. In fact, the faster the fan speed, the quicker dust will build up because the blades interact with more dust particles!
Conclusion
Depending on the type of fan, there are a couple of natural forces you can blame as to why it periodically adds to your list of chores. Whether it is static electricity or fluid dynamics at work, what is certain is that your ceiling fans will always be dustier than you think, no matter which setting you put them on!
If you have any other physics mysteries you’ve encountered while spring cleaning, you can find the answers you seek by signing up for our classes here at Tuition Physics! Beyond just satiating your curiosity about the workings of the known universe, our IP and O-level physics tuition in Singapore also guarantees you achieve overall excellence in the subject and ace even the most daunting exams. Feel free to contact us today for more details on our syllabi and class schedules.
