EE first draft

Saturday 12 September 2020

Last Sunday was the deadline for the EE first draft, I am supervising 4 extended essays and got 4 first drafts, so far so good. As usual there is some interesting work being done as the students get to grips with their research questions and start attempting to evaluate their data. I thought I'd share some of their findings.

Hector

Hector is researching the relationship between the number of blades on windmill and the maximum angular velocity of the blades without load. Without a load, the maximum velocity should be when the resistive forces on the blades equals the propelling forces. Both forces are dependent on the number of blades so it could be concluded that the velocity is independent of the blade number, this is in conflict with several internet sources which just makes it more interesting. I always advise my students to use the physics they know and understand in the development of their hypothesis, it may well turn out to be wrong but that's all part of the fun.

You can't write about something you don't understand.

To solve the problem properly requires some advance aerodynamics way beyond the course, I know it's an extended essay but there are limits.

Hector performed an experiment with a cardboard windmill and an electric fan and discovered that the speed was not constant, the resistive forces were not increasingly at the same rate as the propelling forces. The main resistive force is the drag on the rotating blade, this is normally taken to be proportional to v² but the velocity of the tip is not the same as the middle. Ho hum, more assumptions and simplifications. A second experiment was carried out to determine the relationship between the blade number and drag. Another factor might be that as the blades get closer they will start to pass through air that has already passed by the previous blade, you can't take the energy out of the air twice. Again simple treatment but it leads on to the next experiment keeping the number of blades constant but making them closer to each other. To be continued.

Nifemi

Nifemi, whose name is an anagram of I'm fine, wanted to write about magnetic braking this led to an experiment measuring the time taken for a magnet to fall through a roll of aluminium foil for different numbers of foil layers. This is a typical step in the EE process, from area of interest to focussed research question. The context is magnetic braking but the research is about a falling magnet. This can lead to some confusion in the final paper, student's tend to write too much about their original idea and not enough about their actual area of research. This is particularly noticeable if they write their introduction before they have done their experiment. I advise them not to write until they have done the experiment. How will they know what to introduce before they have done it? The final essay should have a clear focus, the research question.

Nifemi's experiment went well giving results that should be possible to interpret using the principles of electromagnetism learnt in the HL course, Slight problem, we haven't done electromagnetism yet so some extra tutorials were necessary. Up to first daft stage I give students all the help they need, after first draft I withdraw.

A useful hint for anyone repeating this experiment, start with a full roll and unwind, this way the layers will be in close contact with each other.

Roberto

Roberto plays the saxophone so wanted to do some research into the physics of it. Preliminary research revealed that the frequency of the reed is related to the length of pipe but is not quite the same as the frequency of the note, interesting but how can you measure the frequency of the reed when its in your mouth. A bit of lateral thinking led to the idea of using a drinking straw attached to a pipe. If the paper reed was coated in metal its vibration could be monitored using a guitar pick up. This didn't work, the pick up wasn't sensitive enough, a small magnet was added to increase the signal from the pickup but then it wouldn't fit the pipe. Suck it and see. It worked. Sucking the open end instead of blowing the reed end produced a note and the vibrating magnet produced enough EMF to be recorded. The results were good but not as expected. Students are often disappointed when their results don't match the theory, I'm not.

Jay

Jay was keen to do some research into the relationship between angle of attack and lift of a wing. Warning bells, aerodynamics. How can this be turned into something do-able. Well, the way angle of attack produces lift can be explained without complicated aerodynamics, the air simply bounces off the wing causing an upward force (Newtons 3rd). The experiment is not easy without a wind tunnel and not that interesting but how about modeling it in Algodoo. This enables you to change more of the parameters, wind speed, molecular mass, air density etc. and reveals more about the physics.