Confusing Conventions

I learnt at my grandfather's knee that electric current goes from positive (plus +) to negative (minus -). Obvious enough batteries have plus and minus connections. This idea got me a long way. In electrical engineering it lets one work out the direction of magnetic fields. This is known as conventional current.

The problems started as soon as I picked up a book on electronics, the authors of these find it impossible to explain anything without getting into electrons, and electrons the common carrier of current are negatively charged and (since opposites attract and likes repel) travel from negative to positive.

After that I always wonder is that conventional current being talked about or electron current. Look at the symbol for a diode, a big arrow; that arrow is the direction of conventional current. The same applies to the junction transistor symbol.

It's quite possible to learn basic electronics without knowing anything about electron flow, the symbols make more sense.

Car gears are numbered from 1 upwards to maybe 4,5 or 6. If that's all you know you're fine. However the junior engineer also knows about gear ratios. How many times the output shaft rotates for one rotation of the input. As one works through the gears from 1 upwards, the ratio decreases. In sixth gear the wheels go round faster for the same engine speed as in first gear. So what does that sign saying "select low gear" mean. Is it low ratio or low gear number.

In photography the aperture is the hole the light passes through. Make that hole smaller and less light gets to the sensor but the amount that is in focus increases. Photographers measure aperture in "f stop numbers". So given the instruction "increase the aperture" one would think change from f6 to f7. But no, f stop numbers are inverses (the focal length of the lens divided by the diameter of the aperture) so f7 is a smaller aperture than f6.

My lawn mower throttle has a symbol, at one end is tortoise at the other a hare. Obviously tortoise slow, hare fast. But what about the fable of the race between the tortoise and the hare, which the tortoise wins. Are the engineers thinking about that.

Page last modified on October 13, 2019, at 12:55 AM
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