Is It Just Me?

[Nowhere-Man]

......That keeps needing to revise old ground. I find I tend to forget stuff that 3 months ago I had well understood. Last night I had to go over FM all over again as it had all gotten murky. There is a saying of Einstein's I liked a lot when I heard it quoted. He said if you can't explain something technical to a cleaner so it's understood, you haven't understood it yourself. I think being a teacher, this would have been clear to Einstein as you really need to be clued up to explain to others. Anyway, I sometimes think I have something truly sunk in and then I may be explaining myself. And at that point I think, "Hold on a bit, now this seems a bit wobbly." Another thing I forgot recently was how to work out the value of a cathode resistor by first referencing the current and voltage bias required. I had to revise it. So tonight, after plumbing my boat all day, I will further revise FM modulation.

[Nebogipfel]

No, it's not just you but I suspect like lots of us with an interest in this old technology you are no longer in the first flush of youth?

I can remember a time when I could read something and it was logged and filed. In the last several decades Lots of brain cells are gone forever and things just do not "sink in" the same and I certainly find it harder to retain new information BUT..... much of the stuff I learned 40+ years ago is still available to me.

 I am now trying to grasp a bit about electronics/radio and I'll admit it's a struggle. If it's any consolation I dare bet you know more than I ever will and the stuff which is a bit hazy probably only needs a quick refresh

[ppppenguin]

They say that an education is what remains when you've forgotten everything you were taught.

Coming back to matters electrical. There's plenty that's deeply embedded in the brain cells. I'm not going to forget ohm's law, KCL/KVL, Thevenin/Norton, basic complex numbers, basic trig. That will give me a good start on understanding many circuits and is enough for me to look up anything extra I need to know.

I don't have the details of feedback theory and many other things at my fingertips but I know enough to find out when necessary.

On the digital side, basic boolean algebra is instinctive, inculding de Morgan's rule. In practice when designing in VHDL I'm using this knowledge implicitly. The computer does all the heavy work. Likewise synchornous logic and what to do when life isn't truly synchronous.

I never did learn Smith charts properly, I suppose I once knew a bit of 3 phase theory and could mug it up again, getting the answers wrong by a factor of 3 or SQRT(3) as usual. much other stuff I did at uni is a dim memory from the mid 1970s. I've rarely explicitly used any calculus though understanding the principles makes it much easier to understand a lot of things.

Coming back to FM, a subject that vexes the OP. I understand it in simple terms and certainly won't forget it. When you start to delve deeper the maths gets heavy. Bessel functions aren't something I've ever studied but fortuantely you don't need to understand them to fix a radio or even design an FM TX or RX.

[Nowhere-Man]

I suspect when you read stuff, it's easy to just accept it at face value without grinding it into the cerebral cells. Last night I tried comparing AM with FM. Maybe I will try explaining what I read to an imaginary third party. Random questions too when walking the dog. Even Ohms law I can forget bits of if I don't use it often. Even now I would struggle to recall power factors if asked to give an example. Indeed even the resonant frequency sum has been known to slip my memory.

[Nowhere-Man]

I randomly tested myself on power factors and failed. I'd forgotten how to add a series reactance to a resistance. So, sure, I'd forgot. I revised it again. You square the resistance value, square the reactance, add the total and then use the square root button. The square root of the sum of the squares was a rule I plain forgot. So I find I don't retain what I learned.
Here is the example in the book for anyone else who feeels rusty. A pure resistance of 75 R is to be added to 100 X reactance. Thus, 10000 + 5625 = 15625 the square root being 125 Ohms. Voltage is said to be 250 volts and current 2 amps. Apparent power would be 500 watts but the real power is 75 R times current squared 300 watts. Power factor will be 300/500 = 0.6 All of this I just revised and marine engineers also tend to do power factors. However, I'll probably forget it again. Anyway that is the rule I forgot: You don't just add the X and R but you square each value, add total and get the square root on the calculator. Maybe I should set myself imaginary questions.

[Nowhere-Man]

Maybe age is a factor although I tend to exercise my brain a lot and watch little TV. I suppose it's always been the case students cram for an exam, peak, pass and then just forget. I just feel though I'd like to improve understanding and be a bit sharper on overall electrical basics. Or does this happen to others? Are our brains simply forced to reference information after lapses?

[reset]

 I'd like to improve understanding and be a bit sharper on overall electrical basics. Or does this happen to others? Are our brains simply forced to reference information after lapses?

About 50 years ago one of my teachers said it wasn't necessary to remember everything. We need to know which book to refer to and refresh our memory when needed. I can't remember which teacher it was but I do remember he said that!

If you can remember the basics such as DC paths, biasing, and signal paths you can look at a circuit and have some idea how it would work. Being able to name the circuits may be a good thing but not necessary. 45 years ago when I taught troubleshooting I told the kids to measure the plate voltage and go from there. If the plate voltage is correct the bias and DC currents are good. If you can't drive a signal through an amp with proper bias look at the AC signal path.

If you are like me you can read and understand the theory today but in a month or two not be able to explain the circuit. Build the circuit and make it work and you can remember the circuit a year later.

[Murphyv310]

I think you have answered your own question.
In a hobby situation you may have days or even weeks off the bench. In industry you would have been young when taught the theory, and of course you'd be doing practical work, design, or servicing. This keeps your mind active in what you have been taught, perhaps not the deep and involved stuff but it is easier to remember when you work in electronics day in and day out. Since I finished in my shop in 2006 I have forgotten a lot and need to refresh the memory every now and then.

[Aerodyne]

There's no doubt in my mind that 'practical' learning by doing - putting theory into practice - is by far the best method. I learned more in my first few weeks in the TV trade than I did from years of 'hobby' studying.
But the saying is that you can't teach an old dog new tricks. Actually, in the case of my daughter's old dog, that is literally true! But in my case, I have to admit that the advancing years has limited my ability to absorb new facts and figures. This concerns me not at all, it is simply an unavoidable effect of age.

Tony

[Craig]

Apparent power would be 500 watts

Ah. There is a nest of vipers in Apparent Power. If you rigorously derive the current flows in a three phase system, which is inherently balanced, you get 3EI for the Apparent Power. So far so good, straight text book stuff.

But if you then set two phase currents equal to zero, in other words a single phase supply, you get, wait for it, root(2)EI. In other words not the text book EI. It is all down to the fact that a single phase supply is inherently imbalanced.

You needn't invoke a three-phase trick - you can derive the root(2)EI from first principles.

I got heavily into this about a decade ago when we were assisting in setting up a new startup with an Electrical Engineering Professor. He said that all the text books have single phase apparent power wrong, and it should be root(2)EI. We were all super skeptical until he derived it. This was all to do with there being no accepted IEC method for defining Apparent power in the general system of a three-phase supply with non-linear loads and distorted waveforms. His method produced more realistic numbers in those situations - somewhat important for power-hungry companies when it comes to pay the electricity bill.

The uncomfortable, but entirely correct corollary was the root(2) factor in single phase Apparent Power. This manifests itself directly when considering rotating machines like a motor. A single phase motor is always less efficient that an otherwise identical motor but wired for three phase - because of the root(2) in Apparent Power.

To prevent confusion the Prof in question re-named his method as deriving Universal Apparent Power (UAP) rather than the incorrect text book AP.