Resistor hot in bathroom extract

[DetlefSchmitz]

the speed with which individual electrons/'holes' actually move.

Our physics master demonstrated a calculation showing that electron flow was around a 'furlong a fortnight'.

 

[JohnW2]

Analogies can be a very good way for us to imagine things but we must always bear in mind they may only be helpful in a certain way and by no means totally accurate and often have the ability to mislead us.

True. However, I think we need tio remember that our 'understanding' of many of these things (e.g. gravity, 'centrifugal force', the 'structure of an atom' etc. etc.) are really just theories which reasonably explain what we can observe, even though we don't really know the truth about the 'underlying facts'. In that sense, many of these theories are not much different from 'analogies'.

Regarding power factor. I have found that even some electricians etc tend not to grasp it, some even make some big mistakes.
Years ago I too struggled with the concept of power factor.

It seems that a lot of people have difficulty with PF and I think it is probably easier for those who have an understanding of the underlying maths.

However, although it is in some senses a flawed analogy, I've had a fair degree of success in explaining PF and related concepts, by using a ('mechanical') analogy which helps people to understand how the amount of energy (c.f. electrical power x time) required to make something move a certain distance (c.f. electrical current flow) is critically dependent upon the 'phase relationship' between the direction of the force being worked against (c.f. electrical voltage) and the direction of the movement (c.f. direction of current flow).....

Consider a fairly heavy object (hence a fairly large 'force of gravity' trying to 'pull it down') sitting on an extremely low friction surface.

First consider the situation in which the 'force being worked against' and the direction of movement caused by the applied force) being 'in phase' - i.e. one wants to lift the object upwards, against the force of gravity trying to resist that movement. One would apply an appreciable ('upwards') force, and an appreciable amount of energy would be required to do that moving.

Now move to the situation in which there was a 90° 'phase difference - i.e. one wants to move the object sideways. This would require very little ('sideways') force (hence energy) since the only thing resisting the movement would be the (very low) friction afforded by the surface. Most people can understand that as the friction gets smaller and smaller, one approaches the ultimate situation in which it would require zero force (and energy) to move the object sideways.

If one gets that far (with 'understanding') then it's usually not too difficult to get people to understand that as the direction of the applied force (hence movement) moves from being ';in phase' with the movement (i.e. zero 'phase difference') to 90° out-of-phase, the amount of energy required to move the object a given distance progressively decreases from a fairly high value (c.f. "PF=1) to reach 'near zero' (true zero if no friction at all) when the phase difference is 90° (c.f. "PF=0").

Reduced to a conceptual statement, this means that the 'true power' depends to the extent on which the direction of movement (current flow) is 'in sync' with the force resisting the movement, being at its greatest when they were totally 'in sync' (as they would be with a resistive circuit) but approaching zero as the approached being "90° out-of-sync' (i.e. 'perpendicular to one another).

However, that analogy does not work for everyone, so it may not work for you!".

Obviously it is totally wrong, not least because I am thinking in DC and this subject only concerns AC. However if you think in DC then think of quickly disconnecting the battery then connecting again in the opposite polarity I suppose it makes it a little bit more acceptable.

Indeed - similar to what I recently wrote.

When I went to school we were taught about Centrifugal Force. Nowadays we are told that Centrifugal Force does not exist. But the calculations we used to make about Centrifugal Force still hold us in good stead. I wonder if we can think of those as an analogy?

You are saying exactly what I did at the very start, above. Our 'understanding' about these things are theories, that reasonable fit the observed facts (hence the calculations based on the theories 'work'), even if the theories do not correspond to 'the truth' - so, yes, as I said, I think such theories can be described as a type of ('useful') 'analogy'.

Kind Regards, John

 

[JohnW2]

Our physics master demonstrated a calculation showing that electron flow was around a 'furlong a fortnight'.

Exactly - but when an electron moves (even though 'not much') one (a different one!) will "pop out of the end of the conductor 'almost immediately' ", no matter how long the conductor!

Of course, if your mind fancies getting boggled, we are told (the maths tells us) that electrons can be regarded as 'waves', not 'particles' at all

Kind Regards, John

 

[bernardgreen]

This would require very little ('sideways') force (hence energy) since the only thing resisting the movement would be the (very low) friction afforded by the surface

Inertia has an input in the real world. The acceleration for a given force is the inverse of the inertia.

 

[JohnW2]

Inertia has an input in the real world. The acceleration for a given force is the inverse of the inertia.

True, but one doesn't have to do work/expend energy in order to overcome inertia, does one? If one did, that would presumably seriously complicate what are otherwise very simple 'Laws of Motion'.

Kind Regards, John

 

[bernardgreen]

The force that accelerates the object puts kinetic energy into the object.

Work = Force x Distance moved ( I think )

 

[JohnW2]

The force that accelerates the object puts kinetic energy into the object.

Indeed it does, but the kinetic energy gained is determined only by the object's mass and the increase in velocity (no matter how long the acceleration takes to achieve that increase in velocity), so I can't see how inertia could affect that.

Bear in mind that, in context, I'm talking about the 'steady state' reached when a certain current is flowing (continuously) in a circuit - or, in the mechanical analogy, the 'steady state' when the object is travelling at constant velocity, following its initial acceleration to that velocity from rest.

Kind Regards, John

 

[Harry Bloomfield]

Our physics master demonstrated a calculation showing that electron flow was around a 'furlong a fortnight'.

I think even that might well be an over-estimate of the speed..

 

[JohnW2]

I think even that might well be an over-estimate of the speed..

You may well be right - it's certainly very slow.

Edit: I might have added ... that one is fairly easy to deal with 'by analogy', in terms of balls within pipes/tubes, or even people or cars in a queue etc. - and the latter examples help a bit to explain the "almost" in ".... the electrons come out of the distant end of the conductor almost immediately they start moving into the other end, even if the conductor is extremely long, and even though the electrons are moving very slowly".

Kind Regards, John

 

[ebee]

Yep it is all fascinating stuff. and we keep learning then finding out more questions ad infinitum. truly amazing, theories, facts, conjecture, analogies, myths, legends, good conclusions, bad conclusions.
Exciting or what? I think yes

 

[bernardgreen]

Exciting or what? I think yes

The gyro bus

Gyro Bus - The All Electrical Bus from the 1940s

Public transportation is intended to be a form of transportation that connects cities and countries together and at the same time reduces air pollution and emissions. But today, most public transportation uses petroleum-based power sources.

engineerine.com

 

[JohnW2]

Yep it is all fascinating stuff. and we keep learning then finding out more questions ad infinitum. truly amazing, theories, facts, conjecture, analogies, myths, legends, good conclusions, bad conclusions. Exciting or what? I think yes

Definitely yes, at least as far as I am concerned.

As for the 'theories', I think there is sometimes a place having more than one being 'taught as if it were facts' at the same time, if both 'serve a purpose' and one is easier to get one's head around. I suspect, for example, that, as ehen I was at school, students are probably still initially taught about "the Bohr atom" (essentially looking at an atom as a 'solar system') but then, a year or three later, are taught a theory which is probably closer to 'the truth'.

Kind Regards, John

 

[ebee]

The gyro bus

Gyro Bus - The All Electrical Bus from the 1940s

Public transportation is intended to be a form of transportation that connects cities and countries together and at the same time reduces air pollution and emissions. But today, most public transportation uses petroleum-based power sources.

engineerine.com

Yes I remember something about those Swiss buses from a long time ago. Makes a Happy Man feel very old Bernard

 

[ebee]

Definitely yes, at least as far as I am concerned.

As for the 'theories', I think there is sometimes a place having more than one being 'taught as if it were facts' at the same time, if both 'serve a purpose' and one is easier to get one's head around. I suspect, for example, that, as ehen I was at school, students are probably still initially taught about "the Bohr atom" (essentially looking at an atom as a 'solar system') but then, a year or three later, are taught a theory which is probably closer to 'the truth'.

Kind Regards, John

Bohr Atom ? Hmm I`ll have to read that one John, thanks.

 

[JohnW2]

Bohr Atom ? Hmm I`ll have to read that one John, thanks.

I would be very surprised if, like me, you weren't initially 'brought up' with the Bohr (strictly, I think, Rutherford-Bohr) model ('theory') of the atom - i.e. one in which there was a central 'nucleus', positively charged because of proton(s), with negatively-charged electrons 'orbiting' around the nucleus in nice neat discrete 'shells' - with a maximum of one electron in the first shell, 2 in the second, 4 in the third and 8 in the fourth shell etc. etc. - as I said, a bit like a 'solar system', with more than one planet orbiting at the same distance from the nucleus in all but the first (closest to nucleus) shell.

However, a year or three on in one's education it gets 'a bit' more complicated than that, particularly when quantum mechanics/physics rears its head (and we then probably have to try to get our heads around the concept of something being in two or more places {let alone 'an infinite number of places!} at the same time ).

Kind Regards, John