is voltage everyhwere?

digme

Is voltage in the air around me?

Professor_Fink

digme wrote: »

Is voltage in the air around me?

That question doesn't make sense. Voltage is the total energy required to move an electric charge between two points, so it isn't defined for a single location (such as a point in the air). So for a question to be meaningful, it needs to be of the form "What is the voltage between A and B?".

FarmerGreen

http://en.wikipedia.org/wiki/Sky_voltage

im invisible

another name for voltage is potential difference, if that makes it easier to visualise

himnextdoor

digme wrote: »

Is voltage in the air around me?

Sort of. The air is filled with charged particles so there will be small potential differences between any two arbitary points.

Plug

Voltage is electrical pressure.

Professor_Fink

Plug wrote: »

Voltage is electrical pressure.

Your joking, right?

thecatspjs

Even though calling it electrical pressure is off the mark, some of my lecturers use the whole water in a pipe analogy to represent voltage and current. When thought of in this way I think it makes a bit of sense

himnextdoor

digme wrote: »

Is voltage in the air around me?

Yes.

Professor_Fink

thecatspjs wrote: »

Even though calling it electrical pressure is off the mark, some of my lecturers use the whole water in a pipe analogy to represent voltage and current. When thought of in this way I think it makes a bit of sense

The analogy breaks very very easily. It doesn't even remotely begin to explain induction or capacitance, for example, and you can't go far in circuit theory without meeting these. It only works if you only consider resisters, and even then, it's not robust.

himnextdoor

Professor_Fink wrote: »

The analogy breaks very very easily. It doesn't even remotely begin to explain induction or capacitance, for example, and you can't go far in circuit theory without meeting these. It only works if you only consider resisters, and even then, it's not robust.

For Capacitors:

Different sections of a canal are at different heights and in order to move from one level to another, a barge has to make use of 'lochs'.

The water on each side of the loch is at a different height and can be thought of as a 'potential difference' and each side of the lower loch wall can be thought of as the two plates of a capacitor. The height of the barge in the loch is proportional to the 'voltage' across the capacitor (the electrostatic force between the plates) and the rate of change of the height of the barge in the loch represents the 'current' flowing into the capacitor.

When the sluice gate is opened, water begins to fill the lock, a current flows and the barge begins to rise; the capacitor begins to charge. The loch fills up until the barge is at the same height as the higher water-level; the electrostatic force between the plates increases until it is equal to the potential difference across the capacitor and the capacitor becomes charged, the loch is full.

Inductors:

A header-tank filled with water is at a height 'h' and a pipe transports water directly to the bottom of the tower from where a 'connecting pipe' conducts it along the ground on a level path to drain into a hole.

Water flows into the hole at a rate determined by the height of the tower. Here 'h' represents Potential Difference, the rate of flow of the water into the hole represents current and the 'connecting pipe' represents the inductor. The entire system in this state represents a DC configuration in that the only resistance encountered is due to the bore of the pipes.

If the outlet end of the connecting pipe is raised, there will be a corresponding reduction in the rate at which water flows out of the pipe. The system now represents an AC configuration and accounts for the effect of the inductor.

The slope of the pipe can be equated to the frequency of the voltage or to the 'inductance' of the inductor and the reduction of flow-rate equates to a reduction of current due to impedance; the higher the outlet (upward slope of connecting pipe), the higher the frequency, or inductance, and the higher the frequency, or inductance, the higher the impedance.

I think that electronic principles lend themselves particularly well to water analogies.

Professor_Fink

himnextdoor wrote: »

I think that electronic principles lend themselves particularly well to water analogies.

That was a series of water based analogies, not a single way of understanding circuit theory.

himnextdoor

Professor_Fink wrote: »

That was a series of water based analogies, not a single way of understanding circuit theory.

You're welcome.

Professor_Fink

himnextdoor wrote: »

You're welcome.

Sorry to come across as so negative on this, but dodgy physics analogies are the bain of my life. Analogies don't convey understanding so much as the illusion of understanding.

FarmerGreen

Oddly enough, when I have to do plumbing I think about it in electronic terms.
Doesnt explain cavitation and supersonic flow through a nozzle, but what the heck, it does for the simple stuff.
Apple.. foot.. gravity.

FISMA

Plug wrote: »

Voltage is electrical pressure.

Professor_Fink wrote: »

Your joking, right?

Professor Fink,
I see what Plug is on about, it is a colloquial but powerful analogy.

Like it or not, it works wonders to help new students understand circuits.

One of the major problems with teaching E&M is the inability to point to a real world example with which people are familiar.

In optics and mechanics, we deal with such phenomena every day and it is easy to relate material to our everyday experience.

It is easy to teach the Law of Conservation of Momentum, Energy, and Newton's Laws because we see them every day.

It is difficult to teach Gauss, Faraday, or Ampere without resorting to mathematics and abstractions especially when teaching newbies.

Here is what Plug is on about. Scroll to 5:30 if you do not have the time. It is from Modern Marvels, a good show.

https://www.youtube.com/watch?v=GchDzA60cdk

Also, I absolutely love the water circuit analogy. Within 40 minutes I can have students understanding:

1. resistors in series
2. resistors in parallel
3. Voltage Law
4. Current Law
5. capacitors in series
6. capacitors in parallel
7. why electricity appears to work at the speed of light when it actually goes quite slow
8. Ohm's Law Vs Poiseuille's Law
9. potential drops and rises
10. where V's are constant - throughout lengths of wire between devices.
11. DC i's vs AC i's.
12. Current isn't being used up, but voltage.

and much more.

Additionally, it is difficult to get newbies to understand what a capacitor is and its function.

In teaching students that a capacitor is like a water tank, they see that the capacitance of the capacitor only depends upon geometry.

Just like the water tank at home, since water is [virtually] incompressible, the amount of water in the water tank does not depend upon the water pressure, but only the geometry of the tank. The tank is a place to store water and use at a later time when demand dictates.

It really is a great teaching tool. Have a look at this site, they do a good job.
http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html

Have you not come across this analogy?

Ground = reservoir
Battery = water pump
Wires = pipes
Large resistor - low current - like a showerhead
Low resistance - high current - like a toilet
Water Tank - capacitor
Resistance is proportional to length and inversely proportional to surface area

I'll have to make a YouTube video on this someday.

Professor_Fink

FISMA wrote: »

I see what Plug is on about, it is a colloquial but powerful analogy.

Like it or not, it works wonders to help new students understand circuits.
...

Also, I absolutely love the water circuit analogy. Within 40 minutes I can have students understanding:
...

I guess I'm not going to win anyone over here, but I absolutely hate physics-by-analogy. I honestly don't think people really learn anything meaningful from it (though it may certainly be a good tool for getting students to remember tuff for tests). The reason I say this is that is because it gives them a get out clause for thinking about the actual system. Instead of thinking, "if electrons are negatively charged, and there is a gradient ..." they start thinking "Well if water=electrons and gradient=hill..., and water goes down hill...". This works fine for remembering things, but you can't use it to reliably solve an EM problem using it. For example, it is far from clear that the Hall effect makes any sense without adding a new purpose built part to the analogy. The same can be said for the thermal behaviour of resistence, or anything else that hasn't been given a specific rule in the analogy, but which can be determined by reasoning about the underlying physics.

Again, this isn't to say its not useful for getting students through exams (I suspect it's pretty good for this) but I honestly don't believe it fosters real understanding. The reason I'm getting worked up about this is that I spend so much of my life dealing with people who think they understand some particular area of physics, because they have heard of an analogy (like the rubber sheet analogy for GR). It's extremely painful trying to get beyond the analogy.

himnextdoor

Professor_Fink wrote: »

I guess I'm not going to win anyone over here, but I absolutely hate physics-by-analogy. I honestly don't think people really learn anything meaningful from it (though it may certainly be a good tool for getting students to remember tuff for tests). The reason I say this is that is because it gives them a get out clause for thinking about the actual system. Instead of thinking, "if electrons are negatively charged, and there is a gradient ..." they start thinking "Well if water=electrons and gradient=hill..., and water goes down hill...". This works fine for remembering things, but you can't use it to reliably solve an EM problem using it. For example, it is far from clear that the Hall effect makes any sense without adding a new purpose built part to the analogy. The same can be said for the thermal behaviour of resistence, or anything else that hasn't been given a specific rule in the analogy, but which can be determined by reasoning about the underlying physics.

Again, this isn't to say its not useful for getting students through exams (I suspect it's pretty good for this) but I honestly don't believe it fosters real understanding. The reason I'm getting worked up about this is that I spend so much of my life dealing with people who think they understand some particular area of physics, because they have heard of an analogy (like the rubber sheet analogy for GR). It's extremely painful trying to get beyond the analogy.

Ah well, on the upside, if a student fails as an electronics engineer, at least he/she would have a good chance of becoming a plumber.

SOL

Okay not to get too far off topic, but that video is absoloute nonsense...

There main "expert" is some guy who works in an "effects company" that uses tesla coils to do special effects...

it is just awful...

It's basically all that is wrong with modern documentaries...

Professor_Fink

SOL wrote: »

Okay not to get too far off topic, but that video is absoloute nonsense...

I didn't bother watching it

FISMA

Professor_Fink wrote: »

but I absolutely hate physics-by-analogy. I honestly don't think people really learn anything meaningful from it (though it may certainly be a good tool for getting students to remember tuff for tests)...

... but I honestly don't believe it fosters real understanding. The reason I'm getting worked up about this is that I spend so much of my life dealing with people who think they understand some particular area of physics, because they have heard of an analogy (like the rubber sheet analogy for GR). It's extremely painful trying to get beyond the analogy.

I am not sure if we are in total agreement or disagreement. I think the analogy bit is causing confusion. I think our audience is different, I appear to be speaking of analogies in the classroom between students and teachers, you appear to be speaking of analogies and conversations between laymen and Physicists. Correct? Close?

If what you are trying to express is a dislike for "dumbing-down" the content in favor of analogies without proper supporting material, math, theories, and all, then I agree. No one is suggesting that analogies take the place of proper Physics. Conversely, I dislike the opposite route - total mathematics and abstraction without any analogy.

If what you are saying is that analogies have no place in the classroom, I could not disagree more. I think it is the duty of a teacher to demonstrate that Physics is not some bizarre, unintelligible language that relies heavily on mathematics and abstractions. Instructors that do so are at least in part the reason why Physics classes are so small.

I once had a student tell me that Physics was math in words. I was actually offended but happy as it really helped me see the over-reliance that some instructors have on math. It is far easier to show mathematically how something is true, false, or exclusive than to come up with a real world example to do so.

Physics is the study of the natural world and using analogies to relate the abstractions to real world phenomena is good class room practice as long as the students understand the constraints therein.

A one minute analogy from the real world can tie up more loose ends than hours of rigorous mathematical proofs.

One of the reason upper level Physics courses are so scarcely populated is because students lose interest in the early years and because too many instructors rely heavily on mathematical abstractions and lack the ability to demonstrate how/where the laws of the natural world reveal themselves.

Keeping students interested and connected is essential. A student disconnect is not always an instructors fault, but it can be.

Also, it is important to have students believe they understand. Sometimes having an instructor that is able to put a high level Physical abstraction into more manageable terms, such as the water circuit analogy analogy can ease student apprehension and allow learning. Such analogies can break down self imposed mental hurdles and allow students to overcome their fear of learning.

For example, my students currently happily believe that an electron is an extremely small ball of negative charge that has no internal structure, and has quantum numbers like spin.

Should I ruin their world by teaching the mathematics and theory behind: quantizations, excitations, fermion fields, or doublets? Maybe some day:D, But not now.

Perhaps, analogies are like water - you need them to survive, but too much can cause dilution, you may get sick, or even die!

(oops, no pun on the water - circuit analogy)!

Slan

kowloon

This is why I chose Mechanical. Electricity is arcane trickery and it's only a matter of time before everyone realises this and has you all burnt at the stake. And before you get all smart on me; my netbook works through an intricate system of cams and pushrods.

Fringe

Professor_Fink wrote: »

Again, this isn't to say its not useful for getting students through exams (I suspect it's pretty good for this) but I honestly don't believe it fosters real understanding. The reason I'm getting worked up about this is that I spend so much of my life dealing with people who think they understand some particular area of physics, because they have heard of an analogy (like the rubber sheet analogy for GR). It's extremely painful trying to get beyond the analogy.

I hate that analogy as well. Mass will move like a ball on a rubber sheet. But the ball on the rubber sheet moves because of gravity... You can't use the thing itself as an analogy.

FarmerGreen

I think Einstein dumbed it down with the E=MC squared,
true enough mostly, but it hardly explains everything.

Professor_Fink

FarmerGreen wrote: »

I think Einstein dumbed it down with the E=MC squared,
true enough mostly, but it hardly explains everything.

That's because the correct equation is E^2 = p^2 c^2 + m^2 c^4. E=m c^2 is a special case.

Professor_Fink

FISMA: What level are your students?

FISMA

Professor_Fink wrote: »

That's because the correct equation is E^2 = p^2 c^2 + m^2 c^4. E=m c^2 is a special case.

... and the end result of a proof that is anything but dumbed down.

kowloon

Half the task is keeping the student interested, using analogies makes that much easier for secondary teachers.
At third level they're expected to motivate themselves.
If teachers were to start using methods that bored students I guarantee the numbers and grades would drop through the floor.
The standard reaction to these things is to dumb it down and liven it up to get the students to sign up.

Professor_Fink

kowloon wrote: »

The standard reaction to these things is to dumb it down and liven it up to get the students to sign up.

The main problem with this is that standards have dropped through the floor in recent years. A lot of students starting university these days are extremely ill-equipped for their course. It's a problem that is getting worse over time.

himnextdoor

kowloon wrote: »

The standard reaction to these things is to dumb it down and liven it up to get the students to sign up.

I'm not sure that simplifying a concept is the same as dumbing it down.

Simple analogies invite genuine interest in deeper questions - they can form a 'stepping-stone' to knowledge; knowledge that they can pass on to others and thus reinforce it in themselves.

In my opinion, unless one wishes to maintain a mystical air to a particular subject, which is simply a form of snobbery, then analogies are one of the most powerful teaching-aids in an instructor's arsenal.

Anonymo

himnextdoor wrote: »

I'm not sure that simplifying a concept is the same as dumbing it down.

Simple analogies invite genuine interest in deeper questions - they can form a 'stepping-stone' to knowledge; knowledge that they can pass on to others and thus reinforce it in themselves.

In my opinion, unless one wishes to maintain a mystical air to a particular subject, which is simply a form of snobbery, then analogies are one of the most powerful teaching-aids in an instructor's arsenal.

I'd agree with this. Professor Fink, I understand your point but it's a little too extreme to suggest that analogies have no place in teaching physics (possibly not your intention). Many of the things we learn we do so by reference to what we already know. So long as an analogy is indicated as such and not pushed too far I think this is a logical way to teach.

Professor_Fink wrote: »

The main problem with this is that standards have dropped through the floor in recent years. A lot of students starting university these days are extremely ill-equipped for their course. It's a problem that is getting worse over time.

This is a separate point to the usefulness of analogies, though has been conflated with it here. Yes standards at second level are dropping and it is ridiculous that they are. Kids today have so many resources and are much more tech savvy. With maths and science becoming more technical it makes little or no sense to dumb things down. If instead better use of computational resources as an aid to understanding were made then so much more could be taught with better depth.

FISMA

I think we are all in agreement here, just looking at the subject from different points of view.

I believe that Professor Fink does not like Physics by analogies, but I am confident the Professor uses analogies when appropriate.

I think the Professor is probably speaking to a higher level of Physics than I. If you are teaching advanced courses, then analogies may be dangerous.

For example, in teaching Quantum Mechanics, it could be dangerous, and inappropriate, to use the large scale world we visually see to make analogies.

This kind of analogy may give a false sense of security and the totally incorrect picture of the phenomena. In this respect, advanced students may do more harm by learning analogies than the fundamental physical situation under study.

I doubt that the Professor would have any problem in the analogies I use at the lower levels to help students understand physical abstractions. Analogies that are physically correct and accurate. There's nothing that needs to be un-learned at a later date.

The Professor's argument seems to be saying don't teach something that is incorrect and gives the illusion of understanding instead of teaching the actual phenomena.

Professor_Fink

Anonymo wrote: »

This is a separate point to the usefulness of analogies, though has been conflated with it here. Yes standards at second level are dropping and it is ridiculous that they are. Kids today have so many resources and are much more tech savvy. With maths and science becoming more technical it makes little or no sense to dumb things down. If instead better use of computational resources as an aid to understanding were made then so much more could be taught with better depth.

My remark on that was not linked to the rest of the conversation. It was indeed a seperate point, and I'm not sure where you are suggesting the two distinct points have been conflated.

Professor_Fink

FISMA wrote: »

I think we are all in agreement here, just looking at the subject from different points of view.

I believe that Professor Fink does not like Physics by analogies, but I am confident the Professor uses analogies when appropriate.

I think the Professor is probably speaking to a higher level of Physics than I. If you are teaching advanced courses, then analogies may be dangerous.

Actually, I don't think I do use analogies. But that is hardly surprising, as at university the way physics is thought is often quite different than at second level.

My point was simply that I don't think analogies are good for helping people understand physics, though they can have many other advantages: making subjects more accessible by making them seem less daunting, helping students to remember material, etc. So I am not suggesting that analogies aren't a great tool for primary and secondary level teaching, they may well be. I am simply suggesting that they do not necessarily help with understanding what is really happening, but this is not really the goal of most second level curricula anyway so I don't see the conflict.

It's annoying to me not because it isn't a good teaching tool, but because analogies often make people think they understand something which they do not in fact understand, and this translated into teaching problems for me as well as me getting lots of crackpot emails.

himnextdoor

Professor_Fink wrote: »

It's annoying to me not because it isn't a good teaching tool, but because analogies often make people think they understand something which they do not in fact understand, and this translated into teaching problems for me as well as me getting lots of crackpot emails.

Of course, one should always keep in mind that electricity and water do not mix.

Anonymo

Professor_Fink wrote: »

My remark on that was not linked to the rest of the conversation. It was indeed a seperate point, and I'm not sure where you are suggesting the two distinct points have been conflated.

The points are clearly mixed in together - the suggestion appears to be that, because standards have dropped, it is necessary to use analogies to teach topics that the students may not be prepared for. From your other emails you seem to suggest that crackpot emails are as a result of explanation through analogies. Crackpots will always exist. Taking the viewpoint that science should not be explained in simpler terms (while highlighting where the analogy may break down) because some people will jump on the simplification doesn't appear to me to be a reasonable position.

digme

Just wanted to say thanks to all of you who have posted on this thread.I went away and found
MIT lectures online and have learned a lot since then.
I agree fully with Professor_Fink last post. Most teachers are terrible
at communicating things and uncovering things and so need to use analogies.

himnextdoor

digme wrote: »

Just wanted to say thanks to all of you who have posted on this thread.I went away and found
MIT lectures online and have learned a lot since then.
I agree fully with Professor_Fink last post. Most teachers are terrible
at communicating things and uncovering things and so need to use analogies.

A teacher uses analogies to help students to learn and so from all your teachers, from whom you have learned, YOU'RE WELCOME.

*sigh... Why do we bother?*

Professor_Fink

himnextdoor wrote: »

A teacher uses analogies to help students to learn and so from all your teachers, from whom you have learned, YOU'RE WELCOME.

*sigh... Why do we bother?*

In fairness, I remember 'learning' from one of my teachers that we stuck to the earth because it was spinning. On the other hand, it was a primary school teacher.

Not that I have any animousity towards teachers. I had some great ones to whom I am very grateful, but are a few bad ones out there too.

himnextdoor

Professor_Fink wrote: »

In fairness, I remember 'learning' from one of my teachers that we stuck to the earth because it was spinning. On the other hand, it was a primary school teacher.

Not that I have any animousity towards teachers. I had some great ones to whom I am very grateful, but are a few bad ones out there too.

Well, suppose I put it like this: If the existence of the universe could be condensed into one 'simple' law, then everything in existence would be analogous to everything else in existence. Even with four fundamental forces, a lot of things in existence are going to be like a lot of other things in existence.

Professor_Fink

himnextdoor wrote: »

Well, suppose I put it like this: If the existence of the universe could be condensed into one 'simple' law, then everything in existence would be analogous to everything else in existence. Even with four fundamental forces, a lot of things in existence are going to be like a lot of other things in existence.

Have we started going back over a previous part of the conversation? The best we have for 'one simple law' is the standard model Lagrangian, which takes an entire page to write down, and that doesnt include the basic quantum mechanics you need to calculate anything physical from it. But that said, there are certainly somethings which are analogous to other things, but it doesn't mean that for every complex concept there is a much simpler analogous system where the analogy holds up to any level of scrutiny.

himnextdoor

Professor_Fink wrote: »

Have we started going back over a previous part of the conversation? The best we have for 'one simple law' is the standard model Lagrangian, which takes an entire page to write down, and that doesnt include the basic quantum mechanics you need to calculate anything physical from it. But that said, there are certainly somethings which are analogous to other things, but it doesn't mean that for every complex concept there is a much simpler analogous system where the analogy holds up to any level of scrutiny.

LOL. Of course not.

All I am saying is that if you understand why an apple falls from tree to ground then you are halfway to understanding potential difference, temperature gradients, atmospheric pressure, osmotic pressure, alpha-, beta-, gamma-radiation, etc.

digme

himnextdoor wrote: »

A teacher uses analogies to help students to learn and so from all your teachers, from whom you have learned, YOU'RE WELCOME.

*sigh... Why do we bother?*

Why do you bother? You get paid like any other job.Learn?You mean regurgitate don't you?

im invisible

Professor_Fink wrote: »

The best we have for 'one simple law' is the standard model Lagrangian, which takes an entire page to write down,

I wanna get that on a T-Shirt,^^

Professor_Fink

himnextdoor wrote: »

All I am saying is that if you understand why an apple falls from tree to ground then you are halfway to understanding potential difference, temperature gradients, atmospheric pressure, osmotic pressure, alpha-, beta-, gamma-radiation, etc.

I notice this seems to be turning a little anti-teacher, and that certainly wasn't my intention. However, I would be very interesting to hear how having a basic understanding of gravity helps you with understanding the strong and weak interactions responsible for nuclear decay. They are moderated by massive exchange particles, which makes them entirely different to gravity or EM. This isn't meant as criticism, I simply don't see a good analogy, so would be interested to hear if you have one.

himnextdoor

Professor_Fink wrote: »

I notice this seems to be turning a little anti-teacher, and that certainly wasn't my intention. However, I would be very interesting to hear how having a basic understanding of gravity helps you with understanding the strong and weak interactions responsible for nuclear decay. They are moderated by massive exchange particles, which makes them entirely different to gravity or EM. This isn't meant as criticism, I simply don't see a good analogy, so would be interested to hear if you have one.

Well, here goes.

The load of the apples represents nuclear instability, an internal tension, and when an apple hits the ground it represents an atomic nucleus in a lower energy state and the tree 'relaxes' a little which represents, dare I say it, increased entropy. Gravity, of course, represents an energy gradient, high to low and not low to high. The wind in the branches causes oscillations and this dynamic can represent vacuum fluctuations. When a particular apple will fall is unpredictable but there is a rate on average at which apples will fall.

I hope you can accept that this is a little tongue in cheek.

I really didn't mean to imply that the whole of existence can be understood in terms of falling apples but I'm glad I did.

(p.s. I'm not a teacher. I don't get paid for helping people to (mis)understand.)

Professor_Fink

himnextdoor wrote: »

Well, here goes...

Ah I see, so your not actually trying to explain the forces.

(p.s. I'm not a teacher. I don't get paid for helping people to (mis)understand.)

Ok. The "from all your teachers" line led me to believe you were. Sorry for the confusion.