when will they ever learn?

[peterboat]

A wiring diagram generally shows simply what is where.

 

Direction of current flow (+ve to -ve) is all you need to concern yourself with. Electron flow is only of relevance to physicists.

 

Tony

 

The direction of current flow is why we now have -ve earth cars and boats.

positive earthed cars were thought to corrode faster than negative earthed cars, personally i always thought it was down to lack of protection on the cars

[nicknorman]

No the amp is the internationally defined basic unit. (See Kaye and Laby). Charge is derived from it. AFAIR A is the convention for the dimension of current.

 

ETA but Wikipedia agrees with you, Nick. So I am now feeing all weepy and insecure.

 

N

I looked here:

 

https://www.physics.uoguelph.ca/tutorials/dimanaly/

 

just because it was the first thing I came across that gave the dimension for current (I) as opposed to the SI unit which of course is A.

 

However it does seem as though you are right in maintaining that current is the dimension, not charge. But personally I think that the world is wrong and I'm right, but not to worry it happens often!

[Loafer]

I looked here:

 

https://www.physics.uoguelph.ca/tutorials/dimanaly/

 

just because it was the first thing I came across that gave the dimension for current (I) as opposed to the SI unit which of course is A.

 

However it does seem as though you are right in maintaining that current is the dimension, not charge. But personally I think that the world is wrong and I'm right, but not to worry it happens often!

 

I had 'charge' down as the basic unit, which is a large crowd of electrons (with a fixed number of them) waiting to be driven around by a voltage innit?

 

Edit to add a '

Edited February 21, 2016 by Loafer

[nicknorman]

I had 'charge' down as the basic unit, which is a large crowd of electrons (with a fixed number of them) waiting to be driven around by a voltage innit?

 

Edit to add a '

Thank you for joining my one man club, it's us against the world (as usual).

 

Yes if you have something that is the rate at which some other thing moves, the something is a derivative (wrt time), the other thing is the fundamental thing that is moving.

 

I suppose we can't be too suprised, all those mutually back-patting academic scientists can't even merge relativity with quantum mechanics (which means both are wrong) and so if they can't even get that right, what hope is there for them to understand whether charge or current is the fundamental thing?

[Loafer]

Thank you for joining my one man club, it's us against the world (as usual).

 

Yes if you have something that is the rate at which some other thing moves, the something is a derivative (wrt time), the other thing is the fundamental thing that is moving.

 

I suppose we can't be too suprised, all those mutually back-patting academic scientists can't even merge relativity with quantum mechanics (which means both are wrong) and so if they can't even get that right, what hope is there for them to understand whether charge or current is the fundamental thing?

 

I thought Wotever was signed up too?

 

In terms of miles per hour, I think the mile is the fundamental unit.

 

Coulombs per second being an Amp, then the coulomb must be the fundamental unit.

 

Well that's what I believe, and I'm too old to learn all over again!

[Murflynn]

 

I thought Wotever was signed up too?

 

In terms of miles per hour, I think the mile is the fundamental unit.

 

Coulombs per second being an Amp, then the coulomb must be the fundamental unit.

 

Well that's what I believe, and I'm too old to learn all over again!

miles and hours are no longer allowed at this level of technical discussion.

 

metre and second shall be used, but special dispensation may be allowed for using light-year, although it doesn't compute dimensionally.

 

[Loafer]

miles and hours are no longer allowed at this level of technical discussion.

 

metre and second shall be used, but special dispensation may be allowed for using light-year, although it doesn't compute dimensionally.

 

 

here's an amps per hour joke:

 

Police Officer: 'Madam, I've just clocked you at over 40 miles per hour and you're breaking the speed limit!'

 

Madam: 'I'm sorry Officer, I can't possibly have been. I only left my house half an hour ago!'

[dmr]

 

I had 'charge' down as the basic unit, which is a large crowd of electrons (with a fixed number of them) waiting to be driven around by a voltage innit?

 

Edit to add a '

 

So when did this Amp (or charge) stuff happen? I was taught the universe had only 3 dimensions (MLT) and not Amps.

 

Volts is not a fundamental unit, neither is power, and W=VI so why can't amps be reduced to MLT dimensions?

 

wow, its all gets a bit technical on CWDF sometimes.

 

...............Dave

[Batavia]

The fundamental units are;

Metre,

Kilogram;

Second

Ampere;

Candela;

Kelvin;

Mole.

 

To return to the use of "centimetre" for capacitance....

 

They may be centimetres, but not as we know them.

 

The bit missing from Nick's "derivation" of capacitance from the dimensions of capacitors is permittivity. The CGS system assumes that the permittivity of free space is 1 - but this 1 has dimensions of M^-1 L^-3 T⁴ A²

While the answer in centimeters is numerically sensible, it is dimensionally wrong to use normal centimetres.

 

Don't let's start on statvolts......

 

Chris G

 

 

[WotEver]

 

I thought Wotever was signed up too?

Nope, I'm working on my own equations based on kWhkGC/(M1L1T−3Θ−1)*T

 

Basically how much power will be consumed per kilogramme of food kept at a fixed temperature in a freezer dependant upon the ambient temperature and the thermal conductivity of the insulation...

 

Or maybe I've just made that up

 

Tony

[Murflynn]

doesn't it also depend on the level of greed of one (or more) of the fridge/freezer proprietors who replaces cold food with warm food without owning up?

Edited February 22, 2016 by Murflynn

[WotEver]

doesn't it also depend on the level of greed of one (or more) of the fridge/freezer proprietors who replaces cold food with warm food without owning up?

I can see I'll need to include a G in that equation somewhere. Not g of course, because that would be gravity and that's just silly.

 

Tony

[John V]

Though possibly applicable to the force exerted on the increased mass of the purloiner

[WotEver]

I'm going to have to give this some deeper thought...

[nicknorman]

The fundamental units are;

Metre,

Kilogram;

Second

Ampere;

Candela;

Kelvin;

Mole.

 

To return to the use of "centimetre" for capacitance....

 

They may be centimetres, but not as we know them.

 

The bit missing from Nick's "derivation" of capacitance from the dimensions of capacitors is permittivity. The CGS system assumes that the permittivity of free space is 1 - but this 1 has dimensions of M^-1 L^-3 T⁴ A²

While the answer in centimeters is numerically sensible, it is dimensionally wrong to use normal centimetres.

 

Don't let's start on statvolts......

 

Chris G

 

Careful, what you quote as "fundamental units" are in fact SI units. They are a convention, there is nothing fundamental about them. It is important to distinguish between units (SI or otherwise) and dimensions eg mass, length, time. The latter are the "fundamental" concept.

 

As to permittivity that is what I meant when I mentioned dielectric but yes the dielectric constant is just a factor applied to the permittivity of free space. I was trying not to get too bogged down!

 

As to your final point about cm, even though cm may not be the SI unit, it is a unit in common use and anyway I would disagree with your final phrase that it is "dimensionally wrong to use normal centimetres" since clearly it isn't - the correct dimension is length and cm is a measure of length - and anyway it begs the question "what is an abnormal centimetre?".

[MtB]

 

I thought Wotever was signed up too?

 

In terms of miles per hour, I think the mile is the fundamental unit.

 

Coulombs per second being an Amp, then the coulomb must be the fundamental unit.

 

Well that's what I believe, and I'm too old to learn all over again!

 

 

Not sure that follows, given that speed can also be expressed as hours per mile and current as seconds per couloumb...

[Loafer]

 

 

Not sure that follows, given that speed can also be expressed as hours per mile and current as seconds per couloumb...

 

 

Hmm. I'll have to wait for somebody to think about that one!

[nicknorman]

Not sure that follows, given that speed can also be expressed as hours per mile and current as seconds per couloumb...

No you are missing the point about dimensions. The dimensions of current is charge/second. What you are saying is charge / second (current) is another way of saying seconds/charge. One is just the reciprocal of the other, there is nothing fundamentally different dimensionally except the sign of the power of the dimensions which are swapped. Bottom line is that charge stands on is own, it only has one dimension. Current is a derivative of time (ie has 2 dimensions).

Hmm. I'll have to wait for somebody to think about that one!

Sorted!

[Batavia]

Perhaps I should have used the term Base Units, which are believed to be the smallest set of independent units required to describe/define all "properties" of "stuff". You need more than M. L & T to do this.

 

As to your final point about cm, even though cm may not be the SI unit, it is a unit in common use and anyway I would disagree with your final phrase that it is "dimensionally wrong to use normal centimetres" since clearly it isn't - the correct dimension is length and cm is a measure of length - and anyway it begs the question "what is an abnormal centimetre?".

I was just trying to make the point that using centimetres as a measure of capacitance is straying well away from the normal understanding of what a centimetre is - and the fact that is dimensionally wrong as well - see my previous post for what the dimensions of capacitance should be - when a centimetre has the dimension of only L! All the other complicated dimensional bits are embedded in the permittivity of free space, which you can't really ignore just because it has a value of 1.

 

Finally, a question - why Leyden Jars? I would have thought that a cylindrical arrangement (with access to both ends) or a stack of glass plates & conductors would have been much easier to make (even if flat glass was itself made from blown shapes). Was it to stop the galvanism from leaking out?

 

Chris G

[John V]

If....(as I seem to remember being taught)....the charge of a capacitor is stored by the distortion of atomic orbits in the dielectric does that mean that the plates of a capacitor with no physical dielectric (ie. in a pure vacuum) will not store a charge whatever the Pd between ?

[Batavia]

No, there will be a capacitance between two metal plates in a vacuum - but if you introduce a dielectric, the electric field (and hence the potential) will be reduced for a given charge due to polarisation of the dielectric.

 

I remember diagrams with pluses and minuses at the interface between the plate and the dielectric.

 

All this stuff is pretty unbelievable really.

 

Chris G

[Loafer]

No you are missing the point about dimensions. The dimensions of current is charge/second. What you are saying is charge / second (current) is another way of saying seconds/charge. One is just the reciprocal of the other, there is nothing fundamentally different dimensionally except the sign of the power of the dimensions which are swapped. Bottom line is that charge stands on is own, it only has one dimension. Current is a derivative of time (ie has 2 dimensions).

 

Sorted!

 

Knew I could rely on you!

[Theo]

The fundamental units are;

Metre,

Kilogram;

Second

Ampere;

Candela;

Kelvin;

Mole.

 

 

The bit missing from Nick's "derivation" of capacitance from the dimensions of capacitors is permittivity. The CGS system assumes that the permittivity of free space is 1 - but this 1 has dimensions of M^-1 L^-3 T⁴ A²

 

Chris G

 

 

So the units of permittivity are

 

cubic seconds seconds square Amperes per kilogramme per cubic metre. What a useful unit that is!

 

N

[Theo]

I had just written a lengthy post saying that I had capitualed and agree with Nick that charge should be the dimension rather than current for all the reasons that he stated.

 

Now in that case why should the Ampere be the funtamental unit?

 

The reason is that the Ampere is easy to measure in terms of the other fundamental units "One ampere is the constant current that will produce an attractive force of 2 × 10⁻⁷newtons per metre of length between two straight, parallel conductors of infinite length and negligible circular cross section placed one metre apart in a vacuum"

 

I got that from Wikipaedia. You can define the size of 1N in terms of kilogrammes, metres and seconds. Metres are defined, so the size of the ampere can be standardised.

 

The problem with the Coulomb as the fundamental unit is that it has to be defined in terms of the Ampere.

 

N

Spring is on the way and we will soon be too busy to bother with all this nonsense!

 

N

[Batavia]

 

Spring is on the way and we will soon be too busy to bother with all this nonsense!

 

N

Wishful thinking!

 

It took some of the finest minds of the 19th Century quite a while to figure this lot out, so I imagine that it will take us rather longer.

 

Chris G