Is a Coulomb dimensionless?

[James Meyer]

To refresh my memory (something I have to do more and more frequently)
in order to get a handle on MOSFET gate drive requirements, I did a Google
search for "coulomb". One of the hits defined the Coulomb as a dimensionless
number equal to 6.24 times 10 to the 18th power.

I didn't think that was quite right. I seem to remember that that
number of electrons is equal to a Coulomb but that doesn't make it
"dimensionless", does it?

Another definition for Coulomb is one Amp*second. That seems reasonable
but not dimensionless to my way of thinking.

Am I not understanding the definition of "dimensionless"?

Getting back to MOSFET gates, is there a good reason why Coulombs of
gate charge is a better or more easily worked with specification? About half of
the data sheets give a gate capacitance in (pico)Farads. Why are there two
standard ways to express the gate characteristics of a MOSFET?

Another thing I learned was that MOSFETs are sometimes ranked for
quality by the product of their saturation resistance and their gate charge.
Lower is better. I thought that was interesting. Now if they would just figure
in MTBF and price along with that, it would be really easy to pick the "best"
one.

Jim

 

[J. Coady]

Now don't take this as gospel, cause i'm sick as a dog today so I'm not
thinking properly, but if a coulomb is the number of electrons needed to
make a certain charge, then it would indeed be dimensionless. Just like
saying that 12 eggs make a dozen is also dimensionless. 12 is a dozen, but
it doesn't have to be a dozen eggs, it could be a dozen people etc, just a
different way of saying a certain number. Same for a coulomb. You could have
a coulomb of eggs.... i guess.. with a lot of hens..

 

[John Popelish]

To refresh my memory (something I have to do more and more frequently)
in order to get a handle on MOSFET gate drive requirements, I did a Google
search for "coulomb". One of the hits defined the Coulomb as a dimensionless
number equal to 6.24 times 10 to the 18th power.

I didn't think that was quite right. I seem to remember that that
number of electrons is equal to a Coulomb but that doesn't make it
"dimensionless", does it?

Another definition for Coulomb is one Amp*second. That seems reasonable
but not dimensionless to my way of thinking.

Am I not understanding the definition of "dimensionless"?

Getting back to MOSFET gates, is there a good reason why Coulombs of
gate charge is a better or more easily worked with specification? About half of
the data sheets give a gate capacitance in (pico)Farads. Why are there two
standard ways to express the gate characteristics of a MOSFET?

Another thing I learned was that MOSFETs are sometimes ranked for
quality by the product of their saturation resistance and their gate charge.
Lower is better. I thought that was interesting. Now if they would just figure
in MTBF and price along with that, it would be really easy to pick the "best"
one.

Jim

Something is dimension less if it is pure quantity without saying
quantity of what. 6 is dimension less. Pi is dimension less.
Avagadro's number is dimension less. So the count of electrons that
exhibit a coulomb of charge is dimension less, since it is a count, a
pure number. The charge those electrons carry is not dimension less,
since charge requires dimensions (descriptive labels) to define it.
An amp second is one way to dimension a coulomb of charge. Avagadro's
number times the charge (in amp seconds or some other equivalent
units) of one electron is another.

 

[[email protected]]

Your doubts are well founded, a Coulomb is most definitely not
dimensionless. It is a unit of charge (the charge of the electron is
1.602E-19 Coulombs; incidentally, multiply that by the number you found in
that web site and see what you get).

A current is by definition a a flow of charge (charge per unit time), thus
an Ampere is a Coulomb/second.


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[James Meyer]

Now don't take this as gospel, cause i'm sick as a dog today so I'm not
thinking properly, but if a coulomb is the number of electrons needed to
make a certain charge, then it would indeed be dimensionless. Just like
saying that 12 eggs make a dozen is also dimensionless. 12 is a dozen, but
it doesn't have to be a dozen eggs, it could be a dozen people etc, just a
different way of saying a certain number. Same for a coulomb. You could have
a coulomb of eggs.... i guess.. with a lot of hens..

OK, that makes sense. Except that a Coulomb is also an Amp*second.

Hmmmmmm... Perhaps it's that a Coulomb OF CHARGE is an Amp*second and a
Coulomb of eggs would be the worlds largest omelet.

Jim

 

[Roy McCammon]

To refresh my memory (something I have to do more and more frequently)
in order to get a handle on MOSFET gate drive requirements, I did a Google
search for "coulomb". One of the hits defined the Coulomb as a dimensionless
number equal to 6.24 times 10 to the 18th power.

I didn't think that was quite right. I seem to remember that that
number of electrons is equal to a Coulomb but that doesn't make it
"dimensionless", does it?

Jim,
I believe you could come up with a dimension system
in which charge was dimensionless, but in the MKS system
it has the dimensions of amp-seconds.

 

[James Meyer]

Something is dimension less if it is pure quantity without saying
quantity of what. 6 is dimension less. Pi is dimension less.
Avagadro's number is dimension less. So the count of electrons that
exhibit a coulomb of charge is dimension less, since it is a count, a
pure number. The charge those electrons carry is not dimension less,
since charge requires dimensions (descriptive labels) to define it.
An amp second is one way to dimension a coulomb of charge. Avagadro's
number times the charge (in amp seconds or some other equivalent
units) of one electron is another.

I think I see now. It was the "of charge" part that I was overlooking.
You can have a Coulomb of anything. A Coulomb isn't exclusively an electrical
metric unless you say a Coulomb of charge.

Jim

 

[Winfield Hill]

James Meyer wrote...

Getting back to MOSFET gates, is there a good reason why Coulombs of
gate charge is a better or more easily worked with specification?
About half of the data sheets give a gate capacitance in picofarads.
Why are there two standard ways to express the gate characteristics
of a MOSFET?

It's because FET capacitances change with voltage, sometimes in a very
extreme way, by factors of 10x or more in the case of Crss (gate-drain
capacitance). The data-sheet values are at a specific voltage, say 25V,
which is not very useful in practise, except as a reference point for
various graphs, and for comparison to other FETs. Normally charge is
q = i*t = C * V. When C changes it becomes awkward to try to use this
equation. However, even as capacitance changes, charge is conserved.
This means that it may be better to just measure and specify the charge.
Actually, most MOSFETs specify both, which is useful. Another point,
Ciss, which is Cgs + Cgd, and is mostly gate capacitance, doesn't change
much with voltage. As a FET switches, the voltage across Cdg changes
dramatically, and the charge associated with Cgd becomes important.

Another thing I learned was that MOSFETs are sometimes ranked for
quality by the product of their saturation resistance and their gate
charge. Lower is better. I thought that was interesting. Now if
they would just figure in MTBF and price along with that, it would be
really easy to pick the "best" one.

Hahahaha. Yep, large FETs have lower Rds(on), but higher capacitance.
So if one parameter goes down at the expense of the other, the multiple
of the two will clearly be a figure of merit (FOM) for the process.

Thanks,
- Win

whill_at_picovolt-dot-com

 

[John Popelish]

I think I see now. It was the "of charge" part that I was overlooking.
You can have a Coulomb of anything. A Coulomb isn't exclusively an electrical
metric unless you say a Coulomb of charge.

No, I think referring to a coulomb refers to charge.

 

[Spehro Pefhany]

No, I think referring to a coulomb refers to charge.

I agree.

http://www.bartleby.com/61/charts/M0182500.html


Best regards,
Spehro Pefhany

 

[Guy Macon]

No, I think referring to a coulomb refers to charge.

I just checked six dictoinaries, and they all say that a
coulomb refers to electric charge.

 

[Tom Del Rosso]

In John Popelish typed:

No, I think referring to a coulomb refers to charge.

That's what I thought too, but it sounded like you were saying the
opposite in the previous message.

 

[John Woodgate]

I read in sci.electronics.design that Roy McCammon

Jim,
I believe you could come up with a dimension system
in which charge was dimensionless, but in the MKS system
it has the dimensions of amp-seconds.

IIRC, that was the case for an early version of the CGS system.

 

[BFoelsch]

Oooh, no, a Coulomb is NOT a mole of electrons. Avogadro's number is 6.023 x
10e23. Off by a factor of almost 100,000.

 

[Roy McCammon]

I think I see now. It was the "of charge" part that I was overlooking.
You can have a Coulomb of anything. A Coulomb isn't exclusively an electrical
metric unless you say a Coulomb of charge.

Jim

I'm with John Popelish on this. Me thinks "Coulomb" has the
units of charge and is _NOT_ dimensionless. Its like Ohm.

 

[Fred Bloggs]

To refresh my memory (something I have to do more and more frequently)
in order to get a handle on MOSFET gate drive requirements, I did a Google
search for "coulomb". One of the hits defined the Coulomb as a dimensionless
number equal to 6.24 times 10 to the 18th power.

I didn't think that was quite right. I seem to remember that that
number of electrons is equal to a Coulomb but that doesn't make it
"dimensionless", does it?

It's not right- most idiots who post web tutorials on EM are retarded
swine who should take up basket-weaving.

 

[Fred Bloggs]

Something is dimension less if it is pure quantity without saying
quantity of what. 6 is dimension less. Pi is dimension less.
Avagadro's number is dimension less. So the count of electrons that
exhibit a coulomb of charge is dimension less, since it is a count, a
pure number.

I take issue with this generality because we do assign dimensions to
pure numbers- think of things like nepers, dB, and octave:_) These are
dimensions assigned to dimensionless entities.

 

[John Woodgate]

I read in sci.electronics.design that Fred Bloggs <[email protected]>

I take issue with this generality because we do assign dimensions to
pure numbers- think of things like nepers, dB, and octave:_) These are
dimensions assigned to dimensionless entities.

Those aren't dimensions, any more than 'kangaroo' is a dimension in '12
kangaroos'.

 

[Bob Stephens]

To refresh my memory (something I have to do more and more frequently)
in order to get a handle on MOSFET gate drive requirements, I did a Google
search for "coulomb". One of the hits defined the Coulomb as a dimensionless
number equal to 6.24 times 10 to the 18th power.

I didn't think that was quite right. I seem to remember that that
number of electrons is equal to a Coulomb but that doesn't make it
"dimensionless", does it?

Another definition for Coulomb is one Amp*second. That seems reasonable
but not dimensionless to my way of thinking.

Am I not understanding the definition of "dimensionless"?

Getting back to MOSFET gates, is there a good reason why Coulombs of
gate charge is a better or more easily worked with specification? About half of
the data sheets give a gate capacitance in (pico)Farads. Why are there two
standard ways to express the gate characteristics of a MOSFET?

Another thing I learned was that MOSFETs are sometimes ranked for
quality by the product of their saturation resistance and their gate charge.
Lower is better. I thought that was interesting. Now if they would just figure
in MTBF and price along with that, it would be really easy to pick the "best"
one.

Jim

Isn't the Coulomb unit itself a dimension?

 

[Glenn Ashmore]

I have found that almost any number that is named after somebody is
dimensionless.


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