Why Does Mike Holt say heaters use high resistance

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ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
This is one of the few forums I know of where one will encounter multiple longwinded posts discussing simple algebraic concepts. :D

V = IR
P = IV = I^2R = V^2/R
 

aarena

Member
Location
morristown, nj
150110-2032 EST

Time limit.

Edited version.

150110-1614 EST

aarena:

When you provide a reference to something specific that I should look at, as you did in your first post, then provide sufficient information that I can easily find the point where the information is located. Reference to a many page document is much too broad. I think I found your reference location at paragraph 1.18 .

Components that are very good conductors have a low resistance between two measurement points. You need to understand that resistance and conductance are relative measurements when trying to compare different materials and applications. Some materials with relatively low resistance for a given mechanical shape are: silver, copper, aluminum, gold, iron, and stainless steel.

In the "Handbook of Chemistry and Physics". 40th Edition 1958-1959 starting at page 2587 is a table of Resistivity for various materials. This probably can be found in the Index in any edition under "Resistivity of metals".

Code:
All the following are * 10^-6 ohm-cm

Silver 99.98%        at  0 C     =   1.468
Copper hard drawn    at 20 C     =   1.77
Gold pure drawn      at 20 C     =   2.44
Iron 99.98%          at 20 C     =  10
Mercury liquid       at 20 C     =  98.5
Mercury solid        at -183.5 C =   6.97
Nichrome             at 20 C     = 100
Steel vanadium       at 20 C     = 121

Graphite             at  0 C     = 800

See http://en.wikipedia.org/wiki/Electrical_resistivity_and_conductivity note that the units are different so the exponent changes.

What Mike was saying is that material for heater heating elements is of a higher resistivity than the material in wiring conductors because they serve two different purposes. For a heater you want high resistivity material to minimize size and cost, and in distribution wiring you want low resistivity material to reduce mass, cost, and wasted energy.

Suppose you have a perfect source voltage of 100 V that does not vary with a change in load current. Assume copper wire resistance is constant with temperature, and that #14 copper wire has a resistance of 2.5 ohms per 1000 ft. Connect 1000 ft to the 100 V source and the current is 40 A, and the power dissipated is 4000 W. Change the material to nichrome and the resistance is about 100/1.77 = 56.5 times greater, or 2.5 * 56.5 = 141 ohms. Now the current is only 0.71 A, and the power dissipated is 71 W. To make the nichrome #14 wire produce 1500 W we need to shorten it by 1500/71 = 21.1 times. This becomes a length of 1000/21.1 = 47.4 ft. Because of the physical characteristics of nichrome wire we can reduce its diameter to increase the resistance per foot and make a practical 1500 W heater with only a few feet of wire.

On the other hand we don't want to waste power in the distribution of energy to the heater so low resistance copper wire is used for the conductors from the voltage source to the heater. Silver would be better, but it is much more scarce and in turn higher in cost. For the 1500 W heater you might use #12 instead of #14 for distribution copper wiring.

In the 1870s Edison understood that to build a practical power distribution system that the loads needed to be in parallel and the distribution of energy needed to be at high voltage and have a low source impedance. Many theoretical people at the time wanted to operate in a maximum power transfer mode.

However, today in a solar system you do want to operate in a maximum power mode. This is a different criteria. As an excerise determine what value of source resistance will produce maximum power transfer to a fixed resistance load.

.

I will be sure to include the reference in the future. Thank you for the detailed response. I think it is starting to make sense....if you are constrained on the mass of your resistive element and the current of your source (50ft of #14 copper would require 960 amps), a higher resistivity is required.
 

Besoeker

Senior Member
Location
UK
This is one of the few forums I know of where one will encounter multiple longwinded posts discussing simple algebraic concepts. :D

V = IR
P = IV = I^2R = V^2/R
Kinda saddens me that some of the very basic concepts are not understood.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
I will be sure to include the reference in the future. Thank you for the detailed response. I think it is starting to make sense....if you are constrained on the mass of your resistive element and the current of your source (50ft of #14 copper would require 960 amps), a higher resistivity is required.
A good summary with one exception: it is not that the mass of the element is constrained.
Given a fixed amount of metal you can get any resistance you want by shaping it correctly. By drawing it out finer and longer you increase the resistance up to the point where the wire no longer holds its shape or you run out of space.
One of the advantages of nichrome wire, beyond its high resistivity is that it is stiffer and stronger than copper, making for a more stable and durable element. Especially at high temperature.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
Let's go back a bit...

voltage = 120
...
resistor 2 = 5ohm
...
current for resistor 2 = 120/5 = 24
...
power for resistor 2 = 24^2*5 = 2,880

Let's say your 24A heater element is 2 feet long. That's 2.5ohms per foot.

For this circuit you need #10 wire. The resistance of #10 wire is 3.9 ohms per thousand feet, or 0.0039 ohms per foot, according to Table 9.

The resistance of the heater element is 641 times higher than copper wire. Does that help you understand Mr. Holt's point?
 

ActionDave

Chief Moderator
Staff member
Location
Durango, CO, 10 h 20 min from the winged horses.
Occupation
Licensed Electrician
This is one of the few forums I know of where one will encounter multiple longwinded posts discussing simple algebraic concepts. :D

V = IR
P = IV = I^2R = V^2/R

Kinda saddens me that some of the very basic concepts are not understood.
As one who has to work hard to understand anything dealing with math beyond 2+2 and as one who has no formal training in electrical theory I see things differently.

If I did not have access to this forum I know I would not know near as much as I know now.

I asked obvious questions when I joined. Obvious questions and long winded answers drew me here and keep me coming back.
 

Besoeker

Senior Member
Location
UK
As one who has to work hard to understand anything dealing with math beyond 2+2 and as one who has no formal training in electrical theory I see things differently.

If I did not have access to this forum I know I would not know near as much as I know now.

I asked obvious questions when I joined. Obvious questions and long winded answers drew me here and keep me coming back.
Fair points, I agree. And I didn't mean it as a downer on the OP.
I maybe come at it from a different perspective having had a different background.
Maybe.

I went to school, regular school that is, until I was about seventeen. This was prior to any formal education on electrical matters.
In regular school I was taught enough physics to know about Ohm's Law. Maybe I was just lucky and had a good physics teacher.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Fair points, I agree. And I didn't mean it as a downer on the OP.
I maybe come at it from a different perspective having had a different background.
Maybe.

I went to school, regular school that is, until I was about seventeen. This was prior to any formal education on electrical matters.
In regular school I was taught enough physics to know about Ohm's Law. Maybe I was just lucky and had a good physics teacher.
Likewise, I didn't mean to cast any aspersions. In the past, I myself have contributed to a significant number of the "longwinded" threads.

Everything is simple when you understand it and a mystery when you don't.
 

GoldDigger

Moderator
Staff member
Location
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Occupation
Retired PV System Designer
Likewise, I didn't mean to cast any aspersions. In the past, I myself have contributed to a significant number of the "longwinded" threads.

Everything is simple when you understand it and a mystery when you don't.
And trying to provide an explanation without the ability to wave and point, and more importantly to see when the other guy's face is screwing up or his eyes are glazing over is hard. :)
 

Besoeker

Senior Member
Location
UK
Likewise, I didn't mean to cast any aspersions. In the past, I myself have contributed to a significant number of the "longwinded" threads.
I think there is merit in keeping posts and explanations as simple as possible. Precise, concise, and unambiguous.....neat.
But achieving that often requires skill and effort.

In the words of my late, great Friend, Tommy:
"I wanted to write you a short letter but I didn't have time so here's a long one."
Perhaps more profound that it at first appears.

Everything is simple when you understand it and a mystery when you don't.
Yes, quite. It just sometimes surprises me that, on an electrical forum, some basics are not known.
 

PetrosA

Senior Member
Yes, quite. It just sometimes surprises me that, on an electrical forum, some basics are not known.

I both agree with you and sympathize with those who don't "get" it. I'm smart. I speak two languages fluently, I can play moderately complicated music on the piano (for lack of practice, not ability...), I intuitively understand a lot of electrical theory, I can fix almost anything you put in front of me and supposedly my IQ is above 140, but the math gives me problems (and so does reading sheet music). Even simple math is difficult for me. I was helping my 6-year old daughter today and had to check 100 subtraction problems and the numbers were swimming in front of my eyes. I know this is simple stuff, but my brain has never adapted to it and I have to work very hard to actually work through any of the formulae I see here and some of them are just completely beyond my ability.
 

Besoeker

Senior Member
Location
UK
I both agree with you and sympathize with those who don't "get" it. I'm smart. I speak two languages fluently, I can play moderately complicated music on the piano (for lack of practice, not ability...), I intuitively understand a lot of electrical theory, I can fix almost anything you put in front of me and supposedly my IQ is above 140, but the math gives me problems (and so does reading sheet music). Even simple math is difficult for me. I was helping my 6-year old daughter today and had to check 100 subtraction problems and the numbers were swimming in front of my eyes. I know this is simple stuff, but my brain has never adapted to it and I have to work very hard to actually work through any of the formulae I see here and some of them are just completely beyond my ability.

I understand all of that - I think......
I was taught music and played violin in an orchestra so I learned to read music - just one of those things.
Mathematics, I know, is a bit of a b?te noire for many. I think that this may be partly because it is perceived by many as a difficult, even esoteric subject and that turns them off in the first place. Perhaps even to the point of not trying.

I see something of a parallel between reading sheet music and mathematics, particularly algebra. Both are written in symbols. I think you get used to the patterns, particularly the common ones.
For example, if you see a chord written CEG, you know that it's C major and what it will sound like. How? I guess the trite answer is I just do, it just is - familiarity.

Something the same with electrical formulas.
v=iR
Again, familiarity. It just is what it is. Explaining why is another matter.
 

templdl

Senior Member
Location
Wisconsin
I understand all of that - I think......
I was taught music and played violin in an orchestra so I learned to read music - just one of those things.
Mathematics, I know, is a bit of a b?te noire for many. I think that this may be partly because it is perceived by many as a difficult, even esoteric subject and that turns them off in the first place. Perhaps even to the point of not trying.

I see something of a parallel between reading sheet music and mathematics, particularly algebra. Both are written in symbols. I think you get used to the patterns, particularly the common ones.
For example, if you see a chord written CEG, you know that it's C major and what it will sound like. How? I guess the trite answer is I just do, it just is - familiarity.

Something the same with electrical formulas.
v=iR
Again, familiarity. It just is what it is. Explaining why is another matter.
That's why went through an expaination of ohms law as to how voltage resistance and current are related before relating that to watts law resulting in power in watts in my previous post
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
I both agree with you and sympathize with those who don't "get" it. I'm smart. I speak two languages fluently, I can play moderately complicated music on the piano (for lack of practice, not ability...), I intuitively understand a lot of electrical theory, I can fix almost anything you put in front of me and supposedly my IQ is above 140, but the math gives me problems (and so does reading sheet music). Even simple math is difficult for me. I was helping my 6-year old daughter today and had to check 100 subtraction problems and the numbers were swimming in front of my eyes. I know this is simple stuff, but my brain has never adapted to it and I have to work very hard to actually work through any of the formulae I see here and some of them are just completely beyond my ability.
Your problem with symbolic math and arithmetic may be a perceptual problem analogous to dyslexia. And, like dyslexia, it may best be compensated for by specifically targeted exercises rather than just "practice." The learning methods used by the majority simply may not work well for you.
How are you at spoken arithmetic problems?
Or do you deal with those by visualizing them?
 

PetrosA

Senior Member
Your problem with symbolic math and arithmetic may be a perceptual problem analogous to dyslexia. And, like dyslexia, it may best be compensated for by specifically targeted exercises rather than just "practice." The learning methods used by the majority simply may not work well for you.
How are you at spoken arithmetic problems?
Or do you deal with those by visualizing them?

I'll have to think about the written vs. spoken arithmetic comprehension. I never thought about it before so I can't answer the question. I'm definitely going to look into mathematical dyslexia in the meantime.
 

PetrosA

Senior Member
I understand all of that - I think......
I was taught music and played violin in an orchestra so I learned to read music - just one of those things.
Mathematics, I know, is a bit of a b?te noire for many. I think that this may be partly because it is perceived by many as a difficult, even esoteric subject and that turns them off in the first place. Perhaps even to the point of not trying.

I see something of a parallel between reading sheet music and mathematics, particularly algebra. Both are written in symbols. I think you get used to the patterns, particularly the common ones.
For example, if you see a chord written CEG, you know that it's C major and what it will sound like. How? I guess the trite answer is I just do, it just is - familiarity.

Something the same with electrical formulas.
v=iR
Again, familiarity. It just is what it is. Explaining why is another matter.

Part of my math problem was that my teachers in primary school had already been told I was "gifted" so I didn't get much help with the basics until about 4th grade thanks to a stern old Mennonite teacher. With music, I was about six years into the piano lessons before my teacher realized I hadn't learned to read music. I had been playing everything by ear. I'm able to transpose pretty much any piece I can play into any other key, but don't try to get me to read sheet music for even a church hymn. It's just not happening.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Part of my math problem was that my teachers in primary school had already been told I was "gifted" so I didn't get much help with the basics until about 4th grade thanks to a stern old Mennonite teacher. With music, I was about six years into the piano lessons before my teacher realized I hadn't learned to read music. I had been playing everything by ear. I'm able to transpose pretty much any piece I can play into any other key, but don't try to get me to read sheet music for even a church hymn. It's just not happening.
I, too, was determined to be "gifted" as a child. It was both a blessing and a curse.
 
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