Author Topic: how exactly resistor works  (Read 7973 times)

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Offline james_s

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Re: how exactly resistor works
« Reply #50 on: September 30, 2018, 06:35:50 pm »
Don't get too caught up in analogies, they are useful for helping one to visualize a concept on a very simple level. A water analogy is useful in grasping basic electrical concepts because water flowing through a pipe behaves in a roughly similar way to electricity flowing through a wire except it's not really the same. It's helpful because you can see/feel/touch water and you can't really do that with electricity but beyond that it's just a simple analogy, it's not perfect.
 

Offline tester43Topic starter

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Re: how exactly resistor works
« Reply #51 on: September 30, 2018, 10:12:40 pm »
Hi,

Thank you everybody for this very active conversation.
I stopped at the conclusion that:
1. current is limited by resistor to the value described by the Ohm's Law
2. but, current above the limit of resistor will be changed into heat
3. I do not understand why if current over limit is changed into heat then why "battery" life is different for each resistor from my example ( and I know "because different current" - I am talking about heat generated).
4. To understand p3 above, I need to read on material conductivity and how Ohm's Law was formulated.
 

Offline rstofer

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Re: how exactly resistor works
« Reply #52 on: September 30, 2018, 10:39:33 pm »
Hi,

Thank you everybody for this very active conversation.
I stopped at the conclusion that:
1. current is limited by resistor to the value described by the Ohm's Law
I'm not sure I care for the idea that current is limited unless you add 'for a given voltage'.  The resistor drops a voltage based on the current flowing through the resistor, nothing more, nothing less.  The resistor itself may not be the only component in a circuit and, therefore, other components impact the current flow.

I don't think I would use the term limit.  The resistor simply doesn't do that all by itself.  Think a little harder about E=I*R and the other permutations: R=E/I or I=E/R

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2. but, current above the limit of resistor will be changed into heat

No!  Every single bit of voltage dropped across the resistor is turned to heat.  If you want to think in current, every ampere through the resistor causes heat.  P=I*E, P=E2/R, P=I2R.  P is in Watts, E is in Volts, I is in Amps and R is in Ohms.

There's no such thing as a limit.  You are adding a discontinuity or asymptote (a limit, some kind of flattening in the graph of current versus voltage) to a linear relationship.  There simply  isn't some magic 'limit'.  Ohm's Law is a linear, continuous function for all practical purposes.

E=I*R for all reasonable values of the variables.  Obviously there are other design considerations like flash-over voltage and maximum allowable dissipation.  That's why there are resistors with different dissipation ratings.  1/8W, 1/4W, 1/2W...100W and so on.

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3. I do not understand why if current over limit is changed into heat then why "battery" life is different for each resistor from my example ( and I know "because different current" - I am talking about heat generated).
4. To understand p3 above, I need to read on material conductivity and how Ohm's Law was formulated.

The battery is NOT a perfect source.  If it is rated, say, 1 Ah, that doesn't mean that it can provide 3600 Amps for 1 second.  1 hour = 3600 seconds.  There is a very non-linear graph of output current versus depletion time.  If you take the current out at a high value, the charge life will be a lot shorter and the Ah number doesn't apply.  Usually the rate is given for some current and/or some time.  It might actually be capable of providing 0.1A for 10 hours.

Read the second paragraph here:
https://www.allaboutcircuits.com/textbook/direct-current/chpt-11/battery-ratings/

« Last Edit: October 01, 2018, 12:20:47 am by rstofer »
 

Offline IanB

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Re: how exactly resistor works
« Reply #53 on: September 30, 2018, 10:39:46 pm »
Unfortunately, you do not have this correct yet.

Hi,

Thank you everybody for this very active conversation.
I stopped at the conclusion that:
1. current is limited by resistor to the value described by the Ohm's Law

The current is not limited, it is resisted. The current cannot be less than the value described by Ohm's Law, nor can it be greater. The current is always exactly equal to the value given by Ohm's Law.

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2. but, current above the limit of resistor will be changed into heat

There is no current "above the limit" of the resistor (see above).

Every time current flows through a resistor heat is generated. This is a fundamental property of resistors. There are no exceptions.

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3. I do not understand why if current over limit is changed into heat then why "battery" life is different for each resistor from my example ( and I know "because different current" - I am talking about heat generated).

There is no "current over limit". The current is exactly what Ohm's Law says it shall be. Also (for an ideal battery) the battery life is not different for different resistors. If the battery is "perfect" than the capacity of the battery in milliampere-hours and heat generated will be the same for every resistor. (Note that real batteries are not perfect, but for suitably high value resistors this statement becomes approximately true.)

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4. To understand p3 above, I need to read on material conductivity and how Ohm's Law was formulated.

Some more reading will help. Not only Ohm's Law, but also concepts like conservation of energy, electrical power, and conversion of energy from one form to another.
 

Offline rstofer

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Re: how exactly resistor works
« Reply #54 on: September 30, 2018, 11:52:51 pm »
This electronics game is all math!  You simply can't do one darn thing without using, at least, Ohm's Law and its permutations.  The math goes all up hill from there.

To see the equation E=I*R or P=I2*R doesn't leave an intuitive feeling about what is going on.  It's only when you plug in sample values and solve for the unknowns that any real familiarity occurs.  Hence homework...

Take a piece of paper and draw a horizontal line about half way down the page - the is the X axis and for this project it will represent current through a resistor in amps.  Draw a tick at the left end of the line and 5 more ticks to the right, equally spaced.  Then label the ticks 0..5, left to right.

Now draw a vertical line on the left end of the line going up the page and draw a tick at the intersection and up through 10, equally spaced.  This is the Y axis and represents voltage dropped by our resistor.

Assume a resistor value of 1 Ohm (EE textbooks like simple numbers, so do I).  With 0 Amps flowing through a 1 Ohm resistor, it will drop 0 Volts according to E=I*R.  This is at the intersection of the two axes at the lower left corner.

Then assume 1 Amp on the X axis and crunching through the equation you come up with 1V so draw a dot at X=1 and Y=1 (current on X axis is 1 and voltage on Y axis is also 1).  Continue up through 5 Amps on X axis.

Connect the 6 dots with a straight line and that graph shows how a 1 Ohm resistor reacts to various currents in the range of 0..5V.  Look at 1.5 Amps (half way between X=1 and X=2) and you should get a voltage of 1.5V - exactly what the equation says you should get for a 1 Ohm resistor.  Just draw a line from 1.5 on the X axis up until it intersects the graph and then horizontal until it intersects the Y axis where you should be midway between 1 and 2 or 1.5V.

Now, repeat the process with a 2 Ohm resistor.  The maximum 5 Amps will now produce 10 Volts and the line will be twice as steep as the first line.  If you look at 1.5 Amps, you should get 3 Volts.

Repeat the entire process for different resistors until Ohm's Law is part of your DNA.  It should be as automatic as breathing because it will be used just about as often when playing with electronics.

Graphs!  Engineers can't discuss much of anything without drawing a picture.  Some of the great inventions started as a doodle on a napkin at some after-work party.  Most of my career in electrical and project management revolved around napkin plans.  I made a sketch, the contractor built the project.  Pretty simple.

Consider graphing power dissipation.  Excel should become your new best friend.  Charts and graphs, that's what this game is about.  BTW, plotting power dissipation is hard because you have E on one axis and I on another and you want a point in space equal to the product.  What you wind up with for, say, a 1 watt value is a line of the various combinations of E and I that result in a given value of P - the line itself.  There will be similar lines for other dissipations.  You will see these kinds of graphs in datasheets.  A family of curves, if you will.

 

Offline ArthurDent

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Re: how exactly resistor works
« Reply #55 on: September 30, 2018, 11:54:33 pm »
tester43 “1. current is limited by resistor to the value described by the Ohm's Law”

No, that isn’t what everyone has been telling you over and over again. Ohm’s law is an equation that explains proportional relationships between terms, there is no limit involved and it is baffling why you won’t accept this fact.
 
tester43 “2. but, current above the limit of resistor will be changed into heat”

Very wrong! This has also has been explained to you multiple times. Again many others and I have explained this to you. Way back in post #39 I said:
“If you use a 4 ohm resistor the current has to be 1A and if you use a 400 ohm resistor the current will be 0.01A. the power that the resistor has to dissipate as heat is given by the formula of P=EI so in the first case you have 4VDC x 1A or 4 watts. In the 2nd case you have 4VDC x 0.01A or 0.04 watts. In the 1st case, dissipating 4W as heat means the resistor will get far hotter than the 2nd case where you will be dissipating 0.04 watts. In the 2nd case you won’t be able to tell by feel that the resistor is ever so slightly warmer than the ambient air temperature.

There is NO LIMIT above which heat is produced in a resistor but below which there is no heat. You are the only one who is misrepresenting what is being said. ALL energy in a resistor is given off as heat. Just because you don’t have the equipment or knowledge to measure the amount of heat produced at lower levels doesn’t mean it isn’t there. 

In your first post you misrepresented what you claim to have read in books: “is the resistor a way to radiate overflow of energy as heat (wasted energy) -  and that's how books are explaining it.” No book would explain heat radiated by a resistor as “overflow” and heat from a resistor is sometimes exactly the product you are looking for as with a toaster, stove or electric heater and not wasted at all.

My conclusion is that everyone here has been very patient explaining to you multiple times in multiple ways the correct interpretation of Ohm’s law and you admit reading books which I’m sure said the exact same thing. Because your RESISTANCE to take any sound advice has LIMITED your ability to understand these concepts, I’ll leave you on your own. Good luck.
« Last Edit: September 30, 2018, 11:57:18 pm by ArthurDent »
 

Offline a59d1

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Re: how exactly resistor works
« Reply #56 on: September 30, 2018, 11:55:05 pm »
They also have kind of an aura or field emanating from them.

 :palm:
 

Offline ArthurDent

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Re: how exactly resistor works
« Reply #57 on: October 01, 2018, 12:04:23 am »
They also have kind of an aura or field emanating from them.

 :palm:

I assumed you knew that was sarcastic although the glow of a space heater or light from a incandescent light bulb does appear to be an aura.  ;)
 
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Offline Brumby

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Re: how exactly resistor works
« Reply #58 on: October 01, 2018, 01:04:20 am »
To the OP:

You seem to have a fixation about the limit of a current - and I would like to address three "limits" that are associated with resistors...

Voltage limit:  This is where excessive voltage (more than it was designed to handle) across the resistor is so high, that materials in the resistor break down and sparks can jump.  Keep the voltages below this limit for the resistor to perform its job without damage.
Power limit:  This is where the power being dissipated by the resistor is higher than it is capable of physically withstanding and the materials will overheat.  Keep the power dissipation below this limit for the resistor to perform its job without damage.
When a resistor is used within its specifications, neither of these limits play any part in how the resistor operates, so please feel free to ignore them.
Current limit:  This is a simple mathematical calculation where, for a given resistance, the current flowing through the resistor is a function of the voltage available - Ohm's Law.  For any circuit with this resistor and voltage, there will be a maximum current that can flow through the resistor.  When that circuit contains ONLY the resistor, then the current will be what comes out of Ohm's Law.
Where this can change is where you have an additional component in series with the resistor.
Take an LED, for example.  For an LED with a Vf (forward voltage drop) of 2V, in series with a resistor in a circuit which has 5V available, the resistor has 3V across it.  Using that 3V, apply Ohm's Law and you will get the current that will flow.  Since you will not get any more current than this flowing, it could be classified as a maximum current, which implies it is a limit - but it is only a limit for this particular circuit.  Without a resistor, the above LED connected directly to the 5V supply would draw excessive amounts of current and would soon bake itself into oblivion.  IN THIS ROLE, the resistor performs the function of limiting the current to a value defined by Ohm's Law - and is often called a "current limiting resistor".

Such "current limiting" is a function of the resistance of the resistor AND the voltage applied.

I will also place emphasis on the fact that a resistor is, for all intents and purposes, LINEAR.  Put the smallest of currents through it and it WILL dissipate some heat.  There is NO magic point where things suddenly change ... that's the sort of thing LEDs will do - but not resistors.
 

Online CatalinaWOW

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Re: how exactly resistor works
« Reply #59 on: October 01, 2018, 02:58:21 am »
Physical examples seem to speak to you more than even simple mathematics.

One example of using resistors to dissipate small amounts of heat is the arrays of resistors used to generate artificial infrared images for testing infrared imaging systems.  In order to generate real scenes they have to be able to generate continuously varying temperatures (no large steps in temperature between adjacent resistors) and to test good imagers they need to adjust temperatures by small fractions of a degree Centigrade.  The voltages across each resistor are adjusted to give the appropriate current to dissipate the power required to raised that resistors temperature by the appropriate amount. 

Even this simple (in concept) device requires a fair amount of math to operate properly.  The power dissipated by the resistor is not linear with voltage (P=E^2/R) and the temperature rise is not linear with power having terms proportional to temperature difference with nearby objects and other terms proportional to the differences of the fourth power of the temperatures involved.
 

Offline rfeecs

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Re: how exactly resistor works
« Reply #60 on: October 01, 2018, 04:01:00 am »
I'm not sure it's helpful at this point, but here's another model for how a resistor works, the Drude model:
https://en.wikipedia.org/wiki/Drude_model

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The model, which is an application of kinetic theory, assumes that the microscopic behavior of electrons in a solid may be treated classically and looks much like a pinball machine, with a sea of constantly jittering electrons bouncing and re-bouncing off heavier, relatively immobile positive ions.
The electrons are accelerated by the electric field, but keep bumping into the fixed atoms in the material, so that limits their net velocity.  They transfer energy in the process, which warms up the material.

This is not 100% accurate, but basically works, especially with some adjustments.  Ohm's law falls right out of this model.
 

Offline Brumby

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Re: how exactly resistor works
« Reply #61 on: October 01, 2018, 04:58:14 am »
That's the same concept as my cricket ball and bat - except there are lots of balls and bats.
 


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