Electronics > Beginners
how exactly resistor works
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ArthurDent:
djacobow: “Finally, let me just state for the record that the OP's original question "how, exactly a resistor works" was absolutely an invitation for a nitpicky explanation. To borrow from your analogy above, the OP literally asked "how, exactly, do airplanes work?" and the answer he got was "you buy a ticket and it takes you where you want to go."”

If we want to get nitpicky, the OP DID NOT ask "how, exactly a resistor works". That is not a quote, although you presented it as a quote. A quote is exactly what the person said, like the following.

•   is the resistor a way to radiate overflow of energy as heat (wasted energy) -  and that's how books are explaining it.
•   is the resistor like a narrow tube between two large tubes limiting the water flow (no wasted energy).
Which is is true?”
That is a quote, preserving any spelling or grammar the OP used .

To wrongly equate your question of "how, exactly, do airplanes work?" (which can easily be explained by difference in pressure on the bottom/top of the wing and only takes a couple of short sentences) with “how do I hire someone to use their airplane to get me where I want to go”  is entirely different and is a non sequitur.

To answer your question of “"how, exactly, do airplanes work?" with an answer like you need to go to college and get an advanced degree in aeronautical  engineering then work for years to learn how to apply the theoretical knowledge to practical applications would not be responsive.

So as to how resistors work the answer is: They will follow Ohm’s law until someone proves that for all practical applications it’s been repealed.
tester43:
soooo :)
Maybe it would be better to rephrase.
If I would take a battery... LiPo 3,7V 500mAh battery. Fully charged is 4,2V - nominal working value 3,7V.
Then I am adding load: resistor 470Ohm to battery terminals.
From Ohm's Suggestion ( :-DD ) we can estimate the current going through R to be around 8mA.
500/8 gives around 60Hours of battery power (without other factors typical for battery).

using the same battery recharged with resistor 47Ohm gives current around 80mA.
500/80 gives around 6 hours of battery life.

<here comes the problematic part - do not hate please :) >
Call me stupid but the way of my thinking is: if battery would be pushing constant current through resistor (by increasing voltage or other method if exists) then battery life would be always the same, ignoring the value of resistance. It's like battery would say: "i'm giving 1A no matter what - I want it to be over in 30 minutes" - this would lead to putting the resistor on fire. But it does not happen. It means that resistor is in fact stopping battery from pumping the whole current it could deliver. It limits the current flow. But then why Heat? So the question is: HOW resistor is limiting the current?
IanB:
This is two questions.


--- Quote from: tester43 on September 29, 2018, 10:39:56 pm ---But then why Heat?
--- End quote ---

Why heat? Because when the battery is charged it contains energy. If you discharge the battery through a resistor you are letting the energy out of the battery and the energy has to go somewhere. Heat is a form of energy. The chemical energy in the battery gets turned into heat energy in the resistor.


--- Quote ---So the question is: HOW resistor is limiting the current?

--- End quote ---

Because resistors offer resistance to current flow. Ohm's Law says that if the resistance is higher for the same voltage, then the current is lower in inverse proportion. How does the resistor limit the current? Because it is a material property of the substance that resistors are made of. If you put a voltage across a resistor, then more resistance gives less current and less resistance gives more current. This is just what resistors do.
rstofer:

--- Quote from: tester43 on September 29, 2018, 10:39:56 pm ---Call me stupid but the way of my thinking is: if battery would be pushing constant current through resistor (by increasing voltage or other method if exists) then battery life would be always the same, ignoring the value of resistance. It's like battery would say: "i'm giving 1A no matter what - I want it to be over in 30 minutes" - this would lead to putting the resistor on fire. But it does not happen. It means that resistor is in fact stopping battery from pumping the whole current it could deliver. It limits the current flow. But then why Heat? So the question is: HOW resistor is limiting the current?

--- End quote ---

A battery can be considered a voltage source (as opposed to a current source, more in a moment).  It produces a certain voltage E and it produces a voltage drop across the one and only resistor which, obviously, drops all of the battery voltage.  Now, we know E across the resistor, and we know R, the value of the resistor.  Now all we need to do is drop the values in E=I*R and crunch.

Say we had a 1V battery (assume perfect voltage source) and we had a 1 Ohm resistor.  From the E=I*R equation, we can figure that 1A is flowing.  Furthermore, the resistor is dissipating I2*R Watts - in this case, 1 Watt.

Now use a 1K Ohm resistor.  From E = I * R we get 1 mA of current and I2*R = 1 mW

There is a pretty dramatic change in power dissipation.

Low value resistors -> higher current for a given voltage -> higher power dissipation for a given voltage.

Current sources are a different animal - they want to deliver a specific amount of current and they will change their voltage to make it happen.  Think about what happens when a 1A current source is left with no load.  The voltage rises to near infinity and everything burns to the ground.  Something like that...

Voltage sources will deliver any required current and attempt to maintain a specific voltage.  At some point, the load is more than the source can deliver and voltage drops off.  Study Norton Equivalent Circuit for a battery and note that maximum power transfer occurs when the load resistance is the same as the source internal resistance.

That perfect 1V battery above, if it had a 1 Ohm internal resistance would only deliver 1/2V to the 1 Ohm load because you now have 2 Ohms total resistance. 1V = 1/2A * 2 Ohms  Try Excel and put in different load resistors, calculate the terminal voltage given the load resistance and internal resistance and then convert the load voltage and load current to Watts.  Extra credit:  Make a graph.
ArthurDent:
Just looking at your problem mathematically, you have ohm’s law (or more of a very strong suggestion) which says E=IR. if you have a battery of about 4VDC and assume the voltage is going to remain fairly constant, if one term on the right hand side of the equation goes up, the other term must go down for the equation to balance.  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.

A little generalization here but if the 4VDC battery has a 4AH capacity it means that with a 1A load it will discharge in 1 hour. In the second case the 4VDC battery at 0.01A will last 100 hours. Everything is mathematically related so using just a few simple formulas will allow you to calculate what you need to know.   

As to heat, the energy released into a resistor has to go somewhere, in some form. In a resistor it is heat, just like the resistor element in an electric space heater, in an electric car it is the power to propel the car. Basically (at our level) energy is not created or destroyed, it is only converted in form. The same holds true with charging your battery, the electrical power going into the battery is chemically converted and stored as potential energy (which is why its voltage is referred to as potential), ready to be used if you complete the circuit by putting a resistor across the battery.

Ohm's law is like religion, you just have to believe it is true.
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