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| how exactly resistor works |
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| james_s:
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. |
| tester43:
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. |
| rstofer:
--- Quote from: tester43 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 --- End quote --- 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 --- Quote ---2. but, current above the limit of resistor will be changed into heat --- End quote --- 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. --- Quote ---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. --- End quote --- 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/ |
| IanB:
Unfortunately, you do not have this correct yet. --- Quote from: tester43 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 --- End quote --- 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. --- Quote ---2. but, current above the limit of resistor will be changed into heat --- End quote --- 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. --- Quote ---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). --- End quote --- 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.) --- Quote ---4. To understand p3 above, I need to read on material conductivity and how Ohm's Law was formulated. --- End quote --- 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. |
| rstofer:
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. |
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