- Apr 20, 2008
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What determines the amount of heat given off by a resistor?
- Thread starter TurtleCrusher
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- Apr 20, 2008
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Barfo
Lifer
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PieIsAwesome
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i would say both A and B would be acceptable based on the wording of the question
A because the calculation for power (which is translated as heat) is P = I*I*R
B because the size of the resistor dissipates different amount of heat based on their package size. ie a larger cap will feel colder than a smaller cap with the same amount of power going through it
A because the calculation for power (which is translated as heat) is P = I*I*R
B because the size of the resistor dissipates different amount of heat based on their package size. ie a larger cap will feel colder than a smaller cap with the same amount of power going through it
DrPizza
Administrator Elite Member Goat Whisperer
Shouldn't your question read "which of the following is the largest factor affecting the..."? A 10 ohm resistor connected to a 9V battery is going to give off a lot more heat than either a 5 ohm or a 20ohm resistor connected to a 1.5 volt battery, given that all three are connected for the same amount of time.
If two resistors with the same resistance are connected to the same voltage, then the one that's connected for the most time will give off the most heat.
And, I don't care about any of your four poll choices. By changing the amount of time, I can make time more important than any of your choices.
And, someone in here referred to I²R. That's power. Power is the rate at which heat would be given off, not the amount of heat given off.
If two resistors with the same resistance are connected to the same voltage, then the one that's connected for the most time will give off the most heat.
And, I don't care about any of your four poll choices. By changing the amount of time, I can make time more important than any of your choices.
And, someone in here referred to I²R. That's power. Power is the rate at which heat would be given off, not the amount of heat given off.
DrPizza
Administrator Elite Member Goat Whisperer
Oh, this was a test question? Best answer: time. There's a huge difference between "heat given off" and "rate at which heat is given off." Poorly written question (imho.) But, all things else being equal, ohmic value.
I would say "ohmic value".
If it's the wrong value, I'll just leave it in the box and it won't give off ANY heat, whereas if it's the right value, I'll put it in the circuit and it has a chance to give off heat once I finish wiring everything up.
If it's the wrong value, I'll just leave it in the box and it won't give off ANY heat, whereas if it's the right value, I'll put it in the circuit and it has a chance to give off heat once I finish wiring everything up.
- Apr 20, 2008
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To give the correct answer: Size!
However I completely disagree. Ohmic value is for sure it. If there is little to no resistance, less heat is produced, regardless of size. Resistance goes up? Way more heat.
The question should have been asked about heat dissipation, but I digress...
However I completely disagree. Ohmic value is for sure it. If there is little to no resistance, less heat is produced, regardless of size. Resistance goes up? Way more heat.
The question should have been asked about heat dissipation, but I digress...
This has to be a question from a physics class. This is the only explanation I can think of for the horrible wording of the question and for the "correct" answer.
is this HS or college?
is this HS or college?
- Apr 20, 2008
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Main answer: VERY poorly worded question. As given, NONE of the answers are correct, and the best one can do it give a "best" answer - meaning, the least wrong.
"Amount of heat given off ... " I assume actually meant to ask the heat output RATE, or WATTS of heat being given off. Actually, Watts x time gives the real "amount of heat", but I really expect the questioner thought in terms of heating rate.
IF we are asked about heat rate, or Watts, the formula for Power in Watts is P = V x I. We are not given any V or I values, or even an option to give one of those as an answer! If voltage V is connected across a Resistor R, then the P = V^2/R. Or, if one applies whatever voltage across a Resistor R to achieve a current flow of I amps, P=I^2R. So, the Power (heating RATE) is determined by any TWO of: Voltage, Current, Resistance. None of these choices is available for an answer.
Among the answers available, which is "best"? Well, neither Tolerance nor Temperature Coefficient has any direct impact on the heat output rate. Resistance alone does not determine that, either. BUT, from another perspective, the amount of heat that a resistor can give off WITHOUT breaking down due to overheating is determined mainly by its physical size. A larger unit but with the same Resistance value can work at larger voltages and currents without overheating.
One clue to the "best" answer is not in the question - it's where the question was asked: in a Military Technical School test. From the very practical perspective of an electronics tech working on equipment, "where does the most heat comes from" as you examine a circuit board relates to things like:
Where and how do we need to ensure good cooling?
What components are most likely to be hot enough to burn you when touched?
And the easy answer you can literally SEE as you work is: the LARGER resistors, because they have to be large to dissipate all the heat being generated without being destroyed themselves.
"Amount of heat given off ... " I assume actually meant to ask the heat output RATE, or WATTS of heat being given off. Actually, Watts x time gives the real "amount of heat", but I really expect the questioner thought in terms of heating rate.
IF we are asked about heat rate, or Watts, the formula for Power in Watts is P = V x I. We are not given any V or I values, or even an option to give one of those as an answer! If voltage V is connected across a Resistor R, then the P = V^2/R. Or, if one applies whatever voltage across a Resistor R to achieve a current flow of I amps, P=I^2R. So, the Power (heating RATE) is determined by any TWO of: Voltage, Current, Resistance. None of these choices is available for an answer.
Among the answers available, which is "best"? Well, neither Tolerance nor Temperature Coefficient has any direct impact on the heat output rate. Resistance alone does not determine that, either. BUT, from another perspective, the amount of heat that a resistor can give off WITHOUT breaking down due to overheating is determined mainly by its physical size. A larger unit but with the same Resistance value can work at larger voltages and currents without overheating.
One clue to the "best" answer is not in the question - it's where the question was asked: in a Military Technical School test. From the very practical perspective of an electronics tech working on equipment, "where does the most heat comes from" as you examine a circuit board relates to things like:
Where and how do we need to ensure good cooling?
What components are most likely to be hot enough to burn you when touched?
And the easy answer you can literally SEE as you work is: the LARGER resistors, because they have to be large to dissipate all the heat being generated without being destroyed themselves.
Last edited:
SKORPI0
Lifer
- Jan 18, 2000
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