Yes. And every person that is in the first few pages of an electronics text and just learning what a resistor is and about Ohm's Law and about power in a resistor should simultaneously be expected to deal with power derating.

Never knew that a requirement for posting here was to assume everybody was just learning Ohm's law.

It actually is very important to teach newbies that ratings have conditions associated with them. Leads to less shrapnel.

 

Never knew that a requirement for posting here was to assume everybody was just learning Ohm's law.

It actually is very important to teach newbies that ratings have conditions associated with them. Leads to less shrapnel.

Hey, I'm sold.

Every newbie should be expected to learn derating methods as part of their first exposure to resistors. They should also be expected to deal with the different kinds of resistors and the effects at different frequencies from that first lesson.

Every introductory physics course should likewise present static and dynamic friction, including the effects of temperature on various materials plus air resistance as part of the first lecture on Newton's laws. After all, it's very important to teach newbies that physical objects experience frictional forces and have other conditions associated with them.

This whole notion of letting students start with simple basics and then layer on additional complexity as they become profient at the previous level is outdated.

 

It doesn't take a very complex circuit to put more than 1/4 W of power through a 1K resistor powered by a 15V battery.

Place the resistor across a large inductor. Connect the battery, disconnect the battery. Poof!

Bob

 

It doesn't take a very complex circuit to put more than 1/4 W of power through a 1K resistor powered by a 15V battery.

Place the resistor across a large inductor. Connect the battery, disconnect the battery. Poof!

Bob

A 1/4 W resistor can dissipate much more than 1/4 W in a transient event. There's a maximum peak voltage or current rating, which is almost never given, and a thermal model that would allow you to determine the temperature during a transient event and compare that to the absolute max temperature. But neither of those are readily available, either.

And a good exercise is to consider what the constraints would need to be in terms of the resistor, inductor, and series resistance of the inductor given a thermal model of the resistor and max instantaneous temperature.

But it is not a good exercise for someone that is just getting the point implied by the question at hand.

 

Yikes! You guys really know how to make a mountain out of a mole hill.

What's wrong with P = V^2/R

 

Yikes! You guys really know how to make a mountain out of a mole hill.

What's wrong with P = V^2/R

I agree!

Title= very simple question

And then: exceed the power rating of 1/4 watt resistor

Don't believe op asked to keep temperature under 1000 deg.

 

What do you use for V, considering that you must handle any voltage that can be created in any circuit powered by a 15V battery?

Bob

 

Yikes! You guys really know how to make a mountain out of a mole hill.

What's wrong with P = V^2/R

Apparently... everything.

 

I'm on the side of those saying this is a badly phrased question.

Beginners can't be expected to know that a circuit can be built that will produce an increased voltage from a battery, but it is wrong to state that such a circuit is not possible.

 

I'm on the side of those saying this is a badly phrased question.

Beginners can't be expected to know that a circuit can be built that will produce an increased voltage from a battery, but it is wrong to state that such a circuit is not possible.

But that basically means that every question has to be caveated with two pages of all of the possible things that could potentially need to be taken into account.

 

Not at all, something as simple as this would do:

Show that it is not possible to exceed the power rating of 1/4 watt resistor of resistance greater than 1k that is connected across the terminals of a 15 volt battery.

 

Every question is contextual. But why make something more difficult than it needs to be?

"Everything should be made as simple as possible, but not simpler." - Albert Einstein

 

Not at all, something as simple as this would do:

Show that it is not possible to exceed the power rating of 1/4 watt resistor of resistance greater than 1k that is connected across the terminals of a 15 volt battery.

Op question was more clearly posed. imho
By prefacing that it was a VERY SIMPLE question.

A 15 volt battery could reach higher voltage during charging. No mention of de-rating, ambient, or thermal resistance.

It's as easy to nit pick the nit pickers as the op.

" This is getting silly. Next sketch."
M.P.F.C.

 

What do you use for V, considering that you must handle any voltage that can be created in any circuit powered by a 15V battery?

Bob

May win me over yet!

The more I analyze the original question the fuzzier it becomes.

Like when you think about a common word too much, it just sound silly.

 

Not at all, something as simple as this would do:

Show that it is not possible to exceed the power rating of 1/4 watt resistor of resistance greater than 1k that is connected across the terminals of a 15 volt battery.

Ah, but what if the battery is fully charged and has an actual terminal voltage in excess of the nominal battery voltage? After all, how many 12V car batteries are more like 13.8V when fully charged.

No matter how you phrase it, someone is going to maintain that it doesn't quite cover this situation or that situation. The never-ending debate over the Monty Hall problem demonstrates that!

Besides, this wording is almost certainly too narrow for what the question is trying to get at. The entire point is almost certainly for students to recognize, on their own, what the limiting case is. As you've phrased it, you've given them the limiting case as the starting point and they don't have to recognize it as such at all.

 

May win me over yet!

The more I analyze the original question the fuzzier it becomes.

Like when you think about a common word too much, it just sound silly.

The question should be evaluated within the context of both the material it is a part of and the audience it is aimed at. It was stated early on, though not in the original post, that this is from the first few pages of AoE. Hence, the material at that point is in the context of a world that consists of DC voltage sources and ideal resistors. The notion of power and power ratings on resistors was probably just introduced. Prior to that, any resistor could carry any current whatsoever. They've just moved out of that world into one in which resistors have power limitations. Soon they will move into the next world that has both capabilities and limitations compared to where they are now. As we learn more and more, we are constantly finding that we can do things, like generate higher voltages, that we couldn't do in our prior world. Similarly, we are constantly learning about new considerations, such as thermal effects and skin effects and you name it, that were not part of our prior world and so they weren't mentioned. People that are learning things should be allowed to focus on a small handful of new concepts at a time and not be inundated by all of the things that they will encounter down the road but that they aren't ready to tackle today.

It's the approach that we almost always take with anything. When teaching someone to drive and it's a bright sunny day and they are working a clutch for the first time, do we make sure that they are aware that how you are going to have them do it right now to get the car moving in an open parking lot is different from how they will need to do it on hills or in the snow or on ice, or in mud? No. The usual approach is to let them think that how you are showing them to do it now is the one and only way to do it. You let them focus on the new skill in front of them without risking them getting confused about the six other possibilities you've inundated them with. Once they get the hang of that down, then you take them to a hill and explain that what they've learned up to the point has limitations and here's one of them, so let's now deal with this one.

 

Ah, but what if the battery is fully charged and has an actual terminal voltage in excess of the nominal battery voltage?

In my experience in the world of college exam questions, a "15V battery" would be taken to mean literally that - a battery that is at 15V at the moment the question is to be answered. This would be true UNLESS the question was specifically about batteries, nominal versus actual voltage, internal resistance and so forth. For an Ohm's law question like this one, a 15V battery is 15V. My 2¢.

The question is a bit vague but, as you have argued, it's good to keep it simple early on. I'll wager that most students encountering this question get it right, or at least get the premises right. That's the measure of a good textbook question; does it help the student absorb the point. This one does, in my opinion. It's a worthwhile concept that a high enough resistance drops current so much that power ratings "don't matter" beyond a point.

 

In my experience in the world of college exam questions, a "15V battery" would be taken to mean literally that - a battery that is at 15V at the moment the question is to be answered. This would be true UNLESS the question was specifically about batteries, nominal versus actual voltage, internal resistance and so forth. For an Ohm's law question like this one, a 15V battery is 15V. My 2¢.

The question is a bit vague but, as you have argued, it's good to keep it simple early on. I'll wager that most students encountering this question get it right, or at least get the premises right. That's the measure of a good textbook question; does it help the student absorb the point. This one does, in my opinion. It's a worthwhile concept that a high enough resistance drops current so much that power ratings "don't matter" beyond a point.

I completely agree -- and I think you have phrased it rather well, here.

 

So, just to be sure I understand. The concern is that someone might get the impression that a 1/4 W resistor can always safely dissipate 1/4 W. So, by pointing out that a 15V-battery powered circuit can be designed so as to apply more than 15V to a 1k? resistor and thereby raise its power dissipation above its 1/4 W rating will ensure that no one will get the impression that a 1/4 W resistor can always dissipate 1/4 W safely.
...

Please let me clarify. I know you spend a lot of time and effort in the homework forum, and I respect that. You have obviously treated this exactly like a homework question (which most of us here also suspect it was).

I am not arguing with your position of answering this as a homework question, my point was to ADD to that, bringing some new information which IS relevant in this section of the forum where beginners interested in the abilities of a "1/4 watt resistor" may have searched and found this thread to learn what their resistor can safely do.

In that context, the point that a 1/4 watt resistor cannot be relied upon to dissipate 1/4 watt in every situation is a valid point and may be of benefit.

Interesting. I don't quite see the reasoning, but interesting chain of logic.
...

I don't think you can always follow my reasoning. Mastering logic is a slightly different skill set than being an educator.

 

Hey, I'm more than willing to learn.

So please explain how giving an example of how it is possible to EXCEED the 1/4 W rating of a resistor somehow gets the point across that the resistor can't always be relied up to be safely operate even when it is NOT exceeding the 1/4 W rating?

Note that I am not stating that it CAN always dissipate 1/4 W and do so safely, merely that I don't see how an example that shows it's rating being exceeded makes the point that it can't always do so.