Electronics question

Aero787 · 03-01-2007 01:07PM #1

I want to check my answers to this.

You are given a circuit with a non-ideal power supply, switch, ammeter, voltmeter, and an NTC resistor. The circuit is at room temp. When the switch is closed the temp of the NTC changes. V across and I through the NTC are measured at regular intervals until the temp of the NTC remains constant.

1) Is V higher at the start or at the end of the experiment?

2) Describe how you would determine the internal voltage and the internal resistance of the power supply.

3) Given graphs of V versus I, and Power versus Temp, how would you calculate the temp of the NTC at a point on the graph of V versus I?

4) Given graphs of V versus I, and Power versus Temp, how would you calculate the change in temp per sceond when its temp is X degrees C?

5) Finally, while the NTC is still connected, it is heated with a small lighter. If the lighter is removed when the temp of the NTC reaches 90 degrees C, what will the temp of the NTC be a long time later? Explain.

I need to check my answers for this. I'm sure it'll be easy for you guys!

Cheers!

Boston · 03-01-2007 01:19PM

A seriously now, this is your homework right for the leaving cert or first year?

1) I don't know what NTC means(can't remember), but normally resistance decreases as heat increases. Now remember ohm's law for voltage.

2) Use what you know to find what you don't. I.E. use a resister you know the value of, with a current you know the value of, compare the expected result for voltage to the result found. Internal resistance, I remember something called norton and thevlin(spelling?) theorems.

3) You're taking the piss. Temp -> power -> current * voltage.

4) Calculus.

5) Room temperature. How could it be anything else? The circuit is a room temp as outlined in the question.

This is leaving cert stuff.

Aero787 · 03-01-2007 04:32PM

So you'll give me abuse for typing in the whole question? Grow up. I know most parts are obvious, but I thought someone's answers (to one part in particular) might enlighten me on something. I have never dealt with internal voltage and internal resistance before. We haven't covered it in school. I don't think it's on the course. I know practically nothing about it.

Never mind, I worked it out for myself. It had nothing to do with Norton's and Thévenin's theorems.

Boston · 03-01-2007 04:53PM

I thought you said you wanted to check your answers? Caught in a lie I see.

From what I remember Thevenin’s & Norton's thereoms where used to develope equilvalent circuits from which internal voltages and resistances could to readily calculated. Just because you didn't need them doesn't mean you couldn't have used them.

Aero787 · 03-01-2007 05:01PM

I did want to check my answer relating to internal V and internal R. I didn't want to be spoonfed everything about how to do it. It turns out that I didn't need to.

Boston · 03-01-2007 05:09PM

Well I told you how to get the internal voltage. The "Thank you" must have been lost in all the bitching.

Aero787 · 03-01-2007 05:12PM

It had no relevance to the question. Good effort though!

Boston · 03-01-2007 05:31PM

If by no relavence, you mean valid answer. Some people just don't know they are born.

Aero787 · 03-01-2007 05:49PM

No, I mean no relevance. That didn't apply to the question.

Boston · 03-01-2007 05:55PM

I guess you're right, I mean you clearly know so much.

Aero787 · 03-01-2007 06:02PM

Why are you personalising the argument? I have the question in front of me, your solution wasn''t relevant, your post didn't provide any help. Why can't you accept that?

Boston · 03-01-2007 06:19PM

I can accept it. What could I tell someone who knows everything. The opinion of a first year is definitly bang on about my answers. Good work. You'll go far in life, I'm proud of you.

Aero787 · 03-01-2007 06:27PM

Again, well done on personalising the discussion. Is this what you do when you're fighting a losing battle because it seems like you done this before on other threads.

I obviously don't know everything, I never said anything of the sort. I didn't post all the details of the question, those theorems do not apply. I was hoping to gleam an idea from somebodies answers to the above, but I managed to learn about internal V and internal R through another means and apply that to a problem.

Boston · 03-01-2007 06:42PM

I fully accept that I didn't help you. But I was the only one to even try to help you and you're too damn ignorant to say thank you.

I've had enough of you.

Aero787 · 03-01-2007 06:58PM

It took you a while to accept it. I was working on a different problem, hoping that somebodies thoughts on those simple questions (simple apart from Q2) might give me a brain wave. Maybe if you hadn't insulted me when you replied I would have been more grateful. If you want to have the last word you can have it.

cast_iron · 03-01-2007 07:14PM

Aero787 wrote:

2) Describe how you would determine the internal voltage and the internal resistance of the power supply.

Aero787 wrote:

It had no relevance to the question.

Ok, I'm not getting into another childish spat, but the above is pretty clear. To argue otherwise defies belief.

gubbie · 04-01-2007 02:46AM

LiouVille wrote:

I thought you said you wanted to check your answers? Caught in a lie I see.

From what I remember Thevenin’s & Norton's thereoms where used to develope equilvalent circuits from which internal voltages and resistances could to readily calculated. Just because you didn't need them doesn't mean you couldn't have used them.

If he's a leaving cert student then he will never have come across either Norton or Thevenin, if 1st year then not Norton and just a bit of Thevenin... tho I do like his theorem

Boston · 04-01-2007 11:12AM

Sure he seems to know all about both since he told me they have no relavence to the question. I'm guessing at least a second year from that.

Aero787 · 04-01-2007 12:06PM

It wasn't a leaving cert question I was doing, and it wasn't the question in my first post. I studied Thevenin's Theorem (not in as much detail as elec heads) and I know how it could be applied to the idea of question 2 above, but it wasn't needed for what I was working on. It's a pretty cool theorem though. I'm sure I'll be called a know-it-all by you-know-who.

Mellor · 06-01-2007 08:28PM

LiouVille wrote:

5) Room temperature. How could it be anything else? The circuit is a room temp as outlined in the question.

This is leaving cert stuff.

I dont do any advanced engineering, and don't claim to know much at all. Just the basic leaving cert stuff. So if I am wrong here please be nice. As the question is far more advanced than leaving cert engineering.

Just in relation to question 5. The question states that the circut is at room temp when the circut is open. When closed the NTC heats up. And then reaches a constant.
A lighter is applied to increase it to 90 degrees. When the lighter is removed surely the NTC will return to the constant temp as before and not room temp.

Boston · 06-01-2007 09:15PM

Mellor wrote:

I dont do any advanced engineering, and don't claim to know much at all. Just the basic leaving cert stuff. So if I am wrong here please be nice. As the question is far more advanced than leaving cert engineering.

Just in relation to question 5. The question states that the circut is at room temp when the circut is open. When closed the NTC heats up. And then reaches a constant.
A lighter is applied to increase it to 90 degrees. When the lighter is removed surely the NTC will return to the constant temp as before and not room temp.

Right you are, over looked that bit. It would return to whatever the constant temperature was.

As for the others. The kid (as indicated from other posts) doesn't seem to be in college yet, so their probably leaving cert or A level.

Aero787 · 06-01-2007 10:30PM

LiouVille wrote:

The kid (as indicated from other posts) doesn't seem to be in college yet, so their probably leaving cert or A level.

The questions I asked in the original post were LC/A-level standard. The stuff I was doing was essentially first year college stuff. Considering I mentioned having studied Thevenin's Theorem, obviously not in as much detail as eng students, and it's not on the LC or A-Level syllabus (99% sure), I must have been doing something a bit more advanced. Of course, there's still LC (I'm not doing the elec option) and A-level stuff I haven't learned. I'm working through different topics bit by bit. Alrite kid?

Good job Mellor! On the ball. I didn't even spot his mistake.

All the best
huí tóu jiàn

Electronics question

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