Physics or electronics? Help!
Could anyone please help with some vague time constant calculations? I know that T=CxR.
The questions set, are: 470mf x 100k (squiggle like an "n")
220mf x 1m ( " )
680mf x 2m ( " )
I assume the m's for million? but I don't even understand what's required!!
Thank you to anyone who can help!
Ice.Maiden Thurs 09/10/08 23:24
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whats the exact question icey??
if u asked me in 2006 then i wouldda helped ya :D cos i did this kinda stuff on Degree Level!!... but my brains aint made of physis or electronics im afraid :( sorrweeee x
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Hi,
.000470 x 100,000 = 47 seconds
.000220 x 1,000,000 = 220
.000680 x 2,000,000 = 1360
thats if i understand your question properly ?
sqiggle like an n ? do you mean more like a u
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Are you sure that you've typed the letter 'm' correctly in every case? A small 'm' means 'milli' (which is one thousandth), whereas a capital 'M' means 'mega' (which is one million. k, of course, means Kilo = 1000).
The time constant formula is t = CR, where t is the time in seconds, C is the capacitance in Farads and R is the resistance in Ohms. (The symbol for ohms is the Greek letter omega, which is your 'squiggle').
I'll assume that you've correctly typed the letter 'm' in every case. So for question 1:
t = 470 x 1/1000 x 100 x 1000 = 470 x 100 = 47000 seconds
(That's a ridiculously large time constant but that's definitely what you get from those figures)
For question 2:
t = 220 x 1/1000 x 1 x 1/1000 = 220 microseceonds = 0.22ms
(which is a far more realist answer for a time constant).
For question 3 we then get:
t = 680 x 1/1000 x 2 x 1/1000 = 1360 microseconds = 1.36ms
Chris
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Question Author
Thank you very, very much red-forest. Someone else's just sent me a link to a calculator, which was very good, but I wasn't sure where to enter the figures I gave here.
The squiggle, is like an 'n', but curls out, to the right and left of the bottom of each column of the letter - if that's of any help!
Strangely, I sat working these probs out myself, and also came to 1360 for the final question, and then discarded it. I feel embarrassed to say that I got a decent grasde in physics, but just can't remember how to work these things!
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well i am more confused now where about does the squiggle come in the equation are you sure its not more of a u with curls at the bottom which means micra which is one millionth ( 0.000001) if its a n it would mean nano thats 0.000000001
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Question Author
Gosh! Oh dear!!!
The figures on the paper appear to be as I've typed above, and actually, my first workings-out fell into line with both of your answers.
Can I just ask you to explain the "220 microseconds = 0.22ms" please? Are they the same thing? I wasn't sure whether it was 22 microseconds, or ).22 microseconds - and this is where i was getting confused. the more I tried to work it out, the more I was getting mixed up!
Very kind of you Chris. x
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Red-forest:
Given the context (i.e. that R represents resistance, which is measured in Ohms) I remain confident that the 'squiggle' is omega, Ω )
Chris
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Question Author
It looks like the 'n' red-forest. Well it IS - it's not a 'u' with the squiggles at the top.
Which do you all think it's more liable to be? This is some physics work for a 14 year old girl, and the electronics part of the lesson's only just got underway. She's been working on simple capacitors.....
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A microsecond is one millionth of a second. It's written as µs (using the Greek letter µ, which I hadn't worked out how to type when I posted previously!)
A millisecond (written as ms) is a thousandth of a second, so 1 ms = 1000 µs.
Or putting it the other way round, 1 µs = 1/1000 ms
So 220 µs = 220/1000 ms = 0.22 ms
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The squiggle is omega (Ω) for ohms.
I too would have expected capacitance to be in microfarads (μF) rather than millifarads (a term so rarely used you tend to see that value expressed as 1000 μF) but then I suppose a school exercise does not have to follow "real world" conventions, although in general I hope they do.
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Question Author
Ah! I couldn't type that - yes, that's more the shape!
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Question Author
Oh guys....can we have a vote on which you think it'd be please?
You don't know how much I appreciate your help, but I'm now struggling to set things out simply in my head. Seems I DID get the answers - in a way - but my friend's daughter WAS talking about farads of course, and all of a sudden, it's gone double-dutch to me again!
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0.001 = one milli (one thousand'th) symbol little m
0.000001 = one micra ( one million'th) symbol big M
220 micra farad is expressed as 220uf (with a double curl to bottom of the u) 220uf = 0.000220
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Icey:
Put your original question alongside my initial post. You'll see that all I've really done is to retype your original question but rewriting 'm' as '1/1000' and 'k' as 1000.
As long as you've correctly typed the question (i.e. you've not typed 'm' instead of 'M' or 'µ') you should be able to see how it all comes together and (hopefully!) my answers should be correct.
Like Kempie, I would expect real-life situations to have much smaller capacitances (and much higher resistances) but, as long as you presented the correct information I should have provided you with the correct answers. (At least I hope so. My degree's in Maths but my teaching certificate is in Maths and Physics, and I taught both subjects)
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Question Author
The information's correct Chris.
Thank you very much - and thank you for ALL your answers, because it's just jogged the old brain cells here, so you've helped more than one person. xxx
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Buenchico,how did you put the micra symbol in your answer ? can you do any others ie. omega etc.
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Question Author
Have fun guys - I wondered that myself actually!
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Red-Forest:
Normally when I want to type special characters I refer to this list, which I keep a printed copy of next to my desk:
http://www.tedmontgomery.com/tutorial/ALTchrc. html
For example, to type this ¶ I held down the Alt key and typed 20 on the numeric keypad. (You have to have the number lock on but that's the default keyboard setting on most PCs anyway).
However, with some of the more obscure symbols, different fonts seem to require different Alt codes and that list isn't infallible. For example, AB's character set doesn't produce Ω when I use the code on that page. So I 'cheated' and Googled 'omega'. It wasn't hard to find a page that included the symbol, so I simply copied it and pasted it into my post.
Chris
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When people describe me as 'clever' it's usually code for 'a right boring know-it-all' ;-)
Chris
PS:
Red-forest: For the 'micro' symbol, I Googled 'mu' and then copied and pasted again.
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