I am writing a Physics test on Thursday, and am kind of stuck on this point.
We are doing (among others) high voltage cables and why we use them. Now, I have a question. The resistance of the cables would be Resistance = rho * length/area (R=rho * l/A), rho being the resistance factor of the material. Now, you also calculate the resistance by Resistance = Voltage/Amperage (R = U/I). However, of course, if you change the voltage the resistance of the cables would change, but that is impossible because they are a fixed length, area and material! HELP!!! What changes? What IS the resistance I got in the second computation?

------------------
http://imagehosting.bidbay.com/image.php3?imgID=614236 http://www.ironworksforum.com/ubb/no...iles/hero2.gif

Sapphire Dragoness of ALSB
Waitress at Cloudy's Cafe
Guardian of the Temple of Aerie (specialized in GenCon and BG)
Magess of Lady Lioness' Court
Saint of the SoS
Historian of IW
Most Illustrious Arch-Magess of the Illuminati
Lux omnis est
Once Upon A Paper Writer's Forum

That would be the resistance of my thick skull, and your trying to run this question through it http://www.ironworksforum.com/ubb/no...es/biggrin.gif


------------------
http://www.usflag.org/animate/flagwave1.gif

"The Martyr" (excerpt)

There is sobbing of the strong,
And a pall upon the land;
But the People in their weeping
Bare the iron hand:
Beware the People weeping
When they bare the iron hand.

--Herman Melville (written after the assasination of President Abraham Lincoln)

http://www.wizardrealm.com/images/rb.gif

It's been a few thousand years since I had to deal with this, but if you change voltage the current changes.

I wish you good luck since I can't help you.
I'm in the second class of high school you knowhttp://www.ironworksforum.com/ubb/no...es/biggrin.gifhttp://www.ironworksforum.com/ubb/no...es/biggrin.gifhttp://www.ironworksforum.com/ubb/no...es/biggrin.gif!

------------------
http://www.angelfire.com/rpg/castlet...ifs/knight.gif
http://members.fortunecity.com/agrid...s/byronas2.gif

Where there are monsters and barbarians there will be Byronas fighting against them.

I'm in 10th - when does High School start in Greece? I hope I didn't mess the whole thing up - I had to translate all the technical terms from German into English, I may have gotten some wrong.

A man starts high school in Greece after 6 years in primary school.

------------------
http://www.angelfire.com/rpg/castlet...ifs/knight.gif
http://members.fortunecity.com/agrid...s/byronas2.gif

Where there are monsters and barbarians there will be Byronas fighting against them.

Ack! Not physics! I'm allergic to that! Save meeeeee! I'm meeeeeeeeeltiiiiing!

I DID say *physics* in the title, if you read it it's your own fault. http://www.ironworksforum.com/ubb/no...miles/nono.gif http://www.ironworksforum.com/ubb/no...miles/wink.gif *fetches bucket* Here, melt into that.

------------------
http://imagehosting.bidbay.com/image.php3?imgID=614236 http://www.ironworksforum.com/ubb/no...iles/hero2.gif

Sapphire Dragoness of ALSB
Waitress at Cloudy's Cafe
Guardian of the Temple of Aerie (specialized in GenCon and BG)
Magess of Lady Lioness' Court
Saint of the SoS
Historian of IW
Most Illustrious Arch-Magess of the Illuminati
Lux omnis est
Once Upon A Paper Writer's Forum

Yes, Kaz, what Avron says here is true, the resistance of the wires are always the same the U/I = R is just a way to calculate the value of the resistance if you don't have any other information. so shortly put, R (resistance) is always the same, if the voltage (U) changes it only affects the current (I) and if the current (I) changes, the voltage (U) is likely to change.
Hope this helps, if you have more physics questions, just shoot, I'm in this business


------------------
http://www.wizardrealm.com/images/angel2.jpg http://www.angelfire.com/rpg2/mirac/...sigs/Done5.gif
Lady Menqel, Protector of IW's, smiter of evil

http://www.angelfire.com/rpg2/mirac
- Priest of Helm
- Fellow Table Dancer of the Jerome's Table dancer group
- Spell formulater of the IW peace-keeping force
- Magical advisor of MiLady Lioness
- Thanks to Sazerac for the great sig picture

Hm... but if I have both U and I and they do not add up to the R I got by rho*length/Area? To be more exact, something like this

R = rho * l/A
R = U/I
rho = 0.017
l= 100km
A = 0.8 cm2
U = 150kV
I = 533.33 A
Therefore
R = rho * l/A
R = 0.017 Ohm per whatever * 100km/0.8cm
R = 212500 Ohm

HOWEVER
R = U/I
R = 150000V/533.33A
R = 281.25 Ohm

WHAT THE HECK? THE RESISTANCE CAN'T BE 212500 Ohm AND 281.25 Ohm AT THE SAME TIME!!!! *sob*



------------------
http://imagehosting.bidbay.com/image.php3?imgID=614236 http://www.ironworksforum.com/ubb/no...iles/hero2.gif

Sapphire Dragoness of ALSB
Waitress at Cloudy's Cafe
Guardian of the Temple of Aerie (specialized in GenCon and BG)
Magess of Lady Lioness' Court
Saint of the SoS
Historian of IW
Most Illustrious Arch-Magess of the Illuminati
Lux omnis est
Once Upon A Paper Writer's Forum

Kaz - Sorry that I can't help much... the only thing I noticed is that unit length in the numerator and denominator of the first formula (R = rho * l/A) is inconsistent. Ie, the area should be converted into square metres since you've changed the length from km to m.

Gee, sorry I'm not much help... let's hope Mirac returns soon to help you out with this. http://www.ironworksforum.com/ubb/no...iles/frown.gif

I did, I just skipped that step. What I noticed was that I'm not exactly sure what unit rho is measured in...

Kaz:

I'm doing Electronic Engineering at Cambridge 1st year:

R changes ONLY with material, temperature, length and area.

I think http://www.ironworksforum.com/ubb/no...miles/wink.gif I ought to know...

Voltage (potential difference) is adjustable and the current THROUGH the resistor changes acoordingly. THE REISITOR can heat up as a result of this...

------------------
<UL TYPE=SQUARE> <LI>Creator and Diamond Bladesinger of ALSB
<LI> Body Guard of Lioness
<LI>Spam Sentinel
<LI>Saint of Ironworks
<LI> Winner of EPSRC 2000 Science Website is @
http://www.epsrc-sciencerace.net/schools2000/534121/
<LI> My own Forgotten Realms Website is @
http://www.forgottenrealms.main-page.com[/list] http://www.asnsoup.com/a.jpg

Whoohooo Physics http://www.ironworksforum.com/ubb/no...miles/wink.gif
Don;t you have Chemestry Questions?
I love thise http://www.ironworksforum.com/ubb/no...miles/wink.gif

oh wait the question right...
Well Kazzy Honey
It's quite easy
If you higher the voltage and the resistance stays the same
What happens?
The Amperage highers aswell (the current through the resistor as Avatar calles it)

Simple in'it
so basicly U/I=l/A (i'm sure that th area ain't A but have no idea what it IS)




------------------
http://www.phpshop.net/images/rikard/rikard2.gif

Hello fellow Electronic Engineer (I supposed to do second year, but I delayed for a half year).
Anyways, Rikard, that is NOT right what you've said about U/I = l/A. Kaz, forget that, not right!!
About your problem with the calculations, I did the calculation as well, this is how it should be :

rho = 0.017
l = 100 km = 100.000 m
A = 0.8 cm2 = 0.008 dm2 = 0.00008 m2
R = rho * l/A
R = 0.017 * (100.000/0.00008) = 21250000 ohm

This is totally different from the formula U/I = R !!!!!

I'll give a an example, try to solve this one :

What is R (using R = rho * l/A ) when
A = 10 dm2
l = 30 m
rho = 0.5
R =????

Solve R

With the R you calculated here
you go on with this question (using U/I=R is equal (if rewritten) to R*I =U ) :

U = ?????
I = 5000 mA
R = what you calculated in the first question.

I'll check in later this day, if you still can't solve it email me.
Hope this helps

------------------
http://www.wizardrealm.com/images/angel2.jpg http://www.angelfire.com/rpg2/mirac/...sigs/Done5.gif
Lady Menqel, Protector of IW's, smiter of evil

http://www.angelfire.com/rpg2/mirac
- Priest of Helm
- Fellow Table Dancer of the Jerome's Table dancer group
- Spell formulater of the IW peace-keeping force
- Magical advisor of MiLady Lioness
- Thanks to Sazerac for the great sig picture

[This message has been edited by Mirac Honorguard (edited 10-22-2001).]

O....M....G....Beam me back down, Scotty. There IS intelligent life down there! http://www.ironworksforum.com/ubb/no...miles/wink.gif Haven't seen the likes of this in years! Still hurts my head! http://www.ironworksforum.com/ubb/no...miles/wink.gif

------------------
http://home-3.12move.nl/~se021070/dragon2.jpg http://www.ironworksforum.com/ubb/no...iles/hero2.gif
Alureth...Dragon friend of Breanna, Mage and Advisor in the Court of Lady Lioness

Co-Owner/Operator of the Evil Petting Zoo and devout member of the HADB clan.
Commander of the Dragon Fleet, IW Peacekeeping Force
Sublime Sister of the Illuminati - may the Light shine forever!
Second Historian and Scribe of IW. Mother Hen of IW.
Mage extraordinair, Pin-Up Girl for the OLD-COOTS
Magey-Wagey: performer in the Nekked Dancing Trio of the ORT.

Hopeless Romantic *sigh*

Well quoted from Scott Adams's Dilbert Principle..... http://www.ironworksforum.com/ubb/no...les/tongue.gif http://www.ironworksforum.com/ubb/no...les/tongue.gif http://www.ironworksforum.com/ubb/no...les/tongue.gif


------------------
<UL TYPE=SQUARE> <LI>Creator and Diamond Bladesinger of ALSB
<LI> Body Guard of Lioness
<LI>Spam Sentinel
<LI>Saint of Ironworks
<LI> Winner of EPSRC 2000 Science Website is @
http://www.epsrc-sciencerace.net/schools2000/534121/
<LI> My own Forgotten Realms Website is @
http://www.forgottenrealms.main-page.com[/list] http://www.asnsoup.com/a.jpg

AAAAAAAARRRRRRGGGGGG... physics, must run.. and hide.. and... groan... http://www.ironworksforum.com/ubb/no...iles/smile.gif

------------------
Revived I Am to hunt this world... Banish ye evil or face my wrath...

http://www.ironworksforum.com/ubb/no...les/tongue.gif http://www.ironworksforum.com/ubb/no...les/tongue.gif

Well, I hope you didn't think I was implying that no one else posting here was intelligent. http://www.ironworksforum.com/ubb/no...les/tongue.gif Mirac's post just brought back terrifying memories of Electrical Engineering classes an old b/f of mine was taking, gee, 15 years ago. I would help him with the homework sometimes. *ouch, my brain*

------------------
http://home-3.12move.nl/~se021070/dragon2.jpg http://www.ironworksforum.com/ubb/no...iles/hero2.gif
Alureth...Dragon friend of Breanna, Mage and Advisor in the Court of Lady Lioness

Co-Owner/Operator of the Evil Petting Zoo and devout member of the HADB clan.
Commander of the Dragon Fleet, IW Peacekeeping Force
Sublime Sister of the Illuminati - may the Light shine forever!
Second Historian and Scribe of IW. Mother Hen of IW.
Mage extraordinair, Pin-Up Girl for the OLD-COOTS
Magey-Wagey: performer in the Nekked Dancing Trio of the ORT.

Hopeless Romantic *sigh*

I think I can think again today - so if R(cable) = rho * l/A; then R = U/I would be the resistance of the ENTIRE circuit (or whatever it's called in English). Right?
Also got my other physics thing sorted out, with a little help from the teacher. Logical, actually.

------------------
http://www.phpshop.net/images/rikard/rikard2.gif

[This message has been edited by Rikard (edited 10-22-2001).]