Electricity Question

por rhoudini Seg 13 Jul 2015, 12:55

Hi! I didn't understand how to solve this question. Can you help me?
I saw it here: http://studentzoneonline.com/Physics/tabid/67/catid/10/Current-Electricity.aspx

Here is the question:

29. When a piece of aluminium wire finite length is drawn through a series of dies to reduce its diameter to half its original value, its resistance will become

( ) 2 times	( ) 8 times
( ) 4 times	( ) 16 times

Thanks for the support since now! ;)

por **Euclides** Seg 13 Jul 2015, 13:42

Since resistance is proportionally inverse to the area, being the area reduced to the fourth part, the resistance will increase four times.

por **Elcioschin** Seg 13 Jul 2015, 17:19

Dear Euclides and rhoudini

When a wire (L, d) has its diameter reduced (d' = d/2), its volume kept constant, so, its lenght (L') increases.

V = pi.(d/2)².L = pi.[d'/2]².L' ---> pi.(d²/4).L = pi.[(d/2)/2]².4.L ---> L/4 = L'/16 ---> L' = 4.L

R = p.L/A --> R = p.L/(pi.d²/4) ---> R = 4.p.L/pi.d²

R' = p.L'/A' ---> R' = p.4.L/pi.(d'/2)² ---> R' = 4.p.L./pi.(d²/16) ---> R' = 16.(4.p.L/pi.d²) ---> R' = 16.R

(x) 16 times

por **Euclides** Seg 13 Jul 2015, 17:22

Increased lengh... you're actually the Hawk-eye!

por **Elcioschin** Seg 13 Jul 2015, 17:26

These industrial technics is named, in portuguese, trefilação (I don't know the name in english).

Only a mini Hawk-eye!!!

por **Euclides** Seg 13 Jul 2015, 17:31

Num dicionário técnico encontrei:

- wire drawing
- wire stretching

por **Christian M. Martins** Seg 13 Jul 2015, 18:00

Elcio, I noticed some mistakes in your English spelling during your explanation of the question. Since I admire you alot and the opportunity to help you turned up, I'll correct your sentences. I really hope it helps you with English.

"~~When a wire have your diameter reduced~~" should be spelt as "When a wire has its diameter reduced", since you're referring to "it" (wire).

The same thing happens in the sentence "~~your volum maintains constant, so, your lenght will increase~~.", since it should be spelt as "its volume is kept constant, so, its lenght increases".

por **Elcioschin** Seg 13 Jul 2015, 18:17

Christian

Eu agradeço muito pela correção; é sempre tempo de aprender.

por rhoudini Ter 14 Jul 2015, 19:56

Wow, you are amazing, guys! I was afraid to post this question here in the Physics section, I didn't know if I should.

Thanks a lot for everything, since the good explanation until the english used for it Surprised

By the way, when solving some exercises today, I noticed in my physics book one question that mentions this Forum as the "publisher".

Keep it up! :bball:

Kind regards!