Annabeth986
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A high voltage wire for transmission of electricity across the country is made of 10 aluminium wires (resistivity of 2.5×10−8Ωm) wound together with 15 copper wires (resistivity of 1.5×10−8Ωm). If all of the wires have a radius of 2.0mm, calculate the overall resistance of 20km of cable to two significant figures (The aluminium is there to give strength to the cable.)
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Kozmo
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We must use the p=RA/L equation (the resistivity equation) to find the answer to this.

We can rearrange it to find R - R=pL/A, and then sub in the appropriate values:

We know resistivity (p)
We know length (20km - Must be in metres)
We know the radius and thus can work out cross-sectional area
We also know the amount of wires.
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Annabeth986
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So do you add the resistivity for the 2 wires together?
Because I tried and did not recieve the correct answer
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Kozmo
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(Original post by Annabeth986)
Could someone show me how to do the actual working out?
We're not supposed to show full answers here.

What does the text book / source say is the correct answer?
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Annabeth986
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The website Isaac physics lets you type in your answer and it only lets you know if your answers right or wrong
Consequently, I do not know what the correct answer is.
I was wondering how to do the working out and if all the wires are in series or parallel
If so, would the length change if the wires are in series?
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Kozmo
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(Original post by Annabeth986)
The website Isaac physics lets you type in your answer and it only lets you know if your answers right or wrong
Consequently, I do not know what the correct answer is.
I was wondering how to do the working out and if all the wires are in series or parallel
If so, would the length change if the wires are in series?
I'm pretty sure the idea of them being in series/parallel shouldn't matter. It's making out as though those wires making up the entirety of 'a cable' (what we consider the wire).
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Joinedup
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(Original post by Annabeth986)
The website Isaac physics lets you type in your answer and it only lets you know if your answers right or wrong
Consequently, I do not know what the correct answer is.
I was wondering how to do the working out and if all the wires are in series or parallel
If so, would the length change if the wires are in series?
work out the resistance for the individual wires then treat them as resistances in parallel

the question is trying to describe something like this...

Image
https://en.wikipedia.org/wiki/Overhe...ylon)_line.jpg
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Annabeth986
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Thank you
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TheAZGuy
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Hi, did you ever find out what the answers was?
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FusionNetworks
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Anyone got an answer for this?? I got 76000 ohms, but it says its wrong!
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Qinge
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I'll do a walk-through of how to get the answer.

First of all convert units:
R = ?
ρ = Cu = 1.5x10^-8 and Al = 2.5x10^-18 Ωm
l = 20x10^3
r = 2x10^-3

Work out area:
A = πr^2 => πx(2x10^-3)^2 = 1.26x10^-5 m^-2

The small wires all effectively make two big wires:
You have 10 Al wires => 1.26x10^-5 x 10 = 1.26x10^-4 m^-2
You have 15 Cu wires => 1.26x10^-5 x 15 = 1.89x10^-4 m^-2

Now calculate the resistivity of each big wire:
Al = (2.5x10^-8 x 20x10^3) / (1.26x10^-4) = 4Ω
Cu = (1.5x10^-8 x 20x10^3) / (1.89x10^-4) = 1.6Ω

The wires are in parallel:
So use 1/RT = 1/R1 + 1/R2 => 1/4 + 1/1.6 = 0.875

1/0.875 = 1.1 Ω
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egfmnhg
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A fly in a room is flying on a bearing of 204∘ at a speed of 0.36ms−1. Sunlight streams horizontally westward across a room, forming a shadow of the fly on the west wall. How fast does the shadow move?
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laiba.2000
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Complete combustion of compound N occurs in a stoichiometric ratio of 1:6 1 : 6 with oxygen gas. Complete combustion of 4.2g 4.2 g of compound N produces 13.2g 13.2 g of carbon dioxide and 5.4g 5.4 g of water.
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ItsTamilTiger
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******
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Kimberley_99
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In an experiment, a current of 3.5Ais being passed through a copper sulphate solution in a 10cm cubical container, with the electrical terminals being opposite faces. This contains equal numbers of Cu2+ and SO2−4 ions which have respectively +2 and −2 electron charge units. Assuming that the two ions have equal speed in the solution, and that there are 6.0×1026 of each per cubic metre of the solution, work out their mean speed to 2 significant figures.


Help!!!
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i2thepowerof4
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Answer is 9.1e-7 ms^-1

(Original post by Kimberley_P)
In an experiment, a current of 3.5Ais being passed through a copper sulphate solution in a 10cm cubical container, with the electrical terminals being opposite faces. This contains equal numbers of Cu2+ and SO2−4 ions which have respectively +2 and −2 electron charge units. Assuming that the two ions have equal speed in the solution, and that there are 6.0×1026 of each per cubic metre of the solution, work out their mean speed to 2 significant figures.


Help!!!
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LuciusW
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(Original post by i2thepowerof4)
Answer is 9.1e-7 ms^-1
You absolute legend
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A4M1R
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(Original post by Qinge)
I'll do a walk-through of how to get the answer.

First of all convert units:
R = ?
ρ = Cu = 1.5x10^-8 and Al = 2.5x10^-18 Ωm
l = 20x10^3
r = 2x10^-3

Work out area:
A = πr^2 => πx(2x10^-3)^2 = 1.26x10^-5 m^-2

The small wires all effectively make two big wires:
You have 10 Al wires => 1.26x10^-5 x 10 = 1.26x10^-4 m^-2
You have 15 Cu wires => 1.26x10^-5 x 15 = 1.89x10^-4 m^-2

Now calculate the resistivity of each big wire:
Al = (2.5x10^-8 x 20x10^3) / (1.26x10^-4) = 4Ω
Cu = (1.5x10^-8 x 20x10^3) / (1.89x10^-4) = 1.6Ω

The wires are in parallel:
So use 1/RT = 1/R1 + 1/R2 => 1/4 + 1/1.6 = 0.875

1/0.875 = 1.1
You a real one
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