resistance question

[PoorFish]

Greetings all

I have a question here and I need to know if im on the right track

A length of copper conductor 75m long, having an area of 1.5mm2 has a specific resistance of 1.67 x 10-8ohm/m.

What is the resistance?

What I have done is this

R = pl / A

= 1.67 x 10-8 x 75 / 1.5

= 0.0000000167 x 75 / 1.5

= 0.0000012525 / 1.5

R = 8.35 x 10-7

what do ya reckon?

many thanks in advance

P

 

[SPECIAL LOCATION]

Hello P Feeny

are these just made up numbers??

bit confused by your values here ???

e.g. Copper conductor 1.5mm

 

[PoorFish]

Morning

Yeah I imagine this are made up numbers from the college

sorry about confusion

Pat

 

[oneofthedamned]

Greetings allI have a question here and I need to know if im on the right track

A length of copper conductor 75m long, having an area of 1.5mm2 has a specific resistance of 1.67 x 10-8ohm/m.

What is the resistance?

What I have done is this

R = pl / A

= 1.67 x 10-8 x 75 / 1.5

= 0.0000000167 x 75 / 1.5

= 0.0000012525 / 1.5

R = 8.35 x 10-7

what do ya reckon?

many thanks in advance

P

I believe the answer is 835x10-3.

The rho value for copper is 16.7 nΩm

You have to change all units in the equation to the same value i.e mm or M.

I have used mm as I find it easier.

16.7x10-6 x 75x10+3 / 1.5 all units now in mm

16.7x10-3 x 75 / 1.5 the +3 from 75m is subtracted from -6 giving -3

16.7x10-3 x 50 50 = 75/1.5

16.7 x 50 = 835

answer 835x10-3 0r .835

.835 is a reading you might expect to see for 75m of copper cable and is close to special locations .9075

Hope this helps.

 

[Apache]

Chaps - I'm not sure I believe either of you. I think Specs method is the right method just he's not using the numbers given in the question.

If resistance of the 1.5mm CSA wire is 1.67X10-8 Ω/m the CSA doesn't feature in the calculation

so you have

0.0000000167 X 75 = 12.525 X 10-8 Ω

Is my thought. But I'm no spark

 

[binky]

Chaps - I'm not sure I believe either of you. I think Specs method is the right method just he's not using the numbers given in the question.If resistance of the 1.5mm CSA wire is 1.67X10-8 Ω/m the CSA doesn't feature in the calculation

so you have

0.0000000167 X 75 = 12.525 X 10-8 Ω

Is my thought. But I'm no spark

Cocurr completely

 

[davetheglitz]

Greetings allI have a question here and I need to know if im on the right track

A length of copper conductor 75m long, having an area of 1.5mm2 has a specific resistance of 1.67 x 10-8ohm/m.

What is the resistance?

R = pl / A

Hi,

You need to be consistant with your units when doing equations like this.

Resistance - ohms

Length - metres

Area - metres squared

Resistivity - ohm.metres (not ohms/metre!)

If you don't do this then you will be way out - and it's more understandable if you go to the SI units rather than getting confused with mm etc.

Step 1 - Get your area in the correct units 1.5mm2 = 1.5 x 10-6m2

Now plug into your equation

R = pl/A

= 1.167 x 10-8 x 75/1.5 x 10-6

= 1.167 x10-2 x75/1.5

= 0.5835 ohms.

Remember to quote your units at the end!!! Better marks. Also write down any steps along the way. You can easily muck up the maths - but if everything else is written down you should get most of the marks available.

If you just plug the units into the equation you should be able to see how the formula works:-

R = pl/A

= ohm.m .m /m2

= ohms

As an aside you can rearranging to work out the units resistivity:-

R = pl/A

RA = pl

pl = RA

p = RA/l

= ohms . m . m /m

= ohms . m

Used that to help with my exams, as you can always work backwards to get to the formula if it wasn't given!

Good Luck

Dave

​

 

[oneofthedamned]

Greetings allI have a question here and I need to know if im on the right track

A length of copper conductor 75m long, having an area of 1.5mm2 has a specific resistance of 1.67 x 10-8ohm/m.

What is the resistance?

P

Dave the figure is 1.67 not 1.167 rework using your method and compare results.

 

[oneofthedamned]

Check this out

 

[Apache]

Check this out

What we are now discussing is the exact wording of the question. If the OP means (as he has written) ohm/m then I am correct.

Now if he means electrical resistivity (rho) then you are, or course, correct.

But that's not what he's asked in OP. We may be overcomplicating a simple multiplication! Specs quotes the OSG giving ohm/m!

 

[oneofthedamned]

What we are now discussing is the exact wording of the question. If the OP means (as he has written) ohm/m then I am correct.Now if he means electrical resistivity (rho) then you are, or course, correct.

But that's not what he's asked in OP. We may be overcomplicating a simple multiplication! Specs quotes the OSG giving ohm/m!

Agree totally Apache to calculate the resistivity of cable just look in the OSG.

But the City and Guilds require that we learn this formula the attain the certificate even though it will never be used again.

 

[davetheglitz]

Dave the figure is 1.67 not 1.167 rework using your method and compare results.

Oops finger trouble - but the advice and methodology are sound!

Agree with your method - it certainly works - but the more reliable method is to bring everything in line with SI units - saves confusion!

Cheers

Dave

​

 

[PoorFish]

Greetings folks

Thanks for all the input and help

I have written the question as it was put to me

1.67 x 10 -8 ohms.metre - rho;

is what im trying to communicate here .

Apologies for confusion - this is all pretty new to me and am still getting to grips with it all

I came up with this after your advice

.0000000167 * 75/.0000015 = 0.835ohms

one other

R = pl/A

= 1.167 x 10-8 x 75/1.5 x 10-6

= 1.167 x10-2 x75/1.5

= 0.5835 ohms.

excuse the ignorance but to get 1.67 x 10-2

did you just subtract 10-8 - 10 -6

also isnt 10-6 a micro????

Cheers

Pat

 

[Apache]

Think the moral of the story is read the question!

The answer of 0.835 is correct.

Dave's idea of getting everything down into basic units is spot on

I try to avoid micro/milli/pica etc - X10-3, X10-5 lot simpler!

 

[Apache]

If you find it hard working out the base units, do it in stages:

75m --> (100cm in 1 m) 7,500cm --> (10mm in 1 cm) 75,000mm

but remember squared units go in 2 dimentions so 1m2 = 10,000cm2 (100X100) and therefore 1000000mm2 (1000X1000)

simple?

 

[Apache]

I came up with this after your advice.0000000167 * 75/.0000015 = 0.835ohms

what you've done is convert M into mm, which is fine - the other way round would have worked equally, they must both be the same

 

[Apache]

one other this is the same as you wrote above!

R = pl/A

= 1.167 x 10-8 x 75/1.5 x 10-6

= 1.167 x10-2 x75/1.5

= 0.5835 ohms.

You put the wrong number in (from above post)

Correct to 1.67X10-8 X 75 / 1.5X10-6

=0.835

excuse the ignorance but to get 1.67 x 10-2

did you just subtract 10-8 - 10 -6

All you are doing is tidying up the sums, but it really doesn't matter, just wack it into a scientific calculator as it is

Have we got you sorted now ?

 

[PoorFish]

greetings folks#

Thanks loads for all the help

Have been a bit quiet since last night as Im really struggling on something

converting the 1.5mm2 to 1.5 x 10 -6m2 has kind of stumped me at the moment

Im sure its simple to grasp but at this moment in time im not getting it.#

Is this a given thing ...

I will get there I just need to understand it 150 per cent

cheers

Pat

 

[davetheglitz]

Hi Pat,

Don't know if this helps - but looking at it this way:-

1mm = 1/1000 m = 0.001m = 1 x 10-3 m

The 10-3 moves the decimal point to the left 3 places

Now the squared thing.

1mm2 = 1mm x 1mm

Feeding in what you found above:-

1mm2 = 1/1000m x 1/1000m = 1/1000000m2

= 1x10-3 x 1x10-3m2

= 1 x 1 x 10-3 x 10-3m2 (you can do a multiplication in any order!)

= 1 x 10-6m2 the -6 moves the decimal point 6 places (3 + 3) to the left

Don't worry - something will suddenly gel and it'll be easy!

Cheers

Dave

​

 

[Zeespark]

Pat not meaning to make you look like an idiot here, my college lecturer is a maths genius and old school. He taught me that anything on the plus side of zero, u just do the power of 3, if smaller than zero -3.

Andy