Resistance Level 2 - HELP NEEDED!!

[james1985]

hey,

I am currently completing City & Guilds level 2 and stumbled across the below questions and scratching my head...

1. Electric Fire element has a length of 10m, a cross section area of 0.5mm2 and resistance of 50 ohms - calculate resistivity of material used.

Thanks

 

[Des 56]

R = U x L (SorryI don't know how to use the correct symbol for resistivity)
......... A
Resistance is equal to resistivity x length divided by the area

Any material has a natural resistance.over longer distances the resistance gets higher
If the area is increased,the resistance gets lower,distance and area are proportional

From the information given and the values you can transpose the formulae to work out the resistivity

 

[james1985]

Des, from using that formula - I could not locate the material used. Not matched the resistance level provided...

 

[Pete999]

hey,

I am currently completing City & Guilds level 2 and stumbled across the below questions and scratching my head...

1. Electric Fire element has a length of 10m, a cross section area of 0.5mm2 and resistance of 50 ohms - calculate resistivity of material used.

Thanks

Have you not been taught how to calculate the resistivity of a material, it's quite a simple formula

 

[Pete999]

R = U x L (SorryI don't know how to use the correct symbol for resistivity)
......... A
Resistance is equal to resistivity x length divided by the area

Any material has a natural resistance.over longer distances the resistance gets higher
If the area is increased,the resistance gets lower,distance and area are proportional

From the information given and the values you can transpose the formulae to work out the resistivity

Rho

 

[Des 56]

Des, from using that formula - I could not locate the material used. Not matched the resistance level provided...

This is taking me back decades
If I were given the information I would look up what material most heating elements are made from (mostly nichrome) then find out its value of resistivity and go from there

 

[Des 56]

Rho to you from me as well Pete

 

[james1985]

Nichrome is listed as 0.15...still does not come to the resistance level

 

[james1985]

For example - trying 0.15 x 10 then devide by 0.5 which comes too 3?

 

[Des 56]

I only gave an example of a material if it was not known
Again I say you have the information to work out the problem

 

[james1985]

I have gone through the entire list ha ha

 

[Pete999]

Not going to tell you the answer James, google how to calculate the resistivity of a material? the answer is there.

 

[james1985]

Ok....

Resistivity = resistance x area/length

50 x 0.5 then divide by 10 = 2.5

 

[Pete999]

No Rho= LxA/resistance, watch out for the csa part, you will need to find the sq root of 0.5

 

[james1985]

Cheers Pete, appreciated

 

[davesparks]

No Rho= LxA/resistance, watch out for the csa part, you will need to find the sq root of 0.5

CSA is area, the square root of it will give the length of one side of a square with that area.
The formula for resistivity is:
Rho=R*L/A

 

[james1985]

Dave your a legend, can you break it down so I can see the full calculation....

 

[Pete999]

CSA is area, the square root of it will give the length of one side of a square with that area.

So a csa of 2,5mm2 is ?

Dave your a legend, can you break it down so I can see the full calculation....

I had better stay out Dave has it covered.

 

[telectrix]

So a csa of 2,5mm2 is ?

.

rthe csa of 2.5mm is 2.5sq. mm.

 

[james1985]

Ok so am incorrect in saying the answer is 2.5?

 

[Pete999]

Ok so am incorrect in saying the answer is 2.5?

Think so yes, just thinking I used to be good at this sort of thing, age certainly take it's toll on the memory. Hang on til Dave gets back don't want to add to the confusion.

 

[telectrix]

p = (R x L)/ A

 

[The Ghost]

Its fairly difficult to write the formula and/or a worked example so here is one. Hope it helps. It is simple but you need a sci. calculator.

 

[telectrix]

or here:
Resistance and Resistivity - http://hyperphysics.phy-astr.gsu.edu/hbase/electric/resis.html

bear in mind that in that example, csa is in sq.cm.

 

[The Ghost]

Sorry it's sideways but you can download it and turn it upright

 

[james1985]

Thanks once again gents, certainly saved me a massive head ache