Calculating Material Thickness: 5k Ohms CSA 10mm x 20mm

[Kev1n]

1. A conductive sheet of material has a CSA of 10mm x 20mm. Reisistivity of 10^{3 (10 to the 3, 10,000)} ohms M. Calculate thickness of the material required to give resistance of 5k ohms



2. Ro = RA/l , where l would be thickness, l = RA/Ro



3. Thickness = 5,000 x 0.2 / 10,000 = o.1m therefore 100m

Comments appreciated

 

[Redbelly98]

There are some problems with your calculation:

10³ is not 10,000.

The area is not 0.2 (is that supposed to be in m²?). Try converting the mm values to meters first. Remember, 1 mm is 10^-3 m.​

 

[Kev1n]

There are some problems with your calculation:

10³ is not 10,000.

The area is not 0.2 (is that supposed to be in m²?). Try converting the mm values to meters first. Remember, 1 mm is 10^-3 m.​

Thanks:

Thickness = 5,000 x 0.0002/ 1,000 = 1 x 10-3m therefore 1mm

or do I have to sq the .ooo2 giving:

=5,000 x 4x10-8 / 1,000 = 2 x 10-7

 

[Redbelly98]

One of those is correct! Here are 2 questions to help you decide which:

1. How did you get the 0.0002 value for the area, and therefore what are the units associated with this value?

2. Are these units (from my previous question) correct units for an area, or would you need to square the units to to get an area?

Note: correct units for area can be expressed as m², m·m, mm², or mm·mm.

 

[Kev1n]

One of those is correct! Here are 2 questions to help you decide which:

1. How did you get the 0.0002 value for the area, and therefore what are the units associated with this value?

2. Are these units (from my previous question) correct units for an area, or would you need to square the units to to get an area?

Note: correct units for area can be expressed as m², m·m, mm², or mm·mm.

Hi, thanks for the reply, think I am having a christmas moment.
The area should be 10mm x 20mm = 200mm in m 200 x 10-3 therefore 0.2m
Thickness = 5,000 x 0.2/ 1,000 = 1mm

How does this seem

 

[Redbelly98]

Hi, thanks for the reply, think I am having a christmas moment.
The area should be 10mm x 20mm = 200mm ...

I'll stop your post here and make a comment about units.

Just as you multiplied 10 x 20 to get 200, we also need to multiply mm x mm, (since mm appears twice in the values being multiplied) and that gives us mm². So the units will be mm², not simply mm as you wrote.

Since mm² is a unit of area, we have A=200 mm² for the area. Do not square this value to do the calculation; that would result in units of mm⁴ which is not a unit of area.

Of course, if you seem to be working in MKS units, we do need to convert from mm² to m²