Eddy currents with steel

[Njguy]

When you run a magnet across a plate of aluminum you can easily feel the eddy currents creating an opposing magnetic field which resists your movement. I tried this with a plate of steel and noticed that there was hardly any resistance. Was wondering why this?

 

[crutschow]

Likely because the steel has about an order of magnitude higher resistance than aluminum and thus the eddy currents are much smaller. The opposing magnetic field is proportional to this current.

 

[jpanhalt]

I tried this with a plate of steel and noticed that there was hardly any resistance. Was wondering why this?

That's odd. When I try to slide a magnet across a plate of steel, it sticks. Were you using a "non-magnetic" (Austenitic) alloy of stainless?

John

 

[Njguy]

That's odd. When I try to slide a magnet across a plate of steel, it sticks. Were you using a "non-magnetic" (Austenitic) alloy of stainless?

John

I am not referring to the vertical attraction but the horizontal resistance.

 

[Njguy]

do you have any numbers for this because I keep finding charts online that say steel has a lower resistance than aluminum which makes no sense.

 

[magvitron]

for steel it's 1.611 to 7.496 × 10-7 Ωm
for aluminum its 2.82 × 10-8 Ω·m
so it depends on which kind of steel that you use. The attrative force will be always stronger than eddy currents, so we may no 'feel' anything on cast steel.
I think this is the reason.

RESIST.    COND.                SOURCE
ohm-m    SIEMENS/m  % IACS  CODE MATERIAL
1.611E-07 6.206E+06    10.70  1  Steel, Cast
5.945E-07 1.682E+06    2.90  1  Steel, High Alloy
6.897E-07 1.450E+06    2.50  1  Steel, 304 Stainless
6.897E-07 1.450E+06    2.50  2  Steel, 304 Stainless
7.184E-07 1.392E+06    2.40  1  Steel, 347 Stainless
7.184E-07 1.392E+06    2.40  2  Zircaloy - 2
7.496E-07 1.334E+06    2.30  1  Steel, 316 Stainless

 

[steveB]

do you have any numbers for this because I keep finding charts online that say steel has a lower resistance than aluminum which makes no sense.

You must be reading a conductance chart, not a resistance chart. Conductance is higher in better conductors.

 

[Njguy]

You must be reading a conductance chart, not a resistance chart. Conductance is higher in better conductors.

I am getting major conflicting information. I am finding charts saying that the resistance of Aluminum vs Steel is not that different, but also charts which show that the conductivity of aluminum is way better?? I thought resistivity and conductivity were inversely proportional...?

 

[jpanhalt]

I am getting major conflicting information. I am finding charts saying that the resistance of Aluminum vs Steel is not that different, but also charts which show that the conductivity of aluminum is way better?? I thought resistivity and conductivity were inversely proportional...?

Can you screen print or snip one of those charts that says the electrical resistance is the same? As crutschow noted, there is an order of magnitude (i.e., 10 fold) difference. Aluminum is the better electrical conductor.

John

 

[Njguy]

Can you screen print or snip one of those charts that says the electrical resistance is the same? As crutschow noted, there is an order of magnitude (i.e., 10 fold) difference. Aluminum is the better electrical conductor.

John

For the resistance I was going by what magvitron said in the post above. His numbers seem to suggest that aluminum has similar resistive properties to steel. Unless I'm reading it wrong.

 

[Njguy]

Nevermind I found a really awesome chart here https://www.azom.com/article.aspx?ArticleID=1298

Harder to find straightforward info like than I would have thought.

 

[jpanhalt]

for steel it's 1.611 to 7.496 × 10-7 Ωm
for aluminum its 2.82 × 10-8 Ω·m

10-7 is 10-fold greater than 10-8. There is no inconsistency. Magvitron's post agrees with everyone else's.

John

 

[Njguy]

10-7 is 10-fold greater than 10-8. There is no inconsistency. Magvitron's post agrees with everyone else's.

John

yeah but why would there be a 10 fold increase in resistance for Aluminum?

 

[MrAl]

Hi,

1e-7 is a larger number than 1e-8 so re-read that.

Also, how are you preventing the magnet from touching the steel during your 'steel' test? Remember the distance to the object has to be the same for both materials or the test is not comparable.

If the magnet touches it creates drag.

 

[crutschow]

It would be difficult to do an eddy current test comparison between aluminum and steel unless it was non-magnetic stainless steel.

 

[jpanhalt]

Nevermind I found a really awesome chart here https://www.azom.com/article.aspx?ArticleID=1298

Harder to find straightforward info like than I would have thought.

Here's the chart:

That information might lack some of the exponents the SI units have, but the resistance value for copper seems to be a little (i.e., 10^3) too high.

John