u/DependentDoubt6108

I've been doing a small experiment with eddy current braking and I'm confused about the theory.

The setup is pretty simple: an aluminum disk is spun using a rotary motion sensor, and I hold a stationary rectangular neodymium magnet above the disk. The only thing I change is the vertical distance between the bottom of the magnet and the top of the disk (2 cm to 4 cm). I then measure the angular deceleration over the same angular velocity interval each time (20–25 rad/s).

The average decelerations I got were roughly:

• 2.0 cm → 20.24 rad/s^(2)
• 2.5 cm → 12.63 rad/s^(2)
• 3.0 cm → 5.99 rad/s^(2)
• 3.5 cm → 3.09 rad/s^(2)
• 4.0 cm → 1.92 rad/s^(2)

The weird thing is that an exponential trendline fits almost perfectly (R^(2) ≈ 0.99).

I understand the basic mechanism:

• changing flux induces eddy currents,
• the eddy currents create a magnetic field opposing the change,
• the interaction with the permanent magnet creates the braking torque.

I've also seen derivations that the braking force is proportional to vB^(2), so the magnetic field is clearly the important quantity.

What I don't understand is where an exponential dependence on separation would come from physically.

Most explanations I find assume the magnet behaves like a dipole and use something like B**∝**1/r^(3), but my magnet is a rectangular neodymium magnet and the distances are only 2–4 cm from the disk, so I'm not sure that approximation is even valid.

Is there a better expression for the magnetic field of a permanent magnet in this regime? Or is it more likely that the exponential fit is just approximating the actual field over a small range of distances?

I'm mainly looking for the physics behind it rather than curve-fitting advice. If anyone knows a derivation or a good reference on eddy-current brakes with permanent magnets, I'd really appreciate it.

I'm doing an IB Chemistry IA involving the oxidation of oxalic acid by potassium permanganate in acidic conditions and I'm trying to understand whether my results make chemical sense.

I'm using a colorimeter and measuring absorbance vs time.

For each trial, my mixture is:

• 2.0 mL oxalic acid (variable concentration: 0.1 M, 0.4 M or 1.0 M)
• 0.5 mL H₂SO₄ (1.0 M)
• 0.5 mL KMnO₄ (0.002 M)

Total volume = 3.0 mL.

The 0.1 M, 0.4 M and 1.0 M oxalic acid solutions were all diluted from the same 1.0 M stock solution.

I originally increased the KMnO₄ concentration because my absorbance curves were almost linear and I was trying to observe autocatalytic behaviour.

The issue is that 0.4 M consistently behaves oddly. The 0.1 M and 1.0 M trials behave relatively normally, but 0.4 M repeatedly gives a slower absorbance decrease than 0.1 M.

I've repeated the 0.4 M trials multiple times and obtained similar results.

I'm aware this reaction is autocatalytic due to the formation of Mn²⁺, so I'm wondering:

Is there any chemically plausible reason why an intermediate oxalic acid concentration (0.4 M) would consistently react more slowly than both 0.1 M and 1.0 M, or is this most likely experimental error?

Also, could insufficient acidity (0.5 mL of 1.0 M H₂SO₄) contribute to this behaviour, or would that not explain a single concentration behaving anomalously?

I'm trying to determine whether this is a chemically explainable trend or whether oxalic acid concentration is simply a poor independent variable for my IA.

(I'm attaching the absorbance vs time graph as well.)