Why can you do this?
It can be done if B, as a function of x & t, and its first and second partial derivatives are all continuous.
How would you know if a function's first and 2nd partial derivatives are continuous?
If you have a formula for B, you can try and evaluate them.
If B is the magnetic field, you can assume so, because our empirical laws of physics say so.
(Most naturally occurring phenomena are infinitely differentiable.)
Yes, B is the magnetic field! Thanks! :)
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