Obviously a biologist will have lots of knowledge and skills a physicist doesn't have. Mathematical models can be very useful even if they don't model the real world faithfully, as long as they capture part of the behavior to some precision. Physicists spend an extraordinary amount of time studying toy models to build up intuition, which together with an understanding how to proceed in more complicated solutions can be very useful. For example even though most real world EM problems won't have the symmetries required to come up with a closed form solution, the intuition gained from studying toy examples is still invaluable.

In the case of neurons you can model the membrane of the neuron and the interior and exterior to various degrees of accuracy as an EM problem, if you wish you can even add surface defects. Since the temperature is high, all quantum effects are essentially washed out, as are the need to model individual calcium ions etc., at least to first approximation. After the dust settles you can come up with a few effective parameters that model one neuron and an ODE that models its dynamics. As it turns out the ODE indeed exhibits dynamics similar to real world neurons in response to electric stimuli.

If you then couple those ODEs into larger system and try different coupling configurations (the coupling coefficients model the synapses between neurons), you have come up with a simplified model for parts of a brain.