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In EM radiation, the magnetic field is $ 3*10^8$ times smaller than the electric field, but is it valid to say it's "weaker". These fields have different units, so I don't think you can compare them, but even so it seems like we only interact with the electric field of EM radiation, not the magnetic field. Why is this?

ACuriousMind
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aPhysicist
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2 Answers2

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As you already indicated, physical units need to be considered. When working in SI units, the ratio of electric field strength over magnetic field strength in EM radiation equals 299 792 458 m/s, the speed of light $c$.

However, the numerical value for $c$ depends on the units used. When working in units in which the speed of light $c=1$, one would conclude that both fields are equal in magnitude.

A better way to look at this is to consider the energy carried by an electromagnetic wave. It turns out that the energy associated with the electric field is equal to the energy associated with the magnetic field. So in terms of energies electric and magnetic fields are equals.

Johannes
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It's a quirk of units: notice that the conversion between them is dimensionful, and has the value $3\times 10^8\,\mathrm{m/s}$, which is the speed of light. In the CGS system both fields have the same units, and field-squared is an energy density.

In SI units, the energy density for a configuration of fields is given by \begin{align} \frac{dU}{dV} &= \frac12\left( \epsilon_0 E^2 + \frac1{\mu_0} B^2 \right)\\ &= \frac{\epsilon_0}2 \left( E^2 + c^2 B^2 \right) \end{align} which tells you that, given the field strength ratio $c$ you have calculated for electromagnetic waves, the energy of an EM wave is shared equally between its electric and magnetic components.

We do tend to usually think about the interaction between EM waves and materials in terms of the electric field component, but that's a bias because it's relatively easy to liberate free electric charges, and magnetic interactions only occur at second order. Spend some time around folks thinking about plasma physics, or wrap your head around the behavior of the sun's magnetic field reversal cycle, for a very different perspective.

rob
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