# Does the earth has any electric field as the earth's magnetic field is changing over time? [duplicate]

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Earth's magnetic field changes over time because it is generated by a geodynamo -Wikipedia

A time-varying magnetic field can produce electric field. So does the earth has electric field due to changing magnetic field?

## marked as duplicate by Kyle Kanos, Jon Custer, ZeroTheHero, peterh, WolpertingerAug 18 '17 at 7:35

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## 2 Answers

There are various ways to relate the electric and magnetic fields. Semoi gives one example, but in this context perhaps it is simpler to use an alternative relation:

$$\mathbf E = -\nabla\phi - \frac{\partial\mathbf A}{\partial t}$$

This expresses the electric field $\mathbf E$ as the sum of two contributions. The first term:

$$\mathbf E = -\nabla\phi$$

is the contribution to the field from any charges present. The quantity $\phi$ is simply the electrical potential. As it happens the Earth has a net negative charge due to charge separation between the ground and the atmosphere, so it has an electric field regardless of what its magnetic field does.

The second term:

$$\mathbf E = - \frac{\partial\mathbf A}{\partial t}$$

gives the contribution to the electric field from the changing magnetic field. The quantity $\mathbf A$ is a type of magnetic potential energy called the magnetic vector potential.

And you are quite correct that any changes in the magnetic field of the Earth will indeed result in a contribution to the total electric field. However the sorts of changes that Wikipedia is describing happen on very long timescales and the rate of change $\partial\mathbf A/\partial t$ is so small that any contribution it makes to the electric field is negligable.

On the other hand the Earth's magnetic field changes on much shorter timescales due to changes in the solar wind. Indeed a very large solar flare can cause large and rapid changes in the magnetic field that produce large electric fields. These fields are large enough to cause large currents to flow in electrical transmission lines and can cause power outages as a result.

A time-varying magnetic field can produce electric field.

I think this statement is unclear. Rather: A time-varying magnetic field does produce an electric field. This is one of Maxwell's equation (maybe with some prefactors) $$\nabla \times E = - \frac{dB}{dt}$$ If the right-hand-side does not vanish, the left-hand side does not vanish.

Therefore, the change in the earth's B-field will produce an E-field.