Fieldex

Meet Fieldex, a spark who knows that empty space between charged plates is not really empty.

An electric field stores energy, even in a vacuum. When you charge a capacitor, the work you do ends up stored in the field between its plates. The energy per cubic metre depends only on how strong the field is.

The energy density is u = ½ε₀E². It goes with the square of the field, so doubling the field stores four times the energy. Dry air breaks down and sparks at about 3 × 10⁶ V/m. At that field u ≈ 40 J/m³, about the energy needed to lift a 4 kg bag of shopping by one metre.

That limit is why capacitors use special insulating materials, which can hold much stronger fields without breaking down.

The same idea works for magnetic fields, and together they explain how light carries energy across space from the Sun to the Earth, all through fields.