How earth behaves as an electromagnet?

Because the inner core of Earth is a solid sphere suspended in the gravitational center of the liquid outer core. Both cores are an alloy made up of mainly nickel and iron. This alloy is conductive and the inner core is spinning at a different rate in the outer core. This difference in angular momentum creates a huge electric dynamo, which in turn creates a strong magnetic field. If and when the core of the earth completely cools to a solid the dynamo effect will cease, and our magnetic field will collapse. If there is any atmosphere that remains at this time, it will be boiled off into space by our suns solar wind. The water will eventually follow the way of the atmosphere and the surface of earth will begin to look like the surface of Mars.