声音简介
One of these, of course, is gravity, the force between objects that have mass.
Another important force is the electromagnetic force.
The electromagnetic force is responsible for chemical reactions.
Without it there would be no atoms or molecules.
Up until a couple of hundred years ago, very little was known about this force.
Learning about the electromagnetic force and how to use it was a major step forward.
It revolutionized communication, transportation and manufacturing.
Imagine what life would be like without electricity!
The electromagnetic force has two parts, the electrical force and the magnetic force.
The electric force is the force between electrically charged particles such as electrons and protons.
Electric charges attract or repel each other with a force that depends on the distance between them.
The magnetic force is the force between magnetic poles, such as the north and south pole.
Why does the Earth have a magnetic field?
Many scientists believe that the Earth's magnetic field is due to the Earth's rotation.
Deep inside the Earth is a solid iron core, surrounded by a liquid metal outer core.
The moving electric currents inside this liquid metal create a magnetic field.
The Earth's magnetic field is important because it protects us from the solar wind.
The solar wind is a current of charged particles coming at us from the Sun.
When the solar wind nears the Earth, the Earth's magnetic field deflects it away.
This protects the Earth from the radiation, which can be dangerous to life.
It also protects the satellites that we use for communication.
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