There are several fundamental forces that hold our universe together.基本的，
One of these, of course, is gravity, the force between objects that have mass.
重力;万有引力 [物理学]质量;
Another important force is the electromagnetic force.电磁的;
The electromagnetic force is responsible for chemical reactions.电磁力是化学反应的原因。
Without it, there would be no atoms or molecules.
原子 分子
Up until a couple of hundred years ago, very little was known about this force.到;
Learning about the electromagnetic force and how to use it was a major step forward.
It revolutionized communication, transportation and manufacturing.使彻底变革
Imagine what life would be like without electricity.
The electromagnetic force has two parts, the electric force and the magnetic force.电磁的
The electric force is the force between electrically charged particles, such as the electrons and the protons.质子 微粒 电子
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.
In fact, the Earth is like a large bar magnet with a north and a south pole.
Many animals use the Earth's magnetic field to navigate during their migrations. 航行
We can also see this magnetic field when we use a compass.指南针;
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.被…环绕着的
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.使转向；使偏斜
This protects the Earth from the radiation which can be dangerous to life.
It also protects the satellites that we use for communication.人造卫星
