Electromagnetic Force
There are several fundamental:基本的 forces that hold all universe together.
One of these,of course,is gravity,the force between objects that have mass:块.
Another important force is the electromagnetic force.
The electronmagnetic force is responsible for chemical reactions:反应.
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:革命性 communication,transportation and manufacturing.
Imagine what life would be like without electricity.
The electronmagnetic force has 2 parts,the electric force and the magnetic force.
The electirc force is the force between electrically charged particles:带电粒子 such as electrons:电子 and protons:质子.
Electric charges attract or repel:驱除 each other with a force that depends on the distance between them.
The magnetic force is a force between magnetic poles:磁极 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:导航 during their migrations.
We can also see this magnetic field when we use a compass.
Why does the earth have a magnetic field?
Many scientists believe that the earth's magnetic field is due to the earth's routation.
Deep inside the earth is a solid iron core:实铁芯 surounded by a liquid metal outer core:液态金属外核.
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:太阳能.
The solar wind is a current of charged praticles coming at us from the sun.
When the sloar wind nears the earth,the earth's magnetic field deflects it away.
This protects the earth from the radiation:辐射 which can be dangerous to life.
It also protects the satellites:卫星 that we use for commnication.
Magnetic Field Reversal
Scientists now believe that the earth's magnetic field is about to reverse.
This is called magnetic field reversal.
Every 200,000 to 300,000 years ,the earth's magnetic field reverses.
In other words,the north and south magnetic poles switch places.
This is happened to many times in earth's history,but it has been happening more lately.
Recent rearch provides evidence that there have been 5 magnetic reversals during the past 500,000 years.
The most recent event happened approximately:约 41,000 years ago.
Recent studies have found that the earth's magnetic field has weakened about 5% per decade:每十年.
As its current rate,the next reversal could happen within the next century.
Some people worry about the effects of a magnetic field reversal.
They think there is a possibility that it could cause many problems.
For example,without a magnetic field,communications could be disrupted.
Birds and other forms of life could become confused and get lost.
Another effect would be an increase in some forms of cancer,especially skin cancer.
This would be because of an increase in the amout of radiation:辐射 absorbed by our bodies.
The possibility that a field reversal would be an extinction:灭绝 event is very low.
There is very little historical evidence to show that previous reversals have caused extinctions.
Humans have been on the earth for millions of years,during which there have been many reversals.
Therefore:因此 ,scientists believe that any changes would be temporary:临时暂时.
Another theory:理论 is that the earth's outer layer could rotate:回转 around the liquid metal outer core that makes up most of the earth.
So in the next reversal,continents:大陆 could shift:转移 positions from north to south.
Imagine if that happened,Austrilia would be in the northern hemisphere:半球.
With so many theories,how can we know what to believe?
This is important because some of events require us to act well in advance.
Otherwise:除此之外 it will be too lante to prevent:避免 a catastrophe:灾难.
This underlines:强调 the importance of getting reliable information:可靠信息 backed by strong evidence.
(An increase in radiation causes some forms of cancer.)