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If things are made from atoms, why do<br>different things have different properties?

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different things have different properties?)

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What do we use to measure mass?

How fast do electrons move?

As fast as you can get them going! Well not quite. One of the facts of life discovered in the 20th century is that the speed of light (300,000 kilometers per second) is the ultimate speed limit. As you add energy to the electron, it will go faster, but as you get it to go close to the speed of light, you find that you have to add even more energy just to bump it a bit faster. For example, with just over 220,000 eV (which stands for a convenient unit of energy called the "electron-volt"), you can get the electron up to 90% of the speed of light. But to get it to 99.9% (just another 9.9%), you need a total of over 11 million eV! One way of looking at this is that the electron gets "heavier" (more massive) as it goes ever faster. So it's harder to push it faster. At Jefferson Lab, a typical energy for the electrons in the beam is 4 GeV which is 4 billion eV. That means the electron is traveling at 99.9999992% of the speed of light. Close but still not 100%.

You may wonder how fast the electrons are whizzing around in the atoms around you. A good example (and the most simple to calculate) is the hydrogen atom which is in all our water. A calculation shows that the electron is traveling at about 2,200 kilometers per second. That's less than 1% of the speed of light, but it's fast enough to get it around the Earth in just over 18 seconds. Read up on what happens when nothing can go faster than the speed of light.


Carl Zorn, Detector Scientist (Other answers by Carl Zorn)