Frequently Asked Questions

The following are questions submitted to us. Helen Quinn, content provider for this web site, offers answers to the questions.

1. Why is so much energy produced when an atom is split or fused?
2. More about time dilation
3. How does a cyclotron work?
4. Have quarks been observed and isolated in the laboratory?
5. Is mass always conserved?
6. How can the exchange of a photon attract a proton and an electron, yet repel two electrons?
7. More about the speed of light.
8. I was looking for a SU3 chart of the quark model.
9. Why is the visual spectrum continuous if it is produced by electrons going from one quantum state to another within the atom?
10. Time, microphysical processes, and probability.

FAQ3: How does a cyclotron work?

First you have to understand two basic points about electric and magnetic fields and their effects on charged particles.

1. When a charged particle is in a electric field it feels a force that accelerates it in the direction of the field (or in the direction opposite to that direction if it is a negatively charged particle). If this force is in the direction that the particle is already traveling then clearly this acceleration speeds up its motion and thus adds energy (and this is what we want our accelerator to do).
2. When a charged particle is moving through a magnetic field region it feels a force that is perpendicular to its direction of motion (and also perpendicular to the magnetic field). Such a force makes the particle change direction but does not change its speed. This means that in a large enough region of magnetic field the particle will travel in a circle. The size of the circle depends on the speed of the particle and the strength of the magnetic field.

Now how can we use these two facts to design an accelerator --a cyclotron is one example. We make the region of magnetic field by having a pair of large flat magnets, one above the other, with opposite poles facing, so there is magnetic field pointing down from the upper magnet towards the lower one. We arrange two such regions, each one D-shaped (when looked at from above) with the straight sides of the two D's facing one another (i.e. one D is backwards). Now we have a place where a moving electric charge (or rather a bunch of such charges) goes around half a circle in one D, then goes straight ahead till it reaches the other D, and makes another semicircle in that one, and so on.

So now what we have to do is arrange to have an electric field turn on in the right direction (and at the right time) to give the charges a bit of a push each time they cross the gap between the two D's. You can see that the electric field has to reverse its direction while the charge is going around the semicircle inside the D, so that when the charges cross the gap again in the opposite direction they are again accelerated a little.

You also need to build a chamber that you can evacuate to very low air pressure in the entire region where your charged particles are traveling -- between the two pairs of D-shaped magnets and in the gap between them. This is because you will keep losing your accelerated particles if they collide with air molecules, so you want as little air (or anything else) as possible inside your accelerator.

Because the particle is speeding up each time it crosses from one D to the other it travels in a spiral path with increasing radius. So the limit on what energy you can get with such a machine is given by the size of the D-shaped magnets, and the vacuum-chamber between them. This limitation makes it very expensive to build a high energy cyclotron and so modern high energy circular accelerators are built using a different design, known as a synchrotron.

The basic physics principle is the same, you use magnet to make the particle go in a circle, and regions with electric field in them (usually radio frequency or microwave cavities) to accelerate the particles. You make the vacuum chamber a tube that goes in a circle. Then you must adjust the magnet strength (these are electromagnets) as the particles speed up to keep the same radius for their circular path. There is a limited range of energy over which this can be achieved, so if you look for example at the Fermilab accelerator (see www.fnal.gov) you can see they have a series of rings of increasing radius and then feed the particles from a smaller ring to a larger one once they reach the highest energy that can be made to circulate in the small ring.