Van de Graaff & the Particle Accelerator

Vine video of the GSC’s Van de Graaff generator:

Who was Van de Graaff?

Robert Jemison Van de Graaff was born in Alabama in 1901. He eventually made his way to college, earning degrees in mechanical engineering and physics. It was during this time that Van de Graaff recognized the need for the development of a particle accelerator, prompted by the fact that many of his fellow scientists were seeking ways to disintegrate nuclei. Recent discoveries had shed light on the elusive atom, and by probing even deeper into the atom, scientists hoped to gain a better understanding of these building blocks of matter. By 1929, he had successfully constructed the first working model of an electrostatic generator (80,000 volts); improvements were made and by 1931, he had constructed a 1,000,000 volt generator. This second generator was two stories tall and is still on display in the Boston Science Museum where it is used daily to teach public audiences about lightning and electricity.

About the Van de Graaff generator:

There are two basic Van de Graaff generator designs: one charges the belt at the rotors, and the other uses friction to strip off electrons. The one we house at the GSC is of the latter variety. Our generator uses a pulley attached to an electric motor to move electrons from its lower end. A rubber belt passes over this pulley; as the pulley turns, friction occurs between it and the rubber belt. This resulting friction strips off electrons from the pulley, causing the belt to have a net negative charge. Why? When two insulators are rubbed together, one loses electrons to the other. Electrons are negatively charged, so that which gains the electrons becomes electrically negative while the other becomes electrically positive.

These charges are then carried by the belt to the top of the machine where, because they are attracted to the metal brush, they collect on the metal dome (which is a conductor). Referring back to the video: since the dome and the metal plates sitting atop of it have the same charge, the plates are repelled away.

Why does it matter?

In general, generators are used to power any electrical device. More specifically, however, the Van de Graaff generator was designed to power particle accelerators. In fact, the very first accelerators were made possible by Van de Graaff’s generator because they required large voltages that only his generator could produce.

Since the days of Van de Graaff, particle accelerators have gradually been modified and improved upon, each subsequent design capable of generating more energy than the ones that came before it, ultimately allowing scientists to probe matter on increasingly smaller scales. The most recent accelerator is called the Large Hadron Collider (LHC) and is located in Geneva, Switzerland. Every year the LHC uses 1.3 Terawatts of electricity to accelerate particles, enough to power 300,000 homes in the same amount of time.

Large Hadron Collider

Large Hadron Collider

Why accelerate particles at high speeds? In order to understand the largest parts of the universe, we must first look to its smallest components. To do this, we must harness the energy of those large components. Particle accelerators exist to shatter atoms, thereby showing us what the tiniest indivisible blocks of matter are. For example, the LHC has recently verified the existence of a particle that explains why all matter has mass in the first place, confirming a half-century old theory that was previously untestable. This is yet another practice that contributes significantly to the goal of understanding the world around us using science.

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