Magnetic Fields and CERN

Basic rule of Thumb:-

1. Current flow in Scotland =  Flow of Electrons (negative to positive)

2. Current flow for England and others = Conventional flow (positive to negative) or Proton beam


Why is this the case? Does it matter? For most electrical applications - No. But there is exceptions to this rule...

Fleming's Hand Rules

A charged object will produce its own electric field. If that charge moves it will then also produce a magnetic field. If the moving charge is in another magnetic field then it will experience a Force.

The direction of the Force can be determined if you know the sign on the charge and the direction of the magnetic field. Magnetic field by definition runs from north to south.

Electron Current = Fleming's Right Hand Rule

Conventional Current =  Fleming's Left Hand Rule

How the Rules Work...

First finger (index finger) = (Magnetic) field

Second finger (middle finger) = Current

Thumb = Direction of Force ('Thrust')

The video below shows a demonstration of a Current carrying piece of wire in a magnetic field and its resultant motion. (Warning - which version of Current is this?)

Applications of Fleming's Rules

As these rules are designed to work in 3 dimensions, in order to describe these accurately on paper (in 2 dimensions) we must use the following convention:-

To understand the above convention, it helps to imagine an arrow. With the arrow moving away from you (into the page) then what you can see is the arrow's flights. With the arrow moving towards you (out of the page) then what you can see is the arrow's tip. 

Fleming's Rules and CERN

To understand how physicists at CERN on the French-Swiss border create and control beams of charged particles, we must simply apply Fleming's rules. 

The first particle accelerators were linear accelerators. The original LINAC designed by Cavendish Lab in 1930 used a potential of up to 800kV to accelerate Protons down an eight foot long vacuum tube. This gave the protons an energy of 800keV ( 800,000 electron-Volts ). The beam of particles can then be directed at a target. Modern Linear accelerators are much more powerful, for example the Stanford Linear Collider give Protons an Energy of 80 GeV. 

Linear accelerators are useful but can be cumbersome due to the large length required to accelerate particles to high energies. In order to reach high energies with limited space, the Cyclotron was developed. 

In a Cyclotron, charged particles are injected into the centre of the device. An alternating Voltage between the 'dees', accelerating the charged particles. Once inside the 'dees', a magnetic field causes the charged particles to move in a circular path. This repeats over and over with the accelerating particles moving in ever-increasing radii circles within the device until the desired energy is reached and the particles are extracted. 

The final stage in the evolution of particle accelerators is the Synchrotron. 

Synchrotrons use electric and magnetic fields to accelerate charged particles around a ring. This is the technology behind the Large Hadron Collider at CERN. 

A Synchrotron use a continuous series of electromagnets to accelerate charged particles around the ring. As the particle's speed increases, the electromagnet's strength is increased to provide enough force to hold the charged particle in a circular path. At very high energies, the charged particles are moving so fast that Relativistic effects (increased mass) must be taken into account. 

Synchrotrons are used to probe the Standard Model through extremely high energy collisions of particles. As of June 2015, collisions were being performed with a combined energy of 13 TeV (13x1012eV).