# Cyclotron, why do they not use electric fields?

1. Dec 8, 2004

### gillgill

In a cyclotron, why do they not use electric fields to move them in circles?

2. Dec 8, 2004

### Diane_

I suppose you could. A charged particle moving through a magnetic field experiences a force perpendicular to the velocity - this is tailor-made for moving something in a circle. Since the force is always perpendicular to the velocity, a magnetic field will not change the speed of the particle. All it will change is its direction. An electric field perpendicular to the velocity of the particle would do the same thing, but as the particle started to turn, you'd either have to change the direction of the electric field to keep it perpendicular to the velocity of the particle or you'd have to accept that the speed of the particle would change. In addition, if you didn't change the field, the component of the force perpendicular to the velocity would gradually be reduced (as the velocity changed) and so the radius of curvature of the particle's path would change. This could seriously complicate things, depending on what you were trying to do.

Given a particle stream of a particular speed, it's pretty easy to figure the strength of a magnetic field that will give a particular curve. Since the point is to keep the particles inside the cyclotron, the whole engineering thing is made easier (it seems to me) by the use of magnetic fields.

Is that enough?

3. Dec 8, 2004

### gillgill

yea...thats enough...thanks...^^

4. Dec 8, 2004

### dextercioby

Though experimental particle physics is not my cup of tea,lemme try to give a justifiction of why they don't use in cyclotrones an electric field.
The electric field tends to drag the particles along the field's direction and to accelerate them.Let's assume for simplicity that both the magnetic field,and the elctric one are of constant magnitude.For obvious reasons,the particles need to be accelerated up to immense energies BEFORE ENTERING THE CIRCULAR RING.Why's that...??Because had there been an electric field which changes the magnitude and the speed of the particles which acted in the ring,no collision between the particles inside the ring would possible,since THERE WOULD NO PARTICLES LEFT,AS THEY ALL,UNDER THE INFLUENCE OF THE ELECTRIC FIELD WOULD BUMP INTO THE RING'S WALLS AND BE ABSORBED.
Form what i know,nobody has produced a circular loop of electric field.You could be having a standing/stationary em wave,but the necessity that the direction of the magnetic field be perpendicular to the circle of movement would require an electric field in the plane of the movements along the radius,since the wave vector should be kept tangent to the circle of movement.A radial electric field would definitely make the particles bump into the ring's walls.

5. Dec 8, 2004

### gillgill

thanks very much