The Large Hadron Collider (LHC) is a ring about 17 miles around, buried under the border between France and Switzerland. Its job sounds simple: make protons move incredibly fast, then make them hit each other head-on. But nothing in the ring ever touches the protons. So what speeds them up, and what steers them?
You have already seen both tools this machine needs. A charge in an electric field feels a force. A moving charged particle in a magnetic field feels a force.
A proton carries a charge of +1, so an electric field exerts a force on it. The LHC places electric fields along the ring. Each time a proton passes through one, the field's force speeds it up.
A fast proton would fly off in a straight line. But a moving charged particle in a magnetic field feels a force. So magnets placed around the ring steer the protons along the circle. Two beams circle the ring in opposite directions, steered into head-on collisions.
In an accelerator, which field raises the protons' speed, and which field holds them on the circular path?
Electric fields speed charged particles up.
Magnetic fields steer moving charged particles.
The electric fields provide the speed; the magnetic fields provide the steering.
Why can an accelerator not speed up neutrons?
An electric field exerts a force only on charged particles.
A neutron carries a charge of 0, so the electric field exerts no force on it.
A neutron feels no force from the accelerating fields, so the machine cannot speed it up.
A proton circles an accelerator ring. Which field speeds it up?
In the LHC, what holds the protons on the circular path around the ring?
Accelerators can speed up protons and electrons, but not neutrons. What do protons and electrons have that a neutron lacks?
In an accelerator, one set of devices raises the protons' speed and another set bends their path into a circle. Which fields do the two jobs, in that order?
An accelerator works only on charged particles. Why?
A hospital accelerator speeds electrons up for cancer treatment. Partway along, the beam must turn a corner. Which field belongs at the corner?