This episode requires students to apply their knowledge of charged particles and fields.

Lesson Summary

• Discussion and worked example: Acceleration in an electric field (15 minutes)
• Student activity: Researching accelerators (30 minutes)
• Demonstration: Electrical breakdown (15 minutes)
• Discussion: How a linear accelerator works (15 minutes)
• Discussion: Particles in a magnetic field (10 minutes)
• Demonstration: Fine beam tube (20 minutes)
• Student questions: Calculations (30 minutes)
• Discussion (optional): Relativistic effects and Bertozzi’s experiment (15 minutes)
• Visit (optional): Take a trip to CERN (A long weekend)

Discussion and worked example: Acceleration in an electric field

Why accelerate particles? Following Rutherford’s alpha-scattering experiment, physicists wanted to probe matter with beams of particles that were more energetic, more intense and purer.

How can particles be speeded up? (use an electric field.) Won’t a magnetic field do? (particles are accelerated, but the force is centripetal, so their speed does not increase.)

Calculate the speed of an electron (or proton) accelerated through 10 kV. What equation to use?

12  ×  m  ×  v² = e  ×  V

where

e = 1.6 × 10^-19 C

and

m = 9.1 × 10^-31 kg

v =     2qV m approximately equal to 6 × 10⁷ m s^-1

Take care! This is approaching speeds where relativistic effects need to be taken into account.

Will a proton travel faster or slower than this? (slower, because charge is the same but mass is greater.)

In the largest research accelerators, energies are so great that they recreate the conditions minuscule fractions of a second after the Big Bang (typically ~ 10^-10 s for LEP and a planned ~ 10^-12 s for the Large Hadron Collider (LHC) opening in 2007).

Student activity: Researching accelerators

Find out about the development of linear and circular accelerators. Identify important spin-offs (e.g. the development of www, computer graphics, body scanner magnets, isotope production for medicine and industry, material processing etc.)

Episode 518-1: Some information about LEP at CERN (Word, 30 KB)

Demonstration: Electrical breakdown

In linear accelerators, the approach is to get as large a voltage as possible, and to apply it to the particles several times. A practical limit to voltage difference is set by the ability of materials to withstand the electric fields involved. You can demonstrate electrical breakdown.

Episode 518-2: Electrical breakdown (Word, 61 KB)

Discussion: How a linear accelerator works

Explain the construction of the linear accelerator. The drift tubes get longer as the particles move faster. But at the highest speeds approaching that of light, increase in energy makes very little difference to the speed, so the drift tubes are the same length.

Episode 518-3: The linear accelerator (Word, 64 KB)

Discussion: Particles in a magnetic field

There is an advantage in making the particles travel around in a circular path – they can be accelerated time and again. Discuss how the particles trajectories are bent into a circular path with a magnetic field to bring them back to the accelerating electrical field many times. Compare with an electric field.

Recap the equation for this

mv²r = Bqv.

Episode 518-4: How a magnetic field deflects an electron beam (Word, 37 KB)

Episode 518-5: How an electric field deflects an electron beam (Word, 158 KB)

Demonstration: Fine beam tube

Do this if you haven’t previously done so in

episode 413

Episode 413-2: Measuring the charge to mass ratio for an electron (Word, 214 KB)

Show the fine beam tube with Helmholtz coils to provide a magnetic field.

Episode 518-6: The fine-beam tube (Word, 34 KB)

Student questions: Calculations

Your students now know the equations needed to solve many problems relating to accelerators. You may have covered these questions in Episode 413, if not students should try them now.

Episode 413-3: Deflection with electric and magnetic fields (Word, 89 KB)

Episode 413-4: The cyclotron (Word, 49 KB)

Episode 413-6: Charged particles moving in a magnetic field (Word, 65 KB)

Also try:

Episode 518-7: Fields in nature and in particle accelerators (Word, 66 KB)

Discussion (optional):Relativistic effects and Bertozzi’s experiment

Your students should be aware that, at relativistic speeds, things become more complicated. One way to present this is to discuss Bertozzi’s experiment.

Accelerators such as the synchrotron are designed to compensate for the effective increase in m by controlling the frequency of the accelerating voltage as the particles speed up.

Episode 518-8: The ultimate speed – Bertozzi's demonstration (Word, 48 KB)

Episode 518-9: Principle of the synchrotron accelerator (Word, 37 KB)

Visit (optional): Take a trip to CERN (a long weekend)

You can organise a trip to CERN.

Visit CERN

This episode discusses the idea of particle detectors and how to explain tracks, accompanied by a cloud chamber demonstration.

Lesson Summary

• Discussion: The idea of particle detectors (10 minutes)
• Demonstration: Cloud chamber (and spark detector) (15 minutes)
• Discussion: Explaining tracks (10 minutes)
• Student questions: Interpreting tracks (20 minutes)
• Particle tracks: Try analyzing some particle tracks (time permitting)

Discussion: The idea of particle detectors

What particle detectors do you know of? (Spark Counters, Geiger counters.) What do these tell us? (They count particles of ionizing radiation.)

What else might we want to know about particles? How could we tell which particle we have detected? (Need to know a range of properties: e.g. mass/energy, electric charge, momentum, lifetime, etc.)

In general, detectors work by analyzing particle collisions using conservation laws (momentum, energy, charge).

Demonstration: Cloud chamber (and spark detector)

Cloud and bubble chambers make visible the invisible: alpha diameter, d ~ 10^-14 m gives a visible track 0.1 mm wide, a factor of 10¹⁰ increase in size!

If you have access to a spark detector, you could also demonstrate this at this point.

You may have demonstrated a spark counter before?

Episode 509-4: Rays make ions (Word, 71 KB)

Episode 518-2: Electrical breakdown (Word, 61 KB)

Episode 519-1: Range of alpha particles with a cloud chamber (Word, 43 KB)

Discussion: Explaining tracks

Students may be familiar with the patterns made by particles in detectors, and so you could discuss the basic ideas behind analysing the tracks. The length of track is related to the energy of the particle, and also to its lifetime if it decays with a very short half life. Magnetic fields deflect charged particles and so bend their tracks. The curvature depends on momentum, charge and the strength of the field.

How could you tell whether a particle had positive or negative charge? (Curving to left or right; Fleming’s rule.) If a track is a spiral, what does this tell you about the particle’s motion? (It is slowing down; charged particles radiate as they are accelerated, so they slow down.

Episode 519-2: Measuring the momentum of moving charged particles (Word, 42 KB)

Student questions: Interpreting tracks

Students can apply their knowledge to the interpretation of tracks from a bubble chamber.

Episode 413-6: Charged particles moving in a magnetic field (Word, 65 KB)

Particle tracks

Episode 519-3: Particle tracks (Word, 612 KB)