CCEA A Level Physics
CCEA A Level Physics 5.6 Deflection of Charged Particles and 5.7 Particle Accelerators Exam Questions
Free CCEA A Level Physics revision for 5.6 Deflection of Charged Particles and 5.7 Particle Accelerators: what you need to know, common mistakes and exam-style questions with answers. Covers forces on charges in electric and magnetic fields, circular paths, synchrotrons, relativistic mass increase, antimatter and annihilation.
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About CCEA A Level Physics 5.6 Deflection of Charged Particles and 5.7 Particle Accelerators
These sections in Unit A2 2: Fields, Capacitors and Particle Physics bring the field topics together. You need to calculate the forces on charged particles in electric fields (F = qE) and magnetic fields (F = Bqv), and explain why a magnetic field makes a charged particle move in a circle. Particle accelerators then use both fields: electric fields to accelerate and magnetic fields to steer. You need to describe how a synchrotron works, explain the effect of relativistic mass increase at speeds close to c, and describe antimatter, annihilation and the conservation laws that apply. Expect radius of path calculations and explanations of synchrotron design.
What you need to know
- Electric field force F = qE acts parallel to field lines (towards the negative plate for a positive charge), so a particle entering a uniform field at right angles follows a parabolic path.
- Magnetic field force F = Bqv for a particle moving perpendicular to B. The force is always perpendicular to velocity, so it changes direction but not speed, producing circular motion with Bqv = mv^2/r, so r = mv/Bq.
- Synchrotron Particles travel in a ring of fixed radius. Electric fields in accelerating cavities increase their energy each lap, while bending magnets steer them round. As the particles gain momentum the magnetic field is increased to keep the radius constant.
- Relativistic mass increase As a particle's speed approaches the speed of light its mass increases, so each further increase in energy produces a smaller increase in speed. Its speed can never reach c.
- Antimatter and annihilation Every particle has an antiparticle with the same mass but opposite charge. When a particle meets its antiparticle they annihilate, converting their mass into energy, usually two gamma photons moving in opposite directions, so that charge, energy and momentum are conserved.
Common mistakes in 5.6 Deflection of Charged Particles and 5.7 Particle Accelerators
- Saying a magnetic field speeds up a charged particle; the force is perpendicular to velocity so no work is done and speed is unchanged.
- Using F = Bqv when the particle moves parallel to the field; then the magnetic force is zero.
- Saying the electric field in a synchrotron steers the particles; electric fields accelerate, magnets steer.
- Predicting a single photon from electron-positron annihilation at rest; two photons are needed to conserve momentum.
Try three 5.6 Deflection of Charged Particles and 5.7 Particle Accelerators exam-style questions
1. An electron moving at 3.0 x 10^6 m s-1 enters a uniform field of 2.0 mT at right angles. Calculate the radius of its path. (3 marks)
Show the mark scheme answer
Bqv = mv^2/r so r = mv/Bq; r = 9.11 x 10^-31 x 3.0 x 10^6 / (2.0 x 10^-3 x 1.6 x 10^-19); = 8.5 x 10^-3 m.
2. An electron and a positron, both at rest, annihilate. Calculate the energy of each photon produced. Electron mass = 9.11 x 10^-31 kg. (3 marks)
Show the mark scheme answer
Total energy = 2mc^2 = 2 x 9.11 x 10^-31 x (3.0 x 10^8)^2 = 1.64 x 10^-13 J; two photons share it equally; each = 8.2 x 10^-14 J (0.51 MeV).
3. Explain why the magnetic field in a synchrotron must be increased as the particles gain energy. (3 marks)
Show the mark scheme answer
Radius r = mv/Bq = p/Bq; as particles are accelerated their momentum increases; to keep r constant within the fixed ring, B must increase in proportion to the momentum.
5.6 Deflection of Charged Particles and 5.7 Particle Accelerators FAQs
What does CCEA A Level Physics 5.6 and 5.7 cover?
Forces on charged particles in electric and magnetic fields, circular paths in magnetic fields, how a synchrotron works, relativistic mass increase, antimatter, annihilation and conservation laws.
Which unit are 5.6 and 5.7 examined in?
Both are in Unit A2 2: Fields, Capacitors and Particle Physics.
What is the key equation for charged particles in a magnetic field?
Set the magnetic force equal to the centripetal force, Bqv = mv^2/r, which gives r = mv/Bq. Most deflection and accelerator questions start from this.
