16 CHARGE AND FIELD Accelerating charges
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Transcript of 16 CHARGE AND FIELD Accelerating charges
16 CHARGE AND FIELDAccelerating charges
• Derive relativistic expressions for momentum and energy
• Solve problems involving particles accelerated to close to the speed of light
Electric fields
• Define the term electric field
• Investigate uniform electric fields and plot equipotentials
Uniform fields and Millikan’s experiment
• Derive the relation E = -V/d for a uniform
electric field
• Explain the principles of Millikan’s experiment for determining the charge on an electron
Deflection by electric fields
• Observe, model and explain the deflection of an electron beam in a uniform electric field
Deflections of electron beam by electric field
electron gun deflection plates
zero potential
hotcathode
spacing d
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anode+ +V
acceleratingpotential difference
vertical force F = eE = e Vd
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horizontalacceleration
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constant velocity
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vertical accelerationconstant horizontal velocity resultant velocity
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force
F = e Vd
electric field E = Vd
Deflection by magnetic fields
• Explain why charged particle beams follow circular paths when injected perpendicular to a magnetic field
• Determine e/m from experimental data
The bubble chamber track shown above represents a charged particle entering from the left. 1. The magnetic field is directed into the screen. Is the charge on
the particle + or -?2. Sketch the track for another particle of opposite charge to the
first entering from the right.3. Sketch the track of a particle entering from the left, with the
same charge as the first one, but greater kinetic energy.
Particle accelerators
• Explain principles of circular accelerators
• Explain the need for large radius accelerators (synchrotron effect)
What is the main drawback of a linear particle accelerator? (p168)
What is main the advantage of a circular accelerator over a linear one?
For really high energy particle beams in a circular accelerator, why does the ring need to have a very large radius?
How can scientists take advantage of the energy radiated when charged particles are centripetally accelerated? (Hint: the answer is nearby)
Crossed electric and magnetic fields
• Explain the principle of, and model, an ion velocity selector that uses perpendicular electric and magnetic fields
Non uniform fields
• Learn how to characterise field strength in terms of density of field lines
• Use symmetry and geometry arguments to determine field strengths
Field-potential relations and Coulomb’s Law
• By analogy with the gravitational case, deduce the relationship between electric field and potential
• Learn and apply Coulomb’s Law