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Magnetic fields (A-level only)

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Question 15

In a mass spectrometer, a beam of singly-charged sodium ions (23Na+^{23}\text{Na}^+23Na+) enters a velocity selector. The selector contains a uniform magnetic field of flux density B=0.25 TB = 0.25\text{ T}B=0.25 T and a perpendicular uniform electric field of strength EEE.

1.

Calculate the electric field strength EEE required so that sodium ions travelling at a speed of 1.8×105 m s−11.8 \times 10^5\text{ m s}^{-1}1.8×105 m s−1 pass through the selector undeflected.

[2]
2.

After emerging from the selector, the ions enter a deflection chamber containing only the uniform magnetic field of flux density B=0.25 TB = 0.25\text{ T}B=0.25 T. Show that the radius rrr of this circular path is given by:

r=mvBq r = \frac{mv}{Bq} r=Bqmv​
[3]
3.

Calculate the distance between the entry aperture and the point on the detector surface where these 23Na+^{23}\text{Na}^+23Na+ ions strike.

Given values:

  • Charge of a sodium ion, q=1.60×10−19 Cq = 1.60 \times 10^{-19}\text{ C}q=1.60×10−19 C
  • Mass of a 23Na+^{23}\text{Na}^+23Na+ ion, m=3.8×10−26 kgm = 3.8 \times 10^{-26}\text{ kg}m=3.8×10−26 kg
[4]
Markscheme

Magnetic fields (A-level only) Questions

  1. A Level
  2. /Physics
  3. /Magnetic fields (A-level only)

66 exam-style questions on AQA A Level Physics Magnetic fields (A-level only), covering Magnetic flux density (A-level only), Moving charges in a magnetic field (A-level only), Magnetic flux and flux linkage (A-level only), Electromagnetic induction (A-level only), Alternating currents (A-level only), and The operation of a transformer (A-level only). Each one has a worked solution and a mark scheme showing where the marks go.

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