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Motion in Electromagnetic Fields · Paper 2 — Free Physics HL Practice Questions

What this topic covers

Fields is where gravity, electricity and magnetism finally get shown to be the same trick played three times: a source (mass, charge, or moving charge), a field that falls off as $1/ r^{2}$ or sits uniform between plates, and a potential that tells you the energy cost of moving through it. HL bolts on two whole subtopics that SL never sees — motion in combined electric and magnetic fields, and electromagnetic induction — and a huge fraction of the marks in this theme come from just two calculation types: transformer turns ratios and induced emf from $ΔΦ/Δ t$ . Get those automatic, then spend the rest of your revision on the sign conventions ( $V_{g}$ is always negative), the vector-vs-scalar traps ( $g$ vs $V_{g}$ , $E$ vs $V$ ), and the direction rules (Lenz's law, $q v \times B$ ) that separate a 6 from a 7.

Read the full Fields lesson notes →

1MasterySAQ-SCharge in magnetic fields7 marksPaper 2~11 min

A research facility uses a velocity selector to filter charged particles. A beam of alpha particles (

\text{He}^{2+}

) enters a region where a uniform magnetic field of

0.20\,\text{T}

acts perpendicular to the beam, and a uniform electric field acts perpendicular to both the beam and the magnetic field. The alpha particles travel straight through without deflection at a speed of

5.0 \times 10^{5}\,\text{m s}^{-1}

\text{Charge of alpha particle} = 3.20 \times 10^{-19}\,\text{C}

\text{Mass of alpha particle} = 6.64 \times 10^{-27}\,\text{kg}

(a)

State the condition the net force for an alpha particle to pass through the selector undeflected. - [1 mark]

(b)

The alpha particles travel horizontally. The magnetic field points vertically upward and exerts a force on the particles directed to the left. Explain the direction the electric field must point, and justify why this arrangement produces straight-line motion. - [2 marks]

(c)

Calculate the magnitude of the electric field strength required. - [2 marks]

(d)

After leaving the selector, the electric field is switched off and the alpha particles continue into the same magnetic field of

0.20\,\text{T}

. Calculate the radius of the circular path followed by the alpha particles. [2 marks]

Solutions

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