IB Physics SL D 3 2 Charge in Magnetic Fields Questions

Calculate magnetic forces on moving charges and use circular-motion relationships to analyse particle paths, radii, frequencies and mass separation.

Exam points

calculate the magnetic force on a moving charge using charge, speed, magnetic field and the angle between velocity and field
determine force direction and explain circular motion, constant speed and constant kinetic energy when a charge moves perpendicular to a uniform magnetic field
apply radius and frequency relationships to mass spectrometers, cyclotrons and particle paths, comparing the effects of mass, charge, speed and field strength

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Study D.3.2—Charge in magnetic fields conceptsOpen the corresponding Concept lesson for explanations, examples and exam connections.

[Maximum number: 4]

(a)

A proton moves on a circular path in a region of uniform magnetic field of magnetic flux density B that is directed into the plane of the page.

[ 4 ]

(i)

On the diagram, draw an arrow to indicate the velocity of the proton at the position shown.

[ 1 ]

(ii)

Show that the frequency of revolution of the proton is given by $f=\frac{e B}{2 \pi m_{\mathrm{p}}}$ .

[ 3 ]

$\begin{aligned} e v B=\frac{m_{\mathrm{p}} v^{2}}{R} f=\frac{1}{T}=\frac{v}{2 \pi R} \end{aligned}$

Both q and e are acceptable for the
charge.

[3]

Algebra leading to required expression $\boldsymbol{<}=\frac{e B}{2 \pi m_{\mathrm{p}}} \boldsymbol{} \boldsymbol{}$

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IB Physics SL D 3 2 Charge in Magnetic Fields Questions question 1

Question (a)

Question (i)

Question (ii)