IB Physics SL D.3 Motion in Electromagnetic Fields Question Bank

Practise IB Physics SL D.3 by analysing charged-particle motion in electric and magnetic fields with force, energy and trajectory evidence.

Exam points

Apply electric force and potential-energy relationships to charged-particle acceleration.
Use magnetic-force direction and circular-motion reasoning for moving charges.
Interpret trajectory, field-strength and energy data while stating vector assumptions.

Covered syllabus points

D.3.1—Charge in electric fields D.3.2—Charge in magnetic fields D.3.3—Crossed electric and magnetic fields D.3.4—Magnetic force on charge D.3.5—Force on current conductor D.3.6—Force between parallel wires

[Maximum number: 4]

Two oppositely charged parallel plates are a distance 8.0 cm apart. The potential difference between the plates is 120 V . An alpha particle is placed on the positively charged plate and released from rest. Gravity is ignored.

Figure for Question D.3 Motion in electromagnetic fields question 1 — IB Physics SL

(a)

Show that the acceleration of the alpha particle is about $7 \times 10^{10} \mathrm{~ms}^{-2}$ .

[ 2 ]

$\begin{aligned} F=<e E=>3.2 \times 10^{-19} \times 1.5 \times 10^{3} \text { OR } 4.8 \times 10^{-16} \mathrm{~N} a=<\frac{F}{m}=>\frac{4.8 \times 10^{-16}}{4 \times 1.67 \times 10^{-27}} \text { OR } 7.2 \times 10^{10} \mathrm{~ms}^{-2} \quad \end{aligned}$

Allow ECF from a).

Award [1] if they use a charge of e and a mass of 2 u obtaining the right result.

[2]

(b)

A magnetic field directed into the plane of the page is now established between the plates. An alpha particle enters the region between the plates with a horizontal speed of $5.0 \times 10^{5} \mathrm{~m} \mathrm{~s}^{-1}$ . The particle is not deflected.

Calculate the magnitude of the magnetic field.

[ 2 ]

$\begin{aligned} F_{\mathrm{m}} =F_{\mathrm{e}} \text { OR } q v B=q E \text { OR } B=F_{\mathrm{e}} / q v \quad B =\ll \frac{E}{v}=\frac{1.5 \times 10^{3}}{5.0 \times 10^{5}}=>3.0 \times 10^{-3} T \end{aligned}$

Award [2] if $3.0 \times 10^{-3}$ « T » is seen as the answer without working.

Allow ECF from a) and b) i)

[2]

[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 $\boldsymbol{<}=\frac{e B}{2 \pi m_{\mathrm{p}}} \boldsymbol{} \boldsymbol{}$

All question bank results loaded

Covered syllabus points

D.3 Motion in electromagnetic fields question 1

Question (a)

Question (b)

D.3 Motion in electromagnetic fields question 2

Question (a)

Question (i)

Question (ii)