Home / IBDP Physics- D.3 Motion in electromagnetic fields- IB Style Questions For HL Paper 1A

IBDP Physics- D.3 Motion in electromagnetic fields- IB Style Questions For HL Paper 1A -FA 2025

IBDP Physics HL Paper 1 -FA 2025- All Chapters

Question

Negative charges move counter-clockwise in a circular coil with uniform magnetic field into the page.
 
 
 
 
 
 
 
 
 
What is the magnetic effect?
(A) Coil rotates clockwise
(B) Coil rotates counter-clockwise
(C) Coil diameter increases
(D) Coil diameter decreases
▶️ Answer/Explanation
Detailed solution

Negative charges moving counter-clockwise = conventional current flows clockwise
Using right-hand rule for magnetic force on current loops:
• Field into page
• Current clockwise
This creates an outward magnetic force on all parts of the loop, tending to expand it
✅ Answer: (C) The diameter of the coil will tend to increase

Question

An electron enters a region of uniform magnetic field at a speed \(v\). The direction of the electron is perpendicular to the magnetic field. The path of the electron inside the magnetic field is circular with radius \(r\).
The speed of the electron is varied to obtain different values of \(r\).
Which graph represents the variation of speed \(v\) with \(r\)?
▶️ Answer / Explanation
Detailed solution

With the velocity perpendicular to the magnetic field, the magnetic force provides the centripetal force:

\(F_B = qvB\) and \(F_C = \dfrac{mv^2}{r}\)

Equate them:
\(qvB = \dfrac{mv^2}{r}\)

Cancel one \(v\) and solve for \(v\):
\(qB = \dfrac{mv}{r}\)
\(v = \dfrac{qB}{m}\,r\)

So \(v \propto r\): a straight line through the origin.

✅ Answer: A

Question

A negatively charged sphere is falling through a magnetic field.
What is the direction of the magnetic force acting on the sphere?
(A) To the left of the page
(B) To the right of the page
(C) Out of the page
(D) Into the page
▶️ Answer / Explanation
Detailed solution

A moving charged particle in a magnetic field experiences a magnetic force given by:

\( \vec{F} = q\,\vec{v} \times \vec{B} \)

The sphere is negatively charged, so the direction of the magnetic force is opposite to the direction given by the right-hand rule for positive charge.

The velocity of the sphere is downward, and the magnetic field is directed from the north pole to the south pole as shown in the diagram.

Using Fleming’s left-hand rule (or reversing the right-hand rule for a negative charge), the magnetic force is directed into the page.

✅ Answer: (D)

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