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Question 3

From Physics HL Paper 1B (May 2025, TZ1)

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  1. A group of students wants to determine the horizontal component Bₕ of the Earth’s magnetic field.
    They place a magnet (in the form of a magnetic needle) midway between two coils.
    When there is no current through the coils, the magnet aligns itself in the north-south direction. When there is an identical current established in the coils, the magnetic field produced deflects the magnet.
Diagram showing two coils (coil 1 and coil 2) with a magnet placed midway between them. Dimensions 0.25r and r are indicated.
Diagram showing two coils (coil 1 and coil 2) with a magnet placed midway between them. Dimensions 0.25r and r are indicated.

(Question 3 continued)
The students have to decide on the horizontal separation of the two coils. Their choices are separations of 2r, r and 0.5r.
The variation with distance of the magnetic field strength due to each coil and the resultant magnetic field strength for both coils are shown for each of these separations. The dotted line represents the magnetic field from each coil, while the solid line shows the resultant magnetic field.

Three graphs showing magnetic field strength due to coils and resultant magnetic field for separations 2r, r, and 0.5r. The dotted line represents the magnetic field from each coil, while the solid line shows the resultant magnetic field.
Three graphs showing magnetic field strength due to coils and resultant magnetic field for separations 2r, r, and 0.5r. The dotted line represents the magnetic field from each coil, while the solid line shows the resultant magnetic field.

(a) State and explain which coil separation the students should choose for this experiment. [2]

(Question 3 continued)
(b) Explain why the students place axis XX′ of the coils in the east-west direction. [2]

(c) The deflection of the magnet is shown.

Diagram showing the deflection of a magnet between two coils, with arrows indicating the magnetic field due to Earth and the magnetic field due to the pair of coils. The magnet is shown deflected at an angle.
Diagram showing the deflection of a magnet between two coils, with arrows indicating the magnetic field due to Earth and the magnetic field due to the pair of coils. The magnet is shown deflected at an angle.

The magnet comes to rest when it makes an angle of 24° to XX′.
Determine, using the graphs, Bₕ. [2]

Textbook topics in this question

Review the exact course sections assessed by each question part.

Mixed-topic question
3(a)18.2 Magnetic Fields from Magnets and Currents
3(b)18.2 Magnetic Fields from Magnets and Currents2.2 Equilibrium and Net Force
3(c)18.2 Magnetic Fields from Magnets and Currents2.2 Equilibrium and Net Force