Current Balance Practical for H2 Physics: Measuring Magnetic Flux Density with F = BIl

Q: How does a current balance measure magnetic flux density?
A: It measures the magnetic force on a current-carrying wire, converts a balance reading change into force, then uses the gradient of force against current to find the magnetic flux density.

TL;DR
A current balance turns the magnetic force on a wire into a small change in apparent weight. Convert the mass change to force using F = delta m g, plot F against current I, and use gradient = B l when the wire is perpendicular to the field. The key evaluation points are active wire length, field uniformity, heating, balance drift, and whether the wire is exactly at 90 degrees to the field.

Quick practical map

• TL;DR: Convert balance change to force, then graph force against current.
• Principle: Use F = B I l only when the wire is perpendicular to the field.
• Data Table and Graph: Calculate B from gradient / l and evaluate active length errors.

Pair this with H2 Physics practical lab mastery, H2 Physics spreadsheet skills, and H2 Physics ACE evaluation.

Status: SEAB H2 Physics 9478 syllabus checked 2026-05-01. Topic 17 includes magnetic force on a current-carrying conductor and measuring magnetic flux density using a current balance. Exact school-lab apparatus may vary.

1 | Principle

For a straight conductor of active length l carrying current I in a magnetic field of flux density B, the magnetic force is:

F = B I l sin theta

If the conductor is perpendicular to the field, sin theta = 1, so:

F = B I l

The current balance measures F indirectly. A force on the wire changes the balance reading by a small mass difference, delta m. Then:

F = delta m g

Concrete example: one row of data

If the balance reading changes by 0.20 g, convert it to 0.00020 kg before multiplying by g. The force is about 0.00020 x 9.81 = 0.00196 N, not 0.20 x 9.81 N.

2 | Apparatus

Typical apparatus may include:

• Magnets or a magnetic yoke

Reviewed by

Chee Wei Jie·Academic Advisor (Physics)

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