Current Balance Practical for H2 Physics: Measuring Magnetic Flux Density with F = BIl
01 May 2026, 00:00 Z
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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 usingF = delta m g, plotFagainst currentI, and use gradient= B lwhen 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.
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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
2 | Apparatus
Typical apparatus may include:
- Magnets or a magnetic yoke
- Straight conductor segment in the field
- DC power supply
- Switch
- Ammeter
- Rheostat or variable resistor
- Balance
- Connecting leads
- Ruler or calliper to measure active wire length
The active length is only the part of the conductor inside the uniform magnetic field. Do not use the total length of wire in the circuit.
3 | Method
- Set up the conductor so the active section is perpendicular to the magnetic field.
