Magnetic flux is defined as what?

• A. The number of magnetic field lines that pass through a surface.
• B. The total magnetic field strength in a volume.
• C. The rate of change of magnetic flux.
• D. The angle between magnetic field and surface.

Answer: (A)

Magnetic flux tells you how much of the magnetic field passes through a given surface. If the field is uniform, the flux through a flat surface is the component of the field perpendicular to the surface times the area, Φ = B A cos θ. More generally, it’s the surface integral Φ = ∫ B · dA, which reduces to the simple form when B is constant and the surface is flat. This quantity becomes the Weber in units, and it connects to dynamics through Faraday’s law: the induced emf equals the negative rate of change of the flux, ε = -dΦ/dt. The idea that flux is simply the total field in a volume or just an angle, or the rate of change itself, doesn’t capture this surface and orientation-dependent measure.