Magnetic flux
How much magnetic field passes through a surface, flux = B A cos theta. The unit is the weber.
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Analogy
Rain falling through a hoop: tilt the hoop and less rain goes through.
Example
flux = B A cos 0= 0.5 x 0.02 x 1= 0.01 Wb
In practice
A 0.5 T field through a 0.02 square metre loop, face-on, gives 0.01 Wb.
Practice
Flashcards
- What is the unit of magnetic flux?The weber (Wb), equal to one tesla square metre.
Interview questions
- Explain how a generator makes electricity using Faraday's and Lenz's laws.
- A coil rotates in a magnetic field
- Flux changes so emf is induced
- Lenz's law explains why more work is needed when current flows
Questions people ask
Why does the angle matter in flux?
Only the part of the field perpendicular to the surface passes through it. A field parallel to the loop's plane slides past and gives zero flux, while a field at right angles gives maximum. That is why flux contains cos theta, with theta measured from the loop's normal.
What changes flux in a loop?
Three things: the field strength B, the loop's area A, and the angle between the field and the loop. Moving a magnet nearer, stretching the loop or rotating it all change the flux. Any of these changes can induce a voltage in the loop.