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Motional emf

Motional emf. Any change in magnetic flux induces an emf opposing that change – induction Motional emf – motion in a stationary B-field. Consider a conductor length l moving with constant velocity through a uniform B-field The electrons feel a force F = qvB directed downward

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Motional emf

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  1. Motional emf • Any change in magnetic flux induces an emf opposing that change – induction • Motional emf – motion in a stationary B-field

  2. Consider a conductor length l moving with constant velocity through a uniform B-field • The electrons feel a force F = qvB directed downward • Therefore positive charge on upper end & E-field is produced • Charge builds up until magnetic force counters electric force Fe = Fm  qE = qvB  E = vB & potential difference in conductor  Δ V = E l Δ V = B l v

  3. Consider a rod moving at v along a pair of conducting rails separated by l, in magnetic field B • Rails are connected to resistance R

  4. v = Δ x / Δ t • The area is increasing, therefore the magnetic flux is increasing, therefore there is an emf induced • Ε = N Δφ / Δ t = N Δ (B A) / Δ t = B l Δ x / Δ t  Ε = B l v motional emf

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