Electromagnetic Induction Flashcards
Define magnetic flux φ. Give equation and SI unit
mag flux thru plane surface is pdt of flux density normal to surface & area of surface
mag flux thru area, φ = BA
SI Unit: weber (Wb)
*Scalar, not vector like mag flux density
Define magnetic flux linkage, Nφ. Give equation
mag flux linkage thru coil of N turns is pdt of no of turns N of coil & mag flux φ linking each turn
If coil of N turns is placed in a region w uniform B field, then flux linkage thru coil Nφ
Nφ = NBA
(B is normal to surface component)
- SI unit still weber (Wb)
What are the factors that determine magnitude of induced e.m.f.?
- change B field
- change orientat n of coil
- change cross-sectional area
- change no of coil
Does induced e.m.f. always mean induced current?
Not necessarily
- There can be induced emf w/o current
(Like battery not in a closed circuit)
- current flows oni when coil is part of closed circuit
Define Faraday’s Law. Give formulae
states that induced emf is directly proportional to rate of change of mag flux linkage
ε=- d(Nφ)/dt
where
ε is induced emf,
N is no of turns in coil
φ is mag flux thru coil
For N turns coil w mag flux φ thru coil, magnitude of induced emf is
|ε|= |N dφ/dt|
Define Lenz’s Law
States direct n of induced emf is such as to cause effects to oppose change producing it
(EMI) What is the 3 step approach for closed loop?
- Direct n of flux linkage? (Thru loop)
Change? (Increase/decrease)
Due to? (N, B, A or θ) - Apply Faraday’s Law
- mag flux linkage change
=> induces emf - If closed circuit, induced emf cause induced current
- Apply Lenz’s Law to determine direct n of current
=> flows in direct n to produce effect to oppose change in mag flux linkage
What is the formula for magnitude of induced e.m.f along straight conductor and along a coil of N turns
where
B, l, v perpendicular to one another
N is no. of turns (for coil)
l is length of conductor
v is speed of conductor
ε | = NBlv
What is the formula for potential difference across ends of an isolated straight conductor?
ΔV = Blv
*on conductor
(EMI) What is 3 step thinking strategy for isolated conductor?
- What motion cause change in mag flux?
- What is direct n of current due to motion of e- in conductor
- Apply Fleming’s left hand rule determine direct n of force on e-
What is formula of magnitude of induced e.m.f. across one spoke of rotating disk?
where
r is length of spoke (radius of disk),
f is frequency of revolut n
ε | = Bπr²f
Explain direction of induced e.m.f. of rotating disc
Consider e- within one of radial sect n as disc rotate, w instant velocity tangential of e- to its motion
-By FLHR, e- experience force Fb toward rim of disc, migrate in that direct n
- if external circuit connected across center & rim, current can flow
- if no external circuit connected, e- accumulate at rim of disc, and set up e- field
- as more e- accumulate at rim, e- field bcome stronger & migrat n of e- stop when Fe=Fb. An eqm is reached
- ctr of disc thus hv net +ve charge while rim net -ve charge
NOTE: Can use Fleming’s Right Hand Rule as shortcut but NOT for expl n
Explain formation of eddy currents
- When thick/broad piece conductor enter/leave B field, induced currents can b generated in separate loop within diff part of conductor
- Such induced current r called eddy currents, which dissipate energy, create B field that tend oppose change in B field
Explain how A.C. generator works
- as loop rotate, mag flux through it change w time, inducing emf & current in external circuit
- ends of loop connected to slip rings that rotate w loop
- connect n to external circuit made by stationary brushes in contact w slip ring
*current reverses direct n every time plane of loop pass vertical pos n
What is formula for maximum induced emf?
ε max = NBAω
where
ω is angular velocity (θ/t)
