Magnetic Fields Flashcards
Which fingers represent which quantities in Fleming’s left hand rule?
- remember FBI
- thumb - force
- first finger - B (magnetic field / flux density)
- second finger - I (conventional current)
What symbol represents flux or current into the page
x
What symbol represents flux or current out of the page?
.
F = BIL when…?
I is perpendicular to B
For a charge travelling perpendicular to B in a magnetic field, what type of motion occurs?
circular motion occurs
Is work done by a centripetal force?
no
For a charge travelling perpendicular to B in a magnetic field, why is it that speed is constant?
- work done = force x distance
- d=0 and r = constant
- therefore speed is constant
For a mass spectrometer, why is it that only ions with the same velocity pass through into the mass analyser? (hint: use formula sheet)
- force due to B-field = Bqv
- force due to E-field = Eq
- pass through if F due to B-field = F due to E-field, thus E = Bv, v = E/B, so must all have the same velocity in order to pass through
What is meant by flux?
total magnetic field that passes through a given area (think of the number of field lines)
What is the unit for flux?
Weber [Wb]
What is meant by flux density?
the amount of magnetic flux per unit area
What is the unit for flux density?
Tesla [T]
What is meant by flux linkage?
the flux in all the loops added together (flux x number of turns)
What is the unit for flux linkage?
Weber turns [Wb]
In the equation: NΦ = BANcosθ, what is θ?
- angle between the axis of the coil and the flux
- or
- angle between flux and NORMAL of the coil
What is Faraday’s law?
Faraday’s law states that the induced emf is directly proportional to the rate of change of flux linkage
Derive that ε = BLv
- F = BIL
- W = FΔs
- W = BILΔs
- Q = IΔt
- ε = W/Q
- ε = BLΔs / Δt
- ε = BLv
What is Lenz’s law?
the induced emf will always attempt to oppose the change in flux linkage making it
What is the ‘recipe’ for explaining electromagnetic induction?
- relative motion between B-field and conductor
- causes ΔNΦ
- causes an emf
- causes an induced I (if there’s a complete circuit)
- causes an induced B-field
- two B-fields interact
- to oppose the ΔNΦ
What is the role of the soft iron core of a transformer?
to guide all the flux from the primary coil through the secondary coil
Explain how eddy currents can form in the soft iron core?
- the core is a conductor which contains ΔNΦ
- therefore an emf and (eddy) current could be induced
Give 2 reasons why eddy currents in the soft-iron core can be a cause of inefficiency for the transformer?
- will waste energy as heat in the core
- will have an associated B-field that opposes the ΔNΦ in the core
How can we limit the inefficiencies caused by eddy currents in the soft-iron core?
laminate the core with insulating glue
What is meant by hysteresis?
where iron atoms (in the soft iron core) rotate with the alternating B-field, friction causes them to heat up
