Electromagnetic Induction Flashcards
What is the magnetic flux?
It he magnetic flux passing through a plane surface in a uniform magnetic field is the product of the flux density and the component area normal to the field.
What is the Weber?
It is the SI unit of magnetic flux. It is the magenegic flux passing though w plane surface of 1m2 placed normal to a uniform magnetic field of flux density 1T.
What is the magnetic flux passing through assurance of area A with normal at angle theta to the magnetic field of flux density B?
Phi = BA cos theta
If the normal to the surface is perpendicular to the field, what is the magnetic flux? Why?
- Component area = Acostheta = Acos90 = 0
For a coil with N turns, how is the flux linkage determined?
N x flux linkage.
Note: flux linkage is used instead of magnetic flux.
What is Faraday’s Law?
The magnitude of the induced emf in a conductor is proportional to the rate of change of its magnetic flux linkage (rate at which it cuts the magnetic flux)
e = change in N phi/ change in t
What is the magnitude of the emf e induced in a wire of length l moving at speed v in a uniform magnetic field of flux density B?
e = Blv
If l or v are not perpendicular to the field lines, then their component perpendicular to the field should be used.
The direction of the induced current can be predicted using the _____.
Fleming’s right-hand rule.
What is Lenz’s law?
An induced current will flow in a direction that produced effects to oppose the change that had induced the current.
Why is the induced current Not in the opposite direction of the change?
The induced current results in a magnetic force to oppose the change that had induced it. Thus the induced current must be perpendicular to create a magnetic force in the opposite direction of the change.
Using Faraday’s law and Lenz’s law, derive the equation for the induced emf.
Faraday’s law: magnitude of induced emf in a conductor is proportional to its magnetic flux linkage. Hence e = Ndphi/dt
Lenz’s law: direction of induced current is such that the force generated opposes the direction of the change in magnetic flux.
Negative sign is added to the eqn.
If the magnetic field across the body is not uniform (e.g. In a copper pipe), how is the induced emf affected?
It will be different in different parts of the body, giving rise to circulating currents within the body called eddy currents. Lenz’s law is applicable to eddy currents, and thus eddy currents give rise to force opposing the motion resulting in the change.
Using the conservation of energy, explain how energy dissipated by eddy currents slow the relative motion generating the eddy currents.
Energy dissipated by the eddy currents as thermal energy from energy from the relative motion, reducing available energy and thus reducing the motion.
What are some applications of electromagnetic induction?
a) Generator in power station, where a large electromagnet is made to rotate inside fixed coils, changing the magnetic flux linage continuously , creating an induced emf in the coils.
b) Transformer, an alternating current in the primary coil produces a varying magnetic field in the core. The secondary coil wound around the core experiencing varying magnetic flux linking it. Hence a varying emf is induced across the secondary coil.
Why is the core of a transformer laminated?
Alternating currents in the coil creates an alternating magnetic flux in the core, resulting in an alternating induced emf in the core. The induced emf gives rise to eddy currents. Laminated cores causes the eddy currents to be smaller by reducing the area for eddy currents. Thus since I is smaller, the energy losses are reduced.
