phy mc - eddy current

Ans: D
I damping a galvanometer
when currents passes through the coil, it experiences a couple and rotates.
the metal frame, on where the coil is winded, cut the field lines provided by the concave pole pieces during its rotation.
to oppose the change of magnetic flux, eddy current is induced such that its direction is to produce an opposing torque in order to achieve critical damping. i.e. the pointer stops very soon after having detected the current.

II braking an electric train
upon the rotation of the wheel, the wheel cuts the magnetic field produced by the electromagnet which is remotedly controlled. to oppose the change of magnetic flux, eddy current is induced in order to provide heavy damping to resist the rotation of the wheel, as rate of rotation declines, the speed of the trin decreases.

III induction furnace
it is a equippment to melt metals. as the melting point of metals are usually very high, great power is needed.
the furnace is winded by a solenoid, which is in turn connected with an a.c. source of high frequency. due to the repeatedly change of magnetic field by the a.c. source, eddy current is constantly induced in the metal to generate heat by joule heating effect.
as the magnitude of eddy current and hence the heat is directly related to rate of change of magnetic flux and consistently the frequency of the source. therefore, by raising the frequency of the a.c., the heating effect is enhanced

All of them
eddy current整停galvanometer支指針同個火車
An induction furnace uses induction to heat a metal to its melting point. Once molten, the high-frequency magnetic field can also be used to stir the hot metal, which is useful in ensuring that alloying additions are fully mixed into the melt