Magnetism and Current Flashcards
At a microscopic level ferromagnetic material is magnetized by:
A. Aligning it with the Earth’s magnetic field
B. Passing a small electric current through it
C. Aligning its unpaired electron spins in a magnetic field
D. Pairing up unpaired electron spins
E. Taking the material close to the equator
C. If unpaired electron spins within microscopic domains are aligned within a magnetic field of sufficient magnitude, then the material is magnetized
A. Incorrect. This is usually insufficient to permanently magnetize the material except near the poles where the magnetic flux density is high.
B. Incorrect. This is usually insufficient to permanently magnetize the material, though a large current might.
D. Incorrect. Unpaired spins cannot be paired.
E. Incorrect. The magnetic flux density at the equator is low, so magnetization is unlikely
What do you understand by ‘hysteresis’ in magnetic terms?
A. Magnetized material has a permanent associated magnetic field
B. A magnetic field can be reversibly applied to a material
C. Residual magnetism will remain on removal of the applied magnetic field
D. There is no energy loss in the magnetization – demagnetization process
E. It represents saturation of magnetic field strength
C. Residual magnetism on removal of a magnetic field creates the irreversibility associated with hysteresis.
A. Incorrect. While this may be true of some materials, it does not define hysteresis.
B. Incorrect. Magnetization is seldom associated with reversibility, lack of which is a manifested as hysteresis.
D. Incorrect. The fact that the magnetization – demagnetization process is irreversible, creates the hysteresis loop in the B – H curve, the area of which represents heat lost in the cyclic process.
E. Incorrect. Saturation in the process of magnetization is not a feature of hysteresis.
A coil of wire is more efficient than a straight wire in producing a magnetic field because:
A. Neighbouring coils produce mutually opposing magnetic flux
B. The magnetic permeability is increased
C. Neighbouring coils induce neighbouring reinforcing magnetic flux
D. An opposing voltage is induced in the coil
E. A larger current is induced
C. This is the reason a coil creates a stronger magnetic field.
A. Incorrect. The neighbouring fluxes reinforce each other.
B. Incorrect. A coil does not affect permeability.
D. Incorrect. While it is true that the voltage induced in the coil opposes the driving voltage, this is not a cause of a stronger magnetic field, but an effect.
E. Incorrect. The opposing voltage will reduce current change flowing in the coil, which will only partially offset the enhanced magnetic field.
What feature does the voltage induced by the magnetic field around a coil have?
A. It enhances the magnetic field
B. It opposes the change in current flowing in the wire
C. It enhances the change in current flowing in the wire
D. It enhances the voltage drop along the wire
E. Its magnitude is maximum with DC
B. This is the characteristic of such an induced voltage.
A. Incorrect. If anything the induced voltage opposes the driving voltage, which will attenuate the induced magnetic field.
C. Incorrect. As the induced voltage opposes the driving voltage it will in fact reduce the change of current in the wire.
D. Incorrect. As the induced voltage opposes the voltage difference down the wire, it therefore reduces it.
E. Incorrect. Since the induced voltage is proportional to the rate of change of current in the coil, it will be zero when that current is DC.
