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Sample Test paper Magnetism Feb 2010
(1). A coil of 200 turns is wound uniformly over a wooden ring having a mean
circumference of 600 mm and a uniform cross sectional area of 500 mm2. If
the current through the coil is 4A, calculate:
(a) the magnetising force
(b) the flux density
(c) the total flux
Ans H = 1333A/m, B = 0.001675T, Ф=8.375x10-7Wb
(2). A mild steel ring having a cross sectional area of 500mm2 and a mean
circumference of 400 mm has a coil of 2000 turns wound uniformly around it.
Calculate:
(a) the reluctance S in the ring.
(b) the current required to produce a flux of 800 μWb in the ring. (μr=380).
Answer: S = 1.675 ×106 A/Wb ,i=6.7 A
(3). A 6000 turn coil is wound around an iron core which contains a small air
gap. The coil carries a current of 20 mA which sets up a flux within the iron
and across the air gap. If the cross section of the iron is 8×10-5m2, the mean
length of flux path in the iron is 0.15m, μr=800 in iron and the air gap is 0.75
mm.
Calculate the flux density in the airgap.
Answer: B= 0.2T
OR
(4). A rectangular shaped magnetic circuit is made up from steel laminations.
The cross sectional dimensions of the iron circuit is 40mm x 50mm, of which
10 percent is taken up by insulation between the laminations. The magnetic
circuit has a mean length of 0.6m and an air gap of 2mm with an effective
area the same as the iron core. μr= 2000.
If a coil is wound with 800 turns, calculate the magnetizing current required to
produce a flux of 0.0025Wb across the air gap.
Answer I=3.178A
(5) An electromagnet has a coil of 1200 turns and an iron path 0.6m long
which produces a flux density of 0,15T in its 1mm air gap.
Calculate the value of the current in the coil. μr for iron is 2760.
(6) A meter movement coil has a resistance of 1kΩ and a current of 100μA
produces full scale deflection.(FSD).
Calculate the values of resistance that are placed in series or parallel with the
coil movement so that the meter range can be extended to read.
(a) 30 Volts (b) 200 mV (c) 30mA
-2Q6 contd
6 (a) Sketch, and fully label, a B-H characteristic for a magnetic iron core of a
transformer when a sinewave voltage is applied to the primary winding.
and identify and label:
- the saturation flux density
- the coercive magnetizing force
- the remanence flux density
- the direction of magnetization
(b) Explain what is meant by the term “hysteresis loss”
(c) State the unit of hysteresis loss.
(7) A current of 1.5 amps is passed through 250 conductors in an electric
motor armature. Each conductor is 150 mm long and lying in, and at, right
angles to a magnetic field of flux density 0.5T. If each conductor is situated 30
mm from the axis of rotation calculate :
(1) the force produced.
(2) the torque produced ( torque = force x radius )
(8) A straight conductor 0.35 m long moves at right angles to a magnetic field
of flux density 0.48T at a velocity of 5m/sec.
Calculate:
(a) the electromotive force e induced in the conductor and
(b) the force acting on the conductor if the conductor has a
resistance of 0.25 ohm.
(9) A coil of 1700 turns has a resistance of 12Ω and inductance of 0.6 henrys.
(a) Calculate the induced emf in the coil when it is linked with a magnetic
flux which changes from 0.03 to 0.012 weber in 0.01seconds.
(b) If the coil carries a current of 10mA which reverses every millisecond,
determine the value of emf e induced across the coil?
© Calculate the energy stored in the magnetic field of the coil when
connected across a supply voltage of 60V.