EMI & AC Board Questions

2018 [SET-1]


The teachers of Geeta’s school took the students on a study trip to a power generating station, located nearly 200 km away from the city. The teacher explained that electrical energy is transmitted over such a long distance to their city, in the form of alternating current (ac) raised to a high voltage. At the receiving end in the city, the voltage is reduced to operate the devices. As a result, the power loss is reduced. Geeta listened to the teacher and asked questions about how the ac is converted to a higher or lower voltage.
(a) Name the device used to change the alternating voltage to a higher or lower value. State one cause for power dissipation in this device.
(b) Explain with an example, how power loss is reduced if the energy is transmitted over long distances as an alternating current rather than a direct current.
(c) Write two values each shown by the teachers and Geeta. (4)


(a) State the principle of an ac generator and explain it’s working with the help of a labelled diagram. Obtain the expression for the emf induced in the coil having N turns each of cross-sectional area A, rotating with a constant angular speed ‘ω’ in a magnetic field B, directed perpendicular to the axis of rotation.
(b) An aeroplane is flying horizontally from west to east with a velocity of 900 km/hour. Calculate the potential difference developed between the ends of its wings having a span of 20 m. The horizontal component of the earth’s magnetic field is 5 x 10-4 T and the angle of dip is 30°. (5)


A device X is connected across an ac source of voltage V=Vo sinωt. The current through X is given as I=Io sin(ωt+π/2).
(a) Identify the device X and write the expression for its reactance.
(b) Draw graphs showing variation of voltage and current with time over one cycle of ac, for X.
(c) How does the reactance of the device X vary with the frequency of the ac? Show this variation graphically.
(d) Draw the phasor diagram for the device X.


2017 [SET-1]


A long straight current carrying wire passesnormally through the centre of circular loop.If the current through the wire increases, willthere be an induced emf in the loop? Justify (1)


(i) Find the value of the phase difference between the current and the voltage in the series L-C-R circuit shown below. Which one leads in phase : current or voltage?
(ii) Without making any other change, find the value of the additional capacitor C1, to beconnected in parallel with the capacitor C, in order to make the power factor of the circuit unity. (3)

(i) Define the term ‘self-inductance’ and write its S.I. unit.
(ii) Obtain the expression for the mutual inductance of two long co-axial solenoids S1 and S2 wound one over the other, each of length L and radii r1 and r2 and n1 and n2, number of turns per unit length, when a  current I is set up in the outer solenoid S2. (3)

(i) Draw a labelled diagram of AC generator. Derive the expression for the instantaneous value of the emf induced in the coil.
(ii) A circular coil of cross-sectional area 200 cmand 20 turns is rotated about the vertical diameter with angular speed of 50 rad s−1in a uniform magnetic field of magnitude 3.0 ×10-2 T. Calculate the maximum value of the emf in the coil. (5)


(i) Draw a labelled diagram of a step-uptransformer. Obtain the ratio of secondary to primary voltage in terms of number of turns and currents in the two coils.
(ii) A power transmission line feeds input power at 2200V to a step-down transformer with its primary windings having 3000 turns. Find the number of turns in the secondary winding to get the power output at 220 V. (5)