Q23 of 26 Page 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.

a) The device used to change the alternating voltage to a higher or lower value is step up or step-down transformer. One of reasons for power dissipation in this device is production of eddy currents in core.


b) Relation between voltage in secondary coil with voltage in primary coil is given as



And relation between current in secondary coil with current in primary coil is given as



In step-up transformer if voltage is getting stepped up, then current in the secondary coil is getting reduced.


Hence the power loss(P = I2R) is reduced considerably while such stepping up is not possible for direct current.


c) Values shown by teacher: Concerned and ready to share knowledge.


Values shown by Geeta : good listener and interested towards technical knowledge.


More from this chapter

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21

Draw the typical input and output characteristics of an n-p-n transistor in CE configuration. Show how these characteristics can be used to determine (a) the input resistance (ri), and (b) current amplification factor (β).

 22

(a) Give three reasons why modulation of a message signal is necessary for long distance transmission.

(b) Show graphically an audio signal, a carrier wave and an amplitude modulated wave.

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(a) Define electric flux. Is it a scalar or a vector quantity ?

A point charge q is at a distance of d/2 directly above the centre of a square of side d, as shown in the figure. Use Gauss’ law to obtain the expression for the electric flux through the square.



(b) If the point charge is now moved to a distance ‘d’ from the centre of the square and the side of the square is doubled, explain how the electric flux will be affected.


OR


(a) Use Gauss’ law to derive the expression for the electric field (E) due to a straight uniformly charged infinite line of charge density λ C/m.


(b) Draw a graph to show the variation of E with perpendicular distance r from the line of charge.


(c) Find the work done in bringing a charge q from perpendicular distance r1 to r2 (r2 > r1).

 25

(a) State the principle of an ac generator and explain its working with the help of a labelled diagram. Obtain the expression for the emf induced in a 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 × 10–4 T and the angle of dip is 30° .


OR


A device X is connected across an ac source of voltage V = V0 sin ωt. The current through X is given as



(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 frequency of the ac ? Show this variation graphically.


(d) Draw the phasor diagram for the device X.