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 alternating voltage to a higher or lower value is a transformer.
Causes of power dissipation in this device are :
1. Core losses due to eddy currents and hysteresis loop due to alternating flux.
2. Copper losses due to resistance of winding in primary and secondary coils.
3. Loss of power due to leakage of magnetic flux in coil.

(b) The loss of power in the transmission lines is I²R, where I is the strength of current and R
is the resistance of the wires. To reduce the power loss a.c. is transmitted over long distances
at extremely high voltages. This reduces I in the same ratio.
Therefore, I²R becomes negligibly low. For the same reason, at the generating stations, the
voltage is stepped up to transmit it over long distances to minimize power loss.
Therefore a.c. is used because stepping up is not possible for direct current.

(c) Two values each displayed by teacher and Geeta