Question

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.

1) Name the device used to change the alternating voltage to a higher or lower value. State one cause for power dissipation in this device.

2) 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.

3) Write two values each shown by the teachers and Geeta.

Answer 1

1) The transformer is a device used for changing the alternating voltage to a higher or lower value. Formation of eddy currents in the iron core of the transformer is one of the causes of power dissipation in this device.

2) In the long-distance transmission of energy through transmission lines, the power loss is mainly due to resistive loss. Now, this resistive loss is proportional to the square of current flowing through the lines i.e

`H prop I^2`

So, to minimise this loss, we need to minimise the current flowing in the wire. In case of dc, if we decrease the current flowing in the wire, though the power losses will decrease but simultaneous power received at the receiving station will also decrease. So, to achieve the required amount of power at the receiving station, we transmit the ac signal by stepping up the voltage with the lower value of current as ac gives the flexibility of stepping up or down the voltage. In this way, the power losses are reduced while maintaining the required amount of power at the receiving station.

3) 

Two values shown by the teachers are:

• Believer of giving practical knowledge
• Good explainer

Two values shown by Geeta are:

• Curiosity of learning
• Good listener