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.

(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

Answer 1

(a) Name of device

One cause for power dissipation

(b) Reduction of power loss in long distance transmission 

(c) Two values each displayed by teacher and Geeta

(a) Transformer

Cause of power dissipation

(i) Joule heating in the windings.

(ii) Leakage of magnetic flux between the coils.

(iii) Production of eddy currents in the core.

(iv) Energy loss due to hysteresis

(b) ac voltage can be stepped up to high value, which reduces the current in the line during transmission, hence the power loss (I²R) is reduced considerably while such stepping up is not possible for direct current.

[Also accept if the student explains this through a relevant example.]

(c) Teacher : Concerned, caring, ready to share knowledge .

Geeta : Inquisitive, scientific temper, Good listener, keen learner (any other two values for the teacher and Geeta)

Detailed Answer :

(a) The device used to change the alternating voltage to a higher or lower value is step up and step down transformer. In a transformer we use two coils one is called primary coil and other is called secondary coil. We know the relation for transformer

 E₁/E₂ = N₁/N₂

where E₁ and N₁ are voltage and number of turns in primary coil and E₂ and N₂ are voltage and number of turns in secondary coil.

The one cause of power dissipation in transformer is due to heat loss by eddy current.

(b) When electrical energy is transmitted over a long distance as an alternating current then power loss occurs due to the heat produced by the a.c. current.

Therefore, to reduce the power loss by heat, we reduced the current by increasing the ac voltage with the help of step up transformer and at the end again sufficient A.C. current is received by converting high voltage to low voltage current with step down transformer as

N₁/N₂ = E₁/E₂ = I₂/I₂

In this case A.C. power (P_ac = V.I. = const.)

But in direct current (dc) we cannot use the step up or step down transformer, therefore power loss cannot be reduced.

(c) Teacher : Concerned and ready to share knowledge .

Geeta : Good listener, keen learner.