Question

Write two disadvantages of transmitting ac over long distances at low voltage and high current.

Hint:

For fixed power P = VI, low voltage means high current. Think of the I squared R heat loss and the IR voltage drop.

Answer 1

Step 1: Understanding the Concept:
A power station sends a fixed power \(P = VI\) along the transmission line. If the voltage \(V\) is low, the current \(I\) must be high to carry the same power.
The transmission line has a resistance \(R\), which depends on its length.

Step 2: Disadvantage 1: large power loss:
The power lost as heat in the line is \(P_{loss} = I^2 R\).
If the voltage is cut to one tenth, the current becomes ten times larger. The loss then becomes 100 times larger.
A large part of the power is wasted as heat in the wires.

Step 3: Disadvantage 2: large voltage drop:
The voltage lost along the line is \(IR\). With a high current this drop is large.
So the voltage that reaches the consumer is much lower than the voltage that was sent. The supply at the far end is weak and poor.

Final Answer:
Two disadvantages are: (1) a large power loss \(I^2R\) as heat in the lines, and (2) a large voltage drop \(IR\) along the lines.