Question

Electrical energy is transmitted over large distances at high alternating voltages. Which of the following statements is (are) correct?

• A. Transmission lines can be made thinner.
• B. Lower current implies less power loss.
• C. For a given power level, there is a lower current.
• D. It is easy to reduce the voltage at the receiving end using step-down transformers.

Answer 1

Answer: (B), (C), (D)

It is easy to reduce the voltage at the receiving end using step-down transformers.

Formula used:

$P=I_{rms}{V}_{rms}cos\left(\varphi \right)$

In an ac circuit power is given by:

$P=I_{rms}{V}_{rms}cos\left(\varphi \right)$

$cos\varphi =\frac{R}{Z}$

where $V_{rms}$ is the 𝑟𝑚𝑠 voltage and $I_{rms}$ is the 𝑟𝑚𝑠 current and $\varphi$ is constant for a particular ac connection.

The power is to be transmitted over the large distances at high alternating voltages, so current flowing through the wires will be low because of given power $\left(P\right)$. Hence, $\left(a\right)$ option is correct.

$P_{dissipated}=I_{rms}^{2}R$

As $I$ is lower, $P_{dissipated}$ is also less. Hence, (b) option is correct.

As resistance is inversely proportional to area, if area is decreased the resistance will increase. Hence energy dissipated will also increase
$P_{dissipated}=I_{rms}^{2}R$
Thus, (c) is incorrect.

We can use transformers to increase or decrease voltages and decrease or increase current respectively. So, at the receiving end high voltage is reduced by using step-down transformers. Hence, (d) option is correct.

Final Answer: Option (a), (b), (d)