Students should be able to explain why the National Grid system is an…

The National Grid is the system that transfers electrical energy from power stations to consumers. It is efficient because it carries that energy with a high potential difference and a small current, so only a few per cent is dissipated to the surroundings. Efficiency here means useful output energy transfer divided by total input energy transfer. Start with transmitted power. The power carried is P equals V I, so the same power can be transmitted at a high potential difference and a small current. The power wasted heating the cables is P equals I squared R. That heating loss depends on the square of the current, so reducing current is especially effective. Now connect the links as a chain of causes. A step-up transformer raises the potential difference, which makes the current smaller for the same transmitted power. A smaller current means less I squared R heating in the cables, so less energy is dissipated to the surroundings. Transformers are typically over 99% efficient, so little energy is lost in making the change. Suppose the current in the transmission cables is cut by a factor of 16. Pause and think: what happens to the heating loss? Heating depends on I squared, so the wasted power falls by 16 squared, which is 256. That is why high potential difference transmission is efficient: it reduces the current and therefore the I squared R loss. Three mistakes to avoid. High potential difference does not make electricity travel faster along the cables; it allows a smaller current for the same power. Transformers do not create extra energy; they change potential difference, and their efficiency is below 100%. Also, efficiency cannot exceed 100%, because useful output cannot be greater than total input. Put the chain together: high potential difference, smaller current, much less I squared R heating, and efficient transformers. Only a few per cent of the generated energy is dissipated between station and consumer. Efficiency is useful output energy transfer divided by total input energy transfer. Transmitting at 230 volts would need enormous currents and waste far more energy.