The National Grid links power stations to consumers
Definition
The National Grid
A nationwide system of cables and transformers that transfers electrical power from power stations to consumers.
- Power stations generate electricity, but they are often far from the homes, shops, offices and factories that need it.
- The National Grid connects the power stations to these consumers so energy can be transferred electrically across the country.
- Its main parts are the transmission cables, which carry electrical power over long distances.
- It also uses step-up transformers, placed near the power stations.
- And step-down transformers, placed between the transmission cables and the consumers.
Transformers change the potential difference, not the energy
Definition
Step-up transformer
A transformer that increases the potential difference.
Definition
Step-down transformer
A transformer that decreases the potential difference.
- A step-up transformer raises the potential difference from the power station before the power enters the transmission cables.
- Step-down transformers lower the potential difference after transmission, eventually to a much smaller value that is safe for domestic use.
- The order of events is: a power station generates electrical power.
- A step-up transformer increases the potential difference.
- The transmission cables carry the power over long distances.
- Step-down transformers decrease the potential difference.
- The power reaches consumers at a much lower potential difference.
- Transformers only change the potential difference; they do not generate electrical energy.
High potential difference means a small current and low losses
- Some energy is transferred from the transmission cables to the thermal energy stores of the cables and their surroundings, because the cables have resistance and warm up when a current flows; this is wasteful, as less energy reaches consumers.
- Power, potential difference and current are linked by P=VIP = VIP=VI, where PPP is power in watts (W\text{W}W), VVV is potential difference in volts (V\text{V}V), and III is current in amperes (A\text{A}A).
- For a fixed amount of power, a higher potential difference lets the same power be transferred with a smaller current.
- A smaller current causes less heating in the cables, so less energy is wasted to the surroundings.
Key Idea
- During transmission the Grid uses a high potential difference and a low current, which reduces heating in the cables and makes the transfer more efficient.
- Step-down transformers then reduce the potential difference to a much lower value before it reaches your home.
Common Mistake
- Do not say a transformer increases the energy or power made by a power station; a step-up transformer increases the potential difference so the current can be smaller for the same power.
- Do not say the Grid stops all energy loss; the cables still have resistance, so some energy is still transferred by heating. The Grid reduces this loss.
Exam technique
- For an efficiency question, give the whole chain: step-up transformer raises the potential difference, so the current falls for the same power, so there is less heating in the cables, so less energy is wasted.
- A high potential difference is more efficient on its own is not enough; explain why it reduces the loss.
Self review
- What two types of component make up the National Grid?
- What does a step-up transformer do, and where is it used?
- What does a step-down transformer do, and why?
- For a fixed power, what happens to the current when the potential difference is increased?
- Why does a smaller current make transmission more efficient?
