Global challenges Revision Notes

What is the National Grid?

The National Grid is a network of cables and transformers that transmits electrical power from power stations to homes and workplaces across the UK. It is designed to ensure a constant supply of electricity to meet demand, distributing it efficiently over long distances. Without the National Grid, power generation would need to be very close to the point of consumption, which is often not practical or economical.

Why high voltage transmission is crucial for efficiency

Electricity transmission involves sending current through wires. These wires have electrical resistance, which causes some electrical energy to be converted into heat (Joule heating). The power lost as heat is calculated using the formula P_loss = I²R, where I is the current and R is the resistance of the cable. To minimise this energy loss, the current (I) must be kept as low as possible. Power stations generate electricity at a relatively low voltage. To reduce the current for long-distance transmission, step-up transformers are used to significantly increase the voltage from the power station.

The role of transformers in the National Grid

Transformers are vital components of the National Grid. Step-up transformers installed at power stations increase the voltage of the electricity to hundreds of thousands of volts (e.g., 400,000 V). This high voltage transmission dramatically reduces the current for a given amount of power (P = V * I). At local substations, step-down transformers then reduce the voltage to safer levels suitable for domestic and industrial use (e.g., 230 V for homes). This two-stage transformation process ensures efficient energy transfer and safe delivery to consumers.

Reducing energy loss

By transmitting electricity at very high voltages and consequently very low currents, the National Grid significantly reduces the amount of electrical energy wasted as heat during transmission. Since heat loss is proportional to the square of the current (I²R), halving the current reduces energy loss by a factor of four. This efficiency is critical for delivering electricity economically and sustainably across the country, minimising fuel consumption at power stations and reducing greenhouse gas emissions.

Functions of Mains Wires

Mains electricity in homes uses three main wires: live, neutral, and earth. The live wire carries the alternating potential difference (voltage) from the power station to the appliance. It is the dangerous wire as it is at a high potential relative to earth, typically 230 V. The neutral wire completes the electrical circuit, carrying current away from the appliance. Its potential is approximately 0 V relative to the earth. The earth wire is a safety wire, providing a path for current to flow safely into the ground if a fault occurs, preventing electric shocks. It is also at 0 V relative to earth.

Potential Differences Between Wires

The potential difference (voltage) is the electrical 'pressure' between two points. Between the live wire and the neutral wire, there is a potential difference of approximately 230 V. This is the voltage that powers appliances. Between the live wire and the earth wire, there is also approximately 230 V. This is why touching a live wire while grounded is extremely dangerous. Between the neutral wire and the earth wire, the potential difference is approximately 0 V, because both are essentially at ground potential.

Safety Role of the Earth Wire

The earth wire is a crucial safety feature, particularly for appliances with metal casings. If the live wire accidentally touches the metal casing of an appliance due to a fault, the casing would become live and dangerous to touch. The earth wire is connected to this casing, providing a very low resistance path for the fault current to flow directly to the ground. This surge of current will immediately cause the fuse to blow or the circuit breaker to trip, cutting off the power supply to the faulty appliance and preventing electrocution.

Identifying Wires by Colour

In the UK, mains wires are colour-coded for easy identification and safety during wiring. The live wire is typically brown, the neutral wire is blue, and the earth wire is green and yellow striped. This colour coding helps electricians and users correctly connect and identify the wires, reducing the risk of wiring errors that could lead to electrical hazards.

What is Red-shift?

Red-shift is the phenomenon where the light observed from distant galaxies appears to have a longer wavelength than it would if the source were stationary. This is because the universe is expanding, and as galaxies move away from us, the space between us and them stretches the light waves they emit. Longer wavelengths correspond to the red end of the electromagnetic spectrum, hence 'red-shift'.

How is Red-shift Measured?

Astronomers measure red-shift by observing the line spectrum of light from distant galaxies. Every element has a unique set of spectral lines (like a fingerprint). By comparing the observed spectral lines from a distant galaxy to the known spectral lines of elements on Earth, astronomers can see how much these lines have shifted towards the red end of the spectrum. The amount of shift indicates how fast the galaxy is moving away.

Red-shift as Evidence for an Expanding Universe

A key observation is that almost all distant galaxies show red-shift, meaning they are moving away from us. Even more significantly, the amount of red-shift increases with the distance of the galaxy. This means more distant galaxies are receding faster than closer ones. This relationship between distance and recession speed is direct evidence that the universe itself is expanding, much like dots on an inflating balloon would move away from each other.

Implications of Red-shift

The discovery of red-shift and the relationship between galactic distance and recession speed forms the cornerstone of the Big Bang theory. It suggests that all matter in the universe originated from a single hot, dense point and has been expanding outwards ever since. The ongoing expansion, evidenced by red-shift, continues to shape our understanding of the universe's past, present, and future.

Understanding the Live Wire

The live wire in a mains electricity supply carries the alternating potential difference from the power station. In the UK, this is typically 230 V. It is inherently dangerous because it is at a high potential relative to the earth, creating a risk of electric shock if touched. This high potential means there is a significant 'push' for electrons to flow if a path to earth is provided.

Danger with an Open Switch

An open switch in a mains circuit only isolates the appliance itself from the live current. The live wire leading up to the switch, and potentially into the appliance before any internal components, still carries the full mains potential (230 V). If a person touches this part of the live wire while also being earthed, a complete circuit is formed through their body to the earth. This can result in a fatal electric shock, as current directly flows through them.

Dangers of Live Wire to Earth Connection

Any direct connection between the live wire and the earth is extremely dangerous. This could occur if a fault develops in an appliance, such as damaged insulation, causing the live wire to touch the metal casing (which is often earthed for safety). If a person then touches the casing, or if the connection is direct and severe, a very large current will flow. This rapid flow of current is a 'short circuit' to earth, which can lead to electric shock, fire due to overheating wires, and can cause fuses to blow or circuit breakers to trip, cutting off the supply. While these safety devices activate, the initial current surge is still hazardous.