Radioactive decay - waves and particles Revision Notes

What is the National Grid?

The National Grid is an extensive network of overhead power lines, underground cables, and transformers that distributes electricity from power stations to consumers (homes, businesses, factories) across a country. Its main purpose is to ensure a reliable and efficient supply of electrical energy wherever it is needed, balancing generation with demand. It connects various power stations, some generating electricity from fossil fuels, others from renewable sources, into one interconnected system.

Why High Voltage for Transmission?

Energy is lost as heat in transmission cables due to their electrical resistance. This heat loss is proportional to the square of the current (P_loss = I²R). To minimise these losses, the National Grid transmits electricity at very high voltages. Using high voltage means that for a given amount of power (P = V × I), the current (I) is much lower. Since the power loss depends on the square of the current (I²), even a small reduction in current leads to a significant decrease in energy wasted as heat, thereby improving efficiency over long distances.

Role of Transformers

Transformers are crucial for the efficient operation of the National Grid. Near power stations, step-up transformers increase the voltage of the generated electricity to very high levels (e.g., hundreds of thousands of volts) for efficient long-distance transmission. Closer to consumers, step-down transformers reduce the voltage to safer and more practical levels (e.g., 230V for domestic use). This stepping up and stepping down of voltage allows for minimal energy loss during transmission and safe use by consumers.

Efficiency of Energy Transfer

By transmitting electricity at high voltages and low currents, the National Grid significantly reduces the amount of energy lost as heat in the power lines. This makes the system very efficient, typically achieving over 90% efficiency in energy transfer from power station to consumer. Without this high-voltage transmission, a much larger proportion of the generated electrical energy would be wasted as heat, making power delivery much more expensive and less sustainable.

Direct Voltage (DC)

Direct voltage sources, such as batteries and solar cells, produce an electrical potential difference that has a constant polarity. This means the positive terminal always stays positive and the negative terminal always stays negative. As a result, the electric current generated by a DC voltage flows in one continuous direction through any given circuit. The magnitude of the voltage can be constant (steady DC) or can vary but its polarity does not change. DC is commonly used in low-power applications and portable electronic devices.

Alternating Voltage (AC)

Alternating voltage sources, like the mains electricity supply to our homes, produce an electrical potential difference where the polarity regularly reverses. This means the positive and negative terminals swap places repeatedly over time. Consequently, the direction of the electric current in the circuit also reverses periodically. In the UK, the mains supply alternates polarity 50 times per second, meaning the current changes direction 50 times per second. AC is particularly useful for transmitting electrical power over long distances due to the ease with which its voltage can be stepped up or down using transformers, making it more efficient for widespread distribution.

Key Differences Summarised

The fundamental distinction between direct voltage and alternating voltage is the constancy of polarity and the resulting direction of current flow. Direct voltage has constant polarity, leading to current flowing in only one direction. Alternating voltage has periodically reversing polarity, causing the current to periodically reverse its direction. Batteries are typical DC sources, while mains electricity is a prime example of an AC source. These distinct characteristics make each type of voltage suitable for different applications in technology and everyday life.

Understanding Deceleration

Deceleration refers to a decrease in velocity, meaning an object is slowing down. It is essentially negative acceleration. When an object undergoes rapid deceleration, its velocity changes significantly over a very short time interval. This rapid change in motion is directly linked to the forces acting upon the object, according to fundamental physics principles.

Force and Deceleration (Newton's Second Law)

According to Newton's Second Law of Motion, the force (F) acting on an object is directly proportional to its mass (m) and acceleration (a): F = m × a. In the case of deceleration, 'a' represents the rate of slowing down. If the deceleration is very large, then the resulting force on the object (and anything inside it, like a person) will also be very large. This large force is what causes harm.

Impact on Human Body

When a person experiences a large deceleration, for example, in a car crash, their body is subjected to a massive force. This force can cause severe damage. Internal organs can be bruised or ruptured as they continue to move forward due to inertia and then abruptly halt. Bones can fracture, and the head can suffer traumatic injuries from impact. The rapid stopping of the body against a seatbelt or dashboard applies immense pressure.

Reducing Dangers: Increasing Impact Time

Safety features in vehicles, such as seatbelts and airbags, are designed to mitigate the dangers of large decelerations. They work by increasing the time over which the deceleration occurs. By spreading the impact force over a longer duration, the peak force experienced by the person is significantly reduced, thus reducing the severity of injuries. This is based on the impulse-momentum theorem, where a change in momentum equals force multiplied by time; increasing the time reduces the force for a given change in momentum.

The Live Wire Hazard

The live wire in a mains circuit carries the alternating potential difference, typically around 230 V in domestic circuits. It is always at a high potential relative to the earth. Even if a switch is open, disconnecting an appliance from the neutral wire, the live wire remains 'live' up to the switch or the appliance if the switch is in the neutral line. Touching the live wire can result in an electric shock because your body can complete a circuit to the Earth. The potential difference across your body can be the full mains voltage, causing a dangerous current to flow through you leading to injury or death.

What 'Open Switch' Means

An open switch means that the circuit is broken, and no current can flow through the appliance under normal operating conditions. However, it does not mean the entire circuit is de-energised. If the switch is placed only in the neutral wire, the live wire leading up to the appliance remains at a high potential. Even if the switch is in the live wire, components before the switch are still live. For complete safety, an appliance should be unplugged from the wall socket to ensure no part of it is connected to the mains live supply.

Dangers of Connecting Live to Earth

The earth wire is a crucial safety feature. Its purpose is to provide a safe, low-resistance path for current to flow to the ground if the live wire accidentally touches the metal casing of an appliance (a fault). This causes a large surge of current which rapidly melts the fuse or trips the circuit breaker, disconnecting the power and preventing an electric shock. If the live wire is directly connected to the earth wire intentionally, it creates a permanent short circuit. This would immediately cause a very large current to flow from the live wire straight to the earth, without any fault occurring. This would instantly blow the fuse or trip the circuit breaker. If the protective devices fail, this short circuit would lead to overheating of wires, which could cause a fire and serious damage to the electrical installation.