Global challenges Study Guide

The temperature of any body, including the Earth, is determined by a delicate balance between the rate at which it absorbs energy and the rate at which it emits energy. Energy transfer primarily occurs through electromagnetic radiation. All objects above absolute zero emit thermal radiation; the hotter the object, the greater the intensity of the radiation emitted and the shorter the average wavelength of this radiation. Simultaneously, objects absorb radiation from their surroundings. If a body absorbs more energy than it emits, its internal energy increases, leading to a rise in its temperature. Conversely, if it emits more energy than it absorbs, its internal energy decreases, causing its temperature to fall.

A stable temperature is achieved when the rate of absorbed radiation equals the rate of emitted radiation, a state known as thermal equilibrium. For instance, on a sunny day, an object left outdoors absorbs solar radiation. As its temperature rises, it begins to emit more thermal radiation. Eventually, its temperature stabilises when its emission rate matches its absorption rate. The Earth's temperature is a large-scale example of this principle: it absorbs short-wavelength radiation from the Sun, and as a result, its surface warms up and emits longer-wavelength infrared radiation. The presence of greenhouse gases in the atmosphere affects this balance by absorbing some of the outgoing infrared radiation and re-emitting it back towards the surface, leading to an overall warming effect and a higher average global temperature than would otherwise occur.

Power stations generate electricity at relatively low voltages. For efficient transmission over long distances to homes and industries, this voltage must be significantly increased. Transformers are crucial devices that achieve this. A step-up transformer increases the voltage, while a step-down transformer decreases it. Transformers work on the principle of electromagnetic induction, consisting of two coils of wire, called the primary and secondary coils, wound around a soft iron core. When an alternating current flows through the primary coil, it creates a continuously changing magnetic field in the core. This changing magnetic field induces an alternating voltage in the secondary coil.

The relationship between the potential differences (voltages) and the number of turns in the primary and secondary coils of an ideal transformer is given by the transformer equation: Vp / Vs = Np / Ns, where Vp is the primary voltage, Vs is the secondary voltage, Np is the number of turns in the primary coil, and Ns is the number of turns in the secondary coil. For a step-up transformer, Ns > Np, so Vs > Vp. For a step-down transformer, Ns < Np, so Vs < Vp.

In an ideal transformer, with no energy losses, the power in the primary coil equals the power in the secondary coil: Pp = Ps. Since Power (P) = Voltage (V) x Current (I), we have VpIp = VsIs. This means if voltage is stepped up, current is stepped down proportionally, and vice versa. The main advantage of transmitting power at high voltages is to reduce energy loss during transmission. Power loss in cables is primarily due to heating, which is given by P_loss = I^2R, where I is the current and R is the resistance of the cables. By stepping up the voltage, the current in the transmission lines is significantly reduced (VpIp = VsIs, so Is = VpIp/Vs). A smaller current results in a much smaller power loss (due to the I^2 term), making the transmission much more efficient. This is why the National Grid uses very high voltages (e.g., 400 kV) for long-distance transmission, before stepping it down for local distribution.