Energy Study Guide

Energy is never created or destroyed, only transferred from one store to another. When a 'system' (which could be anything from a falling object to a boiling kettle) undergoes a change, its internal energy stores are affected. Describing these changes involves identifying the initial energy store, the energy transfers taking place, and the final energy stores. For example, when an object falls, its gravitational potential energy store decreases as it loses height, and this energy is transferred to its kinetic energy store as it speeds up. When it hits the ground, the kinetic energy is transferred to the thermal energy stores of the object and the ground, and also sound energy. Another common situation is a moving car braking: its kinetic energy store decreases, primarily transferring to the thermal energy stores of the brakes, wheels, and surrounding air due to friction. In an electrical circuit, chemical energy stored in a battery can be transferred to electrical energy, which then transfers to light and thermal energy in a light bulb. Being able to concisely describe these pathways, identifying the specific energy stores involved and the transformations, is fundamental to understanding energy conservation in physics.

In many real-world situations, energy is transferred from a useful store to a less useful store or to the surroundings. This is referred to as 'unwanted energy transfer' or 'energy dissipation'. For example, in a light bulb, electrical energy is primarily intended to be converted into light energy, but some of it is always dissipated as heat to the surroundings. In an engine, chemical energy from fuel is converted into kinetic energy to move a vehicle, but a significant portion is lost as heat due to friction and other processes. Reducing unwanted energy transfer is crucial for improving the efficiency of devices and systems, as it means less energy is wasted and more useful work can be done from the same input.

There are several common methods for reducing unwanted energy transfer, each targeting specific mechanisms of energy loss:

1. Lubrication: When surfaces rub together, friction occurs, generating heat and often sound. Lubricants, such as oil or grease, are applied between moving parts to reduce this friction. By creating a slippery layer, they decrease the resistance between the surfaces, thereby reducing the amount of kinetic energy converted into unwanted thermal and sound energy.

1. Thermal insulation: Heat can be transferred by conduction, convection, and radiation. Thermal insulators are materials that are poor conductors of heat and can help reduce heat transfer. Examples include fibreglass in lofts, foam in cavity walls, and double glazing. They trap air, which is a poor conductor, or prevent convection currents from forming. Reflective surfaces can also be used to reduce heat transfer by radiation.

1. Streamlining: When an object moves through a fluid (like air or water), it experiences resistance called drag. This drag converts some of the object's kinetic energy into unwanted thermal and sound energy. Streamlining involves shaping an object to reduce this resistance. For example, cars and aircraft are designed with smooth, aerodynamic shapes to minimise air resistance, allowing them to move more efficiently.

1. Reducing electrical resistance: In electrical circuits, components like wires have electrical resistance. When current flows through a resistance, some electrical energy is converted into heat (thermal energy). This effect is used in heaters, but in other applications, it's an unwanted energy loss. Using materials with lower resistance or thicker wires can reduce this loss. For instance, high-voltage transmission lines reduce current to minimise heat loss during long-distance power transmission.