PROF M. GOOSEY
Energy is generally considered to have so little mass that, for all intents and purposes, it is negligible. It can of course be considered in the context of Einstein’s famous law, which defines the relationship between energy and mass as follows; where E is energy, m is mass and c is the speed of light.
Using this law and taking the speed of light as being 3x 108 m/s, a single joule of energy has an equivalent mass of around 11 femto grammes, which is indeed very little.
However, if we start to think about how much energy weighs in real life applications, it is not the weight of the energy itself that is important but the medium that is used to store it. In the world of transport this was once, for example, the weight of the coal that was used to boil water in steam engines or, more recently, the petroleum-based liquids that power our ubiquitous internal combustion engines. In the last few years, we have become accustomed to the use of new power sources and technologies, such as the lithium ion batteries that are increasingly used to store and provide energy for powering electric vehicles. In these cases, a key consideration is not just the amount of energy stored but the overall weight. This is very important for transport applications, as moving a large mass of batteries consumes a substantial amount of energy. As a result, this aspect is typically reported in terms of energy density, i.e. the amount of energy stored, or provided, per unit mass and it is where comparisons of the different approaches start to get both interesting and more complex.