Defining Energy

When speaking about energy, what comes to mind? Is it Einstein's famous equation E=mc²? Or the chocolate bar before your work out? Maybe the smart thermostat you installed during the holiday? To different people, energy means very different things simply because it exists in various forms: it could be thermal, photonic, kinetic, electrical, chemical, atomic and even gravitational. 

By its textbook definition, "energy" means "the capacity to do work," and "work" here is defined as the action of moving something against a force. Think this is ambiguous? You are not alone. 

Great intellects in history such as the Greek philosopher Plato and Aristotle, physicist Thomas Young and James Prescott Joule, all attempted to find their explanation behind this perplexing concept. It was not until the mid-19th century did William Thomson formally integrate the idea of eneryg into the field of thermodynamics, a branch of physics concerned with heat and temperature and their relation to other forms of energy and work.  Into the era of particle physics, Einstein tied energy with the mass of matter with his famous mass-energy equivalence equation. 

Source: PBS via Youtube