Prof Brian Cox: Energy is a concept that’s central to physics, but because it’s a word we use every day, its meaning has got a bit woolly. I mean, it’s easy to say what it is, in a sense. Obviously this river has got energy because over the decades and centuries, it’s cut this valley through solid rock.
But while this description sounds simple, in reality, things are a little more complicated. Over the years, the nature of energy has proved notoriously difficult to pin down, not least because it has the seemingly magical property that it never runs out.
It only ever changes from one form to another.
Take the water in that waterfall. At the top of the waterfall, it’s got something called gravitational potential energy, which is the energy it possesses due to its height above the earth’s surface. See, if I scoop some water out of the river into this beaker, then I’d have to do work to carry it up to the top of the waterfall. I’d have to expend energy to get it up there. So it would have that energy as gravitational potential.
I can even do the sums for you. Half a litre of water has a mass of half a kilogram. Multiply by the height, that’s about five metres, andthe acceleration due to gravity is about ten metres per second squared. So that’s half, times five, times ten, is 25 joules.
So I’d have to put in 25 joules to carry this water to the top of the waterfall.
Then if I emptied it over the top of the waterfall, then all that gravitational potential energy would be transformed into other types of energy.
Sound, which is pressure waves in the air. There’s the energy of the waves in the river. And there’s heat. So it’ll be a bit hotter down there because the water’s cascading into the pool at the bottom of the waterfall.
But a key thing is, energy is conserved, it’s not created or destroyed.
So because energy is conserved, if I were to add up all the energy in the water waves, all the energy in the sound waves, all the heat energy at the bottom of the pool, then I would find that it would be precisely equal to the gravitational potential energy at the top of the falls.
What’s true for the waterfall is true for everything in the universe. It’s a fundamental law of nature, known as the First Law of Thermodynamics. And the fact that energy is neither created nor destroyed has a profound implication. It means energy is eternal.
Every single joule of energy in the universe today was present at the Big Bang 13.7 billion years ago. Potential energy held in primordial clouds of gas and dust was transformed into kinetic energy, as they collapsed to form stars and planetary systems just like our own solar system.
That primordial energy was trapped deep inside new planets.
