What is Energy?

Ever wondered how many chocolate bars your average heater consumes in an hour? This page describes how scientists measure energy, starting with the basics, and then getting pretty complicated pretty quickly - such that I'm not sure I understand it fully myself. But hopefully I do understand it enough to explain it a bit.

What is distance?

Distance and length are measured in metres, which used to be defined as the distance between two marks on a stick in France. It is about 39.37 inches.

What is speed?

Speed and velocity (which is like speed but directional) measure the distance travelled divided by the time it took to get there. Scientists measure it in metres/second. 1m/s is about 2.24 miles per hour.

What is acceleration?

Acceleration is about changes in speed. So if a train accelerates from a speed of 0m/s (standing still) to 2m/s in 1 second, then the acceleration was 2 metres per second, per second, or 2m/s2. Gravity accelerates objects towards Earth at 9.8m/s2, which is about 22 miles per hour, per second. If a fast car can zoom from 0 to 60mph in 6 seconds, then it's acceleration was 10mph/s, half of what gravity can do. So for car manufacturers to achieve 0 to 60mph in 3 seconds, they'd be better off dropping the car from a cliff.

What is force?

Force is the continuous pressure you must apply to accelerate something. Force is measured in Newtons, named after Sir Isaac, and is shorthand for kg·m/s2. 1 Newton is the force required to accelerate a 1 kg block from 0m/s to 1m/s in 1 second. If you had a 10kg block or you accelerated it up to 10m/s, the force would have been 10N. On the other hand if it took you 10 seconds, it would have been 0.1N. Gravity exerts a force of 9.8N for every kg, so if you weigh 100kg, gravity is constantly pulling you down with a force of 980N.

What is energy?

Energy is the measure of how much work you have to do to apply a force over a fixed distance. It is measured in kg·m2/s2 or Joules. So if you applied a force of 1N to push something for 1m, you would have expended 1 Joule of energy. Or if you increased the speed of a 1kg block from 0m/s to 1m/s per second, while pushing it one metre, that would be 1J (and it would have taken you only half a second, which I find very confusing, and makes me think I might be wrong about this). Food energy is measured in kilocalories. One kcal is 4186J. An adult woman is supposed to eat 2000kcals per day, which is 8,372,000 Joules, enough to accelerate an 8000 ton block 1m/s2 over a metre.

What is power?

Power measures the rate of consumption of energy and is measured in kg·m2/s3 or Joules/second or Watts for short. I won't try to continue the pushing a block example for power because I can't quite make sense of it. But I can say that a 100 Watt light bulb consumes 100 Joules per second. That's 8,640,000 Joules per day, about the same as an adult woman. By that logic, an adult man needs as much power as a 125 Watt appliance, and a child needs about 90 Watts. Various sites show the power consumption of different appliances.

Back to energy

The measure of energy on electricity bills is the kilowatt hour. A kw·hour is the amount of energy a kilowatt appliance consumes in an hour. As a 1kw appliance uses 1000 Joules per second, it consumes 3,600,000 Joules in an hour. So 1kw·hr equals 3,600,000 Joules. The average 3 bedroom house in Scotland uses 4200kw·hr per year, or 350kw·hr per month, about 11kw·hr per day (40,000,000 Joules). Assuming the average 3 bedroom Scottish house holds 4 people, that's 10,000,000 Joules consumed each per day, a bit more that what they take in from food.

What does this all mean?

I think it is surprising how much electric energy our lifestyles consume. For instance a constantly running 300 Watt computer needs 3 times more energy than me - that's a lot of frozen pizzas. But it can also be encouraging to think in these ways. Turning your office's computer, printer, hub, etc off overnight means 60% less pizzas. And replacing a 100 Watt light bulb with an energy saving 11 Watt light bulb is like going on a 90% diet.