Energy and Power

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What is energy? Why is it such a useful concept? How does it relate to power?

Big Ideas: 
  • Energy is the capacity to do work. Work means displacement against a force.
  • Energy is "conserved"; energy conservation means different things to different people.
  • Energy conservation (for a physicist) means energy can be tracked and traced, even if it changes its form (which is why it is a useful concept.
  • Power is the rate at which energy is transformed from one state to another.

Energy, simply put, is the capability for a system to perform work. Work is the act of moving something with a force. Energy is measured in Joules (or kWh, kcals, BTUs etc).  Work is measured in newton-metres, which in SI units is identical to a Joule (N.m = kg m2/s2). Energy comes in many different forms, such as heat energy, light energy, gravitational energy, kinetic energy and electrical energy. Power is the rate at which energy is transformed from one state into another (in human terms, this often means from a stored state to a useful state), and is measured in Watts (Joules per second, J/s = W = kg m2/s3). 

There are two basic forms of energy: potential and kinetic. Potential energy is stored energy, and kinetic energy is the energy of motion (such as in waves, thermal motion of molecules in a gas and electrons in a metal, the motion of macroscopic objects).

There are several types of potential energy (with examples of the way we exploit them from the C21 webpages):

            Gravitational (hydro-electric electricity)

            Chemical (fossil fuels)

            Nuclear (reactors)

There are also several types of kinetic energy

            Light (solar power)

            Heat (keeping us warm)

            Electrical (generation)


Energy is a useful concept because energy is always conserved, which in the physics sense means that it is never created or destroyed but only passes from one form to another. Some of these forms are more useful than others. The most useful are those that can be readily converted to other forms, for example the chemical energy in gasoline that powers a vehicle or the electrical energy that runs a computer. All this energy ultimately ends up as heat, pumped into the environment through the exhaust or by a cooling fan; this form of energy, although the same amount in joules as we started with, is not much use. Heat, especially if the temperature is not much higher than that of the environment, can be converted into useful work only with very low efficiency. This said, the fact that energy is conserved and all forms can be expressed in the same units means that we can usefully assess total energy needs of individual humans, nations and the entire world population.

In common language, however, "conserving energy" means not using it unnecessarily - very worthy but not the same idea at all. It is also worth bearing in mind that in common language, "energy" and "power" are often used interchangeably, and the two concepts are frequently muddled in the media1.

When considering a single action, like driving a car from A to B, we can talk about the energy used, which relates to the fuel used and thus the cost of the journey. However, when discussing humans and energy in a larger context, it makes more sense to talk in terms of power rather than energy, because averaged over time or human populations, we use energy continuously at a more-or-less constant (or slowly varying) rate. Physicists like to use the watt (one joule per second) as a power unit, but confusingly other groups use a multitude of units. To give two examples, electricity providers talk in terms of kWh per day or MWh per year, and barbeques (at least in North America) are marked in "BTUs per hour". To convert from one unit to another is simple, but a pain.

Consider the energy use of a single human being. Energy is what our bodies need to perform the functions that keep us alive. This energy comes from food, and humans need about 2000kcal/day to stay alive. This works out to a mean "food power" of 100 W, although expressing it like this is not terribly useful if you are trying to keep track of your diet - "2000 kcal/day" makes much more intuitive sense. However, when we are trying to assess to needs of a nation, or the entire global human population, we need to gather together all energy needs (of which food is a tiny part, at least in the west) in the same units so we can add them together and come up with national and global rates of useage, in watts or watts per capita.

In underdeveloped, hot countries such as Bangladesh the power needs per capitais only 200 W, which is only double the amount of power humans need from food to survive. In Canada, the power use per capita is 11 kW (55 times the Bangladeshi level), with the United States just behind at 10 kW2. For the USA, this is 4 times the energy use per capita in 1850, and 3 times the level in 19503.  In developing areas such as the Baltic states, power use per capita is only 3 kW, most of which goes into heating, as they have a cold climate. They do not yet have the excess wealth required to attain North American levels of power consumption.

The total human consumption of energy is currently about 15 TW. About three-quarters of this comes from oil, coal and gas - all non-renewable, carbon-based sources,and only one-quarter comes from renewable sources such as wind and water.  In the future our need for energy likely continue to rise due to increased standards of living and a rise in population. So how will our sources of energy have to change over time? Since fossil fuels are terrible for the global environment, we will need a massive shift to renewable and clean sources in the near future in order to provide increased amounts of energy to the world in a sustainable manner. 

It seems that currently the biggest barrier to innovation in sustainable energy is the trillion barrels of oil remaining (which equals 120 billion tonnes of carbon), and the still-plentiful coal and gas. Taken together there is about one trillion tonnes of burnable carbon left underground. We, humankind, have already added half a billion tonnes of carbon to the atmosphere, and there is little indication that our rate of useage will slow down4.

Our best hopes for clean energy comes from wind, water and the sun (WWS). A recent breezily optimistic article in Scientific American5 starts "Wind water and solar technologies can provide 100% of the world's energy, eliminating all fossil fuels. Here's how." and goes on to explain that enough renewable power is out there for the taking. The Stearn Review6 of 2006 estimates that the greenhouse gases in the atmosphere can be stabilized at 500-550 ppm CO2e by switching to WWS at a cost of -2% to 5% of world GDP (the negative sign meaning we save money), avoiding the worst effects of climate change. To set this amount of money in context, a few % of GDP is what it has been costing countries to bail out their banks in the financial crises of recent years. What is needed is (a) political will and (b) a science-literate electorate willing to face the rise in energy costs and to vote for politicians who have the will. This is a tall order; in comparison, the science and engineering problems to be faced are quite tractable. 


Ancient technology in the service of 21st century problems.


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