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Q & A: Conserving energy

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Most recent answer: 08/01/2013
Can you please explain the importance of conserving energy? Thank you
- Aimee (age 13)
Great Lakes College, Australia
Hi Aimee,

We actually mean two rather different things when we talk about energy and conservation (this is because you're asking physicists, whereas most people only talk about reducing energy consumption).

To a physicist, the conservation of energy is an observed feature of the universe -- energy can be converted from one form into another, and it can be neither created nor destroyed. Energy can be in the forms of potential, kinetic, electromagnetic, chemical, thermal, and other forms, including the energy of just being matter (E=mc^2). We describe this elsewhere on the site (some of these categories of energy really are overlapping descriptions of the same things).

But some kinds of energy are more useful than others in conducting our daily business. The chemical potential energy stored in unburned fossil fuels such as gasoline, oil, and natural gas are tremendously important for moving us about, manufacturing our goods, heating our homes, and generating electricity for doing a huge variety of other useful tasks. The thermal energy in the air on a hot summer day is not as useful for doing anything -- in fact, we often go to great lengths to get rid of that kind of energy. When gasoline burns to make a car go, some of the energy goes to heat up the exhaust, which just escapes, and much of the rest of the energy ends up heating up the engine, the brake pads, and the tires, through friction -- it becomes less useful at that point. The energy in the fuel is much more valuable than the same energy dissipated as heat after the fuel has burned and the work has been done.

The big deal about conserving energy is really about conserving fuels. There is a limited supply of oil underground and under the ocean. By careless waste of this precious resource, we will use it up more quickly than if we are careful about our use. Before then, of course, the oil that we haven't yet pumped out of the earth will become harder to find as we use up the easy-to-find stuff first. The price of oil will go up as we have to dig deeper and deeper, and farther and farther away. Countries with lots of oil reserves may use them as political and economic tools to affect the welfare of countries that use lots of oil but do not have natural reserves (or, more likely, just to make themselves very wealthy).

Burning fossil fuels also has the unpleasant consequence of increasing the amount of carbon dioxide in the atmosphere. Carbon dioxide is a "greenhouse gas" -- it acts a bit like a blanket around the earth, trapping solar heat in the atmosphere and the oceans and the land, by reducing the rate at which this heat is reradiated back to space. If the earth's temperature rises even by a few degrees, the climate will change in many places, glaciers will recede, and the ocean levels will rise. Deserts may expand in size. By reducing our consumption of fossil fuels, we can delay the onset of global warming.

We can use coal instead of oil in many applications (such as generating electricity, and various industrial applications, such as making steel). Burning coal has unpleasant consequences too. In addition to the carbon dioxide, coal burning also produces sulfur dioxide, which combines in the air with water to make sulfuric acid. Sulfuric acid (and nitric acid) dissolved in raindrops have lowered the pH of many lakes in North America and Europe, in many instances killing all the fish in them. Pollutants in the air caused by coal burning have caused many people to have respiratory illnesses. Coal mining is also a hazardous occupation.

Nuclear-generated electricity is not without its hazards as well. Spent fuel remains radioactive for tens of thousands of years and must be disposed of properly. Nuclear power-plant accidents can cause environmental harm -- the Chernobyl fire in Russia made a large area uninhabitable (that reactor was particulary poorly designed). Reducing consumption of electricity reduces the need to build more power plants of all kinds -- coal, gas and nuclear ones. Even hydropower is not without consequences. Rivers dry up below hydropower dams, and the lakes created upstream flood previously useful land, and destroy wildlife habitats. Salmon can no longer swim upstream where a hydropower dam is put in place.

Handling energy in just about any form is hazardous, and reducing consumption can reduce the risks.

Tom J.

p.s. You may wonder whether the difference between 'useful' and 'useless' energy is just a matter of how clever our engineers are. Actually, it is based on a deeper scientific distinction. There's a quantity called 'free energy' which is what we're really trying to preserve.Free energy depends not only on energy but on another quantity called 'entropy'. Unlike energy, entropy is NOT conserved in an isolated system. Entropy keeps increasing, by the Second Law of Thermodynamics. That means that free energy keeps decreasing.

When certain politicians tell you not to worry about using up energy because we can soon convert to hydrogen fuel, they're forgetting that the hydrogen around is either in the fuels we were already using or in water. To get hydrogen out of water, you ALWAYS need to use more energy than you get out. That's not an engineering problem but a law of physics. So those politicians would be lying, if they had any idea what they were talking about.

Fortunately, the Earth isn't completely isolated. Sunshine provides a steady (but not enormous) influx of free energy.

Mike W.

(published on 10/22/2007)

Follow-Up #1: Why conserve energy?

energy cannot be created or be destroyed then why to save it?
- Tanvi (age 13)
nagothane Maharashtra India

That's a very important question. We think that this thread already answers it. In short, the forms of energy that are associated with low entropy are not conserved. Energy keeps turning into the high-entropy forms. When people say we need to conserve energy, they mean we need to conserve the low-entropy types of energy.

Mike W.

(published on 08/01/2013)

Follow-up on this answer.