What is Energy?


What is Energy?

We use the term energy in many ways. There are energy drinks. We are told that energy conservation is important. Our car and home might be wasting energy. It took you a lot of energy to ride your bike 20 miles.  We are told that it is important to conserve energy. There’s green energy and renewable energy. So what is energy?

We are going to start with the Physics definition of energy :

Energy is defined in physics as the ability to do work. Okay, that’s interesting but what does it mean to do work?

If we push a block from point A to point B we did work in moving that block. While the block was in motion it was said to have had kinetic energy which you can think of as the energy of motion.

If we lift that same block from the ground to a point above our head, we did work to get it there and it is now said to have potential energy.  We use the term potential energy, because it has the potential to do work, by falling to the ground or to be more accurate being acted upon by gravity through a distance. It has this energy because of its position and the distance that it can fall. If we release it, all of the potential energy will convert to kinetic energy as the block goes flying towards the ground. Half way to the ground the energy is 50% potential and 50% kinetic and just as it hits the ground the energy is 100% kinetic. When it hits the ground all of the kinetic energy is transformed into deforming the ground and the block. The amount of potential energy that the block has is relative to its position. If we move the block so that it is now positioned above a table, it doesn’t have nearly as far to fall and therefore, relative to the table top, it has less potential energy.

How do potential and kinetic energy relate to energy we use in our everyday lives? What are the practical applications of the physics?

Let’s look at an example of generating electricity using a water wheel. There is water in a stream that is flowing down a hillside and we place a wheel with paddles such that the water flows over the wheel on its way to a lower elevation. The water that is further up has potential energy since its position is elevated. As it flows over the edge its potential energy is converted to kinetic energy based upon the fact that it is in motion as it falls. As it passes over our water wheel the motion of the water causes our wheel to spin due to the force of the water pushing on the wheel’s paddles. We have slowed down the water transferring some if its’ kinetic energy to kinetic energy in our wheel that is now moving. To turn the kinetic energy from the wheel into electrical energy we attach the wheel to a generator that creates spinning magnetic fields to generate electricity which in turns slows down our wheel converting the kinetic energy of the wheel into electrical energy.  As long as our source of water up stream keeps getting refilled by the rain our setup continues to deliver electricity.

To recap:

Our water had potential energy that became kinetic energy as it flowed downhill. When it turned our water wheel we converted the water’s kinetic energy to kinetic energy of our wheel in the form of a circular motion and finally to electrical energy from our generator.

So what’s the big deal? The big deal is that this exact same process is used to create electrical energy on a much larger scale. At Niagara Falls the power generation planets use the water diverted from the top of the falls to hydro-electric plants to generate 4.9 million Kilowatts (see What is a Kilowatt) of power which is enough to power 3.8 million homes! You can express this amount of power as 4.9 Gigawatts (109). A million is 106 and kilo is 103 combined to be 109.

Photo source: http://www.niagaratorontotours.com

That’s cool but you’re probably thinking that there are not too many gigantic water falls out there to create power. And you’d be right so we make our own in the form of dams. A dam is placed in a river creating a reservoir of water (lots of potential energy!) and that water is directed through what is called a hydro-electric plant (same exact concept as our water wheel but on a much larger scale) to generate large quantities of electricity. The Hoover Dam generates 2.1 Gigawatts of power from the water contained in Lake Meade.

Las Vegas Lights

Photo Source: http://www.lasvegastours.info

The Hoover Dam

Photo Source: http://www.visitingdc.com/las-vegas

We have been talking about energy changing from one form to another but energy is never created or destroyed.  All of the energy in the universe already exists and it just changes form from one type of energy to another.  This is a basic fact of the universe and in physics it’s called the law of conservation of energy.

What people are usually talking about when they say that they are conserving energy is that they are conserving energy in a certain form of natural resources. They are using less oil or electricity and electricity is typically created by burning fuels like coal or natural gas or by nuclear reactions.

A car takes a certain amount of energy to move it from point A to point B, say from your home to the store. The weight of the car, how it’s geared, and how the exterior body is shaped all have an impact on how much energy is required to accomplish the task of moving the car from point A to point B. That energy can be generated by burning gasoline in an internal combustion engine or it can be generated by batteries that drive an electric motor or it could even be generated by the passengers all pedaling on a mechanism that drives the wheels but in all cases, the amount of input energy that is required is the same. So electric cars conserve the oil required to make gasoline and hopefully the overall pollution is less although other natural resources are consumed to create the electricity like coal and natural gas.


Might also be of interest: What is Energy?