Conservation of Energy
Conservation of energy Energy is an animating force that makes things move or changes It comes in many guises and may manifest itself as a change in height or speed, travelling electromagnetic waves or the vibrations of atoms that cause heat. Although energy can metamorphose between these types, the overall amount of energy is always conserved. More cannot be created and it can never be destroyed. We are all familiar with energy as a basic drive. If we are tired, we lack it; if we are leaping around with joy, we possess it. But what is energy? The energy that fires up our bodies comes from combusting chemicals, changing molecules from one type into another with energy being released in the process. But what types of energy cause a skier to speed down a slope or a light bulb to shine? Are they really the same thing? ( taming in so many different guises, energy is difficult to define. Even now, physicists do not know intrinsically what it is, even though they are expert at describing what it does and how to handle it. Energy is a property of matter and space, a sort of fuel or encapsulated drive with the potential to create, to move or to change. Philosophers of nature going back to the Greeks had a vague notion of energy as a force or essence that gives life to objects, and this idea has stuck with us through the ages.
Energy exchange It was Galileo who first spotted that energy might be transformed from one type to another. Watching a pendulum swinging back and forth, he saw that the bob exchanges height for forward motion, and vice versa as the speed then brings the pendulum back up again before 11 falls and repeats the cycle. The pendulum bob has no sideways velocity when it is at either peak of its swing, and moves most quickly as it passes through the lowest point. C nilileo reasoned that there are two forms of energy being swapped by the swinging bob. One is gravitational potential energy, which may raise a body above the Earth in opposition to gravity. Gravitational energy needs to be added to lift a mass higher, and is released when it falls. If you have ever cycled up a steep hill you will know it takes a lot of energy to combat gravity. The other type of energy in the bob is kinetic energy - the energy of motion that accompanies speed. So the pendulum converts gravitational potential energy into kinetic energy and vice versa. A canny cyclist uses exactly the same mechanism. Riding down a steep hill, she could pick up speed and race to the bottom even without pedalling, and may use that speed to climb some of the way up the next hill (see box).
Likewise, the simple conversion of potential into kinetic energy can be harnessed to power our homes. Hydroelectric schemes and tidal barrages release water from a height, using its speed to drive turbines and generate electricity. Many faces of energy Energy manifests as many different types that can be held temporarily in different ways. A compressed spring can store within it elastic energy that can be released on demand. Heat energy increases the vibrations of atoms and molecules in the hot material. So a metal pan on a cooker heats up because the atoms within it are being made to wobble faster by the input of energy. Energy can also be transmitted as electric and magnetic waves, such as light or radio waves, and stored chemical energy may be released by chemical reactions, as happens in our own digestive systems. Einstein revealed that mass itself has an associated energy that can be released if the matter is destroyed. So, mass and energy are equivalent. This is his famous E = mc2 equation - the energy (E) released by the destruction of a mass (m) is m times the speed of light (c) squared. This energy is released in a nuclear explosion or in the fusion reactions that power our Sun (see pages 136-43). Because it is scaled by the speed of light squared, which is very large (light travels at 300 million metres per second in a vacuum), the amount of energy released by destroying even a lew atoms is enormous.
We consume energy in our homes and use it to power industry. We talk about energy being generated, but in reality it is being transformed from one type to another. We take chemical energy from coal or natural gas and convert it into heat that spins turbines and creates electricity. Ultimately even the chemical energy in coal and gas comes from the Sun, so solar energy is the root of everything that operates on Earth. Even though we worry that energy supplies on Earth are limited, the amount of energy that can be derived from the Sun is more than enough to power our needs, if we can only harness it.
Energy conservation Energy conservation as a rule of physics is much more than reducing our use of household energy; it states that the total amount of energy is unchanged even though it may switch between different types. The concept appeared relatively recently only after many types of energies were studied individually. At the start of the 19th century, Thomas Young introduced the word energy; before then this life force was called vis viva by Gottfried Leibniz who originally worked out the mathematics of the pendulum.
It was quickly noticed that kinetic energy alone was not conserved. Balls or flywheels slowed down and did not move forever. But fast motions did often cause machines to heat up by friction, such as when boring metal cannon tubes, so experimenters deduced that heat was one destination for released energy. Gradually, on accounting for all the different types of energy in built machines, the scientists began to show that energy is transferred from one type to another and is not destroyed or created.
Momentum The idea of conservation in physics is not limited to energy. Two other concepts are closely related - the conservation of linear momentum and the conservation of angular momentum. Linear momentum is defined as the product of mass and velocity, and describes the difficulty of slowing a moving body. A heavy object moving quickly has high momentum and is difficult to deflect or stop. So a truck moving at 60 kilometres an hour has more momentum than a car moving at the same speed, and would do even more damage if it hit you. Momentum has not just a size but, because of the velocity, it also acts in a specific direction. Objects that collide exchange momentum such that overall it is conserved, both in amount and direction. If you have ever played billiards or pool you have used this law. As two balls collide, they transfer motion from one to the other so as to conserve momentum. So if you hit a still ball with a moving one, the final paths of both balls will be a combination of the velocity and direction of the initial moving ball. The speed and direction of both can be worked out assuming that momentum is conserved in all directions.
Angular momentum conservation is similar. Angular momentum, for an object spinning about a point, is defined as the product of the object's linear momentum and the distance it is away from the rotation point. Conservation of angular momentum is used to effect in performances by spinning ice skaters. When their arms and legs are stretched out they whirl slowly, but just by pulling their limbs in to their body they can spin faster. This is because the smaller dimensions require an increased rotation speed to compensate. Try doing this in an office chair; it works too. Conservation of energy and momentum are still basic tenets of modern physics. They are concepts that have found a home even in contemporary fields such as general relativity and quantum mechanics.