The Basics of Mechanics, Dynamics and Thermodynamics Explained
An informative article on basics in mechanics, dynamics and thermo. In avoiding to make designs and choices that are based on wrong assumptions, wasting time and money, basics are often more important than advanced knowledge. No computer can correct wrong assumptions and thus we have to think twice and thrice, before we do. In this article the backgrounds are explained in layman terms.
Why would you need to have some basic knowledge of mechanics and thermo, you may ask? Well, we live in a technological society and so we are confronted with technological matters and products, that we need to understand the basics of to make proper choices. Ever bought expensive "energy-saving" lamps? Do you think hydrogen is an energy sources? Do you think energy can be produced and consumed? Would you invest money in solar panels for your home? The more these kinds of things apply on you, the more you need to read this article.
The Laws of Newton
The metric, or SI system of units is based on the laws of Newton and so is most of modern mechanics and dynamics. They are essential for basic understanding:
1. A mass object persists in its momentary motion to speed and direction, unless it is forced to change it by external forces working on it.
2. The acceleration a of an object is proportional with the force F working on it and inverse proportional with its mass m . Hence, the acting force is given by: F = m.a
3. A force acting on an object, will yield a counter force of the same strength in the opposite direction: action = reaction.
Power and Energy.
Power and energy are very often mixed up. For example a lightning is very powerful, but it has very little energy, because it lasted very short. Energy is the range of power and time. Power is expressed in Watt and energy in Joule. 1 Watt thus is 1 Joule per second. If you during one hour would apply a power of 1000 Watt, which approximately is what a flat iron takes, the energy involved is 1 kWh and this is thus equal to 3600 kJ. If you instead would develop that energy in one second, the power becomes 3600 kW, or 3.6 MW - a small power plant! If thus a lightning would have a power of say 10 GW (a big power plant) and lasted 1 millisecond (it looks much longer, because of the glowing air around it), it contained an amount of energy of just 10 MJ = 10,000 kJ, or not more than 2.8 kWh to power a flat iron for around three hours! If you in brochures and articles would read dimensions like kilowatt per hour, or horsepower per hour, you can know that the author has no idea what he/she is talking about.
Energy is also the range of force and traveled way. If you lift up a mass of 1 kg to a height of 1 meter, the force needed for that is the range of mass and gravity acceleration. On Earth, gravity acceleration is around 10 meter per second square. The lifting force then becomes 10 kilogram meter per second square, which is called the Newton (N) and the work done is then 10 Nm (Newton meter), which is 10 Joule: 1 J = 1 Nm.
How about temperature and energy? What would you rather have in your hand, a 1 inch red glowing needle, or a 4 inch red glowing bolt? Though both have the same temperature, the needle will just cause you a blister, whereas with the bolt, you won't have a hand any more. The bolt contains much more energy (hot mass) than the needle does and that makes the difference, not the temperature.
The temperature that a solar panel can yield to heat water in your home, is therefore not that important. You pay for energy instead and that is what you want to save on. Collect more energy at a lower temperature (at a higher efficiency) and heat additionally to get your desired temperature - sounds "weird", doesn't it? Right and therefore you allow manufacturers of solar panels to optimize on temperature, without need for additional heating, which is a good selling argument for the energy-unaware public. A water storage tank at higher temperatures becomes smaller also, which sells better as well. They don't talk about efficiency, being the relationship between how much solar energy hits the solar panel and how much of that you can use in the end. They talk about capacity instead and that the solar energy is "free" and that's what sells!
From Newton's third law (action = reaction) follows that on an object in mechanical rotation, two equal forces are working in opposite directions, a centrifugal one, radially away from the center of rotation, and a centripetal one, pointing inwards. If the mechanical contact with the center of rotation suddenly is broken, in that very moment no forces are working on the object any longer and thus it will move away in the direction and with the speed it had in the moment just before losing contact (Newton's 1st law) - tangentially, not radially. Hence, when you are in a car that makes a sharp curve, your body does not push against the inside of the car (centrifugal), but the inside of the car pushes your body into the curve (centripetal) - it's called inertia (of your body).
Largely unknown is the meaning of mechanical impulse (p), being the range of speed (v) and mass (m), which is equal to the range of working force (F) and the working time (t): F.t = m.v = p. An impulse has a direction, which (kinetic) energy has not and therefore impulses can have a positive or a negative sign between opposite directions of motion.
If you consider a mechanical system (machine), that has a certain total mass, but also internally moving parts, the sum of all their impulses will be zero relative the system's center of gravity, but not necessarily relative a resting frame of reference (an observer), in which the whole system (machine) may be moving (at constant speed). The sum of kinetic energy of all the internally moving parts, is of course a positive value (negative energy is less than nothing).
This value is the system's internal (kinetic) energy. Since this internal energy is needed to keep the internal parts moving, there cannot be any energy left to accelerate the system (machine) as a whole. Sadly, there are several patents on according designs, claiming to be "inertial drives" for space-ships or whatever. Their inventors have mixed up reference systems. See some of those unfortunate examples here: http://jnaudin.free.fr/html/IPEmain.htm
Often misunderstood, is that energy can't be "used up". Surely, the gasoline you put in your car is used up, but the energy it developed is still there, to stay around for all eternity. All the chemical energy that was stored in the original fuel, is converted to heat. All energy that we "use" with our technology, finally decays to heat at ambient temperature, even the light from your lamps at home does that.
So is there the term "waste heat", as opposed to "useful heat". What is useful? Take "energy-saving" lamps for example. If you live in a cold climate, where you have to heat your home, a normal cheap hot glowing light bulb actually delivers 100% useful energy, 5% of which is light, the rest is heat that helps heating your home, but this is not what you are told. Only the 5% light is brought forward as "useful" and you are told that you are "wasting" 95% with a normal glow bulb.
The misconception by the public is that useful energy is "consumed" and waste energy is not. Therefore you read everywhere about "energy production" and "energy consumption". The First Law of Thermodynamics says that energy cannot be created (produced), nor destroyed (consumed). However, the scientific definition says that if you add energy to a system to bring it in an other condition, you must remove the same amount of energy to bring it back in the original condition. Naturally, because if we could remove more, energy would be created from nothing and if less, energy would disappear into nothing. This formulation has great consequences, as follows:
Let's consider an ideal hydrogen (water) engine, by which we pour water in it on one side and the same water (steam) comes out on the other side. Then there cannot be a net output on the shaft - it would have been created from nothing. If there is an output anyway, this means that the according energy had to be applied as well, not only the water. Indeed, we must apply energy to split the water in hydrogen and oxygen and a part of that (as per the second law of themodynamics) would appear as mechanical work on the shaft. This then means that the hydrogen only was an energy converter, definitely not an energy source! The same can be said from fuel cells, working on hydrogen. The energy that fuel cells are supposed to "produce", originally came from fossil fuels to manufacture the input hydrogen. Can we call that "non-pollutant" energy?
In everyday life we experience that most things don't happen spontaneously, only accidents do. If we want things to happen, we usually have to do work for it. Hence we could formulate the Second Law as: "for free only the Sun goes up". Oh yes, solar energy is free, but you can't use it for free, why not? Because it is widely spread in Nature and thus the effort to collect it into one point of usage is very large and you have to pay for that effort. There is one exception though and that is hydro-electric power. The forces of nature actually do all the work for us, by collecting rain water in high situated reservoirs, ready for us to use.
Next would be heat pumps. The heat pump, as the name says, pumps up the ambient heat to a higher temperature that we can use. A heat pump can give off between 3 and 4 times more energy than what it takes to run it, also that given off at the higher temperature. If all the billions of dollars that to date and ongoing are wasted on wind propellers and solar collectors of various kinds, would have been used to provide all households with heat pumps, many power plants could have been shut down by now and no more oil would be burned in homes for heating.
This however is a truth with modification. A huge polluting industry, likely using fossil fuels, would be behind all those heat pumps, but that would be the same for wind propellers, solar panels and the production of hydrogen and fuel cells also, all having to be financed by the consumers and making a profit as well - For free, only the Sun goes up!
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ABOUT THE AUTHOR
Rudolph N. J. Draaisma is a double graduated engineer in electrics and mechanics, specialized in energy conversion, refrigeration, waste-heat recovery and alternative energy systems. Read more on The Alternative Energy and Engineering Site