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Friday, August 8, 2014

Heat em up!!

    Heat, Brownian motion, atomic or molecular excitement, thermal energy, call it by any of its names, heat is 'just' another form of energy that can be used for work.  Heat, as with any form of energy, can be added or removed from any given system, extracted to do work, stored for later, concentrated and  transferred. 
   Heat is motion.  Heat is the measured wiggle and giggle of the atoms or molecules in a substance.  That motion can be slowed, and the result is a measured drop in temperature; the temperature rising indicates an excitement or increased activity on a molecular level.  

   On the Celsius temperature scale, which is calibrated to 100 degrees when pure water boils at 1 atm or 101.325 kP of pressure and 0 degrees when water freezes to ice, the lowest possible temperature we can create or measure is -273 degrees, what is known as 'absolute zero'.

   Absolute zero is actually zero degrees on the Kelvin scale and 273 degrees is the point at which ice melts to water, 373 where water boils.  Same degree calibration, just 273 degrees slideways down the scale. 

   Because the rest of the world already switched over to the metric system for easier communication, I am not even going to discuss the Fahrenheit scale.  Its outdated, and although there are more fine gradations of temperature built in, the scale is based on sea water, not pure water.  What sea? Dead sea or Caribbean? Big difference.  Purify your water, calibrate your thermometer.

   Temperature, as we know it, is a direct measurement of the kinetic energy of the individual atoms or molecules.  The more agitated the molecules are the higher the observed temperature.  We cannot measure the individual motion of the molecules in our soup say, but we can measure the average energy of the molecules in motion.

   Heat can be transferred two ways by physical means, and by one method  over a distance.  Lets take care of heating via distance first.   When you put your hand up to the sun you are feeling many wavelengths of electromagnetic (EM) radiation that are absorbed by your skin or deeper tissues and converted to heat by adding kinetic energy to the atoms that make up the tissues of your hand.  This is called radiation, or radiative heating.  An object emits infrared, or heat waves, which are a form of EM, as soon as the temperature of said object rises above absolute zero.  If it is warmer than its surroundings, it will emanate EM radiation out, and receive less coming in from cooler objects.  In this way, objects cool via radiative loss and cooler objects in the path of those EM waves heat up via radiative heating.

    Physically we can move heat from one object to another by conduction, or direct contact from the hotter object to the cooler one.  When you place your hand on a hot object, it feels hot to you because you are feeling the movement of heat from the hotter object into your hand.  An object that is cooler than your hand removes heat and feels cold as a result.  Conduction happens through contact or touch.

    The other way heat can move via physical means requires uneven or spot heating of a fluid, a gas or liquid to operate.  This very interesting and complex transfer of heat is called convection.  Most heated fluids lose density and rise upward away from gravity.  Its that simple.  A flame below a pan of water will cause the water to rise away from it.  A convective flow will always occur when you heat a fluid unevenly.  Cooler fluid sinks to the bottom to replace fluid that is rising from the heat being added and a cycle forms in which heat is added at the bottom and circulated up to give that heat off at the top.

  It is convective flow, combined with the Coriolis effect, that causes our weather and the ocean currents on our little planet.  Water in the oceans and the air above are heated more on the equator and less at higher latitudes-  but only during the day.   That heat is moved around and is always uneven because the earth is only ever half lit by the sun.  Half is giving off heat, half is being heated.  Heat moves from hot side to cold side, but never catches up.  Hot air moves, cold air fills in behind it.  It sounds so simple right?  Not even close.  This is the most simplified and basic model that can be devised.  Basic, but real.  The complexity and detail of a reality based model would be staggering.  Weather modeling is done by massive computing arrays... not geeks on blogs.

   Heat is a form of energy, and in the next article I will go into how heat is used in all differing applications from engines and desalination plants to nuclear power plants and even the pumped hydronic heating systems in homes.  Heat removal, transfer and recollection are the hallmarks of our society.  Well ok, that and really cool silicon gizmos.  Thats for other writers to dissect tho...

Til the next round...  If you found this useful, follow on Twitter @MachineWhisprr or MachineWhisperer on Facebook.



Tuesday, August 5, 2014

Electrical Issues, Short or Open?

To the uninitiated, one electrical issue looks like the next.  'My lights quit working, can you fix it?' is a common question, or some variant thereof... I think its a short'.  I explain that I can't know until I check it over with test equipment, and ask the client if they did the basics and checked the fuses.  Usually that question gets a blank look for an answer...  

If you have never studied electricity, an automotive electrical system can look insanely complex with wires everywhere, switches in all sorts of places, various black boxes and connectors scattered everywhere, and to top it all off, nothing is labeled, which makes it even less friendly.  

All circuits in a car, motorcycle, RV or boat are all very similar.  You have a source of power- the battery or alternator or both, a fuse to protect the circuit, a switch, loads that are being powered (light, fan, etc...) and then the wires to connect it all up and conduct electricity from the source to the load and back.

All loads need two wires, one for the supply or negative side, one for the positive or return side.  Someone a long time ago screwed things up by saying that the positive side of a battery is the source of current, but that is nonsense and we need to kill that old confusing and useless paradigm.

Here comes the physics part of it... you have been warned.

Electrons form the outer, reactive shell of an atom of stuff, lets say metal in this case.  Those electrons, the source of the word electricity, carry a charge we arbitrarily call 'negative' while the inner portion of the atom carries a 'positive' charge.  It is these charges and the polarity difference that keeps atoms together, the positive and negative charges are pulled together by forces created by that polarity.  The weak electromagnetic force in physics is the direct result of these charges and  defines the interactions between those charged particles.

Extra electrons on the negative plates of a fully charged battery is what creates the negative charge on that terminal of the battery, and the lack of electrons is what creates the positive terminal.  Current physically flows from the negative side with excess electrons to the positive side with not enough electrons.  Voltage is considered to be the strength of the force created by the difference in the number of electrons stripped form the positive and dropped to the negative side during charging.  A discharged battery has the same number of electrons in both plates, therefore no voltage or current flow can happen.

A flow of electrons, bumped from one atom to the next on and on down the wire, is what we call electricity.  The voltage is the pressure driving the electrons down the wire, while current or number of amps is the actual measured number of electrons that are flowing in a given time past a given point.

In a standard circuit, the current flow is controlled by the resistance of the wires and load, and by the voltage pushing it down the line.  A fuse is rated for a specific flow of current through it, after which the wire inside melts and burns out stopping the flow of electrons through it.  If the resistance is too low in the circuit, the current goes up in direct proportion to the reduced resistance.  The same resistance can pass more current if the voltage is raised as well.  Either way, some safety device should be installed to prevent that over-current from damaging other components and wiring.

An open circuit is the simplest and most common form of failure.  It is also the hardest to track down.  An open circuit is simply one that no longer connects, thus current does not flow.  Somewhere in the circuit, something broke and no longer is connected and conducting.  It could be a switch that no longer switches, a burned out bulb or other component, or even as simple as an old fuse that finally blew from years of use near its limit.  Then there are the nightmare scenarios where there is a break in a wire somewhere in that massive bundle of wires and now we need to find it.  It could be broken in any of a thousand places along as much as 50 feet of wire in a bigger vehicle.  Like I said... nightmare scenario.  In these cases, I will often run a new wire with the old ones and power that load directly skipping the old wire entirely.

If two wires melt or rub through and touch metal to metal, the result is a connection that was not intended and usually results in over current.  This is a 'short circuit'  Literally, the circuit is too short to be healthy, it skips the load and has found a shorter path from one side of the battery back to the other side.  Hopefully the overloading of the circuit results in blowing a fuse, but if the circuit is one of the main power distribution wires, the fuse may be so high powered that it never blows, even while wires are smoking and catching fire.  

Replacing a blown fuse with one of a higher rating is always dangerous and stupid even for trained and experienced technicians running diagnostic tests.  Find the cause of the short, find it and fix it.  Then replace the fuse with the correct one and carry on.  Intentionally overloading a circuit by using a higher current fuse than the wire is designed for WILL result in a wiring fire.

A short circuit from improperly routed wires, or over-fused, under-wired circuits causing a fire is the biggest danger in self wired vehicles.  The car owner has the option to install stereo systems, additional lights and power sockets, inverters and appliances nowdays.  All these new and interesting electrical add-ons can be bought in the parts stores  or truckstops easily but have no decent instructions to educate the novice installer how to keep it safe.  

Helping your mechanic diagnose electrical problems starts by properly describing the problem.  If you can tell your mechanic that the windshield wipers have an open circuit and don't work, its very different from saying it has a short circuit in which the fuse blows every time and you smell burning plastic.  Correctly stating the problem is the first step to your mechanic not spending hours to determine what kind of fault is happening.  

Hope that clarifies a few things for ya... til next time...

MW outta here :-)