Posts Tagged ‘wire size’

Joule’s Experiment With Electricity

Friday, October 16th, 2015

      In my work as an engineering expert I’ve dealt with all forms of energy, just as we’ve watched James Prescott Joule do.   He constructed his Joule Apparatus specifically to demonstrate the connection between different forms of energy.   Today we’ll see how he furthered his discoveries by building a prototype power plant capable of producing electricity, a device which came to be known as Joule’s Experiment With Electricity.

Joule's Experiment With Electricity

Joule’s Experiment With Electricity

      As the son of a wealthy brewer, Joule had been fascinated by electricity and the possibility of using it to power his family’s brewery and thereby boost production.   To explore the possibilities, he went beyond the Apparatus he had built earlier and built a device which utilized electricity to power its components.   The setup for Joule’s experiment with electricity is shown here.

      Coal was used to bring water inside a boiler to boiling point, which produced steam.   The steam’s heat energy then flowed to a steam engine, which in turn spun a dynamo, a type of electrical generator.

      Tracing the device’s energy conversions back to their roots, we see that chemical energy contained within coal was converted into heat energy when the coal was burned.   Heat energy from the burning coal caused the water inside the boiler to rise, producing steam.   The steam, which contained abundant amounts of heat energy, was supplied to a steam engine, which then converted the steam’s heat energy into mechanical energy to set the engine’s parts into motion.   The engine’s moving parts were coupled to a dynamo by a drive belt, which in turn caused the dynamo to spin.

      Next time we’ll take a look inside the dynamo and see how it created electricity and led to another of Joule’s discoveries being named after him.

Copyright 2015 – Philip J. O’Keefe, PE

Engineering Expert Witness Blog


Wire Size and Electric Current

Sunday, March 13th, 2011
     Whether or not you live or work in a city, you are probably aware of rush hour traffic and how frustrating it can be.  As a matter of fact, this traffic is the number one reason many choose to live within cities providing public transportation.  Instead of watching the cars pile up in front of you, you can be checking your email or reading the paper.  And no matter where you live, you’ve probably encountered a narrow one-lane road at some time.  If this road were to be spotted with traffic lights and double parked cars, the resulting frustration would reach a new high, one which has you craving the freedom of a crowded three-lane expressway.  At least there’s the possibility of movement there.

      Generally, the wider the road and the fewer the impediments, the better traffic will flow.  The problems presented by vehicular traffic are analogous to those present in electrical wires.  For both, obstructions are impediments to flow.  You see, the thicker the metal is in a wire, the more electrical current it can carry.  But before we explore why, let’s see how electric wires are classified.

     If you’ve ever spent any time hanging around a hardware store looking at the goodies, you’ve probably come across wire gauge numbers, used to categorize wire diameter.  American Wire Gauge (AWG) is a standardized wire gauge system, used in North American industry since the latter half of the 19th Century.  Handy as it is, the AWG gauge numbering system seems to go against logic, because as a wire’s diameter increases, its gauge number decreases.  For example, a wire gauge number of 8 AWG has a diameter of 0.125 inches, while a gauge number of 12 AWG has a diameter of 0.081 inches.  To make things easier on those who need to know this type of information, wire diameter is tabulated for each AWG gauge number and readily available in engineering reference books.

      So what does this have to do with electric current?  To begin with, the larger the AWG number, the less current it can safely carry.  If we turn to an engineering reference book, and look up information relating to an 8 AWG insulated copper wire, we find that it can safely carry an electrical current of 50 amperes, while a 12 AWG insulated copper wire can safely carry only 25 amperes.  This information allows us to make important and relevant design decisions regarding a myriad of things, from electrical wiring in electronic devices, to appliances, automobiles, and buildings. 

      So, why are bigger wires able to carry more current?  Well, as you’ve heard me say before, no wire is a perfect conductor of electricity, but some metals, take copper for instance, are better conductors than others, say steel.  But even the best conductors are inherently full of impurities and imperfections that resist the flow of electricity.  This electrical resistance acts much like traffic lights and double parked cars that impede the flow of traffic.  The larger the diameter of the wire, the less electrical resistance is present.  The logic here is simple.  Wire that is larger allows more paths for electrical current to flow around impurities and imperfections.

      The congestion present in rush hour traffic results in travel delays and hot tempers, and heat is also present in electric wires that face resistance to electricity flow.  If the resistance to electric current flow is high enough, it can cause overheating.  Road rage within the wires is a possibility, and if the wires get hot enough, electrical insulation can melt and burn, creating a fire.  Known as the “Joule heating” effect, this phenomenon is responsible for its share of building fires.

      We’ll learn more about Joule heating and how wires are sized to keep electrical current flow within safe limits next week.  Until then, try to keep out of traffic.