Electrical Physics & Shade Tree Mechanics

An Essay by James A Graves, Jr.

Voltage, which is measured with a voltmeter, is the force that causes electrons, which are negatively charged particles that orbit atoms, to flow from the negative terminal of a voltage source, like a 12 Volt battery, through a wire and cause an electrical device to operate, like a car’s headlights, and then return to the positive terminal of the battery.

The tungsten elements inside a car’s headlight bulbs create a resistance to the electron flow. When this happens the tungsten molecules speed up, heat up, and glow, producing light. This is called a law of physics. When the conditions are correct, the electrons must flow in this manner and the tungsten must heat up.

I assume there is some kind of subatomic, particle physics penalty if the electrons and molecules fail to obey the law.

The wire, headlights, fuse, switch and other electrical hardware, all connected from the positive terminal of the battery to the negative terminal of the battery, is called an electrical circuit. Specifically, the headlight circuit. The electron flow within the electrical circuit is called amperage, or amps, and is measured with an ammeter.

If at all possible, it is advisable that humans not become part of an electrical circuit. Primarily due to the fact that doing so can become somewhat uncomfortable. 12 volts, considered low voltage, is typically not at issue here, (although, some people, i.e., shade tree mechanics, can usually find a way to make 12 Volts an issue), however, higher voltage circuits, i.e., 120 Volts AC, should be avoided like the plague.

Now, in the headlight circuit, if the resistance of the tungsten element was not in the circuit, i.e., the element broke or burned out, then the circuit would be open and electrical current would not flow, producing zero amps.

However, if the headlight bulb had no resistance, (i.e., if by chance an idiot shade tree mechanic had touched the tip of a screwdriver across two of the headlight’s electrical terminals, effectively connecting the terminals together, referred to as a “short”), then all of the electrons at the negative terminal of the battery would try at once to flow through the wire, across the tip of the screwdriver, to the positive terminal of the battery, causing maximum, or possibly infinite, amperage.

The result is that the wire would be unable to carry such a high level of amps, the resistance of the wire would be overwhelmed and the wire would heat up, just like the tungsten elements in the headlight bulbs. But since the headlight circuit’s copper wire can only withstand a limited level of amps, it would overheat and melt.

(And the screwdriver tip wouldn’t fare so well either.)

To prevent current overloads due to shorts, electrical circuits usually have an in-line fuse or a circuit breaker, which is designed to allow only a specific level of amperage to flow through it, and will blow, or open, when the current flow exceeds the amperage rating of the fuse (i.e., 10 amps), stopping the current flow and preventing the wire from melting. (Screwdrivers typically do not have an amperage rating.)

If the wire in the headlight circuit was large enough to handle the maximum current flow from the battery (i.e., jumper cables), the wire would effectively short the negative and positive terminals of the battery together. Since the battery is not designed to handle the astronomical level of heat that would be generated by the current flow inside of it, the battery would either melt, or explode, or both.

When this situation occurs, it is typically referred to as a “dead short”.

When it is caused by human error, it is typically referred to as “very stupid”.

The moral to this example is: never “dead short” a charged car battery with something like a wrench, a set of jumper cables, or even a wire coat hanger. The result will not be pretty.

I once performed an impressive current flow demonstration on my dad’s 1956 Ford.

It all began years earlier when the original owner started using non-detergent oil in the engine, which is a very bad idea. The exhaust was smoking and someone suggested that the oil return holes in the engine heads might be plugged, causing the oil that lubricates the rocker arms to build up inside the valve covers and then leak through the valve guides, getting into the cylinders via the valves and causing the smoking exhaust.

Being the conscientious shade tree mechanic that I am, I pulled the valve covers to inspect the situation and found the top of the heads coated with globs of sticky black oil goop. So, I decided to use a straightened wire coat hanger to unplug the oil return holes in the heads, which would allow the oil to drain back into the oil pan.

One of the first maintenance safety steps recommended when working on a car engine is to disconnect the negative terminal from the battery before you begin work.

I did not do that.

Initially, my shade tree mechanic method of unplugging the oil return holes appeared to be working fairly well. But, as I busily worked the coat hanger wire down into one of the oil return holes, the other end of the wire, which was flopping around inside the engine bay like a snake trying to escape a mongoose, touched the positive terminal of the battery, arced and then promptly welded itself to the battery cable terminal.

The result was instantaneous; all of the electrons at the negative terminal of the battery were apparently really anxious to travel through my coat hanger to join the other electrons at the positive terminal of the battery. The current, flowing at a level that would have no doubt maxed out a nuclear power plant ammeter, instantly heated up the coat hanger (that I was holding with both hands) to a temperature approximately that of the surface of the sun, branding me and setting fire to the oil goop inside the oil return hole. Then, performing a magician-like feat, the coat hanger disappeared in a bright flash and puff of white smoke.

All of that happened in less than a second. Afterward, I just stood there in dazed awe and amazement, watching acrid black smoke rise up from the burning oil goop that was quickly becoming a respectable blaze on top of the engine head. But, at that point, my brain finally caught up and notified me about the pain from being branded across both palms by a white hot coat hanger.

After dancing around a bit while expressing my serious personal comments on the event, as well as my considered opinion of the car, battery, coat hanger and my stupidity, I finally came to my senses and extinguished the fire.

Needless to say I never did that again – with a coat hanger. Instead, I’ve used various tools and other electrically conductive materials over the years, but none produced the spectacular results of that coat hanger.

Such are the joys of being a shade tree mechanic.

Back to Essay Index