The two most common terms in electricity are volts and amps. So let's start there.
Electrical force or pressure is measured in Volts.
Electrical current or flow is measured in Amperes or amps.
(Capitals are used because these are the names of people in the modern discovery of electrical phenomenon.)
The Volt is defined as 1 Joule of energy per 1 Coulomb (a very large number) of electrons.
The Ampere is measured in units of Coulombs of electrons passing a point in a wire per second.
When you multiply Volts times Amperes you get Watts - the unit of power.
Joules Coulombs Joules
--------- X ---------- = --------- = Watts.
Coulomb second second
When you divide Volts by Amperes you get Ohms - the unit of resistance.
Really, the more Volts it takes to move one Ampere means the more resistance (Ohms) that you are up against.
When you divide Amperes by Volts you get Mhos - the unit of conductance.
Again the more Amperes that move for each Volt applied means the better a conductor you have.
To review . . .
Voltage pushes.
Current flows.
Insulation resists.
(Well, everything resists. It's just a matter of how much. )
Now, repeat these first three lines 10 times - per day.
Batteries store energy. They pump up the electrons - as it were. They give the electrons their desire to go from the negative terminal to the positive terminal. (I know, I know. But these conventions were defined before we discovered that the electrons were actually what was moving. So conventional current, or positively charged carriers, is thought of as flowing from positive to negative while real current, negatively charged carriers, flow from negative to positive. It can be thought of and talked about either way. But we usually talk in terms of conventional current.)
Wires conduct the energized electrons From one battery post to their destination where work is done - light, heat, motion, etc. - and then carry them back to the other post on the battery again. Wires should have very low resistance as should plugs and splices and such. The destination is where all the resistance should be - that is - it is where all the work should be done. Wires should not get hot. Well, not so much that you notice.
A wire that gets hot is not big enough around for the current that is flowing through it. Insulation that arcs through is not resistive enough (thick enough) to keep the voltage from pushing the current through it. This is almost never a problem unless the wire gets too hot from too much current and the insulation melts or unless the insulation rubs against something and gets thin.
Fuses are there to protect the wires. That's right the wires. Say a wiper motor that goes bad. It can be replaced easily. If the wire going to it gets hot and the insulation melts and "short circuits" to ground somewhere, then you have a much nastier problem.
Or think about your house. You have a 20 Ampere circuit Breaker wired to a to a plug with a 120 Watt lamp (that uses 1 Ampere of Current). (1 Ampere times 120 Volts = 120 Watts.) So why a 20 amp Breaker? Because the wire can safely carry 20 Amperes of current. Fuses and breakers protect the wires.
When the current finds a less resistive (or even a zero resistive) path back to the battery we say it has found a shorter circuit to ground. Or a "short circuit". This is the single most misused phrase in the whole of the English language and perhaps in the history of mankind. Unfused short circuits manifest themselves as magnificent displays of sight and sound and smell and heat and smoke. Remember - all that energy in the electrons has to do some work somewhere before it returns to the battery. It has to give up it's energy doing something. If it bypasses it's destination and goes right to ground this is what happens - if it isn't fuse protected. Unfused short circuits are very easy to find. On the other hand if the fuse blows out it could be due to the motor or light or whatever going bad and drawing too much current. Either way, if a fuse blows out find out why. Disconnect the device and if the fuse still blows then you have an honest to goodness
short circuit on your hands. If it doesn't blow then the device is bad and is the reason the fuse blew.
Unprotected short circuits are very spectacular displays, but don't last very long. The wire usually melts (that is where the energy goes) and leaves a gap between itself and the ground point of contact. This is known as an "open". 99.9999999999999999999999999999999999999% of all electrical problems are OPEN circuits. NOT SHORTS!!!!!!!! If you didn't hear the ark or see the flash or smell the smoke or feel the heat then you probably don't have a short circuit - you most likely have an open circuit. These can be SOBs to find.
And dirty connections are one of the most common forms of "open" circuits. You might have enough of a connection to light a half amp (1/2 amp X 12 volts = 6 watts) test light but not enough to turn a 600 cold cranking Ampere starter. Such a dirty or bad connection will get hot if enough current tries to pass through it. The bad connector actually becomes part of the load resistance. Lights will go dim. Motors will slow down. The plastic shielding around the bad connection will start to melt. 12 Volts is not really a lot of force or pressure. So it doesn't take much in the way of dirt on a connector to make a circuit not work right. Sandpaper and cleaning sprays are your friend here. The edge of a sharp knife is highly regarded as well for cleaning the surface of the connectors.
That green scum on your battery is the devil himself. As it gets between the post and the cable terminal it inhibits the good clean low resistance conduction of current. If you want to see what this can do try this - unbolt every battery cable at both ends and clean them vigorously. Clean chrome or copper or steel or iron is what you want to see. And do this for the heavy cable that goes from the chassis or frame to the engine block. That cable returns the starter current to the battery. And clean the frame and chassis and engine points of contact as well. 12 volts is not a lot of pressure. Clean tight fitting contacts are absolutely essential for good conduction of current. Discolored plugs are a red flag.
Hope this helps.
Bogy.
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--- On Mon, 1/31/11, brian's 83 300d's <bstromsoe@yahoo.com> wrote:
> I can take an engine out, tear it
> down, rebuild it, and all that stuff.
>
> But, I was born with a defect in my brain that prevents me
> from understanding electrcity. I think it is because I can
> pick up a crankshaft and look at it, but electricity is
> invisible and I get shocked by it. If I have an electrical
> problem, I call my son "the electrician" and he does his
> magic act for me.
>
> So, why don't one (or more) of you people who understand
> how electricity works in a car write some exhaustive posts
> for us dummies whose brains are wired incorrectly, start
> simply and build from there. The key word here is simple,
> for simple people. Maybe take it system by system, or part
> by part. I know I have a battery with a positive and a
> negative, but what about fuses, relays, all those shiny cube
> things. Start at the basic beginnings and help those of us
> who are still wandering in the desert after 40=40 years.
>
> brian from laverne
>
>
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