Ohms Law Explained

[That Ozzy Guy]

“The amount of current flowing in a circuit made up of pure resistances is directly proportional to the electromotive forces impressed on the circuit and inversely proportional to the total resistance of the circuit.”

So which one of you electrikiti minded people can simply explain ohms law to a simpleton (read: me)? What is the relationship of the resistance to the circuit?

Reason I ask is my leads. I noticed last Tuesday night that they were arching and after having a chat to a mechanic today he told me to check them with a multimeter. Wanting to learn something instead of open my wallet for others I went to buy one and borrowed a newish set of leads off my friend to use as a comparison.

From what the mechanic told me the leads should have 3k to 4k ohms per 11". My coil lead is about 10" and measures in at 1.59k ohms. I checked other leads taking into consideration the length and they were all 1/5 to 1/8 of what they should be when compared to the control set of leads.

Anyhoo, what do the ohms tell you and can someone possibly compare the ohms in another application so I can understand a bit more about leads?

Thanks guys:classic:

[tanny]

Leads? Those are the little carbon sticks you put into those refillable pencils, aren't they? Lower resistance values would allow more current to flow through a circuit. Voltage = Current X Resistance, so algebraically Current = Voltage divided by Resistance, so the lower the resistance, the higher the current. I would think having lower resistance in your "leads" would be a good thing, but I think resistance is also used to suppress electromagnetic noise in your radio caused by the ignition system. Sounds like maybe there might be cracks in the insulation around your leads. There must be someone here who knows what the theory is behind arcing plug and coil wires. Victor.

[Z Kid]

Electrical current is like a fat chick, it takes the path of least resistance (this is why you do not wear a gold ring when spot welding). So if there is greater resistance inside the lead then what it takes to get to the metal body of the car and earth itself, then it will jump to the body of the car, producing your arcing. Most leads are just a bit of string dipped in carbon, so they can break if they have been bent in a tight corner or twisted etc.

[EScanlon]

Electricity can be deceptively simple and confoundingly complex in one statement. Ohm's Law while being one of the basic precepts of electronics is also one of the hardest to "get down pat".

If you break it down into two statements, like Tanny mentioned, it becomes easier to grasp.

The relationship he mentions is:

Voltage (E) = Current (I) multiplied by Resistance ®

E = I * R

By manipulation:

I = E / R and R = E / I

This all gets combined into the little circle divided into one half piece and then two quarter pieces. With the letters arranged this way:

E

=====

I | R

Now, with all the mathematic gibberish out of the way, we can discuss the basics of ohm's law.

Basically it says that the amount of electrons going THROUGH a wire (current; I ) is inversely related to the opposition (resistance; R ) of the wire's composition AND directly related to the amount of strength (voltage; V or E )

So the MORE power (Voltage) you apply to a given circuit the more Current you will get limited by the Resistance inherent in the material.

Hope this helps

Enrique

[Bambikiller240]

Originally posted by EScanlon

So the MORE power (Voltage) you apply to a given circuit the more Current you will get limited by the Resistance inherent in the material.

Zzzzzzzzzzzzzzzzzzzz Zzzzzzzzzzzzzzz Zzzzzzzzzzzzzzzzzzzz

(Not that Enrique isn't correct on this, but I learned this stuff in a trade school and did well with "calculating on paper" what happens if E, I, or R increases, or decreases; but as soon as we hit the real world I become lost. Pardon me if I go back to sleep while the folks who can troubleshoot discuss this) :stupid:

[drunkenmaster]

I think its pretty well explained above.

The resistance (Ohm's) will govern how much current will flow at a set voltage, since current is determined by both voltage and resistance.

V= IxR

or

I=V/R

or

R=I/V

are all the same expression (Ohms Law) re arranged to find the variable your after.

At a set voltage (like a 12V car battery) the current will be determined only by the resitance (as the voltage is fixed). A small resistance would result in a larger current than a large resistance. It is often compared to water in a pipe, and resistances are like bottlenecks/restrictions, limiting flow.

One more thing worth noting, maybe not in this application, but the more current flowing through a resistance, the more heat it will produce. This is one reason why you fuse long cables as all wires/cables have some form of inherent low resistance, so if the short circuit to ground, there will be a huge current surge (because of the cables low resistance=high current) creating a lot of heat and possibly fire.

Probably hasnt answered your question, but a breif back ground in electronics...

In tomorrows lesson we will discuss power:

P=IxV

or

P=I^2 x R

:cross-eye

[Alfadog]

...and I thought Physics would never come back to get me again...

[tanny]

I like zkid's analogy, but my lead never has broken even when twisted severely by a fat chick. I guess it wasn't in a tight enough corner. Victor.

[Gav240z]

Hey Alfa I actually enjoyed Physics at Uni but I only did first year I hear second is the killer.

What Escanlon and Zkid are saying is correct but I'm still confused as to what you want answered George?

I know your looking for a fault in the circuit and your looking for an Arc which is basically what Z kid said about welding I also did some welding :classic: .

Not sure about your leads though..

[Alfadog]

I loved physics at school too, until I got stuck with a teacher who just wasnt compatible with me, and I ended up dreading going to class...

[tanny]

Geez Alpha, exact same thing happened to me. Maybe it was the spitwads on the back of his shirt. Poor guy. But anyway, the obvious and I believe the only solution is to get a new set of plug and coil wires if they are all exhibiting the arcing. You probably already knew that. Victor.

[That Ozzy Guy]

Originally posted by tanny

But anyway, the obvious and I believe the only solution is to get a new set of plug and coil wires if they are all exhibiting the arcing.

It is the obvious but not the only solution. What I'm dreading is buying new leads and finding that I have the same problem. It could be an earthing problem in the end but I don't know how to prove or test it.

So is this right: my leads have a lower than normal resistance therefore more volts are running through them which is causing the arcing?

If not then: leads have small cuts which is causing the arcing and lowering the resistance. Is that possible, could cuts and poor dirty connectors lower the resistance of the whole lead?

Thanks for your lengthy explanations guys. Much appreciated.