How is a car grounded if the only thing touching the ground are 4 rubber tires?

[JEDI]

jump starting a car.

"connect the black cable to any part of the chasis because it's grounded."

but no conductor is touching the ground???

 

[PottedMeat]

Ground = return path for current. Conventional current flows from positive to negative (ground).

Jumping a car needs 2 cables to complete a circuit ( loop ) -> good battery positive to bad battery positive, then negative bad battery ( bad chassis ) to good battery negative ( good chassis ).

 

[LTC8K6]

Chassis ground, not Earth ground.

 

[Zenmervolt]

In this instance, the explanation that it's seeking a chassis ground and not an earth ground is correct. However, there's more to the overall story.

Believe it or not, tires are conductive. There is enough carbon used in tire tread compounds that the tires are reasonably conductive and do not effectively isolate the car from ground. If the tires truly isolated the car from ground, the car would develop a charge differential while driving (from the friction of the tires against the road, similar to scruffing feet across carpet) that it could not dissipate and you would be shocked by static electricity every time you went through a drive-through.

The reason that you attach the ground cable to a chassis ground and not to the negative terminal of the dead/weak battery is because automotive lead-acid batteries give off hydrogen gas when they charge and there is a potential for sparking when you remove the ground cable after jump starting the car. If you had the cable attached to the negative terminal, there is a chance that it will be sitting in a stream of hydrogen gas when removed and if it were to spark the battery could explode. By telling you to use a chassis ground point (the negative terminal of the battery is attached to a chassis ground point as well, so you are still effectively attaching to a lead from the negative terminal) the instructions are making sure that the sparks, if they occur, are farther from the charging battery and therefore farther from the source of potential hydrogen gas.

ZV

 

[Gibson486]

ground is a zero reference, not a physical one. Its a concept that is supposed to mean path of return current (which is negetive in reference to the original), but they usually just simplify it by refering it to a zero voltage line (which is partially true because ground is a short).

 

[Analog]

Originally posted by: Zenmervolt
In this instance, the explanation that it's seeking a chassis ground and not an earth ground is correct. However, there's more to the overall story.

Believe it or not, tires are conductive. There is enough carbon used in tire tread compounds that the tires are reasonably conductive and do not effectively isolate the car from ground.

Newer technology tires are replacing the carbon with greater amounts of silica for gas mileage reasons. Silica is a great insulator, and that is why you may experience static shock when exiting the vehicle more often with that type of tire. I have witnessed sparks coming off the tire of a car at night as well. But you are essentially correct about most of the tires on the road today.

 

[shabby]

Originally posted by: JEDI
jump starting a car.

"connect the black cable to any part of the chasis because it's grounded."

but no conductor is touching the ground???

Jedi your a comedic genious! Thanks for the laugh

 

[Pacfanweb]

It's grounded ultimately to the battery. That's done through the chassis, engine block, etc. But eventually it makes its way back to the battery.

 

[Xcobra]

Originally posted by: Analog

Originally posted by: Zenmervolt
In this instance, the explanation that it's seeking a chassis ground and not an earth ground is correct. However, there's more to the overall story.

Believe it or not, tires are conductive. There is enough carbon used in tire tread compounds that the tires are reasonably conductive and do not effectively isolate the car from ground.

Newer technology tires are replacing the carbon with greater amounts of silica for gas mileage reasons. Silica is a great insulator, and that is why you may experience static shock when exiting the vehicle more often with that type of tire. I have witnessed sparks coming off the tire of a car at night as well. But you are essentially correct about most of the tires on the road today.

happens to me all the time... :|

 

[soydios]

as many people have said, electrical ground is a relative term in circuit analysis. there's chassis ground which is at the negative terminal of the battery, and earth ground which is obviously at the earth. as others have also said, tires are conductive enough that the difference between chassis ground and earth ground is small, though it is there.