Let's clear up the lightning/car safety issue

[JohnCU]

Not all that familiar with the details of the physics of lightning and not all that experienced with applying all these equations and theories I've learned with the real world, but something that comes up all the time is why it's safe to be in a lightning storm. Some people say it's because of the rubber tires, other say the car is a Faraday cage and others cite the skin effect.

here's my take, many corrections will probably be needed:
1. Rubber tires
There is a potential difference between the cloud and ground and eventually the difference is so great that air breaks down and electrons are ripped away and charge is able to flow, ie current. Now, rubber is a terrific insulator, so the car is not grounded, for all intents and purposes it can be considered a floating point, right? the same potential difference that exists between the cloud and ground is not the same that exists between the car and cloud.

2. Faraday cage
Some sources I looked up say that a Faraday cage only works with static electricity and that lightning is certainly not static. Furthermore, most cars are not made out of metal (fiberglass, etc) so the Faraday cage argument wouldn't apply anyway. But if the car were made of metal completely, and a charge was placed on it, then surely the charge should remain outside...

3. Skin effect
The skin effect is only applicable to signals of some frequency (ie not DC). Knowing very little about lightning strikes, if I had to model it, I would most likely using an impulse and as such the delta function (don't you math people say anything about calling the dirac delta a function ). anyway, the impulse has a constant value at all frequencies if I remember correctly, but it's... weird to me because it doesn't really have a frequency. It's like, BAM! and it's over, although then I guess you would need to analyze the impulse response of the car (should have paid more attention in signals class).

Discuss.

 

[DrPizza]

Cars *do* get hit by lightning. #2 is the correct answer.
As far as reason #1: Sooooo, lightning can travel 3 or 4 miles through the air, but it can't travel the last 6 inches through the air from the car frame to the ground?! So, if the potential difference between the ground and cloud is high enough, the car simply is in the way.

 

[bobsmith1492]

^^-^^
True that. Rubber tires... boooo.
The only thing worse is when people say they can't get shocked because they are grounded. Huh.

P.S. : You are going to take the FE exam? Why? I'm trying to decide whether or not to do it and I haven't heard any good reasons to do so as of yet other than it'll look good on a resume. It's such a huge time commitment... the school wants me to sign up for a (free) prep class that occupies 8 hours a day for 5 Saturdays in a row (!!) and then take the 8 hour exam.... for what purpose?

 

[imported_BigT383]

The lightning has a better chance of hitting the car than hitting the ground next to the car. This is because the metal in the car is a better conductor than the air, so having the big chunk of metal there effectively reduces the distance the lightning has to travel, thus reducing the resistance of the lighting taking that path. As we all know, electricity takes the path of least resistance. So lightning does strike cars.

The reason that people inside are unhurt is not a question of the car blocking the charge (faraday cage), or the car being insulated from the ground (tires), or anything. The reason people inside are unhurt is because generally, with the windows rolled up as they would be during a storm, the people inside are not grounded, and they're not touching the outside of the car. Plus, the metal outside of the car makes a better conductor than the air (or people) inside the car, so the lightning strikes the top of the car, races around the metal exterior, and then jumps the relatively small gap from the bottom of the car to the ground.

The people inside the car are safe because the energy travels through the metal around them. Even though it is a longer route, there is less resistance since metal is a much better conductor.

 

[silverpig]

Originally posted by: JohnCU
Not all that familiar with the details of the physics of lightning and not all that experienced with applying all these equations and theories I've learned with the real world, but something that comes up all the time is why it's safe to be in a lightning storm. Some people say it's because of the rubber tires, other say the car is a Faraday cage and others cite the skin effect.

here's my take, many corrections will probably be needed:
1. Rubber tires
There is a potential difference between the cloud and ground and eventually the difference is so great that air breaks down and electrons are ripped away and charge is able to flow, ie current. Now, rubber is a terrific insulator, so the car is not grounded, for all intents and purposes it can be considered a floating point, right? the same potential difference that exists between the cloud and ground is not the same that exists between the car and cloud.

2. Faraday cage
Some sources I looked up say that a Faraday cage only works with static electricity and that lightning is certainly not static. Furthermore, most cars are not made out of metal (fiberglass, etc) so the Faraday cage argument wouldn't apply anyway. But if the car were made of metal completely, and a charge was placed on it, then surely the charge should remain outside...

3. Skin effect
The skin effect is only applicable to signals of some frequency (ie not DC). Knowing very little about lightning strikes, if I had to model it, I would most likely using an impulse and as such the delta function (don't you math people say anything about calling the dirac delta a function ). anyway, the impulse has a constant value at all frequencies if I remember correctly, but it's... weird to me because it doesn't really have a frequency. It's like, BAM! and it's over, although then I guess you would need to analyze the impulse response of the car (should have paid more attention in signals class).

Discuss.

The car can sort of be considered grounded. It is at the same potential as the ground I believe (hook up a voltmeter to the ground and your car to see if there's any current flow).

However, ground is a relative thing, and you have to have a large enough drain such that your grounding body can take the extra charge and not be charged itself. This won't be the case for a car and lightning.

 

[JohnCU]

Originally posted by: bobsmith1492
^^-^^
True that. Rubber tires... boooo.
The only thing worse is when people say they can't get shocked because they are grounded. Huh.

P.S. : You are going to take the FE exam? Why? I'm trying to decide whether or not to do it and I haven't heard any good reasons to do so as of yet other than it'll look good on a resume. It's such a huge time commitment... the school wants me to sign up for a (free) prep class that occupies 8 hours a day for 5 Saturdays in a row (!!) and then take the 8 hour exam.... for what purpose?

makes me look smarter, really depends on what field you go into. i'm going into nuke power and don't really need it but maybe one day i will.

 

[DrPizza]

Originally posted by: silverpig

The car can sort of be considered grounded. It is at the same potential as the ground I believe (hook up a voltmeter to the ground and your car to see if there's any current flow).

However, ground is a relative thing, and you have to have a large enough drain such that your grounding body can take the extra charge and not be charged itself. This won't be the case for a car and lightning.

IIRC, there are some instances where a really high static charge can build up on a car... I wish I could remember the details... maybe it was a pickup or something... All I can remember is that there was a long belt used to operate some other machine, and it was necessary to lean a metal bar against the vehicle to ground it. I also believe that Car Talk used this situation in one of their weekly puzzlers about 5 or so years ago.

 

[JohnCU]

Originally posted by: silverpig

Originally posted by: JohnCU
Not all that familiar with the details of the physics of lightning and not all that experienced with applying all these equations and theories I've learned with the real world, but something that comes up all the time is why it's safe to be in a lightning storm. Some people say it's because of the rubber tires, other say the car is a Faraday cage and others cite the skin effect.

here's my take, many corrections will probably be needed:
1. Rubber tires
There is a potential difference between the cloud and ground and eventually the difference is so great that air breaks down and electrons are ripped away and charge is able to flow, ie current. Now, rubber is a terrific insulator, so the car is not grounded, for all intents and purposes it can be considered a floating point, right? the same potential difference that exists between the cloud and ground is not the same that exists between the car and cloud.

2. Faraday cage
Some sources I looked up say that a Faraday cage only works with static electricity and that lightning is certainly not static. Furthermore, most cars are not made out of metal (fiberglass, etc) so the Faraday cage argument wouldn't apply anyway. But if the car were made of metal completely, and a charge was placed on it, then surely the charge should remain outside...

3. Skin effect
The skin effect is only applicable to signals of some frequency (ie not DC). Knowing very little about lightning strikes, if I had to model it, I would most likely using an impulse and as such the delta function (don't you math people say anything about calling the dirac delta a function ). anyway, the impulse has a constant value at all frequencies if I remember correctly, but it's... weird to me because it doesn't really have a frequency. It's like, BAM! and it's over, although then I guess you would need to analyze the impulse response of the car (should have paid more attention in signals class).

Discuss.

The car can sort of be considered grounded. It is at the same potential as the ground I believe (hook up a voltmeter to the ground and your car to see if there's any current flow).

However, ground is a relative thing, and you have to have a large enough drain such that your grounding body can take the extra charge and not be charged itself. This won't be the case for a car and lightning.

How can the car and the ground be at the same potential when they aren't touching electrically? If you drew a circuit, the resistance of the tires is so high it would appear as an open circuit, a floating node above ground.

That's like me, floating in the air above a circuit and holding a resistor above the grounded portion. It's not grounded.

 

[Aluvus]

Originally posted by: DrPizza
IIRC, there are some instances where a really high static charge can build up on a car... I wish I could remember the details... maybe it was a pickup or something... All I can remember is that there was a long belt used to operate some other machine, and it was necessary to lean a metal bar against the vehicle to ground it. I also believe that Car Talk used this situation in one of their weekly puzzlers about 5 or so years ago.

This often happens to semi-trucks (tractor-trailers). Because of the large surface area of the sides of the trailor, and the large amount of air passing by at speed during highway travel, a considerable amount of static can build up. Some trucks drag a chain on the ground underneath the trailor to keep it grounded. Or anyway, this is what I've been told.

Originally posted by: bobsmith1492
P.S. : You are going to take the FE exam? Why? I'm trying to decide whether or not to do it and I haven't heard any good reasons to do so as of yet other than it'll look good on a resume. It's such a huge time commitment... the school wants me to sign up for a (free) prep class that occupies 8 hours a day for 5 Saturdays in a row (!!) and then take the 8 hour exam.... for what purpose?

It's generally suggested that you take it while you're in school (or shortly after you graduate) in case you ever decide you want to become a PE (either to take on more authority, or to get more money). Saves you from having to relearn a lot of stuff. My school subsidizes the entire cost of the exam for seniors just to get them to take it.

 

[jagec]

Originally posted by: JohnCU
Some sources I looked up say that a Faraday cage only works with static electricity and that lightning is certainly not static.

Not static? Enlighten me. Electronics aren't my forte, but I thought lightning was generated in a similar manner to just rubbing your feet on the carpet, but on a much larger scale?

 

[f95toli]

Faraday cages work for all electromagnetic fields (which is why is spend much of my time inside a screened room, which after all is just a Faraday cage), at least in theory. In reality there is some frequency dependence because a) real cages are never perfect and b) the walls do not have an infinite conductivity.

Btw, it is very difficult to draw conclusions about lightning based on how it "should" behave. As a matter of fact lightning does NOT behave according to any "obvious" rules and there is much we still do not understand, e.g. lightning strikes the highest building in the area MOST of the time; NOT ALL the time as it "should do".

 

[JohnCU]

Originally posted by: jagec

Originally posted by: JohnCU
Some sources I looked up say that a Faraday cage only works with static electricity and that lightning is certainly not static.

Not static? Enlighten me. Electronics aren't my forte, but I thought lightning was generated in a similar manner to just rubbing your feet on the carpet, but on a much larger scale?

That's what one of the sources said, I dispute it as well.

 

[Eeezee]

If Zeus throws his lightning bolt at your car there's not a damn thing you can do that will save you!

 

[BrownTown]

Originally posted by: JohnCU

Originally posted by: jagec

Originally posted by: JohnCU
Some sources I looked up say that a Faraday cage only works with static electricity and that lightning is certainly not static.

Not static? Enlighten me. Electronics aren't my forte, but I thought lightning was generated in a similar manner to just rubbing your feet on the carpet, but on a much larger scale?

That's what one of the sources said, I dispute it as well.

lightning is created by static electricity, perhaps it was refering to the fact that when lightning strikes the ground it often strikes the same place multiple times, so you would have several strikes. However, it would look more like a sawtooth wave than a sine wave. The reason why a car is supposedly safer is that hte metalic skin is a better conductor than your body, so the current goes threw the metal and not threw your ass. Also, it should be noted that hte majority of people who die by lighning are not actually stuck by lightning, they are standing under trees, and the tree is stuck and the bark splinters and kills them. Sitting in your car is certainly alot safer than standing under a tree (which is the worst thing you can do).

 

[silverpig]

Originally posted by: DrPizza

Originally posted by: silverpig

The car can sort of be considered grounded. It is at the same potential as the ground I believe (hook up a voltmeter to the ground and your car to see if there's any current flow).

However, ground is a relative thing, and you have to have a large enough drain such that your grounding body can take the extra charge and not be charged itself. This won't be the case for a car and lightning.

IIRC, there are some instances where a really high static charge can build up on a car... I wish I could remember the details... maybe it was a pickup or something... All I can remember is that there was a long belt used to operate some other machine, and it was necessary to lean a metal bar against the vehicle to ground it. I also believe that Car Talk used this situation in one of their weekly puzzlers about 5 or so years ago.

Ha, yeah I guess a car wouldn't be considered grounded if you hooked it up to a Van de Graaf generator

 

[silverpig]

Originally posted by: JohnCU

Originally posted by: silverpig

Originally posted by: JohnCU
Not all that familiar with the details of the physics of lightning and not all that experienced with applying all these equations and theories I've learned with the real world, but something that comes up all the time is why it's safe to be in a lightning storm. Some people say it's because of the rubber tires, other say the car is a Faraday cage and others cite the skin effect.

here's my take, many corrections will probably be needed:
1. Rubber tires
There is a potential difference between the cloud and ground and eventually the difference is so great that air breaks down and electrons are ripped away and charge is able to flow, ie current. Now, rubber is a terrific insulator, so the car is not grounded, for all intents and purposes it can be considered a floating point, right? the same potential difference that exists between the cloud and ground is not the same that exists between the car and cloud.

2. Faraday cage
Some sources I looked up say that a Faraday cage only works with static electricity and that lightning is certainly not static. Furthermore, most cars are not made out of metal (fiberglass, etc) so the Faraday cage argument wouldn't apply anyway. But if the car were made of metal completely, and a charge was placed on it, then surely the charge should remain outside...

3. Skin effect
The skin effect is only applicable to signals of some frequency (ie not DC). Knowing very little about lightning strikes, if I had to model it, I would most likely using an impulse and as such the delta function (don't you math people say anything about calling the dirac delta a function ). anyway, the impulse has a constant value at all frequencies if I remember correctly, but it's... weird to me because it doesn't really have a frequency. It's like, BAM! and it's over, although then I guess you would need to analyze the impulse response of the car (should have paid more attention in signals class).

Discuss.

The car can sort of be considered grounded. It is at the same potential as the ground I believe (hook up a voltmeter to the ground and your car to see if there's any current flow).

However, ground is a relative thing, and you have to have a large enough drain such that your grounding body can take the extra charge and not be charged itself. This won't be the case for a car and lightning.

How can the car and the ground be at the same potential when they aren't touching electrically? If you drew a circuit, the resistance of the tires is so high it would appear as an open circuit, a floating node above ground.

That's like me, floating in the air above a circuit and holding a resistor above the grounded portion. It's not grounded.

The same way a piece of ground in Africa can be called grounded if you call the piece of ground under the car in North America is grounded.

I mean, if you attached one end of a resistor to your car's body, and the other end to some +5V lead, you'd get current going through the resistor would you not?

 

[BrownTown]

no...

 

[Aluvus]

Originally posted by: silverpig
I mean, if you attached one end of a resistor to your car's body, and the other end to some +5V lead, you'd get current going through the resistor would you not?

+5 V with respect to what?

 

[silverpig]

Originally posted by: Aluvus

Originally posted by: silverpig
I mean, if you attached one end of a resistor to your car's body, and the other end to some +5V lead, you'd get current going through the resistor would you not?

+5 V with respect to what?

+5V with respect to a 50 foot copper rod buried 45 ft down in the ground

 

[BrownTown]

Answer is still no

 

[AnthraX101]

Well, then you'd get +5 v potential...but no flow ("current").

Remember, all that extra "stuff" has to go somewhere.

AnthraX101

 

[Born2bwire]

Originally posted by: JohnCU

Originally posted by: jagec

Originally posted by: JohnCU
Some sources I looked up say that a Faraday cage only works with static electricity and that lightning is certainly not static.

Not static? Enlighten me. Electronics aren't my forte, but I thought lightning was generated in a similar manner to just rubbing your feet on the carpet, but on a much larger scale?

That's what one of the sources said, I dispute it as well.

I seem to recall that Faraday cages aren't a guarantee with static charges. If you enclose a static charge inside a Faraday cage, then the cage will have no effect, they do protect against external charges. Either way, they most definitely do work for any EM wave , given you have a solid shell of PEC. Real life Faraday cages can be thought of more as excellent conductor shells that are waveguides. However, the waveguides do not support propagation for frequencies less than a desired cutoff. For example, the window on your microwave. The holes in the window are large enough to let visible light through, but are small enough not to support the propagation of the microwaves. So most of the microwave power is not transmitted through.

 

[JohnCU]

Originally posted by: silverpig

Originally posted by: JohnCU

Originally posted by: silverpig

Originally posted by: JohnCU
Not all that familiar with the details of the physics of lightning and not all that experienced with applying all these equations and theories I've learned with the real world, but something that comes up all the time is why it's safe to be in a lightning storm. Some people say it's because of the rubber tires, other say the car is a Faraday cage and others cite the skin effect.

here's my take, many corrections will probably be needed:
1. Rubber tires
There is a potential difference between the cloud and ground and eventually the difference is so great that air breaks down and electrons are ripped away and charge is able to flow, ie current. Now, rubber is a terrific insulator, so the car is not grounded, for all intents and purposes it can be considered a floating point, right? the same potential difference that exists between the cloud and ground is not the same that exists between the car and cloud.

2. Faraday cage
Some sources I looked up say that a Faraday cage only works with static electricity and that lightning is certainly not static. Furthermore, most cars are not made out of metal (fiberglass, etc) so the Faraday cage argument wouldn't apply anyway. But if the car were made of metal completely, and a charge was placed on it, then surely the charge should remain outside...

3. Skin effect
The skin effect is only applicable to signals of some frequency (ie not DC). Knowing very little about lightning strikes, if I had to model it, I would most likely using an impulse and as such the delta function (don't you math people say anything about calling the dirac delta a function ). anyway, the impulse has a constant value at all frequencies if I remember correctly, but it's... weird to me because it doesn't really have a frequency. It's like, BAM! and it's over, although then I guess you would need to analyze the impulse response of the car (should have paid more attention in signals class).

Discuss.

The car can sort of be considered grounded. It is at the same potential as the ground I believe (hook up a voltmeter to the ground and your car to see if there's any current flow).

However, ground is a relative thing, and you have to have a large enough drain such that your grounding body can take the extra charge and not be charged itself. This won't be the case for a car and lightning.

How can the car and the ground be at the same potential when they aren't touching electrically? If you drew a circuit, the resistance of the tires is so high it would appear as an open circuit, a floating node above ground.

That's like me, floating in the air above a circuit and holding a resistor above the grounded portion. It's not grounded.

The same way a piece of ground in Africa can be called grounded if you call the piece of ground under the car in North America is grounded.

I mean, if you attached one end of a resistor to your car's body, and the other end to some +5V lead, you'd get current going through the resistor would you not?

There are several thousands of volts difference between different "grounds" on the earth, IIRC.

 

[jagec]

Originally posted by: JohnCU
There are several thousands of volts difference between different "grounds" on the earth, IIRC.

Several THOUSAND? Got a link? Maybe we should use the transatlantic cable for power transmission

There are some interesting effects near high-power radio towers...you can drive electrodes into the ground at the base of the tower, one a bit further out than the other, and light a light bulb on the potential difference.

 

[JohnCU]

Originally posted by: jagec

Originally posted by: JohnCU
There are several thousands of volts difference between different "grounds" on the earth, IIRC.

Several THOUSAND? Got a link? Maybe we should use the transatlantic cable for power transmission

There are some interesting effects near high-power radio towers...you can drive electrodes into the ground at the base of the tower, one a bit further out than the other, and light a light bulb on the potential difference.

http://en.wikipedia.org/wiki/Ground_loop_%28electricity%29

For example, the electrical potential at different points on the surface of the Earth can vary by hundreds of volts, primarily from the influence of the solar wind.

Ground loops, yuck. My magnitude was off.