MadSci Network: Physics

Re: Do rubber tires have anything to do with protection from lightning?

Date: Tue Jun 29 12:35:08 1999
Posted By: Everett Rubel, Degree in Physics
Area of science: Physics
ID: 929410149.Ph


Thanks for the question.  This question has been asked before, and
while I think that the previous answers make good points there is
more that could be added.  Please see the answers at:

MadSci Article 1
MadSci Article 2
Here is some general information about lightning: Lightning Informationn
Here is a simple diagram of how the current from a lightning bolt might flow through the metallic body of a car while a person that is inside the car is relatively safe with no current flowing through them. There might be millions of volts of potential and thousands of amperes of current in the lightning bolt, but since the body of a car is usually a much better conductor of electricity than a human body, the electricity of the bolt prefers to travel through the metal of the car rather than through the occupant. Lightning current 
flows through the body of the car and not through the occupant, mostly. The above explanation is mostly correct but incomplete. There are two concepts of physics that should be considered to get a more complete picture of what is going on. These concepts are Ohm's Law and induced EMF. Ohm's Law states that voltage equals current times resistance, or that V = I * R. What this law means is that when electricity flows through something there is a voltage different between one end of the thing and the other end. The bigger the flow, the bigger the voltage. Also, the bigger the resistance to flow, the bigger the voltage. For a lightning bolt, the average current might be 50 amperes. However there can be peak currents of 10,000 amperes that only last for a very short time, like a millisecond. Also, even though a car is made of metal it is not a perfect conductor, there will be some resistance. I am guessing here, but it might be 0.1 Ohms (one tenth of an Ohm.) If you multiple 10,000 amperes by 0.1 Ohms you get 1000 volts. This is a voltage that will appear between the point where the lightning bolt hits the car and the point where it leaves the car. Modeling the car 
body as a resistance and the lightning as a current gives the voltage that 
could appear on the car body. The 1000 volts will be present between the top and the bottom of the car if the lightning hits the top of the car and exits out the bottom. This 1000 volts will be present even though all the current of the lightning is flowing through the car body. If you were in the car and touching the roof with your hand and also touching the metal near the bottom of the car when the lightning hit, there would be a brief moment when 1000 volts would be trying to push electricity through your body. The time would be brief, about 0.001 seconds, so the energy would be fairly small, about 0.01 joules. This would not be enough to burn you but it could interfere with your heart contractions. I would not want to experience this myself. There is another way that lightning could hurt you even while you are sitting in the car. This is through induced EMF. Any rapidly changing electrical current will produce a rapidly changing magnetic field. Magnetic fields are generally harmless, however a rapidly changing magnetic field will in turn produce an electric field. If the electric field is induced in a conductor, like the human body, it will cause a circulating current to flow. In a sense, the path of the lightning bolt is like a radio transmitter, and the body could act like a receiving antenna. Changing currents 
create circulating magnetic fields, which then create circulating electric 
fields. The induced EMF should not be a problem except when the lightning current flows close by. Of course, if you are in a car that is hit by lightning, the current is very close by! There is a factor that does help reduce the danger in the car. The fact that the current will flow fairly evenly throughout the body of the car means that there will be cancellation of the induced EMF due to oppositely located currents in the car body. The EMF from current flowing down the left side of the car should cancel the EMF from current flowing down the right side of the car. However, since cars are not perfectly symmetrical, the cancellation won't be perfect. Also, since the EMF is in what is called the "near field" region, the cancellation is only 100% effective in the exact center of the current flow. The closer a person is to the sides of the car the greater the induced EMF they will experience. Like the case with the Ohm's Law scenario, the amount of energy that is potentially delivered to the occupant of the car is fairly small, not enough to cause a burn. However, the circulating induced current could interfere with the heart action. Here is an article that describes how the induced EMF effect may explain the mysterious deaths of some hikers: Article about mysterious deaths possibly related to lightning
In summary, a car is not a perfectly safe place from lightning. The rubber tires really do not have anything to do with the protection that you get inside of a car. The lightning bolt has probably sparked across a mile of air already by the time it reaches a car. If the car is hit by an especially strong bolt, there will be significant, though probably not fatal, electrical effects on the occupants inside the car. Still, a car is a much better place to be than almost any other place that I can think of. Regards, Everett Rubel

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