It is a common notion that cell phones could be a potential device for conducting lightning and sending a voltage surge through the user by providing a less resistive path compared to surrounding. Other notions include the electricity travelling through the radio waves associated with the devices and deliver a shock to the person using them. In this paper we discuss the way a lightning is produced, and explore the possibility of a cell phone providing a lightning path through the user to the ground.
[1] By a mechanism that is not fully understood, charges in a thundercloud are separated such that the lower portion of a cloud takes on a negative charge while the upper portion is positive. [2] This is true in more than 90 percent of the cases with only a relatively few clouds having a reversed polarity charge separation. The charge buildup occurs regardless of what is on the ground. In addition to the negative charge on the base of the cloud, there are small pockets of positive charge on the base. When the charge builds up to a sufficient magnitude, 108 or 109 V with respect to ground, a localized branched breakdown occurs, called a leader, extending from the cloud toward ground for about 50 m. This leader, called the stepped leader, ceases. Then another stepped leader extends from the cloud which encompasses the original step and extends for an additional 50 m or so. This stepping process continues until one of the branches of this stepped leader is within 50 - 100 m of the ground or some object on the ground. A streamer from the ground then joins the leader channel, and a return stroke goes up the leader channel, making all the branches bright. This is the high current bright phenomena called, a lightning stroke. To aid in appreciating speeds, the speed of light is approximately 3 × 108 m/s. Following the first return stroke, the lightning channel luminosity decreases to a great extent when a bright ball of light, called the dart leader, may descend the primary channel, without branching, to the point of contact on the ground. Then the second return stroke ascends this primary channel with great brightness. The totality of all the lightning strokes from a region in the cloud is called a lightning flash. A lightning flash thus consists of the initial return stroke and all subsequent return strokes, if any. Discussing the direction of lightning travel makes little sense unless terms are very carefully defined. In a usual lightning flash, negative charge travels from the cloud to ground, so conventional current direction is from ground to cloud. The stepped leader and dart leader light travel from cloud to ground while the return stroke light travels from ground to cloud. It is probably better to refer to a lightning path rather than indicating that lightning traveled in a certain direction between cloud and round.
Except for the case when tall structures such as radio towers or buildings over 60 m in height are on the ground under the cloud, the lightning stepped leader will start and descend from the cloud regardless of what is on the ground. When one of the stepped leader branches is within 50-100 m of the ground or an object on the ground, a return streamer from the object will reach the stepped leader channel and initiate the return stroke. An object on the ground will only attract lightning that could have struck in the immediate vicinity anyway; thus a sailboat in the middle of a lake will not attract lightning from all over the lake but will only attract lightning that would have struck near the sailboat anyway. For tall structures the stepped leader may start from the structure and travel to the overhead cloud, so that lightning is triggered by an object on the ground. For structures less than 60 m in height, fewer than ten percent of the flashes are upward. A lightning flash produces thunder because of the shock-waves produced by the sudden heating of the air in the lightning path. Thunder is seldom heard more than about 15 mi away from the stroke, so that hearing thunder' indicates lightning was within this distance from the observer. [3]
A tall object will tend to attract lightning that was going to strike in its vicinity. For many years various methods were proposed to evaluate the protection afforded to a lower object by a nearby taller object. One method suggested is that there is a cone of protection around an object like a tall grounded metal pole such that another object will not be struck by lightning if it is within the conical volume whose height is the pole and whose base is a circle on the ground, centered on the pole with a radius equal to the height of the pole. Lightning would tend to strike the pole rather than an object within this volume. Such a cone of protection is not perfect and there are numerous reports of objects within this cone of protection being struck. Other zones of protection, taking into account the striking distance for the last jump of the stepped leader, are also used in the design of lightning protection systems. Using the cone of protection concept, it is evident that the probability of an object being struck .is proportional to the square of its height above the surrounding region. A pole of height h will tend to attract lightning to itself which would normally have struck in the circular area at its base which has a radius h. The area is porportional to the square of h. Objects within a zone of protection can be damaged by lightning even if they are not directly struck. This is caused by sideflash. When lightning strikes, a large voltage to ground can be developed along the path because of the large peak currents and sharp rate of rise of the current. The peak currents cause high voltages to ground because of resistance in its path such as the resistance of the object struck and the resistance at the ground connection. The rapid rate of rise for the current produces voltages to ground because of inductance in the object struck. When the object that was struck has a high voltage with respect to ground, a sideflash over to a nearby grounded object can occur.
The chances of a cell phone being struck by lightning is less likely, it can happen, although probably more as a function of the person using the phone being the tallest thing around when lightning strikes rather than anything having to do with the phone itself. Cell phones (and cordless portable phones) used indoors during electrical storms are perfectly safe because there is no wire through which the electric discharge could travel. (The belief that lightning can follow the radio waves is completely unfounded.) There is also a popular belief that cell phones pose a risk when used outside because lightning is attracted to metal, handsets generally contain insignificant amounts of metal.
© Shiv Agarwal. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] T. Bernstein, "Lightning and Power Surge Damage to Appliances," IEEE Trans. on Industry Applications 20, 1507 (1984).
[2] M. A. Uman, Lightning (Dover, 2011).
[3] M. A. Uman, Understanding Lightning (Bek Technical Publications, 1971).