Wikipedia tells me that a bolt of lightning releases roughly 1 GJ of energy, but I'm guessing that's along the entire length of the bolt and that most of it is dissipated as heat and light to the surrounding atmosphere.

Don't know much about the physics behind this, but assuming the bolt is 20km long that's about 50 KJ per meter, or 90 KJ for an average human. Or am I WAY off on my assumptions here?

  • $\begingroup$ Not an answer to your question but when demonstrating Leyden jars in schools, one should not give pupils shocks of more than millijoules. A discharge of joules can be fatal. There is a case in Norway where a lightning strike killed hundreds of reindeer. $\endgroup$ – Pieter Sep 15 at 19:58

Energy transferred by current is defined as : $$ E = I Q R $$ Where $I$ is current strength in amperes, $Q$ - transmitted charge and $R$ conductor resistance in $\text[ohms]$.
Typical lightning bolt current is about $30~000 ~\text{[A]}$, and transmits $15 ~\text{[C]}$ charge. If lightning passes through internal body structures, then one needs to account for internal body electrical resistance which is about $1000 ~\Omega$. Putting these into equation gives about $\bf {450 ~\text{MJ}}$ of transferred electrical energy.

Above info fits situation when cloud discharges electrons to ground, i.e. so called negative lightning. But 5% of lightning strikes are positive ones, when a cloud discharges positive charges to ground, i.e. electrons move upwards from ground to cloud. Large bolts of positive lightning can carry up to $120~000~\text{[A]}$ current and $350~\text{[C]}$ charge. In this case one gets positive lightning strike energy about $\bf {42~\text{GJ}}$. Thus positive strikes are a lot more dangerous than a negative ones.

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When lightning is fatal, it is usually due to an electrical discharge-induced heart attack. Since lightning is essentially a electrostatic discharge event, we can roughly view it as analogous to a capacitive discharge.

IEC 60470-1 provides threshold values of various physiological effects of capacitive discharge current, including threshold for ventricular fibrillation due to current through the heart, as a function of capacitance and voltage. Using the relationship $E=\frac{CV^2}{2}$ you can compute the energy stored in the capacitor prior to discharge associated with the IEC various physiological effects.

However, you can't assume all of the lightning strike current and energy will be delivered to the heart. Although the available energy of the strike is high, the source cited below states that most people do survive a lightning strike. It says one reason is that lightning rarely passes through the body. Instead, a “flashover” occurs, meaning that the lightning travels over the surface of the body through the conductive sweat (and perhaps rain) on the surface of the body which provides an alternative external pathway around the body for current to flow.

For additional information, see.


Hope this helps.

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