Friday 19 April 2019

homework and exercises - Why do lightning rods have a sharp point at the top?


We know that a lightning rod or lightning conductor is a metal rod or metallic object mounted on top of an elevated structure and, if we look closely, most of them have a sharp point at the top. What is the reason for this sharp point?



Answer



Suppose there is a charged cloud floating over your conductor. Then making your lightning conductor pointy at the edge would facilitate better discharge by setting up a high electric field.



We will take a spherical approximation of the pointed end, then ${\sigma}=\frac{q}{4\pi r^2}$ is the surface charge density of the end. It has a very high surface charge density due to its small radius. Hence, in this case, the electric field over that small part will be $E=\frac{\sigma}{\epsilon_0}$ which is also very high.


Then, for a pointy metal rod, the electric field set up at pointy ends is high. Now for some reason, if the discharge of the cloud occurs, the charge will be easily passed through the lightning conductor and conducted to the ground. Your artifact which you are trying to save is ultimately protected from damage.


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