The tops of the modern buildings have an iron rod that is connected all the way to the earth. What role does such a thing play? Well, for those who are not aware, it insulates the building from a lighting threat. But how does a thin iron rod does this job all on its own? This question leads us to another question of what ‘lightning’ exactly is? How does it get so much energy that it can flash an entire city? The answers for all these questions are discussed in this article.
Lets start with what makes a ‘lightning’. All of us are aware of electricity, the fuel for every electric appliance. The electric appliances require moving charge called current. The movement of charge from point A to point B is induced by the difference in electric potential energy of the charge carrying species between the two points. As we have learnt during our H2 Physics tuition class, every charge carrying species tends to decrease its potential energy and thus tries to move from high-potential point to low-potential point. Now, if you recollect, the two ends of a battery are of two different potentials and hence motion of charge is initiated by this potential difference and this moving charge drives the electric appliances.
A lightning works in a similar way. There are different types of lightning like intra-cloud lightning, cloud-cloud lightning (inter-cloud lightning) and cloud-ground lightning. The physics of each of these is same. A lightning requires separation of charge. When positive and negative charges pile up at different regions separated by non-conducting medium, the potential difference between the media builds up. When it crosses the threshold point, charge moves from high-potential zone to low-potential zone and this process is associated with release of large amounts of energy in the form of light. The two zones can be within the cloud or between two clouds or between cloud and ground. In the case of cloud-ground lightning, air acts like insulator.
When an iron rod or a conducting medium is placed in between the points with difference in potential, the moving charge chooses a path along the conducting medium. So, when an iron rod is placed on top of the building that goes all the way to earth, the charge chooses to move along the rod and does not affect the building. In most of the cases the iron rod gets heated up to extremely high temperatures and gets molten. This is because the moving charge loses energy as it travels between the regions with potential difference and some of the energy is released in the form of heat.
The amount of energy released in the form of heat or light, during lightning process, depends on the number of charged particles that move across the two regions and also the potential difference between them. For the electric current to move through air, it requires very high amount of potential difference and the number of charge carrying species moving across is also very high. So, lightning can momentarily flash a very large region.