As the winter months fall behind us and summer heat starts to rise, we have some visitors in our midst: the all-pervading mosquitos. Everywhere and anywhere, we find these wonderful creatures hovering all around us, in the walkways, in classrooms, at the chai stalls here in IIT Kanpur, where I teach.
Often in the mornings, they are so full of a late night feast that some of them can’t even lift their own weight. Even though they contain some of the most beautiful elements of physics and biology, given their dexterity in stealthily puncturing our skin, it is hard for most humans to love them.
So among all the electronic and chemical technologies humans have ever developed to battle them, perhaps the most impressive is the ‘electric tennis bat’. While someone unaware may mistake it as one of the pieces of sports equipment Indians love, this single-player game is a pleasure to play. You chase and hit an airborne mosquito with the bat. If you succeed, you will hear the sweet sound of the blood-sucker’s body crackling to death.
Complete the circuit
The bat’s working principle is simple. There are three metal meshes. The one at the centre is positively charged and the outer ones are negatively charged. When the layers don’t touch each other, current can’t flow. But now when a mosquito connects them, a current passes through and kills the insect.
Essentially the mosquito receives an electric shock, just like we might if we were hit by lightning on a stormy night. The mosquito bat is a portable thunderstorm for the mosquito. The physics of sparks and lightning is the same, whether it’s in your gas lighter, in the belly of storm clouds, in the mosquito bat.
So the question arises: why do electric sparks – like a bolt of lightning – look white? If this is the ‘colour’ of electric current, shouldn’t all the current flowing through electric wires at home also be white? Yet they aren’t.
Batteries and shocks
Electric current is carried by electrons, the negatively charged fundamental particles that usually revolve around positively charged protons in every atom. Every atom has an equal number of protons and electrons, rendering them electrical neutral.
In any piece of metal such as copper, there is a large number of atoms but every atom also shares some electrons with other atoms. The whole material still remains neutral but these common electrons can freely move from every atom to any other, and conduct current easily.
In an insulator, on the other hand, every atom holds onto its electrons and doesn’t share. The air we breathe is a wonderful insulator – as are most of us. This barrier can be torn down by applying a high voltage, which will force electrons out of atoms. Suddenly, instead of a neutrally charged air, we have a gas made of positively charged atoms and negatively charged electrons floating together.
A battery generates electric force. How much electric force is generated depends on the battery’s volt value. Higher the voltage, greater the force. For example, the pencil battery that powers our wall clocks is usually 1.5 V. A phone battery has a comparable range.
These are strong enough to drive currents through clocks and phones but not strong enough to give humans electric shocks. That’s why you don’t have to worry when holding them in your hand. On the other hand, the current supplied to our household appliances comes with a voltage of 220V, which is enough to electrocute us.
During a thunderstorm, the voltage can cross a hundred million volts, powering electrons to fly through the air.
Atoms’ signature sparks
Strong voltages ionise and pull electrons away from atoms. These unhappy atoms then try to get their electrons back. If the electrons do go back, they need to lose the ‘excess’ energy they have, and they do this by emitting light. This is similar to when agitated children lose their energy by yelling or agitated drivers by honking through the Kanpur traffic.
Every time the electron loses some energy, the light has some wavelength. In the case of air, the light the electrons lose is in the range of wavelengths human eyes can see. And this light is what we see.
The colour of the light and the spark depends on the type of atom. In fact, this emission can be thought of as an atom’s fingerprint – its unique identifier. For example, in air, most of the atoms are of oxygen and nitrogen and so the sparks are white or near-white. On the other hand, on some alien planet with an atmosphere made of neon, the sparks will look red.
Interestingly this is also the physics which goes into working of a tube light but in a slightly different way.
All for one
Circling back to the mosquitos and our mosquito bats: how much voltage do these devices generate?
It’s around 1,400 V – equivalent to approximately a thousand regular batteries, and enough to drive a powerful electric current through the mosquito and at the same time drive electrons out of atoms in the air nearby, thus creating the sparks we see.
In case you are also wondering how certain wavelengths of light can be emitted by certain atoms: it is only by learning quantum mechanics can one understand this. So if you are interested, consider pursuing a course in physics.
And the next time a mosquito troubles you and you end up using the electric bat with crackling success, just remember: it’s not just you. Quantum physics, electrons, and the atoms in the air are all joining in to celebrate your victory.
Adhip Agarwala is an assistant professor of physics at IIT Kanpur.
