Well after a few months of a hectic summer, rains are here. The campus is green and nature is once again abundant. With rains, however, has also come a usual game of hide and seek. Every time I take my umbrella out, the rain is nowhere to be seen. But when I forget it, I am greeted by sudden bouts of rain spoiling my ambition to reach my class dry and tidy.

Umbrellas, however, have something amazing about them. You press a button and it automatically opens up, stretches itself, and unfolds into a shade, growing much larger than it was. Like a magical genie.

Have you wondered how an umbrella works? You only applied a small amount of energy to push the button. Where did it get all this energy to open up so big?

As always, we are on the cusp of some beautiful physics.

Spring forth

If you have ever broken up an umbrella, you will have seen a spring inside the main rod. That is the most crucial piece.

Springs are usually metal wires winded in a spiral, giving it a remarkable mechanical property: it becomes a ‘mechanical battery’. Just like batteries allow us to store electrical energy, springs allow us to store mechanical energy. Once pressed, the springs try to come back to their original size and exert a force on its end.

How much force? That depends on how much you have compressed it. The more you compress, the greater the force. The British physicist Robert Hooke discovered this 350 years ago; the relationship between the compression and the force is today called Hooke’s law.

Incidentally, you must have seen batteries and springs together. Where? Well, everytime you try to fit a battery in the sockets in a remote control. The spring keeps the battery snug and tight, and the fact that it is made of metal allows for electrons to easily flow into a circuit from the battery through the spring.

The strength of springs are often quoted in newtons per millimetre, or N/mm. Here, ‘newton’ is the unit of force and ‘metres’ is the unit of distance. A newton is about the force a medium-sized apple exerts on your hand. You can easily see a millimetre on a regular stationery scale. Now, a usual spring you may have seen in a battery socket faces about 1 N/mm. So when you try to tuck a battery in, that battery compresses the spring and the spring in turn pushes on the battery by about one newton of force, to keep it in its place, so that it doesn’t fall, and keeps the circuit connected.

But that’s just one spring you see. You are in fact surrounded by springs, just that they are hidden. The sofa you sit on while entertaining a guest, the mattress you collapse on after a day of office work, and even your car or your scooter where you spend hours managing rainy-season potholes — all of them have several springs to make sure you are comfortable.

If you have ever paid close attention to our trains or metro rails, you might have noticed large springs under the coaches or hanging on the top. These springs help stabilise the train as it moves over uneven terrain. And they can reach up to strengths of hundreds of N/mm.

Now, even though you may have noticed the spring, the real physics question is why it stores any energy at all. And what type of force does it exert?

Forces in nature

In physics, we think of four fundamental forces. One is the electromagnetic force, which decides how electric charges move and magnets attract. The second is the gravitational force, due to which we don’t fly off and remain stuck to the ground. The two other forces are called strong and weak forces: they work at very small distances, like inside the nucleus of an atom.

The force created by springs doesn’t seem to be any of these four options. So what is it?

Turns out even the spring’s force is secretly an electromagnetic force. Think of any metal, say aluminium. It has billions of aluminium atoms arranged like oranges on fruit carts. These atoms have a massive positively charged nucleus and negatively charged electrons that are very light. Electromagnetism teaches us that positively charged particles repel each other while positives and negatives attract. Now, in a metal, even though the atoms stay in their places, the electrons move around, spending their time near whichever atom they like.

The atoms are like residents in a colony who don’t get along with each other. But then all of them have kids (the electrons), which form a group and love to hangout at all the houses in the neighborhood. And the kids are loved by all and everyone takes care of them. So the kids hold the whole neighborhood together.

But remember: the nucleus of the two atoms still repel each other since they are both positively charged. So the atoms strike a deal. The two nuclei of atoms decide to build homes at some convenient distance where they can tolerate each other even while their electrons can happily move between the houses.

Now, when you try to push any material from one side, the distances between these atoms become squished. The positively charged nuclei come closer than they want to, and since positive charges repel each other, they now push back. This is how the elastic force of any material arises.

Vibrations and sound

Now, while one spring can push back, a lot of them arranged together in a material can do something more: they can vibrate together.

Think of a sofa with many springs beneath. When you jump at one end (of course you need to ignore the possibility of damaging it permanently), you can feel the whole bed vibrating even at the other end. The same thing happens in water when you drop a stone at one point. Water molecules vibrate and push and pull on nearby molecules as well, leading to a ripple.

Even in a solid material like iron or aluminum, when we strike one end, we make small changes to the position of atoms at that point. These atoms push and pull nearby atoms using those elastic forces. These vibrations can move at very high speed throughout the material. They are called sound waves. If a steel plate falls from your hand on the floor, the vibrations and sound you hear moves through the plate at a speed of about 20,000 km/hr and reaches you over air molecules at about 1,000 km/hr.

Sound and superconductivity

It may seem that the vibrations of positively charged atoms and the movement of negatively charged electrons do not bother each other much. And they do not, at least not significantly, when they are at room temperature. This is why metals behave like metals when we see them. But when the temperature drops, atoms and electrons can do funny things. For instance, atoms can conspire to generate attraction between two negatively charged electrons.

Imagine two atoms (neighbours) that move closer together using the spring that connects them, to attract a few kids to their garden for a moment. Now, once one electron (Bunty) moves towards them, another electron (Bubly) notices and also approaches the same neighborhood. Now as they move, another set of neighbours on a different road do the same thing. So Bunty and Bubly will continue following each other and form like a pair even though they did not mean to move together.

This pair of electrons is now called a Cooper pair, named after Leon Cooper, a U.S. physicist who won a Nobel Prize in 1972 for discovering this mechanism.

So what if two electrons start attracting each other? Turns out this formation of Cooper pairs leads to superconductivity. This is an extraordinary phenomenon: where a metal decides to completely change its character and start conducting electric currents without any resistance. Your electricity bills would nosedive if such materials could exist at room temperature.

Most metals become superconductors but at very low temperatures. Aluminum, often used to make your pressure cookers, becomes a superconductor at –271.9 C. Even though scientists discovered superconductivity more than a century ago, even today they don’t understand all aspects of it. It really requires quantum mechanics to understand, and is one of the most active areas of research in quantum condensed matter physics — which is the study of quantum materials around us.

Back to umbrellas

Finally, our most important question: why does an umbrella need that big spring?

The umbrella’s rod consists of multiple cylinders one around the other, and the spring opens them up in succession. When you close an umbrella, you compress the spring and store energy in it — essentially squishing billions of atoms slightly closer together than they want to be. This gets locked by a hook. When you press the umbrella button, the hook releases to let the spring push out again.

Next time, when you open an umbrella and walk in the rain, don’t forget to thank the billions of atoms and their small inter-atomic springs, which are also breathing a sigh of relief.

Adhip Agarwala is an assistant professor of physics at IIT Kanpur.

A good old game of carrom has really stood the test of time. Set up a board and people around will flock to it while practising their finger gymnastics. You may not have noticed but the game often starts with an underappreciated ceremony: a pinch of magic powder strewn over the board that somehow energises the sluggish coins, helping them glide smoothly with just a tiny push.

What did that little bit of powder do?

Fixing friction

Friction is a barely visible force that surrounds us and slows us down every time we move. Isaac Newton told us a long time ago that if something moves, it will continue to do so unless stopped. Then why doesn’t a car just continue moving once it has been pushed? That way we could have just reached from one place to another without spending any money on fuel – and we wouldn’t have to worry if its price is rising or falling.

The reason is friction. Even if we make roads as smooth as possible, there will always be some friction that will slow the car. The invisible air also creates friction for aeroplanes and birds. The tricky part even for our rockets is the phase when they are getting out of the earth’s atmosphere. The air’s friction creates a lot of heat – the same heat you generate when you rub your palms against each other on a cold day. In outer space, there is no air and spacecraft move more easily, often for very large distances with just one push.

Where does this friction come from?

Any surface – such as a carrom board or your desk – may seem to be completely smooth and straight, but that isn’t really the case. In fact, no perfectly uniform surface exists in the universe . If you look at any surface very closely, you will notice tiny protrusions: basically a few places that have bumps and others that have dips, like a camel’s back.

The surfaces of a carrom board surface as well as a carrom coin also have bumps and dips. When the coin tries to move on the board, the bumps and dips on both surfaces will hit each other, slowing the coin. This is the friction at play. Friction can also arise if there are things to stick on both surfaces, like some oily dirt. In short, friction arises because of forces or interactions between two surfaces.

Think of the carrom coin as a car moving on a road. If the road is smooth, the car will move easily. But if there are bumps and dips, the tires will have a hard time moving, and slow down the car. The rougher the surface, the more friction there is.

We fix a broken road by filling up the potholes. Similarly, that little bit of fine powder fills up all the tiny invisible holes on your carrom board, and then the coins move easily.

Dosa pan and air tracks

You may wonder: isn’t there a simple way of removing all friction? Well, people have found amazing ways to do this. The simplest version is something you have been doing at home, and for a long long time. If you have ever made dosa or some besan chilla, you may have sprinkled some water on the hot pan before or after. The water droplets move on the pan as if they have no friction. This is often to check if the pan is hot enough. But why does the water not stick to the pan?

This is because as soon as the water drops hit the hot surface, a thin layer of steam is formed below the drops. So the water drops are really floating on the pan. Given the surfaces are no longer in touch, they have very little friction, allowing the drops to glide around.

In one of the first physics courses at IIT Kanpur, where I teach, new students perform experiments to verify Newton’s laws, like the one I mentioned earlier. But then Newton’s laws are only true when there is no friction. So we need to make objects move without friction first.

One ingenious way is to conduct an experiment in which an object is made to move on an air track. Some amount of air is continuously pumped from within rails using powerful air blowers. If an object is not very heavy, it can ‘float’ above the track by sitting on the air, and move as if there is no friction. As a result, the students can now test Newton’s laws for themselves.

In fact, superfast trains that use the principle of magnetic levitation do the same thing. Here, the whole passenger train floats on a strong magnetic field created by the tracks. There is thus much less friction and the trains can move at a great speed.

The trouble with electrons

Just like cars have a tough time moving over a broken road, electrons find it hard to move through metal, such as a copper wire. That is why we pay our electric bills: so that someone somewhere can continuously apply some voltage to the wires, and the electrons pass into them and come to our houses.

What could be stopping electrons inside a material? Just as roads have potholes, any material around us has impurities. For example, any copper wire will have a trace presence of some other metals, such as zinc or aluminium. The atoms of these metals create ‘friction’ for electrons and make them slow down as they pass through. So if we are able to remove this friction, we will be able to get the wire to conduct an even greater current without applying any voltage.

You may even be thinking that if we can remove all the impurities from a material, we can escape paying electric bills. Unfortunately, this will not be possible. Even though Newton’s laws are sufficient to explain how carrom coins move on a board or cars move on a road, they are not sufficient to explain electrons.

Here, the laws of quantum mechanics are at play. A careful study of quantum mechanics and its applications to materials tells us there is another way of getting rid of friction for electrons – by the help of  a phenomenon called superconductivity. If you are more interested in learning about superconductors and their fascinating properties, a physics course may be worth thinking about. 

So the next time you curse the rising petrol bills, remember that you are not alone. The real problem is friction – which cars, carrom coins, and even electrons are struggling with.

The author is an assistant professor at IIT Kanpur.