solid state Archives - PhysicStuff

What is anomalous expansion of water?

By Pratik Barve | June 1, 2019 | Comments 0 Comment

First of all let us see why is it called anomalous ( meaning unusual ) expansion of water. Raising the temperature of liquid or gas increases the average separation between the molecules because they start vibrating more vigorously, leading to decrease in the density. The same should happen with water , heating it to 373 K (100°Celsius) increases the average separation up to a point where it becomes a gas and lowering it to 273 K (0° Celsius) decreases the average separation and becomes ice. So according to that logic ice should have the highest density, right?

But it is observed that as we cool water, density goes on increasing till 4°C and then starts decreasing again. Weird, right? In-fact this is the reason why ice floats on water. At about 4°C water reaches its maximum density.

A water molecule consists of 2 Hydrogen and 1 Oxygen atom. As we cool the water, the inter-molecular distance decreases and molecules come closer thus increasing the density. At about 4° C the water molecules come close enough that Hydrogen bonding takes place between Hydrogen atom of one molecule and Oxygen atom of other molecule. This type of bonding is not as strong as covalent bonds. Now the H₂O molecule has a specific structure. Due to Hydrogen bonding the atoms begin to arrange themselves in a lattice like structure and at 0°C the state changes to solid when water crystallizes. The water expands after 4°C because the oxygen atoms repel other oxygen atoms of water molecules and hydrogen bonds maintain the lattice structure.

Hydrogen forms a bond with Oxygen atoms of neighbouring molecules.

The positive and negative signs in the above figure denote the effective charge on atoms in water molecules. Water molecule is a polar molecule. It means that there in uneven distribution of electron density in the atoms. In one water molecule, the probability of finding the electron of hydrogen atom is more near the oxygen atom due to the bond between them. So effectively hydrogen atom has effective positive polarity and oxygen atom has negative polarity. An electrostatic attraction between the partial positive charge near the hydrogen atoms and the partial negative charge near the oxygen results in the formation of a hydrogen bond between 2 water molecules.

Effects of anomalous expansion

This very strange but interesting property of water was actually very significant in the survival of aquatic life.

Aquatic Life flourishes even if the top layer of entire lake freezes.

As the water has highest density at 4°C, in very frigid and cool atmospheres the lakes used to have liquid water at bottom. Only the top layer of lake would turn into ice, further insulating the water below ice sheet. This helped aquatic life survive in the depths. Same with ocean caps and icebergs. The density of ice is 9% less than water at 4°C so it floats. So this anomalous expansion of water in-fact helps the aquatic animals to live. Even under icebergs or we can think back to the ice-age where ocean layers froze but deep layers were water due to this property of water.

Other such effect is bursting of water pipelines in really cold regions. Due to extreme cold weather, the water freezes inside pipes, expands and due to this immense pressure the pipes burst. You can observe this really easily by filling a plastic water bottle upto the brim and keeping it in the freezer. The bottle expands or in some cases bursts. (If you are really adventurous try freezing a glass bottle full of water. Cleaning is not fun.)

What is p-n junction?

By Pratik Barve | October 30, 2017 | Comments 0 Comment

The p-n junctions are elementary building blocks of semiconductor electronic devices. As the name suggest it is a junction between two types of semiconductor materials the p-type and the n-type. The p-type is the positive side of junction and n-type is the negative side. The p-type consists of excess holes (missing electrons) and n-type consists of excess electrons. The junction is the boundary or interface between these 2 types inside a single crystal of semiconductor. When a p-n junction is formed i.e when it is fabricated there are excess electrons on n side and holes on p sides so electrons and holes combine at the junction. Departure of electrons from n side to p side leaves with a positive donor ion on n side and likewise hole leaves negative acceptor ion on p side. The uncompensated ions are positive on n side and negative on p side which creates an electric field which provides a force opposing the continued exchange of charge carriers.

When the electric field is sufficient to stop further transfer of charge carriers the depletion layers reaches an equilibrium. When a positive voltage is applied to the p-side with respect to the n-side, a large current will flow through the junction. However, when a negative voltage is applied, virtually no current flows. This “rectifying” behaviour is the most important characteristic of p-n junction. This is a very short description of p-n junction and how it works.

The p-n junction was invented by Russell Ohl an American physicist of Bell Laboratories in 1939. The p-n junction is created by doping (adding impurities) by various methods on any one type of base semiconductor material. Various methods like ion implantation, diffusion, etc. Semiconductors for instance silicon, is doped with boron, phosphorus, or arsenic by ion implantation method. In diffusion method the impurity moves into the semiconductor crystal by solid-state diffusion. In the ion-implantation method, the intended impurity is introduced into the semiconductor by accelerating the impurity ions to a high-energy level and then implanting the ions in the semiconductor.

The p-n junction can be used in electronics circuits in two different configurations, forward bias and reverse bias.

In forward bias, the p-type is connected with the positive terminal and the n-type is connected with the negative terminal. With a battery connected this way, the holes in the p-type region and the electrons in the n-type region are pushed toward the junction and start to neutralize the depletion zone, reducing its width. And thus current flows through the junction.Connecting p-type region to the negative terminal of the battery and the n-type region to the positive terminal corresponds to reverse bias. Very little current will flow until the diode breaks down in reverse bias configuration.

The forward-bias and the reverse-bias properties of the p–n junction imply that it can be used as a diode. A p–n junction diode allows electric charges to flow in one direction, but not in the opposite direction; negative charges (electrons) can easily flow through the junction from n to p but not from p to n, and the reverse is true for holes. When the p–n junction is forward-biased, electric charge flows freely due to reduced resistance of the p–n junction. When the p–n junction is reverse-biased, however, the junction barrier (and therefore resistance) becomes greater and charge flow is minimal. p-n junction is used in many semiconductor devices such as diodes, transistors, solar cells, LED’s, integrated circuits, etc.