Fuel cells can make an electricity from a simple electrochemical
reaction in which oxygen and hydrogen combine to form water. There are several
different types of fuel cell but they are all based around a central design
which consists of two electrodes, a negative anode and a positive cathode.
These are separated by a solid or liquid electrolyte that carries electrically
charged particles between the two electrodes. A catalyst, such as platinum, is
often used to speed up the reactions at the electrodes. Fuel cells are
classified according to the nature of the electrolyte. Every type needs
particular materials and fuels and is suitable for any applications. The
article below uses the proton exchange membrane fuel cell to illustrate the
science and technology behind the fuel cell concept but the characteristics and
applications of the other main designs are also discussed. Proton Exchange Membrane Fuel Cells (PEMFC)
The hydrogen ions permeate across the electrolyte to the
cathode, while the electrons flow through an external circuit and provide
power. Oxygen, in the form of air, is supplied to the cathode and this combines
with the electrons and the hydrogen ions to produce water. These reactions at
the electrodes are as follows:
Anode: 2H24H+ + 4e-
Cathode: O2 + 4H+ + 4e- 2H2O
Overall: 2H2 + O22H2O + energy
PEM cells operate at a temperature of around 80°C. At this
low temperature the electrochemical reactions would normally occur very slowly
so they are catalysed by a thin layer of platinum on each electrode.
<u>Answer</u>: Natural selection.
<u>Explanation</u>:
- The concept of <em>natural selection</em> was given by <em>Charles Darwin.</em>
- According to this concept, in a given environment the organisms that possess <em>better adaptations</em> have<em> higher chances of survival</em> and, leave behind a <em>greater number of progenies</em>.
- When malaria outbreak occurs, the allele frequencies change and only the organisms that possess the alleles which help them to <em>survive</em> this outbreak due to resistance against malarial infection will be selected by nature and<em> hence, this is an example of natural selection.</em>
The red blood corpuscles have a pigment called hemoglobin that binds to the oxygen and carries it to the whole body in both the conditions, when the body is at rest or when the body is exercising. The oxygenated blood from the heart is carried to the various parts of the body and the de-oxygenated blood from various parts of the body is carried to the lungs where the exchange of gases takes place.
