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Moment of truth
The Sun, with a diameter of 1,392,000 km, is made of 94% hydrogen atoms.

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fuel cell Chem.
A voltaic cell that uses hydrogen and oxygen to create electricity via an electrochemical process. It consists of an electrolyte and a porous anode and a cathode. Hydrogen is fed to the anode which then separates into protons and electrons by catalysis. The protons travel through the electrolyte and electrons travel via an external circuit to reach to cathode.

The notable difference between a fuel cell and a normal battery is that, for the former, the supply of fuel (such as hydrogen) from some external source, in contrast to the limited internal storage capacity of a battery.

fumigation Med.
Use of smoke, gas or vapor to kill microorganisms, mould or pests. This is in contrast to spraying and it does not left any residue.

fumigant Chem., Med.
A substance that is used for fumigation. It is usually a low molecular weight and volatile chemical. For example, methyl bromide (CH3Br) is used to kill woodworm, or formaldehyde for killing bacteria.

fusion reaction Phys.
A nuclear process whereby two atomic nuclei fuse to form a heavier nucleus with the release of energy. The amount of energy released is depended on the difference of the mass of the final nucleus and the total mass of the initial nuclei which is given by the Einstein's mass-energy equation E = mc2. Fusion reaction is the main process that powers the Sun and stars. The easiest route to nuclear fusion involves the isotope nuclei of hydrogen, deuterium and tritium. Even then, extremely high temperature will be needed to produce a plasma to jump start the reaction, releasing helium-4 nucleus and a neutron.

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