F - Factary

Fahrenheit (scale of temperature)

A scale of temperature no longer used in science. However in some countries, not least the UK, people still refer to temperatures in degrees Fahrenheit. You can still hear people say, “the temperature's in the 90's today” or “I felt really bad the other day, I had a temperature of over 100”.

Most countries today have adopted the Celsius or Centigrade system.

On the Fahrenheit scale the freezing point of water is 32^o, body temperature is 98.4^o and the surface of the Sun has a temperature of over 12 000^o!

To change a temperature expressed in degrees Celsius into degrees Fahrenheit you multiply by 1.8 and add 32. So 20 °C = (20 x 1.8) + 32 = 68 °F.

If you want to do it in your head and get an approximate answer then double the Celsius value and add thirty. So 20 °C = (20 x 2) + 30 = 70 °F. Pretty close!

To go quickly in the opposite direction (from Fahrenheit to Celsius) try subtracting thirty and dividing by two – in that order!

Fahrenheit, Gabriel Daniel (1686-1736)

The German born instrument maker and physicist who invented the alcohol thermometer. Early in his life he travelled around Europe meeting many other instrument makers but eventually settled in Amsterdam where he remained for the rest of his life.

Faraday, Michael (1791-1867)

Born in 1791, this English scientist is best remembered for his investigations of electromagnetism. When he discovered that electricity could be made by moving a magnet inside a wire coil, he was able to build the first electric motor. He later built the first generator and transformer.

A unit of electricity was named after him. The farad (F) is a unit of capacitance, which is a measure of stored electrical energy.

Femto ( f )

Although it sounds like something medical, femto is actually a prefix indicating 10^-15. That's 0.000 000 000 000 001 of something.

The radius of a proton is about 1 femtometre (1 fm), and what is more there are devices which can measure distances to that accuracy. They are called Superconducting Quantum Interference Devices (SQUIDS).

Scientists at the National Institute of Standards and Technology in the USA have also designed and built a 'femtosecond' clock - one that is accurate to a femtosecond. It would take 30 billion years (twice the current age of the universe) before it lost or gained one second!

Fermi, Enrico (1901-1954)

In the 1930s Enrico Fermi became the first physicist to split the atom. His later research pioneered nuclear power generation. He was born in Rome, Italy and graduated from the University of Pisa in 1922. In 1934 he perfected his theory of beta particle emission in radioactivity. Fermi then experimented by bombarding uranium with slow moving neutrons. This caused reactions which were found later to be atomic fission. With researcher Leo Szilard, he began work on construction of an 'atomic pile'. This made possible the controlled release of nuclear energy, which was finally accomplished in 1942.

He was transferred for a time to the Los Alamos atomic bomb laboratory in New Mexico. Fermi then returned to Chicago in 1945 as a professor at the Institute for Nuclear Studies. At the same time he also became a United States citizen. He was awarded the Nobel Prize for physics in 1938 for his work on nuclear processes.

Filament

A structure in the corona of the Sun consisting of cool plasma supported by magnetic fields. Filaments are dark structures when seen against the bright solar disk but appear bright when seen over the solar limb. Filaments seen over the limb are also known as prominences.

This image shows many nice filaments on the Sun. They show up as dark cloud-like wisps against the bright Sun. They appear dark because the gas in the filament is blocking out much of the radiation from the Sun. When the same features are seen off the limb of the Sun they appear bright because they are emitting a small amount of radiation themselves.

Fission (nuclear)

The splitting of atomic nuclei which releases tremendous amounts of energy. This is achieved by the bombardment of the nuclei by high-energy neutrons. It was first achieved in the laboratory by Enrico Fermi and is the basis of the energy source in nuclear power stations.

Flare (solar)

A rapid and violent release of energy from a small region on the Sun in the form of electromagnetic radiation, energetic particles and motions within the Sun.

Here is a flare seen by TRACE >

Flares are classified according to their brightness at different wavelengths. One common classification is for their brightness in X-rays. Flares are classified with a letter. For example the very brightest are class X and fainter ones are M and C. The origin of this scheme is thought to have been (from weakest to strongest) B=baseline, C= common, M= moderate, X=extreme.

As the sensitivity of instruments improved, it was possible to see much fainter events and so a very weak 'A' class was added. Within each letter group, the strengths are divided into number classes so you will see reference to an M2 or C8 class flare, for instance. Scientists of course like real physical measurements and so this classification scheme is also linked to a direct measure of the X-ray energy output measured in watts per square metre. A C3 flare has a peak X-ray output of 3.0 x 10^-5 W/m² and an M6 flare an output of 6 x 10^-4 W/m².

For comparison see the Factary entry Spectral type for details of the spectral classification of stars and how another seemingly arbitrary alphabetical sequence relates in that case to stellar surface temperature.

Find out more about solar flares in our section on solar explosions

Free Electron

An electron that has broken free of its atomic bond and is therefore not bound to an atom. Also known as a beta particle. See the entry for electron. Free electrons can move along a wire as an electric current.

Frequency

The number of repeats per unit time of the oscillations of a wave. The higher the frequency, the smaller the wavelength. Frequency is measured in hertz (Hz), which is one cycle or wave per second.

There is a link or relationship between the frequency, wavelength and velocity of a wave, given by:

Velocity = Frequency x Wavelength

Velocity is measured in metres per second (m/s), frequency is measured in hertz (Hz) and wavelength is measured in metres (m).

Fuel Cell

The simplest picture of a fuel cell is a magic box that gives you electricity, plus a bit of heat and water, when you feed it with hydrogen gas and oxygen. Ordinary air can supply enough oxygen, so all that is really needed is a source of pure hydrogen gas.

You may be curious enough to want to know what exactly goes on inside such a box. If so then read this detailed account provided by Dr Rob Potter of the Johnson Matthey Technology Centre:

If you pass a direct current through acidified or alkali water, you get hydrogen gas released at the negative (reducing) electrode and oxygen gas released at the positive (oxidising) electrode. With the right type of electrodes, if you turn the power supply off and, instead, connect the two electrodes to e.g. a small light-bulb, you can show that the process is somewhat reversible. The hydrogen and oxygen bubbles now go back into solution as water and you get power back out. This is what Sir William Grove discovered back in the 1830’s using platinum electrodes. This is the principle behind the fuel cell.

But what is actually going on at a molecular level, and how does nature allow us to get electrical energy from chemicals rather than just by burning them in ordinary combustion?

The hydrogen + oxygen reaction

H₂ + ½ O₂ = H₂O …. (1)

is a ‘downhill’ reaction, which means that energy will be liberated during the formation of the product water. Chemists can easily calculate how much free energy should be released – around 237 kJ per mole of water. Both hydrogen and oxygen exist as di-atomic molecules – they prefer to be in pairs hence our formulae in reaction (1) are written as H₂ and O₂. In the gas-phase reaction, hydrogen molecules collide with oxygen molecules, and both hydrogen and oxygen molecules split apart and re-arrange into molecules of H₂O. Energetically, this looks difficult to do as the strength of the dihydrogen bond is around 436 kJ/mol and that of dioxygen some 498 kJ/mol. Nonetheless, the gas phase reaction can occur very readily, often with explosive force.

In a fuel cell, the same reaction as the gas phase one is taking place, only this time the hydrogen and oxygen gas molecules are kept separate by a polymer membrane and are only allowed to collide with electrode surfaces. The same diatomic bonds need to be broken, but the intermediates that are formed at the electrode surfaces are different from the gas phase:

H₂ = 2H⁺ + 2e- ….. (2)

½ O₂ + 2e- = 2O^2- …..(3)

At the oxidising electrode (anode), the dihydrogen gas is ionised to form protons - reaction (2). This is for an acid fuel cell, alkali fuel cells work in a similar way but with different solution intermediates. The protons are dissolved in water and move (by migration and diffusion) through the special polymer membrane towards the reducing electrode (cathode). Here the protons collide with oxygen species made by reaction (3) (note that the oxygen reaction is more complicated than reaction (3) implies, and the mechanism is still not properly understood). Electrons are injected into the oxygen intermediates via the special electrode (catalytic) coating and water molecules are formed. The 273 kJ/mol of energy associated with this reaction now appears as useful electrical work done by the electrons that have gone into the circuit at the oxidising electrode and re-appeared at the reducing electrode. Little heat is evolved in contrast to the gas phase reaction where almost all the energy is released thermally.

This is, as you will realise, a very simplified picture of the way the (acid polymer) fuel cell works at a molecular level. The two most important concepts to appreciate are that:

1) Nature does allow us to extract energy from a chemical reaction in different ways even though the end product is the same, and;

2) In a fuel cell, the reactant gases are not allowed to collide with each other, they collide with separate electrode surfaces and so the collision dynamics and types of key intermediates involved are often very different from those in the gas-phase reaction.

Rob Potter, Johnson Matthey Technology Centre

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