Sun|Trek Adventures Solar Surface Hot Solar Atmosphere Magnetic Sun Flowing from the Sun
Sun Earth Connection Solar Spacecraft Earth and Beyond The Sun as a Star
  Suntrek homepage
Green Loops & Sun
Down Arrow
Down Arrow

P - Factary


Pascal, Blaise (1623-1662)


Image of Blaise PascalPascal was a French mathematician, physicist and religious thinker. He first worked on geometry (the mathematical study of shapes) but he is best remembered for his work on gases and liquids. One famous experiment he did was to record the air pressure at different points up a 1,220m high mountain (the Puy de Dôme near Clermont). He found that the pressure got less as he went higher. That's something we take for granted now but it certainly wasn't obvious before Pascal did the experiment. Pascal was too ill to climb the mountain himself so he sent his brother-in-law Perier instead. Those studies were why the SI unit of pressure pascal is named after him.


At the age of 19, Pascal also designed a mechanical adding machine. Because of that work his name is used for the name of a computer programming language (called PASCAL).


pascal (Pa)


The SI unit of pressure, symbol: Pa. 1 pascal is equal to a force of 1 newton acting on an area of 1 m2 - actually that's a pretty small pressure - try this example for size:


1 pascal = 1N/m2


1 litre (1000 cc) of water has a mass of 1 kg and, near the Earth's surface, this weighs or exerts a force of 10 N


To get a force of 1 newton we only need 100cc of water spread over 1 m2.


100cc of water spread over 1 m2 would cover to a depth of 0.1mm.


So 1 pascal is the pressure of a layer of water 0.1mm deep. Hard to feel the effect of that!


On the other hand what pressure is created when you step barefoot on the point of a drawing pin?


Assume the sharp end of the pin is approximately 0.1 mm across so the area is about 10-8 m2 and your weight = 500N (assuming your mass is 50 kg).


So the pressure your skin feels is 500/10-8 = 50 billion pascals. No wonder the point of the pin forces its way through your skin! Ouch!


Peta (P)


A prefix indicating a million billion of something. As a number it can be written:

1015 or


So it's nothing to do with bread.


Many people are familiar with the terms megabyte or gigabyte when talking about computer disk sizes. A petabyte is a million gigabytes. If you think that no one has that much data, think again.


The Large Hadron Collider [LHC] is a new particle smashing experiment being built at CERN (the European Laboratory for Particle Physics in Geneva). When the LHC is ready in 2007 it will enable scientists to look further into the deep mysteries of the structure of matter and the origins and early evolution of the Universe. The data generated by each of LHC's four detectors will be about


1 petabyte of data per second.


In total that's equivalent to filling a stack of CDs 4 miles high every second.


Photoelectric effect


Waves at sea appear to go on for ever (until they reach the shore!), but it is often necessary to think of electromagnetic radiation, like light, as being made up of small packets of waves acting individually like particles rather than it being one continuous wave. We call these particles, or packets of radiation, photons.


Each photon carries a very specific amount of energy dependent on the frequency - you can think of it as a little packet of energy riding on a wave with a particular frequency. Red light photons have less energy than blue light photons because the frequency of red light is less than that of blue light.


What happens when a photon hits something solid, like a metal, or an atom of silicon, say? We'll stick with silicon because that is what photovoltaic solar panels are made of.


Atoms consist of a dense, positively-charged nucleus, a whole lot of empty space and a bunch of negatively charged electrons. Each of these electrons sits in an orbit around the nucleus at a particular energy level. Let's now imagine a photon interacting with an electron. The electron will absorb the energy of the photon. Therefore, the electron now has more energy than it had before, so it has to change its energy level by moving to a higher energy orbit. If the original photon had enough energy the electron will have sufficient energy to leave the atom of silicon completely not just move orbits. When this happens, this is called the photoelectric effect.


Red-light frequency and energy is too low - no electrons escape

Red-light frequency and energy is too low - no electrons escape


Blue-light frequency and energy is high - electrons escape
Blue-light frequency and energy is high - electrons escape


Einstein nobel prizeSolar panels are made so that when the electron leaves the atom it crosses a one-way junction and then cannot return. Since we have a positively charged nucleus and a negatively charged electron separated from each other, we have created a potential difference.


This can be used to drive a current round a circuit and so then what do we have? Electricity from sunlight!

In 1921, Einstein received the Nobel prize for Physics for his work on the photoelectric effect, not for his much more famous work on relativity.




A discrete quantity of electromagnetic energy. Short wavelength (high frequency) photons carry more energy than long wavelength (low frequency) photons.


A photon is a packet of electromagnetic radiation, which carries energy from one place to another. The proposal that this was the correct way to think of electromagnetic radiation was first made by the German physicist Planck.



Planck proposed that a photon with a wave frequency of f hertz, carries an energy of E joules and that those two quantities are related by the equation:

E = h x f

where h is called Planck's constant and has units of joule seconds (Js).

Planck’s constant has a value of 6.626 10-34 Js - not very big.

For example, to calculate the energy carried by a photon of blue, visible light with a wavelength of 400 nm we have

Wavelength = 400nm = 4 x 10-7 metres
Since frequency = velocity/wavelength
  = 300,000,000 / (4 x 10-7)
  = 7.5 x 1014 Hz
  = 750 million megahertz

A bit higher frequency than your radio can pick up.

According to Planck’s equation, the energy carried by this one photon is therefore:

E = (6.626 x 10-34) x (7.5 x 1014)
  = 4.97 x 10-19 joules

Not a lot! For comparison, approximately 4.2 joules are required to raise the temperature of 1 gm of water by 1 °C.

So if we wanted to use the energy of these blue-light photons to heat 1 gm of water by 1 °C (assuming all the energy could be transferred perfectly) we would need

8,450,000,000,000,000,000 photons. That's more than 8 billion billion photons.




The layer of the Sun where most of the visible light comes from. Deep inside the Sun the radiation cannot escape directly. However, the photosphere is an outer layer in which the material becomes transparent to visible light.


See our section "On and around the surface of the Sun"




Photosynthesis is the process used by plants and some algae to transform the energy of sunlight to the chemical energy stored in sugar. Plants need only light energy, carbon dioxide (CO2) and water (H2O) to make sugar. A special chemical called chlorophyll makes the process of photosynthesis possible by acting as a catalyst in the chemical reaction. Chlorophyll is found mainly in the cells of the leaves of plants. Chlorophyll, and hence the leaves containing it, looks green because it absorbs red and blue light, and so the only light reflected back to our eyes is green. It is the energy from the absorbed red and blue light that is used to perform photosynthesis.

Chemically, the photosynthesis reaction can be written as:


6CO2 + 6H2O (+ light energy)→ C6H12O6 + 6O2.


Carbon dioxide + water + sunlight → sugar (glucose) + oxygen



There are actually two slightly different kinds of chlorophyll, known as chlorophyll-a and chlorophyll-b. Both are complicated chemicals though with chemical formulae of:


chlorophyll-a = C55H72MgN4O5
chlorophyll-b = C55H70MgN4O6


both slightly bigger molecules than either carbon dioxide or water! Can you spot the difference between them?


Image of a chlorophyll moleculeHere's a model of a chlorophyll molecule (courtesy Dr Bennardo,



The atom right in the middle is an atom of magnesium (Mg), which is a great example of why plants cannot live on water and air alone, but need other chemicals to grow healthily – no magnesium, no chlorophyll, no photosynthesis, no sugar, no growth!


Pico (p)


A prefix indicating a million million times as small. Other ways of writing this are:

0.000 000 000 001
or by abbreviating to a letter ‘p’. For example, pm stands for picometre, a million millionth of a metre. This is a useful measurement when discussing the wavelength of X-rays and gamma rays.


Planck, Max Carl Ernst Ludwig (1858-1947)


Image of Max PlanckMax Planck was a German physicist and one of the founders of modern quantum theory. His most famous contribution was to realise that radiation had to exist in the form of small packets of energy - called photons. Even Planck was at first very doubtful whether that idea made any sense, but the results of using the idea agreed with experiments in many areas of physics and the idea was eventually accepted as fundamental to our understanding and description of the physical universe.

The first result of Planck's photon idea was that there was a relationship between the frequency of an electromagnetic wave and the energy it carries. See the entries for Planck's constant and photon.


Planck's constant


Is one of those seemingly magical numbers which define our universe, along with others like the speed of light and Newton's gravitational constant. Usually written h, Planck's constant has the value of 6.626 x 10-34 joule second. It occurs in many equations associated with electromagnetic radiation, but the simplest and best known is E = hf, where f is the frequency of an electromagnetic wave and E is the amount of energy it contains.




Image showing the four different states of matterThe fourth state of matter, the others being solid, liquid and gas: see the diagram. Plasma consists of matter heated to sufficiently high temperatures that some or all of the electrons escape from the atom. The atoms become ions. Most of the matter in the universe, some estimates give 99%, and virtually all of the matter in the Sun is in the plasma state.


In our everyday lives we very rarely come across plasma. Plasma is created as a lightening bolt flashes through the atmosphere and there is a plasma inside a fluorescent light or neon sign, but otherwise it seems we live mostly surrounded by some of the 1% of the universe which is not plasma!



Don't confuse the plasma we're talking about with the 'blood plasma' you hear doctors talking about on TV programmes like Casualty and ER. To a doctor, plasma is the liquid in which red blood cells are carried.


Polar plume


A structure of plasma which occurs in the solar corona along magnetic field lines in coronal holes. Although plumes usually occur at the poles, they can appear anywhere there is a coronal hole. In this image the polar plumes are the faint spikes sticking out at the south pole.


Soloar Plumes




There are lots of definitions of the word 'pole' of course! When studying the Sun or planets, it means the point on the surface of the body which is cut by its axis of rotation. Or if you like, the poles are the points where the stick goes into and comes out of a toffee apple!


Potential Difference


See also, charges and current.


This is the 'push' given to electrons when they leave a cell on their way around an electrical circuit. The bigger the potential difference the bigger the current that flows. Potential difference is measured in volts (V).


Potential energy


See Gravitational potential energy.






A structure in the corona consisting of relatively cool plasma (10 - 20,000 oC) supported by magnetic fields. Prominences are bright structures when seen over the solar limb but appear dark when seen against the bright solar disk. Prominences seen on the disk are also known as filaments. See the section "On and around the surface of the Sun" for more information on prominences.




The idea of 'proportionality' pops up again and again in science so it’s important to be clear what it means.


Everyone has some basic idea of proportionality. Think of an oven. Each time you turn up the control setting, the temperature of your casserole increases. In other words, as one goes up so does the other. If they go up in regular amounts each time then we say that they are proportional.


In science there are many quantities that are proportional to each other. Sir Isaac Newton discovered that the acceleration of an object is proportional to the force applied to it (as long as the mass remains constant). In other words, the harder something is pushed the quicker it will speed up.


Proportionality can work the other way too. As one factor increases so another decreases. This is called inverse proportionality. Pressure and area are inversely related. If you spread a force over a larger area then the pressure will decrease. That's why you should wear snow shoes if you don't want to sink into soft, fresh snow.




A positively charged elementary particle, which has 1836 times the mass of an electron. The nuclei of atoms consist of both protons, and, in all atoms except hydrogen, neutrons. Every element has a different number of protons in its nucleus. In other words, it is the number of protons in the nucleus which defines which element that atom is.

The nucleus of the simplest element of all, hydrogen, is a single proton. So the nucleus of a hydrogen atom, a proton, and ionised hydrogen are all the same thing!


back to top


Sun|trek homepage | Sun|trek Adventures | Solar Surface & Below | Hot Solar Atmosphere | Magnetic Sun | Flowing From The Sun

Sun/Earth Connection | Solar Spacecraft | Earth & Beyond | The Sun our Star | Factary | Gallery | Hot News | Contact Us