O - Factary

Ohm, Georg Simon (1789-1854)

German physicist who was the first to compare electricity flowing in a circuit with water flowing in pipes. He showed that in an electrical circuit, the voltage difference across a component divided by the current flowing through it will be a constant for that component. This constant is known as its resistance.

The law, called Ohm's law, is:

resistance = voltage/current

or

R = V / I

Ohm is remembered mainly for this work although he also did important work on the human perception of sound waves which forms the basis of today's musical synthesisers and samplers.

ohm

The SI unit of electrical resistance (its symbol is Ω - which is the Greek letter 'omega' - an obvious choice!). Whenever an electrical current (electrons) flows through a conductor it experiences some form of 'resistance' to its motion. This resistance is the property of the material which makes it a good or bad conductor. An electrical component is said to have a resistance of 1 ohm when a current of 1 amp causes the voltage difference across its terminals to be 1 volt. That’s a long-winded way of saying:

1 ohm = 1 volt / 1 amp

Long wires have more atoms/molecules in them than short wires. Therefore the electrons/ions experience more 'collisions' as they flow through them so more energy is transformed to heat. In short, the resistance has increased.

'Thicker' wires have a greater cross sectional area, therefore more electrons can pass a particular point every second. This results in a greater current. Hence the resistance is reduced.

'Hot' wires contain more energetic atoms/molecules. This restricts the flow of electrons/ions, increasing the resistance and decreasing the current.

The easy way to remember Ohm's Law:

In the triangle diagram (below), cover up the quantity you want to find and the diagram then shows you how to find that quantity when you know the other two values. So, for example, if you know the current ( I ) and the voltage ( V ) and you're asked to calculate the resistance, cover the R and you're left with V above the horizontal dividing line and I below it, so:

R = V / I.

Optical radiation

Another expression for what we often just call light. It is that electromagnetic radiation which can be detected (seen) by the human eye.

See also, Electromagnetic radiation.

Orbiting Solar Observatories (OSO)

A series of eight satellites launched between 1962 and 1975 to carry out observations of the Sun.

This is a picture of the last satellite in the sequence. The 8th Orbiting Solar Observatory (OSO-8) was launched on 21 June 1975.

Orbit

The path of an object around another object under the influence of gravity or some other force.

See the "Earth and beyond" section for a full explanation of how it is that the Moon is really falling towards Earth all the time but is never going to crash!

Orbital period

The amount of time it takes a spacecraft or other object to travel once completely around its orbit.

The satellites that are used for observing the Earth, such as weather satellites, are only around 500-800 km above the Earth's surface. At that distance, their instruments are able to detect a lot of detail on the Earth. They have to be travelling very fast to stay in orbit and typically go round in about 90-100 minutes. The higher the orbit, the longer the orbital period.

Communications satellites are a very different matter. They need to stay still relative to the Earth - otherwise you'd be constantly re-pointing the satellite dish on the side of your house! The only way this can be achieved is for the satellite to make one complete orbit in a day. These satellites have to be much higher up - at 35,900 km above the Earth's surface. This is known as a geosynchronous orbit.

The orbital period of the Moon around the Earth is just over 27 days - from which we get the Old English word moonth - or to give it its modern version - month.

Ozone

Normally oxygen atoms go around in pairs, but ozone is created when ultraviolet radiation (sunlight) with a wavelength shorter than 240 nm strikes the upper atmosphere, splitting the oxygen molecules (O₂) into atomic oxygen (O). The atomic oxygen quickly combines with further O₂ oxygen molecules to form ozone (O₃):

O₂ + ultraviolet photon → O + O
O + O₂→ O₃

Once formed, ozone can then act as an absorber of ultraviolet radiation through the following re-actions:

O₃ + photon → O₂ + O

O + O₂→ O₃

So no ozone is destroyed.

In this way the layer of ozone in the stratosphere helps to protect us from much of the high-energy (and so dangerous) ultraviolet radiation from the Sun. That's the good news!

However, ozone is also found at other levels in the atmosphere where its effects are not so good. In the lower parts of the atmosphere (9-13 km height) ozone is created by natural lightening processes and by aircraft exhaust gases. At this height ozone becomes a significant 'greenhouse gas' and contributes to the global warming phenomenon.

Much nearer the ground, ozone can also be produced by interaction of sunlight with various atmospheric pollutants and even by everyday office equipment (computers etc.). In these circumstances ozone is a harmful pollutant that causes damage to lung tissue and plants.

Ozone hole

The Ozone Hole often gets confused with the problem of global warming. Ozone does contribute to the greenhouse effect in the lower parts of the atmosphere, but the Ozone Hole is another problem.

Ozone high in the atmosphere over Antarctica has been disappearing over the last 15 years at certain times of the year. This is mainly due to the release of man-made chemicals containing chlorine such as CFC's (ChloroFluoroCarbons). CFC's are a common product used in refrigeration systems, air conditioners, aerosols, solvents and in the production of some types of packaging.

Destruction of the ozone layer is important because it decreases the protection we have from dangerous ultraviolet radiation.

This image from NASA shows the hole (blue area) appearing over the Antarctic in spring. Remember September is spring in the southern hemisphere.

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