W - Factary

Watt (W)

The SI unit in which power is measured. Named after Scottish engineer and inventor, James Watt.

Power is the 'rate' at which energy is transferred. Remember that energy can't be created or destroyed, it can only be changed from one type into another. This is known as the law of conservation of energy.

Remember: whenever a 'rate' is mentioned, we are talking about something that happens 'per second' ('per' means 'divided by').

So watts are the number of joules per second (J/s or Js^-1). A 100 watt light bulb uses energy at the rate of 100 J/s.

Watt, James (1736-1819)

James Watt was trained as an instrument maker but his great breakthrough came when he was asked to repair an early version of the steam engine. Watt realised that a simple redesign would greatly increase the speed and efficiency of the engine. Watt went into partnership with Matthew Boulton, an engineer based in Birmingham, to build the new design and it was this engine, together with further improvements in the design, which powered the industrial revolution in the UK.

Watt was also the first person to use the word 'horsepower' to measure the power output of his engines. He had been working with ponies in coal mines and calculated that an average pony could do 22,000 foot-pounds of work in one minute. Strange sounding units, and what would have been wrong with ponypower? All it means though is that the pony could lift 1000 pounds 22 feet, or 22 pounds 1000 feet - or any other combination that comes to 22,000 - in one minute! As a definition, Watt set the horsepower unit to be equal to 33,000 foot-pounds per minute: 50% more than the average pony. Maybe he considered horses were that much stronger than ponies. In SI units, one horsepower translates to 746 watts.

You may have heard car engines described in terms of their Brake Horsepower (BHP). That is a measure of the actual or useful power output of an engine (after all the losses internal to the engine have been accounted for). A powerful sports car can generate about 400 BHP, which from the value given above is equivalent to having a 400 x 746 / 1000 = 298 kilowatt power source under the bonnet - quite a lot!

Wave

A wave is a disturbance or variation which travels through a medium. The medium through which the wave travels experiences local changes as the wave passes, but the particles in the medium do not travel with the wave.

There are many different types of waves: water waves, sound waves, light waves, radio waves. However, they all have one thing common, they all carry energy. There are two basic types of waves: transverse and longitudinal.

Wavelength

The distance from crest to crest (or trough to trough) of a transverse wave (see electromagnetic radiation). It is also the distance from one position of maximum compression to the next in a longitudinal wave.

In fact to find the wavelength it doesn't actually matter where you measure the wave just as long as you measure between identical places on the wave; it's the distance from one point on a wave to the same point on the next wave.

Weber, Wilhelm Eduard (1804-1891)

Wilhelm Weber was a German scientist who did a lot of work on electricity and magnetism. His laboratory and that of another German scientist, Karl Friederich Gauss, were connected by one of the very first electric telegraphs. Without their work, Alexander Graham Bell could not have invented the telephone!

Wilhelm Weber also had a famous brother (Ernst Heinrich Weber 1795 - 1878) who was a physiologist and anatomist. He did valuable work to find out about the differences in the way our body's nervous system responds to a stimulus, like a punch or a kick .

weber (Wb)

The SI unit of magnetic flux whose symbol is Wb.

You probably know about magnetic fields, but what is "flux"? This is a word used to describe how 'strong' an effect is over a certain area. A powerful magnet would be described as having a high magnetic flux.

When translated to the fundamental SI units

1 Wb = 1 m²·kg·s^-2·A^-2

(that’s one metre squared kilogram per second squared per ampere squared)

Thank goodness for Weber!

Weight

This is the force created by gravity. It's an old problem and it probably feels like physics teachers are the only ones that ever worry about it, but there is a big difference between weight and mass.

See also, Kilogram.

Mass is a property of matter. The mass of a bag of apples might be 1 kg and in a shop on Earth we sloppily say it weighs 1 kg - but to a scientist that is WRONG! Since weight is the force of gravity acting on the apples it is actually equal to 10 newtons, because force is measured in newtons not kilograms. Who cares? Well no one much (apart from scientists), except think about the future when trading between planets or between the Earth and the Moon is possible. If you lived on a farm on the Moon and, from Earth, I ordered 1 kg of apples from you, would I be happy with a bag of apples that weighed 10 newtons on the Moon?

Remember, 'I have a mass of 50 kgs' is correct but 'I weigh 50 kgs' is not. You just have to weigh (!) up the pros and cons of being correct and saying 'I weigh 500 newtons' in which case probably nobody will understand what you're talking about, or being (scientifically) incorrect and saying 'I weigh 50 kgs' and being understood. It's a tough life being a scientist in the real world!

See the Earth and Beyond section for examples of how weight changes on the planets of the solar system.

White light

What is white light? Simple, white light is light that looks white to the human eye! What else is there to say? Actually quite a lot. To answer the question 'yes, it looks white, but what IS it?' Let's take it step by step.

1) we see an object because radiation (since we can see it, we call it 'light') is coming from that object and enters our eyes. For the moment it doesn't matter whether the light is being produced by the object or whether it's just bouncing (reflecting) off it.

2) The light is detected by the eye (the retina at the back of the eyeball) and it sends a signal to the brain which then says to us 'OK, that object is white' as opposed to blue or green or brown.

3) The question we are really asking then is 'what light is it that makes the brain say that?'

4) If, instead of letting that light enter our eyes, we send it through a glass prism, something strange happens. It comes out as a spectrum, or rainbow, of coloured lights.

5) From that simple experiment it was Newton who first suggested that 'white' light is just a mixture of different coloured lights and that the prism was a way to split up the white light so that we can see what it was made of.

So when the right mixture of colours falls on the retina of the eye, the brain interprets that as the phenomenon we call 'white light'. Maybe it's a bit like hearing lots of instruments all playing at the same time - we hear an orchestra, not necessarily the contributions of the individual instruments.

Extensions

6) One objection to this interpretation is that the prism itself is not splitting the light but creating the rainbow by some means and it would do the same for any light, not just white light. Newton answered this by showing that if you let pure red light enter the prism, all you get out is .... pure red light! The prism cannot create a spectrum out of a single colour.

7) A prism is not the only way of producing a spectrum. Another is called a diffraction grating. That uses the phenomenon of diffraction rather refraction as in a prism. Most people have access to a simple diffraction grating these days - it's called a CD! Ever noticed the spectrum produced by a CD?

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