These notes are a "bare-bones"
summary of the material in the book.
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Energy is measured in joules (J)
Heat is a flow of energy.
Heat makes the temperature of objects increase. Temperature is measured with a thermometer; the unit for temperature is degree Celsius (oC)
Heat passes from hot objects to cold objects, never from cold to hot.
A thermogram is an image taken with a special camera to show where heat is being lost.
You can tell that the greatest heat loss is from the trunk of the animal. The least heat loss is from the ears.
When materials are heated, the temperature goes up, except when the material is changing state (melting or boiling). We can show this as a graph:
The temperature does not change at all while the material is changing state (melting or boiling).
Latent heat is the heat required to melt (or boil) a certain amount of the material. Its units are joules per kilogram (J/kg). It can be worked out by the equation:
Energy (J) = mass (kg) × specific latent heat (J/kg)
In Physics code:
E = mL
When the temperature is changing, we use a different property of the material, specific heat capacity. This is:
the amount of heat required to raise the temperature of 1 kilogram of the material by 1 degree Celsius.
The units are joules per kilogram per degree Celsius (J/kg/oC)
Energy (J) = mass (kg) × specific heat capacity (J/kg/oC) × temperature change (oC)
In Physics code:
E = mcDq
The strange looking letters are:
q "theta", a Greek letter 'th'; means temperature.
D "Delta, a Greek capital letter 'D'; means "change in".
Don't worry if you can't remember the equations in code form. Use the word form.
There are lots of ways of keeping our homes warm:
Loft insulation;
Cavity wall insulation;
Double glazing;
Some of these are quite expensive. Even low cost methods will help:
draught Excluders;
closing curtains.
If it cost €6000 to put in double glazing and it saves €200a year on the heating bills, it will take 30 years for the double glazing to pay for itself. This is called the payback time.
payback time = cost of insulation
annual saving
We buy energy to heat and light our homes in different units:
Coal - tonnes (t);
Electricity - kilowatt hours (kWh);
Gas - cubic metres (m3);
Oil - litres (l).
1 tonne = 1000 kg.
Good insulators are bad conductors. We can also save heat loss by putting reflective sheets of metal behind our radiators. The heat is reflected back into the room.
The efficiency of a heater is important too. Inefficient appliances waste heat and money.
Efficiency = useful energy output (J)
total energy input (J)
You will notice that this give a fraction, which can be multiplied by 100 to give a percentage efficiency. No appliances are 100 % efficient. If you get more than 100 %, you done something wrong.
Three important processes:
Conduction - heat is passed by atoms of a solid bumping into each other transferring their kinetic energy.
Convection - heat is passed through a liquid or a gas. The fluid getting less dense as and rising to the top. It cools and gets more dense. It falls. A convection current is formed.
Radiation - heat is passed as an electromagnetic wave.
Radiators pass their heat to the room by
convection.
Note:
When any material expands, it gets less dense.
Gases and liquids are bad conductors.
Insulating materials like loft insulation trap air which cannot pass heat by convection.
Radiation of heat is by infra red radiation. Only radiation can pass through a vacuum.
We can work out density by the following equation:
density (kg/m3) = mass (kg)
volume (m3)
In Physics Code:
r = m
V
The strange symbol, r, is "rho", a Greek letter 'r'. It's the physics code for density.
The electromagnetic spectrum is a huge family of waves from radio waves, through light, to gamma rays
Note:
The visible light region is much smaller than shown here.
The wavelength gets smaller as you move from left to right.
The energy of the waves gets higher as the wavelength gets smaller.
The boundaries between different waves is not hard and fast; "hard" X-rays are sometimes called "soft" gamma rays.
All electromagnetic waves can travel in a vacuum.
All electromagnetic waves have a speed of 300 000 000 m/s.
All travel in a straight line unless bent by refraction, or reflected.
Heat is transferred by infra-red radiation. Hot objects give out infra red. Shiny or white surfaces reflect infra-red; black surfaces emit it and absorb it very well.
A conventional oven cooks food by infra-red radiation. The photograph below shows a hotplate. The camera is sensitive to infra-red, which our eyes are not. That's why it has a bright glow.
Infra red is used in TV remote controllers.
Microwaves can cook biological materials by increasing the kinetic energy of water molecules. Microwaves were used for Radar, and birds that crossed the beam dropped out of the sky lightly cooked! Microwaves penetrate about 1 cm into the food. The heat is transferred through the rest of the food by conduction and convection.
Microwaves are also used for satellite, mobile telephone communication, and wireless networks. There are concerns that microwaves could cause localised heating to the sensitive and developing tissues of young people's brains.
Mobile telephones only work well within "line of sight" of the transmitter. Unlike longer wavelength radio waves, microwaves do not go round or diffract around large obstructions like hills.
Mobile telephones are banned in areas where there is a lot of sensitive electronic equipment, as in hospitals.
We use analogue signals to see and to hear:
They are continuously variable, i.e. can have any level we want.
They have an amplitude which tells us the loudness.
They have a frequency or pitch. Frequency is measured in Hertz (Hz).
The picture below shows an analogue signal of orchestral music.
Digital signals are a series of pulses that have two states, ON and OFF (0 or 1, high or low):
Digital signals have two advantages:
They are less prone to interference;
They can be multiplexed. This means that digital signals from several sources can be sent down the same cable. This is shown below:
Radio and light waves can be used to transmit information:
Signal fires and semaphore signals used light to transmit messages in the old days.
Morse code can be transmitted by light or radio. A flashing signal lamp was used to transmit messages in Morse between close-by ships. Pilots of aeroplanes need to know Morse so they can identify navigation aids.
Lasers can be used to send messages. The advantage of lasers is that:
Light is of one wavelength - monochromatic;
The waves are all in step or in phase. The light is called coherent.
Infra-red lasers are used to send digital pulses down fibre-optic cables.
A laser reads the bumps on a compact disc. The reflected light is picked up by a photodiode, and given out as a series of digital pulses. These are converted by a circuit called a digital to analogue converter into electrical signals that are then amplified and played through loudspeakers.
Rays of light can be bent or refracted if they pass from air to glass.
We can see that:
Angles are measured from the normal (the dotted line at right angles to the surface of the block).
The refracted ray going into the glass is bent towards the normal.
The angle of incidence is greater than the angle of refraction;
When the ray comes out the other side, the ray bends away from the normal.
A common bear-trap is to call it defraction.
Or call diffraction defraction. Don't.
We can look at this in more detail:
If we increase the angle of incidence, we get an increased angle of refraction:
At the critical angle the angle of refraction is 90 degrees. This is not always easy to see.
Above the critical angle we get total internal reflection:
Total internal reflection is used in:
periscopes (which have 45-45-90 degree prisms);
single lens reflex cameras;
optical fibres;
endoscopes (which use optical fibres).
In an optical fibre, the light bounces from side to side by total internal reflection.
Radio waves can be reflected, just like light.
Ghosting on a TV set happens because of radio waves reflecting off buildings.
Interference on FM radio can happen as you walk about a room; the waves are reflecting off you.
A satellite dish is a concave mirror that focuses microwaves onto an aerial.
Some transmitters use a concave mirror to send out a focused beam of radio waves.
Radio waves can be refracted. Waves of longer wavelength are refracted more. Longer wavelength radio waves are more likely to be totally internally reflected by the ionosphere, a layer in the atmosphere. Short wavelength radio waves and microwaves refract out into space.
Radio waves can be diffracted. The longer the wavelength, the more they can be diffracted. This is why you can hear a radio broadcast behind a hill, even if you can't see the transmitter. You wouldn't be able to get a mobile phone signal because the wavelength is much shorter ( a few centimetres) so it doesn't diffract so much.
Radio waves and light waves are transverse (like water waves):
All waves obey the wave equation:
waves speed (m/s) = frequency (Hz) × wavelength (m)
In physics code:
c = fl
The strange looking symbol l is "lambda", a Greek letter 'l', the physics code for wavelength.
If we want the wavelength:
l = c/f
If we want the frequency:
f = c/l
Make
sure you read the question. If it's light, c = 3 × 108 m/s.
If it's sound, c = 340 m/s.
An earthquake happens when rocks are stressed and break along a fault line. The breakage point is called the focus. The point on the ground vertically above the focus is called the epicentre.
Earthquakes are measured with a seismometer. The instrument draws out a trace. It can pick up the traces from earthquakes all around the planet.
There are three kinds of earthquake waves:
P-waves (pressure or primary). These are longitudinal and travel fast, between 5 - 8 km/s.
S-waves (shear or secondary). These are transverse and travel less fast, between 3 and 5 km/s.
L-waves (Love waves). These travel on the surface, and are quite slow, about 1 km/s.
A longitudinal wave is like this:
Scientists can't drill down to the centre of the Earth, so they have used data from earthquakes to work out the structure. There are two key facts that help them:
P-waves can travel through the core. They refract into the core and refract out again. There is a shadow zone between 103o and 142o from the point of the earthquake. No P-waves are picked up in this area.
S-waves cannot travel through the core. Beyond 103o no S-waves are picked up.
For S-waves the picture is like this:
The temperature of our planet varies naturally from time to time. The current concern is that the temperature of the planet is increasing rapidly due to the increased release of carbon dioxide and other greenhouse gases, which result from human activity. If plants cannot take up all the carbon dioxide produced, the levels will rise. Other greenhouse gases include:
water;
methane (23 times more powerful than carbon dioxide).
While global warming may cause increased episodes of drought in some parts of the world, in Europe the expected results are:
hotter, drier summers (although the cold and wet summer of 2007 rather went against this trend);
milder and wetter winters.
Weather can be affected also by:
pollution from factories;
pollution from transport, especially aeroplanes;
dust from volcanoes.
These tend to counter the effects of global warming by blocking some of the sunlight out.
The increased level of sunshine will lead people to sun-bathe more. There is an increased risk of sunburn. With that comes an increased risk of skin cancer. Melanoma (the medical term for skin cancer) is a serious condition. If it's left untreated, it can rapidly spread to other parts of the body, proving fatal.
People with darker skins have increased protection against sunburn due to increased levels of a dark pigment called melanin. When we tan, our bodies produce melanin to protect us against ultraviolet rays that lead to sunburn and skin cancer. We can also:
wear loose fitting clothing to cover up our bodies (which also hides some people's flabby and unattractive nakedness!);
use sunscreen.
Sunscreen creams have a sun protection factor (SPF). The higher the number, the longer you can stay out in the sun. Also we should take into account the sun index:
| Sun index | Risk |
| 1 - 2 | Low risk |
| 3 - 4 | Avoid being out for more that 1 - 2 hours |
| 5 - 6 | Burns in 30 - 60 minutes |
| 7 - 10 | Severe burns in 20 - 30 minutes |
We can work out how long it's safe to stay on in the sun:
length of time = SPF number × time in sun index
About 50 km above the earth's surface, in the upper stratosphere, is a thin layer of ozone (O3, a molecule consisting of three oxygen atoms). This helps to filter much of the harmful ultraviolet radiation from the sun.
Chemicals called chlorofluorocarbons (CFC), used as aerosol propellants and refrigerants, have reacted with ozone and reduced the layer. This thinning has been particularly noticeable above the South Pole. Fortunately very few people actually live there!
CFC chemicals have now been banned and the ozone layer is starting to build up again.
The thinning of the ozone layer does NOT result
in global warming.
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