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Light:reflection

We see luminous objects because the light they give out comes to our eyes. Everything else we see because of reflection. Light reflects in very different ways from a smooth object compared to a rough one.

Scattering

Rought objects scatter light. The light comes in parallel but bounces off in all directions. This is why we don't see an image in them.

The law of reflection

Smooth surfaces scatter light according to the law of reflection. To do this the surface must be really smooth, like a mirror, a still lake or a plane of glass.

When this happens the light reflects as in this diagram. The angle of incidence = the angle of reflection when we measure it from the normal (which is an imaginary line at 90 degrees to the reflecting surface).

This means light rays that come in parallel leave the surface parallel and you get an image.

The window reflects some of the light from the flowers so you get a dim image compared to a mirror. But why are the flowers on the right here reflected 3 times and those on the left only once?

Light : Basics

Light is a form of energy. What we call light is a small part of the electromagnetic spectrum which includes ultra violet and infrared as well as radio waves and microwaves. All these things are basically the same but we can only see a small section of the whole lot (spectrum). This make sense, when we were evolving the only radio waves on the planet would have come from the stars or lightning. Not helpful to see only those things when a predator is creeping up on you. The difference between all these parts of the electromagnetic spectrum is their wave length and our eyes evolved to see the most useful wavelengths around on this planet.

Remember light travels in straight lines (unless it goes past a star or black hole when they can be bent). In space light travels at 300,000,000 m/s around a million times faster than sound travels through the air. Luminous objects are light sources they give out light. Materials can be described as transparent (let's through all the light), translucent (let's through some light) or opaque (let's through no light). Remember the thickness of the material is important! One sheet of paper is translucent but 100 sheets will be opaque. We also need to know about reflection, refraction and dispersion which are coming in the next post.

Speed

Speed measures how far we travel in a unit of time.  This is why we measure it in miles per hour.  Miles measure distance per means in every and hours measure time.  Of course there are other units of speed as well such as kilometres per hour and meters per second which we use a great deal in common entrance physics. Fast and slow moving things might need different units. A rocket will move a certain number of kilometres per second (around 11 km/s to escape earth orbit) a glacier might move a few
centimetres/year. It would be silly to measure these movements in meters per second the numbers would be really big or really small and any calculations you do complicated.
So to measure speed we need to know the distance moved and the time taken. Then we divide the distance by the time. In other words:
Speed= distance / time
If you forget this just think of a speed you go in your car. 50 miles per hour. Miles (distance) per (divided by) hours (time).

I am on a train as I write this we are going 60 miles in 2 hours so the speed is
S=d/t
S=60/2=30 mph

Remember this will be our average speed. We stop at each station, speed up and slow down all the way so we won't go at 30mph exactly much of the time.

If my train was travelling for 4 hours at this average speed how far would it go?
Well the distance=speed x time
Distance =30 x 4 =120 miles.
This is because the speed tells us the distance we go each hour if I go 4 lots of 30 that is 120 miles.
So how long would it take my train to do 105 miles?
Here we do time=distance/speed
Time=105/30=3.5 hours.
Take care. Exam question setters love to put in traps. Look at this question.
A horse travels at 8m/s for two minutes how far does it go?
Here I have muddled up minutes and seconds. The first thing we must work out is how many seconds the horse is travelling for. 2x60=120 (as there are 60 seconds in one minute)
Now we can do
distance=speed x time
Distance=8 x120
Distance=960m

Pressure

Pressure in physics tells us about the concentration of a force. Imagine sitting on a chair. Ah! Relax. It's rather different if someone leaves a drawing pin on the seat!  In the first case your weight (gravity force)  is spread out over the whole of the area you press on. When the pin is there you put the same force onto a tiny area. This increases the pressure and the pin sinks into you. 

Many things are designed to have a high or a low pressure when used. A garden fork is made to have a high pressure. The ends of the prongs are sharp. This makes a high pressure so when you press on it it will sink into the earth. The drawing pin is another example the sharp end makes a high pressure so it will sink into display boards. The other end has a bigger area. This gives a low pressure so it will not think into your thumb. Snow shoes are also designed to have a low pressure they spread out your weight so you don't sink too far into the snow.

To calculate the pressure made by something you divide the force by the area it is exerted (made) on. So

Pressure=Force/area

Forces are measured in Newtons and areas are measured in cm2 (centimetres squared) or m2. This means pressure is measured in N\cm2 (Force divided by area) or N\m2 which is also called a Pascal (Pa). Check your bicycle tyres they will probably have this but also lb\in2 or pounds per square inch.

Series and parallel circuits

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Series and parallel circuits have several key differences. Firstly series circuits have only one current path around them. There are no branches. Parallel circuits have two or more current paths. There are different routes around the circuit. See our videos on YouTube to get the idea. Series circuits must have everything turned on at once but parallel circuits can have different things working at different times, this means we can use them to make And and Or circuits as well as many others. In a series circuit the current will be the same wherever you measure it. In a parallel circuit the current will split down different branches. Adding together the current in the different branches will tell you the current leaving the cells or returning to it.

Energy resources, alternative energy

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The alternative energy resources are called that because they offer a different way to generate our energy to the three fossil fuels, coal, oil and gas as well as nuclear energy.  These 4 energy resources are often called 'non-renewable' resources as eventually the supply  of them will run out.  The alternative energy resources are renewable.  That is to say they are renewed in a much shorter timescale.
We are going to cover the following alternative energy resources in this post:  Biomass, Wind, Tidal and Hydroelectric in mountains, Wave, Solar and Geothermal.
All of these have the same advantages over fossil fuels.  They do not cause the pollution that brings climate change or acid rain and they will not run out.  They do come with their own problems however.
Biomass:  in biomass a crop or waste is burnt to heat water into steam, replacing the coal, oil or gas in a traditional fossil fuelled power station.  The steam is then used to spin the turbine which is connected to the generator. However, collecting enough fuel is difficult.  Wood, which can be grown for this has a sixth of the chemical energy in it that coal does.  It also takes away land from agriculture and other uses (some companies are developing a seaweed they hope to farm, dry and burn)

Wind: The wind spins the turbine (the big propeller) and  the turbine spins the generator, which produces the electrical energy.  This is very easy.  However, you need a great many of them to replace one fossil fueled power station, its not always windy and they can be considered ugly.

Tidal: There are several ways we might be able to use tidal power.  At the moment we build a dam across a river estuary.  In the walls of the dam there are tunnels which have the turbines in them.  As the water flows through the tunnels it spins the turbines which are connected to the generators, producing electricity.
Putting turbines and generators very like wind turbines on the bottom of river estuaries is being talked about.  I don't know that this is actually in use anywhere.
The problems are that building the dams is difficult and expensive.  Also the tide only flows hard at certain times of day (but we can at least predict these due to the orbit of the moon).  In between these peaks much less energy is generated.  Damming a river also affects the wetland habitats around it and the migration of some fish.

Hydroelectric in the mountains:  This can be done at any section of a river, not just in mountains.  There has to be a strong flow of water though so having a lake dammed up at the top of a mountain provides a good flow.  The water is allowed to flow down the mountain spinning turbines as it does so.  A small plant has also been build on the River Thames in Abingdon, no mountains there but a strong flow of water through the weir.  Again problems include damming the river and changing the water flow, which can also vary due to rain and bad weather.

Wave:  Wave power has had many different approaches to make it work.  Some of these involve having a floating container on the sea (in the shape of a duck or a snake! Search Salter ducks and wave power snakes to see more).  These contain oil which, as the waves rock the container, flows through a turbine, spinning it.  These have proved to be difficult to get to work well.  There is also the idea of building a chamber on the seashore.  This has no bottom to it so the waves can rise up into the chamber.  In one wall there is a hole and in the hole is a turbine connected to.... yup a generator.  As the wave rises into the chamber the air is forced out through the hole spinning the turbine.  As the wave sinks back out of the chamber the air is drawn back in through the whole spinning the generator again.  This is apparently rather noisy!  Also if there is no wind there will not be waves to make this work.

Geothermal power: This is great for geologically active areas with hot rock close to the surface of the earth.  Water is pipped down to these hot rocks and is heated into steam.  The steam is squired at the a turbine just like in a fossil fuelled power station.
Its not much use in the UK as there are not many areas with these hot rocks.

Solar I have left solar to the end as there is no turbine and no generator involved.  All the other alternatives mentioned here change kinetic energy into electrical energy with a generator.  Solar does not.  It changes the light from the sun directly to electricity. (Exactly how we wont go into here!)  It is getting cheaper to do this all the time but it is still expensive and of course night time presents a real problem particularly during the winter in the UK.

Energy resources

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Energy resources are the things we use to make our electricity. They all in involve changing another type of energy into electrical energy.
Currently we mainly rely on fossil fuels, coal, oil and gas. These fuels are called fossil fuels as they have been formed out of the fossilized remains of plants and sea creatures over millions of years. We use them by burning them to heat water into steam. This steam is shot at a turbine and the turbine spins the generator which puts out electricity (remember when water changes to steam it expands 1600 times which give s the force required to spin big turbines). There are 3 problems with using the fossil fuels.
1. Firstly they will run out. We are using them much faster than they are replaced.
2. Acid rain burning coal in particular produces sulphur dioxide which dissolves into the rain making it acidic. When this rain falls it pollutes rivers, lakes and the soil this kills fish and stops trees taking in water which kills them too. The acid rain can also erode buildings.
3. Global warming. Carbon dioxide is also released when you burn fossil fuels. The molecules of this absorbed heat passing through the atmosphere which would usually escape to space and so the temperature of the planet increases. this will lead to melting ice caps, sea level rise (mainly caused by the water in the sea expanding) which will lead to flooding and extream weather as there is more energy in the atmosphere.
So we need to look at alternatives. See the next post for these.

Density

Density
Density measures the mass of cubic centimetre of a material.  This is an important physical measurement for designers, scientists and engineers.  For instance the density of iron is around 7.8 g/cm3 (so a cube one centimetre in all three directions has a mass of 7.8 grams) but the density of Aluminium is only 2.7 g/cm3.  If you replaced all the aluminium parts of an aeroplane with iron it would not get off the ground because the mass of each part would be around three times higher.

Density is also important in predicting if things will float.  A material will float if it is less dense that the liquid or gas it is in.  Water has a density of around 1 g/cm3 (well exactly 1 g/cm3 at 4 degrees centigrade) so materials with a density of less than 1 g/cm3 will float but those with a density of more than 1g/cm3 will sink.  This is also why Helium balloons float, they are less dense than air.

 If you put a helium balloon into a room full of hydrogen what would it do? Well it would sink. Helium is more dense than hydrogen.

Heating and cooling things can change their density.  Hot air balloons work by this principle heating the gases of the air gives the particles more energy so they move apart.  This means there are fewer of them in the same volume of space so compared to the cold air outside the balloon the density drops and the balloon starts to rise.  This is also how convection cells or current work.  Cooling most things makes them smaller so their density rises.  This does not happen with water!  When water freezes it expands and so its density actually goes down which is why icebergs float.

To work out the density of a material  we need to know how to find it mass and volume.  There are different ways to do that shown on these videos:  Density of a rectangular solid<>,  Density of a liquid<>, Density of an irregular solid<>

Energy types

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When anything happens around us it involves energy being changed from one type to another. This could be you talking, the electricity going into your computer to light up the screen, a log burning on a fire or a weight bouncing up and down on a spring.
Its important to remember the law of conservation of energy.  This is that we can not make energy, or destroy it, but only change it from one type to another.  The key types of energy you need to know are:
Gravitational Potential energy, (gained when lifting object up, for example being lifted on a ski lift)
Elastic potential energy, (stored in things that are stretched, for example in a catupult which has been stretched)
Chemical energy, (stored in materials and released in chemical reactions for example in your food, magnesium about to be burnt or an electrical cell)
Kinetic energy,(the energy of moving things, like a moving car)
Electrical energy  (the energy carried by a current, through the wires in your computer)
Thermal energy (heat energy nearly anywhere!)
Sound energy (the energy released by things vibrating, like a guitar string)
Light energy (the energy carried in electromagnetic waves some of which we can see, from example from a torch or the Ultra Violet rays that burn us when we forget sun cream.)
The first three are all potential energies.  They can be stored and have the potential to be released at any time.
So an battery powered torch stores chemical energy in the cell.  This is moved as electrical energy as the current flows through the wires and is released as heat and light from the bulb (see this video).  So in the torch the light is the useful energy and the heat is wasted.
Or in a fire the chemical energy stored in a log is again released as heat and light, as happens when any fuel is burnt.
The amount of energy involved in these changes can be estimated quite easily.  If a fly crashes into you the feeling is very different if a rugby tackle is made on you at the same speed.  So the mass of the object makes a big difference.  If the rugby tackle happens slowly it is very different to one at full speed, and so on.  We measure the amount of energy in Joules (but of course still use calories as well when we think about the energy in food).

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