IGCSE PHYSIC Topic : The Danger from Radon Gas

 IGCSE Physics Notes: The Danger from Radon Gas

Radon is a radioactive gas given off by certain rocks. Unlike most radioactive materials it can be breathed in because it is a gas, and it produces the most dangerous type of radiation if ingested or inhaled – alpha radiation. Alpha radiation can damage cells and cause cancer. Radon inhalation is believed to be the second most common cause of lung cancer after smoking, causing about 5% of lung cancers.

Alpha radiation can build up in closed spaces and unventilated buildings. Some parts of the the United Kingdom are more prone to high radon concentration than others – Devon and Cornwall especially. This is shown in the 'radon map' of the UK below.

IGCSE PHYSIC Topic : The Digital Future

IGCSE Physics Notes: The Digital Future

Communications are changing. Mobile phone signals, radio and television have, or will soon be switching over from analogue to digital technology.

Analogue signals vary continuously. They are very prone to interference, because it is hard to separate the original signal from spurious signals.

Digital signals consist of a stream of zeros and ones - basically off and on. The signal to be transmitted is changed into a stream of zeros and ones by sampling the signal many times per second. The signal sent does not include only the information that is transmitted, but also extra information (also in the form of zeros and ones) which enables any error in the signal (which shows up as a zero being sent being received as a one) that means this error can be easily detected and corrected. This means no loss in the quality of the signal between being transmitted and received, because although errors may creep in very easily - because as the signal travels along, it gets weaker, so noise becomes relatively more important – digital signals are much easier to process and the errors are easier to detect than with analogue signals.

Radio, television and mobile phone calls all used to be transmitted as analogue signals. This restricted the amount of information that could be transmitted, made them prone to interference, and also meant in the case of mobile phones, that signals were difficult to make secure and easy to eavesdrop. Digital signals are easy to encrypt, so can be made very secure. At the receiving end, the sigmals can be unencrypted as required, and decoded – turned back into analogue signals.

IGCSE PHYSIC Topic : The Vacuum

IGCSE Physics Notes: The Vacuum

A vacuumis is a space with no matter in it. There is not 'nothing' in a vacuum – there may be light passing through it, but there is no matter. In fact, the definition above is a perfect vacuum – no vacuum is ever perfect. We describe how close a vacuum approaches to a perfect vacuum by it's 'quality'. Other things equal, lower gas pressure means a higher-quality vacuum. A typical vacuum cleaner produces enough suction to reduce air pressure by around 20% - not a very good vacuum. Ultra-high vacuum chambers, common in chemistry, physics, and engineering, operate below one trillionth () of atmospheric pressure and evacuate all but 100. Outer space is an even higher-quality vacuum, with the equivalent of just a few hydrogen atoms per cubic meter on average.
A good quality vacuum may be easily produced in the laboratory with mercury, a (long) test tutbe and a glass container. The test tube is filled with mercury then inserted upside down into the glass container, the opening below the mercury level as shown. The spce at the top of the test tube will be a good quality vacuum, able to support a 760mm column of mercury because of the atmospheric pressure on the mercury surface in the glass container. This is called a torricellian vacuum.

Vacuums – or at least low quality, partial vacuums where the pressure is slightly below atmospheric pressure, are widely used. Every incandescent light bulb contains a partial vacuum, and vacuums can be used to form seals so that food stays fresh, for example.

IGCSE PHYSIC Topic : Transmitting Signals Via Optical Fibre Cables

IGCSE Physics Notes: Transmitting Signals Via Optical Fibre Cables

Fibre optic cables are used for many telephone and TV transmissions. Fibre optic cables are long, thin, strands of glass or clear plastic and carry signals in the form of electromagnetic wave – light or infrared. The electromagnetic waves are transmitted as pulses, with the signal being switched on and off many times a second to produce a string of zeros and ones. In the transmitter, electrical signals are changed into light signals by a light emitting diode (LED) or a diode laser. At the receiving end, the light signals are picked up by a photodiode and changed back into electrical signals.

Optical fibre cable is thinner, lighter and cheaper than copper cables and can carry more signals. Signals in copper cables get weaker very quickly – and the loss in signal depends on the frequency of the signal - so many amplifiers are needed if the cable is long. The signals also accumulate noise unavoidably and these are also amplified, reducing the quality of the signal. Signals in optical fibres get weaker much more slowly and are less prone to interference.

The amplifiers can be much further apart. Optical fibres also have the advantage that they cannot be 'tapped', so that conversations are inherently more private.

Most long distance telephone calls are now carried by fibre optic cables, as is most computer data and internet traffic.

IGCSE PHYSIC Topic : Diodes/Zener Diodes

IGCSE Physics Notes: Diodes/Zener Diodes

Diodes allow electricity to flow in only one direction. The arrow of the circuit symbol shows the direction in which the current can flow.

Forward Bias

Electricity uses up a little energy pushing its way through the diode, rather like a person pushing through a door with a spring. This means that there is a small voltage across a conducting diode, it is called the forward voltage drop and is about 0.7V for all normal diodes which are made from silicon. The forward voltage drop of a diode is almost constant whatever the current passing through the diode so they have a very steep characteristic (current-voltage graph).
If the diode is switched around it is said to be in reverse bias. A much larger voltage must be applied to obtain a currnt – maybe 50V. If this is exceeded the diode will fail and pass a large current in the reverse direction, this is called breakdown.

Bridge rectifiers

There are several ways of connecting diodes to make a rectifier to convert AC to DC. The bridge rectifier is one of them and it is available in special packages containing the four diodes required. Bridge rectifiers are rated by their maximum current and maximum reverse voltage. They have four leads or terminals: the two DC outputs are labelled + and -, the two AC inputs are labelled .
The diagram shows the operation of a bridge rectifier as it converts AC to DC. Notice how alternate pairs of diodes conduct.

Zener Diodes

Zener diodes are used to maintain a fixed voltage. They are designed to 'breakdown' in a reliable and non-destructive way so that they can be used in reverse to maintain a fixed voltage across their terminals. The diagram shows how they are connected, with a resistor in series to limit the current.
Zener diodes can be distinguished from ordinary diodes by their code and breakdown voltage which are printed on them. Zener diode codes begin BZX... or BZY... Their breakdown voltage is printed with V in place of a decimal point, so 4V7 means 4.7V for example.
Zener diodes are rated by their breakdown voltage and maximum power:

• The minimum voltage available is 2.4V.
• Power ratings of 400mW and 1.3W are common.
  

IGCSE PHYSIC Topic : Motion In a Circle

IGCSE Physics Notes: Motion In a Circle

Newton's First Law states: A body continues in a state of uniform motion unless acted on by an external force. At any point on the circle below, at any point the moving body will seek to move in a tangent. To keep it moving in a circle a force must act so that the direction of motion is constantly changing

As shown on the right hand diagram, as the the particle moves around the circle it must be making constant changes to it's velocity, always towards the centre. This means that it is always accelerating towards the centre, and implies from Newton's Second Law, F=ma, that it is accelerating towards the centre constantly. The force could be a tension, gravity, some reaction force, an electric force or some other force.

For a planet moving in a circle around a star the speed is always the same. Generally planets move in ellipses. At any point on its orbit the planet has two types of energy: kinetic energy and potential energy.

When the planet is closest to the star most of it's energy is kinetic energy because it is moving faster there. As it moves on its orbit and becomes more distant from the star some of this kinetic energy is changed into gravitational potential energy and the star moves slower. When the planet is on the right in the diagram above it is moving slowest, and it's energy is mostly gravitational potential energy.

IGCSE PHYSIC Topic : Useful Things Worth Knowing

IGCSE Physics Notes: Useful Things Worth Knowing

The pressure of a column of gas or liquid is independent of the area. This is becauseand the area is not mentioned.

In the absence of air resistance, a feather falls with the same acceleration as a cannon ball.

Hence if both are released from the same height at the same time, they will reach the ground at the same time.

If you double the speed of a car, you don't double the braking distance. You multiply by 4. If the speed of the car is tripled the stopping distance is multiplied by 9. This is because

Silver/shiny things heat up more slowly than black/dull things because they are worse at absorbing radiation. Silver/shiny things cool down more slowly than black/dull things because they are worse at emitting radiation.

The resistance of a thermistor or semiconductor decreases if the temperature increases. If the thermistor is in a circuit with a voltage, then the current in the circuit may increase and the voltage across the thermistor may decrease.

Materials with higher specific heat capacities take long to heat up and heat up less for the same amount of energy.

Materials with higher latent heats of vapourization or fusion need more energy to evaporate or melt.

Less of the material is evaporated or melted for the same energy input.

Friction is always present. If energy is lost during a process it is usually because work has to be done to overcome friction or air resistance.

When anything is melting or evaporating the temperature is constant.

IfthenandBE SURE YOU LEARN HOW TO REARRANGE THESE SIMPLE EQUATIONS.

The distance travelled is equal to the area under a velocity time graph. The acceleration is equal to the gradient of a velocity time graph.

Heavier things take longer to slow down than light things, if the braking or resistance force is the same. This means they will travel further before they stop.

A moving magnet will make charge move and produce a current and a moving charge – a current – will induce a magnetic field.

IGCSE PHYSICS Topic : IGCSE Physics Notes: Conservation of Energy – Transforming Energy From One Form to Another

IGCSE Physics Notes: Conservation of Energy – Transforming Energy From One Form to Another

There are lots of different types energy:

Kinetic, gravitational, chemical, heat and many more.

It is one of the the fundamental principles of physics that energy is conserved. When any energy transfer takes place, the energy we start with is transformed into some other form of energy. The energy never goes nowhere. If some of the energy appears to be wasted, then that energy has been sed to overcome forces like air resistance or friction. If this happens some energy has become internal energy of some substance, but it has not gone “nowhere”.

For each form of energy there are equations to tell us how much energy there is. If a body of mass m is moving with velocity v then it has kinetic energy and the amount of kinetic energy is given by

If a body of mass m is at a height h above the floor then it has gravitational potential energy and the amount of gravitational potential energy is given by

In the diagram below, as the skateboarder moves from position 1 to 2, his kinetic energy changes into potential energy. At 3, all the kinetic energy the skateboarder had initially has been changed into kinetic energy. The situation below has been idealised. In fact there is friction which must be overcome, and the skateboarder's potential energy at 3 will be less than that show. In practice this will mean he will not rise to a height of 3.27m.

IGCSE PHYSICS Topic : Black and Silver Bodies – Reflection and Absorption of Radiation

 IGCSE Physics Notes: Black and Silver Bodies – Reflection and Absorption of Radiation

You may know already that shiny things are shiny because they reflect all – or almost all – of the radiation that shines on them, and that a black thing is black because it absorbs all – nearly all the radiation that shines on them. In fact these properties of being able to reflect or absorb radiation extends beyond the light we can see into many other parts of the electromagnetic spectrum.

Black things left in the sun heat up more quickly because they are able to absorb more of the radiation shone on them. Shiny things take longer to heat up because they reflect most of the radiation that shines on them. It works the other way round. Black things cool down more quickly if left in the shade, and silver things cool more slowly.

We can use this in some surprising ways. For example if someone is suffering from hypothermia, it might seem wise to wrap them in something black, so they can get warm quickly In fact this would mean they would radiate away their own body heat. Instead they are wrapped in aluminium foil. Aluminium foil is found in every first aid box at sea for this reason.

Ever wondered why Arabs wear white? It is for the exact reason that they will absorb little of the heat that shines on them and keep the wearer cool.

The white clothes above keep the wearer cool. A patio heater has a silver hood to reflect down the heat from the heater which would otherwise be lost into space.In this way it keeps the patio

IGCSE PHYSICS TOPIC : Conducting Electricity – Metals, Semiconductors and Insulators

IGCSE Physics Notes: Conducting Electricity – Metals, Semiconductors and Insulators

Everybody knows that metals conduct electricity. If you apply a voltage, you get a current. Most elements are metals. However the are two other classes of materials – semiconductors and insulators. Which class a material falls into depends on how free the electrons are to move. In a metal many of the electrons are completely free. In the diagrams below, the electrons can be considered to occupied the green shaded regions, and the regions in which electrons can have enough energy to be able to move are coloured blue. This does not mean there are any electrons in the blue region. For there to be electrons free to move the blue and green regions have to overlap.

They do overlap for metals, hence metals can conduct electricity – the typical resistivity of a metal isThey don't overlap for semiconductors. Some energy may be given to the electrons so they can move from the valence band, where they normally are, to the conduction band, where they may conduct electricity. This energy may be in the form of heat or light. This implies that if you heat a semiconductor, it's resistance falls, since it can conduct electricity better. The typical resistivity of a semiconductor is

 

 For insulators there is a much larger energy gap between the conduction and valence bands. The resistance of insulators is very high. A lot of energy must be given to the electrons to push them into the conduction band. The typical resistivity of an insulator is