Physics in action 

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  • Created by: Olivia
  • Created on: 05-05-13 10:29
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  • Physics in action
    • Imaging
      • Lenses
        • Converging lenses change the curvature of the wavefronts.
          • When waves pass through the lens they are given curvature centered on the focal point.
            • The lens does this by slowing down the light travelling through the middle of the lens for longer than the light at the lens edge.
              • All points of the wavefront take the same amount of time to get to the focal point.
          • The more powerful the lens the more curvature it will add to the wavefronts so, the shorter the focal length.
            • Power and the amount of curvature are essentially the same thing
              • Power=1/f
              • Curvature=1/radius of curvature
        • One can use the lens equation to find where an image will be formed.
          • 1/v=1/u+1/f
            • U is the distance between the object and the lens axis
              • It is negative as the center of the lens is regarded like the center point of a graph.
            • V is the distance between the image and the lens axis
        • A lens can also produce a magnified image.
          • Linear maginification = m=size of image/size of object
            • This is the same as m=v/u
      • Information in  images
        • Computers store information in bits. The system is known as binary and involves only 0 and 1.
          • 8 bits = 1 byte
          • The number of bits storing the data dertermines how many alternatives that string can code for
            • A single bit has 2 alternatives
            • Number of alternatives= 2^number of bits
            • The determine the number of bits needed...
              • Number of bits=log base 2(number of alternatives)
        • Images are stored as a string of bits each represents a pixel.
          • The value of the binary number codes for a shade
            • If one has 8 levels(digit binary number) there are 256 possible alternatives
            • In  coloured images each pixel can be described by 3 binary numbers one for each of the primary colours. The length of the binary number depends on how many shades of the colour are needed.
          • One can improve images in a range of manners
            • Multiplying by a fixed value improves contrast
            • Adding a fixed value makes the image brighter
            • Adding false colour can highlight features
            • Replacing pixels with the median of their neighbours reduces noise
              • One can also do this by using the mean, but this blurs the image. The mean effectively smooths the image.
            • The Laplace rule is used to find edges and detect changes in gradient of an image.
              • One multiplies a pixel by 4, then subtract the pixel values above and either side of it. one is left with just the edges.
    • Sensing
      • Circuits
        • There are two basic distinctions, parallel and series.
          • In a parallel circuit current is split between branches and voltage is a constant, f only one component is in each branch.
          • In a series circuit current is a constant and voltage is split between components.
        • Current is defined as the rate of flow of charge It is measured in Amps.
        • Voltage/ Potential difference is the energy per unit charge. It is measured in Volts.
          • Voltage is conserved around a circuit. Some volts may be lost, due to internal resistance within the battery.
        • Power is the rate of transfer of energy and is measured in Watts.
        • Resistance and conductance
          • Resistance is opposition to the flow of current.
            • If voltage is plotted on the y axis, and current on the x axis, then the gradient is the resistance
              • In  metals as the temperature rises, the atoms move around more vigorously  so resistance increases.
              • The shallower the gradient of the graph, the higher the resistance.
                • For an ohmic resistor resistance is a constant, assuming that the temperature is a constant.
            • Conductance is the inverse/ opposite of resistance
        • A potential divider is two resistors connected in series. This can be used to control voltage over components.
          • The addition of a thermistor or LDR in series with a fixed resistor creates a sensor of certain qualities.
    • Signalling
      • Waves
        • There are two types, transverse and longitudinal.
          • Transverse wave is a wave where the vibrations are at right angles to the waves direction of travel.
            • All electromagnetic waves are transverse
          • Longitudinal waves have their vibrations  in the same direction as its movement.
          • The main type we are interesting in is transverse waves.
            • Transverse waves can be polarised, so they only oscillate in one direction.
              • If two polariod filters are held at right angles to each other, one in front of the other, no light will get through
        • The time it takes 1 wavelength to pass a point is called the period.
          • Frequency=1/period
            • Frequency is the how many waves pass a specific point over 1 second.
            • V=flambda
              • lambda= wavelength
              • V= speed of the wave
      • Sampling
        • Analogue signals vary continuously and can take any value
          • Digital signals can only take a set amount of values, determined by the  number of bits.
            • Digital signals however as they only have a set number of values resist noise.
            • They have 4 advantages over analogue signals
              • They can be sent, received and reproduced more easily because they have a set number of values.
              • They can represent different types of information in the same way.
              • They are easy to process using computers as computers are also digital.
          • Analogue signals can be digitised.
            • To do so one takes the value of the signal at regular time intervals then find the nearest digital interval.
              • Each digital interval is represented by binary numbers. The signal created will never be the same, but may be very close.
            • The quality of the signal depends on its resolution  and the sampling rate
              • If a signal only had a few widely spaced levels then the analogue values sampled will differ greatly from the level values, and the reproduced signal will be different.
              • The higher the number of levels, the more closely the digitied signal will match the analogue one.
                • The greater the number of bits, the more levels so resolution is improved by adding more bits.
                  • However noise limits the number of bits one can use.
                    • If one uses too many bits, noise will also be reproduced in the signal.
                    • Maximum number of bits= Log base 2 ( total variation/ noise variation)
              • Ideally the minimum sampling rate ought to be twice the maximum frequency.
                • A low sampling rate can lead to the creation of aliases (they are not in the original signal)
        • Signals are made up of lots of waves with different frequencies
          • The frequencies that make up a wave are called its spectrum
          • If one wishes to reconstruct a signal you need to know all the frequencies within it.
            • Bandwidth is the range of frequencies. In communications systems the bandwidth determines how many signals can be sent at the same time.
              • Also the amount of information
              • A radio station emits a carrier wave which carries the audio signal which has been converted on it.
                • When the radio is tuned into the right station, it can separate the carrier signal and the audio, then convert it back into noise for you to listen to.
                  • All radio stations have different carrier frequencies so they don't interfere with each other.
                    • There must be a gap between frequencies used, and this is determined by the bandwidth.
                      • The larger the bandwidth the larger the gap must be to stop neighbouring carrier frequencies overlapping.
              • Rate of transmission = samples per second times bits per sample
    • Materials
      • Hooke's law states that extension is proportional to force.
        • F=ke where k is the spring constant
        • Hooke's law only works to a certain point, and past this, linear relationship, the material will be permanently stretched.
          • We this happens when the force is removed, the material will no longer return to its original length.
            • This is plastic deformation.
            • Before this point during the linear relationship the behaviour of the material would be elastic.
              • This means that when the material is put under tension, the atoms of the material are pulled apart from one another.
                • Atoms can move small distances relative to their equilibrium positions without changing position in the material.
                  • Once the load is removed the atoms return to their original positions.
      • Stress, strain and young's modulus
        • Stress is force/cross sectional area
        • Strain is extension/ original
        • Young's modulus tell us the stiffness of the material.
        • A stress big  enough to break the material is called the breaking stress.
      • Structures
        • Metals have a polycrystalline structure, where the atoms in each section are arranged in an ordered repeating, regular pattern.
        • Glass and other ceramics have a amorphous structure
          • This means the structure is random
      • Resistivity and conductivity
        • Resistance depends on 3 things
          • 1) Length, the longer something is, the more difficult it is to make current flow.
          • 2) Area, the wider a wire the easier for electrons to travel through it.
          • 3) Resistivity depend on the materisl, the structure can make it easy of difficult for current to flow. In general resistivity also includeds factors like temperature or light.
            • In metals as temperature increases the  the number of charge carriers stays the same, but the lattice moves more. This means resisitivity increases.
            • In semiconductors  as temperature increases, conductivity rises as the number of charge carriers increases.

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