Physics in action

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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