# Physics in action

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• Created by: Olivia
• Created on: 05-05-13 10:29
• 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
• 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.
• 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.