# 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 lens does this by slowing down the light travelling through the middle of the lens for longer than the light at the lens edge.
- 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

- Power and the amount of curvature are essentially the same thing

- When waves pass through the lens they are given curvature centered on the focal point.
- 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

- U is the distance between the object and the lens axis

- 1/v=1/u+1/f
- 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

- Linear maginification = m=size of image/size of object

- Converging lenses change the curvature of the wavefronts.
- 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.

- The value of the binary number codes for a shade

- Computers store information in bits. The system is known as binary and involves only 0 and 1.

- Lenses
- 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

- If voltage is plotted on the y axis, and current on the x axis, then the gradient is the resistance

- Resistance is opposition to the flow of current.
- 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.

- There are two basic distinctions, parallel and series.

- Circuits
- 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

- Transverse waves can be polarised, so they only oscillate in one direction.

- Transverse wave is a wave where the vibrations are at right angles to the waves direction of travel.
- 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

- Frequency=1/period

- There are two types, transverse and longitudinal.
- 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)

- However noise limits the number of bits one can use.

- The greater the number of bits, the more levels so resolution is improved by adding more bits.
- 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)

- To do so one takes the value of the signal at regular time intervals then find the nearest digital interval.

- Digital signals can only take a set amount of values, determined by the number of bits.
- 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.

- There must be a gap between frequencies used, and this is determined by the bandwidth.

- All radio stations have different carrier frequencies so they don't interfere with each other.

- 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.
- Rate of transmission = samples per second times bits per sample

- Bandwidth is the range of frequencies. In communications systems the bandwidth determines how many signals can be sent at the same time.

- Analogue signals vary continuously and can take any value

- Waves
- 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.

- Atoms can move small distances relative to their equilibrium positions without changing position in the material.

- This means that when the material is put under tension, the atoms of the material are pulled apart from one another.

- We this happens when the force is removed, the material will no longer return to its original length.

- 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.

- Resistance depends on 3 things

- Hooke's law states that extension is proportional to force.

- Imaging

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