# P3

• Created by: jacktd98
• Created on: 19-05-15 17:58

## X-rays

X-rays are part of the electromagnetic spectrum. They have a very short wavelength and cause ionisation.

Properties of X-rays include:

• They affect a photographic film in the same way as light
• They are absorbed by metal and bone
• They are transmitted by soft tissue
• Their wavelength is of the same order of magnitude as the diameter of an atom.

X rays can be used in medicine as, X ray photography, CT scans, X ray therapy, CT scans distinguish between different types of soft tissue as well as bone.

The use of charge-coupled devices (CCDs) allows images to be formed electronically.

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## Ultrasound

The range of human hearing is about 20 Hz to 20 000 Hz.Electronic systems can be used to produce ultrasound waves, which have a frequency higher than the upper limit of hearing for humans.

Ultrasound waves can be produced by a transducer a passed through the body. When a wave passes from one medium into another, some of the wave is reflected and some is refracted. The time for the wave to reach the devise after being reflected can be used to calculate the distance

Calculation of the distance between interfaces in various media.

s   =  v   x    t

• s is distance in metres, m
• v is speed in metres per second, m/s
• t is time in seconds, s

Uses - Breaking down kidney stones, it is painless and no need for surgery - pre natal scanning, the reflections are picked up and shown as a video image on a computor screen.

Ultrasound is safer than X ray but produces a worse picture quality

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## Refractive index

Refraction of light is the change of direction of light as it passes from one medium to another. When waves slow down they bend towards the normal. If a wave hits a boundary at 90*, along the normal, it will not change dirrection but it does slow down.

Every transparent material has a refractive index. when a light ray enters a new medium, the angle of incidence will affect the angle of refraction and this also depends on the refractive index

Refractive index (n) = Sin i / Sin r

• Towards (normal
• Air to
• Glass
• Away
• Glass to
• Air
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## Total internal reflection

Total internal reflection is when the angle of incidence becomes so large that it passes the Critical angle meaning there is no refraction and just reflection, total internal reflection. This can be used in optical fibres to carry visable light over long distances such as those in telecommunications and in endoscopes. In endoscopes two bundles of optical fibres are used to carry light to the area then to create an image on a screen. used for keyhole surgery. This can also be used for lasers

Refractive index (n) =  1 / sin c (critical angle)

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## Lenses

A lense works by changing the direction of light passing through it.

A CONVERGING (convex) lens - makes parallel rays converge to a focus called the principal focus (focal point) - used in a magnifying glass and a camera

A DIVERGING (concave) lens - makes parallel rays diverge, the point where the rays appear to come from is the principal focus - used to correct sight

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## Converging lens

- inverted                                - inverted                                      - virtual

- real                                      - real                                            - right way up

-same size                              - bigger                                         - bigger

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## Diverging lens

- Always virtual

- Always right way up

- Always smaller

- Always on the same side

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## Magnification

When a converging lens is used in a magnifying glass a virtual image is created that is larger than the  real but the object must be closer than the focal length

Magnification = Image height / Object height

Power - The more powerful a lens the more it converges rays of light so the shorter its focal length this is shown as: it is measured in dioptres

Power (D)  =  1 / focal length (m)

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## Eye Functions

Cornea - refracts the light as it enters the eye

Iris - ring of muscle that controls the amount of light entering the eye

Pupil - The hole in the middle of the iris allowing light to enter the eye

Lens - changes shape (thickness) to focus light onto the retina

Ciliary muscles - attatched to the lens by the suspendsory ligaments and control the thinckness of                            the lens

Retina - covered in light sensitive cells which detect light and send signals to the brain

Optic nerve - carries the nerve impulses from retina to brain

The eye can focus between the near point and far point, the average near point is 25cm

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## Correcting vision

Short-sighted people can't focus on distant objects, to correct this a diverging lens is used in glasses

Long sighted people can't focus on near objects, a converging lens is used to correct this.

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## Centre of mass

The centre of mass of an object is that point at which the mass of the object may be thought to be concentrated. A freely suspended object will swing until its centre of mass is vertically below the point of suspension. The centre of mass of a symmetrical object is along the axis of symmetry.

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## Moments

A moment is the turning effect of a force.

Moment = Force  x  perpendicular distance from the line of action of the force to the pivot

M  =  F  x   d

To increase the moment of a force increase the force applied or the distance from pivot

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## Stability and pendulums

If the moments on an object are not equal the object will turn. The most stable objects have a low centre of mass and a wide base. An object will begin to tip over if its centre of mass moves beyond the edge of its base.

A simple pendulum is made by suspending a weight from a piece of string. The time for the weight to swing to the other side and back again is called the time period

Time period  =  1 / frequency

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## Hydraulics

Liquids are virtually incompressible. Pressure in a liquid is transmitted equally in all directions.

Pressure is the force per unit area and is measured in pascals (Pa)

Pressure  =  force  /  area

The pressure in liquids can be used in hydraulics to lift heavy things. They use cross sectional areas with one smaller and one larger cross section. A smaller force has to be applied to the small cross section to get a greater force in the larger one.

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## Circular motion

If an object is travelling in a circle, its velocity is constantly changing not because of speed but because its direction is changing. As the velocity is changing, it is accelerating and it is accelerating towards the centre of the circle.The force that causes this is Centripetal force.

Centripetal force can either be caused by:

Tension - of a rope with a bucket  -  fair ground rides

Friction - a car going round a bend

gravity

Centripetal force increases as Speed increases, Mass increases, and the radius of the circle decreases.

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## Magnetic fields

A magnetic field is a region where Magnetic materials (like iron and steel) and also wires carrying currents experience a force acting on them.

When a current flows through a wire, a magnetic field is created around the wire. the field is made of concentric circles with the wire in the centre.

The magnetic field round a coil of wire is strong and uniform. This is an electromagnet as as soon as the current stops the object loses its magnetic field. The strength of the electromagnet can increased by using more coils, increasing the current or adding an iron bar through the coil.

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## The motor effect

A current in a magnetic field experiences a force. To get the maximum force the wire and magnet must be at 90* to each other

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## Electric motor

The split ring commutator swaps the contacts every half turn to keep the motor rotating in the same direction

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## The loud speaker

Alternating current is needed to swap the direction of the current and therefore the direction of the coil, this makes the cone vibrate making the sound waves.

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## Electromagnetic induction

Electromagnetic induction is the creation of a potential difference across a conductor which is experiencing a change in magnetic field. Moving a magnet in a coil of wire induces a voltange. This happens when a conductor (the wire) cuts the lines of a magnetic field (the magnet). If you move the magnet in and out an AC current will be produced

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## Transformers

Transformers work by electromagnetic induction. The primary coil produces a magnetic field which stays within the iron core. As it is an alternating current going through the first wire an alternating magnetic field is induced in the iron core. This then induces an alternating potential difference in the second coil. This is why transformers only work with AC supply. A step up has more coils on the second wire a step down has more on the first.

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## Switch mode transformers

These are used in chargers and opperate at higher frequencies than traditional ones, they work at 50kHz - 200kHz meaning they are much lighter and more efficient

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