GCSE Physics P3 (AQA)

• Are EM waves, high freq, short wavelength

• Used for medical diagnosis, CT scans, cancer precautions, checking for bone fractures, dental problems

• Transmitted (pass through) healthy tissue, absorbed by denser materials like metal and bone

• Affect photographic film in the same way as light

• Produced when an electron strikes a metal target

• In modern hospitals, xrays are captured by CCDs rather than photographic film.

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• CCDs (Charge Coupled devices) are electronically small sillicon chips divided in into a grid of millions of identical pixels. This is used in digital cameras

• CCDs detect xrays and produce electronic signals which form high res. negative images

• X Rays ionise

• High does kill living cells and can be used to treat cancers (like gamma rays)

• X Rays are focused on a tumor using a wide beam that is rotated around the patient with the tumor at the centre. This will minimise radiation being exposed to normal cells which will reduce the chance of damaging the rest of the body

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• Risks-

• Must be focused carefully

• Radiographers need to minimise their radiation dosage

• To prevent harm

• Patient is put into a cylindrical scanner and an X Ray beam is fired through the body from an X Ray tube, which is picked up by detectors on the opposite side, that are rotated throughout the scan

• A computer interprets signals from the detectors and form a digital image of a 2D slice through the body (cross section). Multiple scans can be put together to form a 3D image

• Body organs made of soft tissue (such as the intestines) can be filled witha contrast medium so that they absorb X Rays and can be seen on an X Ray image

• CT scans use 9x more X Rays than normal X Rays to find any tiny variations in tissue density

• A higher freq. than the humans upper limit, therefore cannot be heard by humans

• Between 20Hz and 20,000Hz

• Uses: Prenatal scanning, removal of kidney stones, imaging ligaments and muscles, cleaning jewellery

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• How it Works-

• Sound waves pass from one medium to another

• Are refracted (change of direction of light)

• Which is called Partial Reflection (you can point a pulse of ultrasound at an object and wherever there are boundaries between 2 substances, some of the ultrasound get reflected back to the detector)

• Adv. of Ultrasound- Breaks down kidney stones (high energy waves at the kidney stones turn them to smaller particles), no surgery, non-ionising, safe, check fetal development (by reflecting a wave once it reaches a boundary that is the fluid in the womb/skin of fetus. Echos are processed by a computer to produce a video image of it)

• Disadv. of Ultrasound- Fuzzy image, therefore harder to diagnose

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• Adv. of X Rays- Produce clear images of bones and metals, relatively cheap and easy to use, can be read by non-radioologists

• Disadv. of X Rays- X Rays are ionising, Ionisng of cells can cause cancer

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• Adv. of CT Scans- Detailed 3D image, can diagnose complex illnesses and plan complex surgery

• Disadv. of CT Scans- Use a lot more X Ray radiation, therefore exposed to more radiation

• Endoscopes-

• 2 thin tubes/bundles which contain optical fibres.

• They allow Keyhole Surgery

• Lasers cauterise (burn and seal) to stop infection, bleeding, burning. Colour of laser is matched to type of tissue to produce maximum absorbtion

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• Optical Fibres-

• Are thin, flexible glass fibres

• Carry light far

• Wave enters one end and bounces repeatedly to the other end

N= 1 / SIN C

N= Refractive Index C= Critical Angle

Critical Angle= Angle of incidence

• When light passes between one medium to another it changes speed

• When light speeds up from one material to another the angle of refraction is greater than the angle of incidence

• Rays of light refract when they cross from glass to air. It is refracted away from the normal.

• TIR can only take place when a wave travels through a more dense substance towards a less dense substance (glass-water/water-air)

• A dense material with a high refractive index= totally internally reflect more light

N= SIN i / SIN r

• N= Refractive Index

• Is the change of direction of light when it passes from one transparent substance to another

• Takes place because waves change speed when they cross a boundary

• Freq of light does not change during refraction

• Refractive Index is a measure of how much a substance can refract a ray of light

• The bigger the refractive index, the slower the light travels in the material

The nature of an image is whether the image is-

• Real or virtual

• Upright or inverted (upside down)

• Larger or smaller

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Oscillioscopes-

S= V X T

• Oscillioscope traces how ultrasound pulses reflecting at 2 seperate boundaries

• The time it takes for the reflections to reach a boundary can be used to measure how far away the boundary is. This is how ultrasound imaging works

• 1 ms= 0.00001s Seconds per division

• Iris- Coloured ring of muscle, controls the amount of light that eneters the eye

• Cornea- Transparent layer that protects the eye, helps to focus light onto the retina

• Pupil- Central hole formed by the iris, light enters eye through pupil

• Ciliary Muscles- Attached to the lens by suspensory ligaments, muscles change the thickness of the lens

• Lens- Focuses light onto the retina

• Retina- The light sensitive cell receptors around the inside back of the eye

• Blindspot- Region whether the retina is not sensitive to light (no light sensitive cells present)

• Optic Nerve- Carries nerve impulses from retina to brain

• Aqueous Humour- Transparent watery liquid that supports the front of the eye

• Vitreous Humour- Transparent jelly like substance

• Cilliary Muscles- Alter thickness of lens

• Lens is automatically designed to become thinner to keep what you see in focus. They are attached to the edge of the lens by suspensory ligaments, which constrict and slacken

• Relax= thinner. Thinner= focus

• Light enters eye through the cornea

(• Cornea protects eye, helps to focus light on the retina

• Retina are a layer of light sensitive cells around the back inside of the eye

• They pupil is the circular opening at the centre of the iris)

• The eye lens focuses light to give a sharp image on the retina

• The image on the retina is inverted (upside down) but the brain interprets it so you see it upright

• Converging Lenses-

• Cause parallel rays oflight to bulge outwards

• The principal focus of a converging lens is the point where rays hitting the lens parallel to the axis all meet

• Real image Convex= Bulges outwards

• Parallel light rays converge (meet) at the principal focus. Any image is upside down.

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• Diverging Lenses-

• Cause parallel rays of light to diverge (spread out)

• The principal focus of a diverging lens is the point where rays hitting the lens parallel to the axis all appeat to meet

• Virtual image Concave= Caves inwards

• Parallel light rays diverge (spread out). Tthe principal focus is where the ray leaves the lens

• For a lens of a given focal length, the grater the refractive index of the lens material, the flatter and thinner the lens can be manufactured

• Long sight (+D) converges light

• Short sight (-D) diverges light

• A axis of a lens is a line passing through the middle of the lens

• There is a principal focus on each side of the lens. The distance from the centre of the lens to the principal focus is called the focal length

• Camera-

• Rays of light from the person are converged by the convex lens. This forms an image on the film of the CCD (in the case of a digital camera). An object close to the lens causes the light rays to enter at a sharper angle. This causes the rays to converge away from the lens. The lens can only bend the light to a certain degree, the image is focusesso that it can fit on the film. This is done by moving the lens away from the film

• If the object is away from the lens the rays enter at a wider angle. The rays being refracted at a sharper angle and the image forms closer the the film to get a focused image. The real image of a closer object forms further away from the lens than the real image of a distant object

• Magnifying Glasses-

• Image is virtual, upright

Magnification- image height / object height

• Short Sight-

• Distant objetcs blurry

• Uncorrected image formed at the front of the retina

• Eyeball too long/ lens too powerful

• Correct- Diverging lens

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• Long Sight-

• Close object blurry

• Uncorrected image formed behind the retina

• Eyeball too short/ lens too weak

• Correct- Converging lens

Converging Lens-

• An incident ray parallel to the axis refracts through the lens and passes through the principal focus on the other side.

• An incident ray passing through the principal focus refracts through the lens and travels parallel to the exis.

• An incident ray passing through the centre of the lens carries on in the same direction

Diverging Lens-

• An incident ray parallel to the axis refracts through the lens and travels in line with the principal focus (so it appears to have come from it)

• An incident ray passing through the lens towards the principal focus refracts through the lens and travels parallel to the axis.

• An incident ray passing through the centre of the lens carries on in the same direction

How to find the centre of mass-

1) Tie the object from a corner to a piece of string and put aplumbline (weighed piece of string) on the other end.

2) Hang this on something (e.g. a clamp or nail in a wall)

3) Because there is no moment, the plumbline hangs straight through the centre of mass. Draw this line along the object. Repeat, but his time rotate the object. Draw another line

4) Where the lines meet is where the centre of mass is

• Orbits

• Mass: the amount of matter in an object

• Weight: the force of gravity acting upon an object with a mass. Measured in Newtons

• Circular orbits have one focus point

• Elliptical orbits have 2 focus points. The sum of the distance between them is equal

• Satellites

• Geostationary: orbits at a certain height above the equator with the same speed as the earth. Looks stationary.

• Low polar orbit: orbits over the poles at a low height. Used for monitoring.

• For an object moving in a circle at a constant speed:

the object accelerates towards the centre of the circle constantly

• the centripetal force increases as:

the mass or the speed increases

the radius of the circle decreases

• Camera-

contain converging lens to get a real image

contain film/pixels that pick up light and form image

• If a wire cuts across a magnetic field, potential difference/a current is induced.

• Direction of flux, current and motion are at right angles to each other.

• Increased 'flux cutting' can be done by:
a stronger magnet
varying the magnetic field (faster)
moving the coil of wire (faster)

• Dynamo Effect: produces electricity from the motion (induction)

• Flemming's Left Hand Rule

• When the coil of wire cuts perpendicular through the magnetic field, it induces a current. Because it cuts the magnetic fierld and the wire is attatched to a circuit, the current that was induce can travel through the complete circuit, powering a motor.

Step up: from power station to pylons. More efficient, less lost as heat and over the distance travelled.

Stepdown: from pylons into homes. Appliences can't handle such voltage, average plug voltage in EU 240v.

• A star to be is called a protostar . Particles of dust and gas (nebula) gather together under their gravity. They merge and become more concentrated at one point.

• As it becomes denser, the temperature increases. The nuclei of hydrogen and other light elements fuse together. The core gets hotter and it shines.

• Our sun shines because of the hydrogen fusion at the core. This is the main stage of a stars life. Can continue for billions of years.

• Radiation flows out of the star steadily. This prevents it from collapsing in on itself. This stays balanced until all the hydrogen fuses in the core.

Death:

•When the star runs out of hydrogen, it expands. It cools down and becomes a red giant.

• Helium and other elements fuse to form heavier elements.

•When there is no more elemetns to fuse, fusion stops and the star collapses in on itself, becoming a white dwarf. This is a small, hot and dense star. They eventually go cold. Red->Yellow->White.

Bigger stars collapse and reach the white dwarf stage. Then in a dramatic explosion called a supernova.

This compresses the core into a neutron star, made from neutrons. If it has a large enough mass it will become a black hole. The gravity of a black hole means nothing can escape it, it is so strong.

Low mass star: protostar > main stage > red giant > white dwarf > black dwarf

High mass star: protostar > main stage > red giant > white dwarf > supernove > neutron star > black hole if mass sufficient

• A lens forms an image by refracting light. Refraction is when light changes direction when it moves from one medium to another