physics unit 3

- high frequency, short wavelength electromagnetic waves 

- they are transmitted by (pass through) healthy tissue but are absorbed by denser materials like bones and metal

- they affect photographic film in the same way as light does (x-rays turn the film black so bones come up white as they dont let any rays through)

- images can be formed electronically on CCD's- charge coupled devices- which are silicon chips divided into pixels which detect x-rays and produce electronic signals to form high resolution images. 

CT SCANS:

- use x-rays to produce high resolution images of soft and hard tissues (soft tissue can absorb small amounts of x-rays). CT scans use more x-rays than normal x-ray scans 

- patient is inside a cylindrical tube and an x-ray is fired from the beam on one side and picked up by detectors on the opposite side, so passes through the body. a computer then interpretes signals from the detectors to form an image. 

TREATING CANCER:

- X-rays can be used to treat cancer as high does will kill living cells. (low doses cause mutations) 

- carefully focus x-ray at right dosage on the tumour using a wide beam. then rotate beam with cancer still at centre to avoid exposure time for normal cells so less damage to them but cancer is still killed 

RADIOGRAPHER PRECAUTIONS:

- X-rays are ionising so need to take precuations to minimise x-ray dose

- lead aprons are worn and they stand behind a lead screen or leave the room 

- lead is also used to shield other areas of the patients body to avoid exposure of normal cells

- an ultrasound is sound with a higher frequency than we can hear. they are formed when electrical systems convert mechanical vibrations to sound waves

- when an ultrasound reaches a boundary some of it is partially reflected and some passes through 

-the time it takes for reflected ultrasounds to reach a detector determines how far away the boundary is. this is how ultrasound imagery works. 

- distance (m)= speed (m/s) x time (s). but remember you need to half the distance as the distance calculated will be there and back. 

uses in medicine:

BREAKING DOWN KIDNEY STONES:

- kidney stones are hard masses that block the urinary tract. an ultrasound beam concentrates high energy waves onto the stones and breaks them down into sand like particles which then pass out of the body in urine. PAINLESS

PRE-NATAL SCANNING:

- whenever some of the ultrasound reaches a different boundary eg between fluid and skin of foetus, some wave is reflected and picked up by a detector 

- the timing and distribution of the waves are processed by a computer to form a video image of the foetus. 

IS IT SAFE?

- ultrasounds are non-ionising so they are safe 

- x-rays are ionising and can cause cancer by mutations. shouldnt be used on a foetus. 

- CT scans use more x-ray radiation than normal x-rays so the patient is exposed to more ionising radiation. so thats why CT scans are rarely done. 

IMAGE QUALITY:

- ultrasound images are fuzzy- harder to diagnose conditions 

-x-ray images are clear but only show bone and metal

-CT scan images are very detailed so can be used to diagnose and plan complicated surgery 

refraction is when waves change direction because they change speed when they enter a different medium due to difference in density's. 

- when light slows down wave bends towards normal, when light speeds up wave bends away

- when light hits a new medium, some light will refract and pass through but some will reflect back. it all depends on the angle of incidence. 

REFRACTIVE INDEX:

- refractive index of a medium is a ratio of speed of light in a vacuum compared to speed of light in that index. 

-SNELLS LAW- when an incident ray passes from air into another medium, the angle of refraction depends on the refractive index of the medium. 

- refractive index (n)= sin(i) / sin(r)

- lenses form images by refracting light and changing its direction. 

CONVERGING LENS:

• convex- bulges out 
• it causes parallel rays of light to converge together at the principal focus 

DIVERGING LENS:

• concave- caves inwards
• it causes parallel rays of light to to diverge (spread out) at a virtual principal focus 

-AXIS- the line passing through the middle of a lens 

-PRINCIPAL FOCUS IN CONVERGING LENS: where rays hitting lens parallel to axis all meet 

-PRINCIPAL FOCUS OF DIVERGING LENS:  point where rays hitting the lens parallel to the axis appear to come from when you trace back the diverging rays to a point where they all meet behind the lens. 

-FOCAL LENGTH- distance from centre of lens to principal focus 

rules for refraction in a converging lens:

• an incident ray parallel to the axis refracts through the lens to pass through the principle focus on the other side. or vise versa 
• an incident ray passing through the centre of the lans continues on in the same direction 

rules for refraction in a diverging lens:

• an incident ray parallel to the axis refracts through the lens to travel in line with the virtual principal focus or vise versa 
• an incident ray passing through the centre of the lens carries on in the same direction 

REAL OR VIRTUAL IMAGES:

- A real image is when light comes together to form an image on a screen. eg in the retina 

- a virtual image is when light is diverging so the light from the object appears to be coming from a completely different place eg behind a mirror or using a magnifying lens 

describing an image:

• real or virtual 
• upright or inverted (upsidedown)
• relative size compared to object 

CONVERGING LENS RAY DIAGRAM:

- Where the object is compared to the principal focus determines the nature of the image 

- the image gets futher away the closer the object gets (unless the object is closer than F when it forms a virtual upright and magnified image the same side as the object) 

- as the object gets closer towards the lens, the image gets bigger 

-the image formed is always inverted and real unless the object is closer than F which then forms and upright and virtual image 

-the object at 2F will form a real, inverted image the same size at 2F 

-the object closer than 2F (inbetween F and 2f) will form a real inverted image, bigger than the object further away from 2F 

DIVERGING LENS RAY DIAGRAM:

- a diverging lens always produces a virtual image 

- the image is always upright and smaller than the object no matter where the object is 

-magnifying glasses work by creating a magnified virtual image 

- the object being magnified must be closer to the lens than the focul length (principle focus) 

-you cant project a virtual image on a screen

-MAGNIFICATION = IMAGE HEIGHT / OBJECT HEIGHT 

POWER OF A LENS:

-the more powerful a lens, the more strongly it converges together the rays of light so the shorter the focal length

- power (D) = 1 / focal length (m)

- converging lens' have a +ve power and diverging lens' have a -ve power

- refractive index of lens material and curvature of the lens determine power. 

- CORNEA- transparent window in a convex shape which has a high refractive index. it does most of the eyesfocusing. 

 -IRIS- coloured part of the eye, made up of muscles that control the size of the PUPIL which is the hole in the middle of the eye. this controls the intensity of light entering the eye. 

-LENS- changes shape to focus light from objects at varying distances in the same place (retina) . its connected to the CICILARY MUSCLES by SUSPENSORY LIGAMENTS. when the muscles contract, tension is released so lesns becomes fatter. when they relax, tension is build and lens becomes thinner 

-RETINA- images are formed on the retina which is covered in light sensitive cells. these cells detect light and send signals to the brain to be interpreted. 

eyes far point- the furthest distance that the eye can focus comfortably. normally its infinity 

eyes near point- the closest distance that the eye can focus on- normally about 25cm.  as the eye focus' on nearer objects the lens becomes more powerful and focal lenth decreases

- when you take a photo light from the object travels to the camera and is refracted by the lens to form an image on the film. 

- it is a real image as the light actually meets there 

- the image is smaller than the object as the object is a lot further away than the focal length. 

- the image is inverted but our brain flips it the right way up (like in our eye) 

- the film or CCD in a camera represent the retina in our eye. 

SHORT SIGHT:

- cant focus on distant objects as their far point is closer than infinity. therefore light converges together infront of the retina instead. 

- caused by eyeball being too long or lens in eye too powerful. 

- to correct, place a diverging lens infront of the eye so that light diverges before entering the eye so that an image can be formed on the retina. 

LONG SIGHT:

-cant focus properly on near objects- their near point is further away than normal. therefore light converges together behind the retina. 

- caused by eyeball being too short or lens not being powerful enough 

- corrected by placing a converging lens infront of the eye so light already starts converging so the image can be formed on the retina. 

LASERS ARE USED TO CORRECT EYE PROBLEMS:

-a laser is a narrow intensive beam of light 

- they can be used to cut through body tissue 

- they can also cauterise (burn and seal shut) blood vessels to reduce bleeding

- they can burn off acne scars and skin scars

- but can also be used to vaporise part of the cornea to change its shape to affect its focusing ability. this can increase or decrease the power of the cornea so that images can be formed properly on the retina. 

OPTICAL FIBRES:

-visible light can be sent along optical fibres over long distances. 

- they work by bouncing waves off the sides of the inner core of glass or plastic. continued reflection allows light that enters at one end to emerge at the other end. 

-happens because of TOTAL INTERNAL REFLECTION:

- it can only happen when a wave travels through a dense substance eg glass towards a less dense substance eg air. 

• if angle of incidence is less than critical angle, most light passes out but a tiny bit is reflected
• if angle of incidence = critical angle, the emerging ray comes out along the surface and theres quite a lot of reflection 
• if angle of incidence is greater than critical angle- no light comes out, it is all totally internally reflected. 

CRITICAL ANGLE- the angle of incidence beyond which light it totally internally reflected

- a dense material with a high refractive index has a low critical angle. 

-if a material has a high refractive index it will totally internally reflect more light 

- diamonds are so sparkly because they have a low critical angle so more light totally internally reflects

REFRACTIVE INDEX = 1/SINC

ENDOSCOPES:

- A thin tube containing optical fibres that allow surgeons to examine a patients body 

- they have 2 bundles of optical fibres; 1 bundle to carry light there and 1 to carry an image back to be viewed.

-the image can be seen through an eye piece or an a screen

- keyhole surgery- only tiny holes are needed. 

-moment is the turning effect of a force

- MOMENT= FORCE X PERPENDICULAR DISTANCE FROM LINE OF ACTION TO PIVOT

-Line of action is just the direction the force is applied 

- the force of the spanner causes a turning effect (moment) on the nut. 

- using a longer spanner, the same force can exert a bigger moment because the distance from the pivot is greater 

- to get a maximum moment you need to push at right angle to the pivot 

-pushing at any other angle means a smaller moment as the perp distance is smaller

COM:

- centre of mass of an object is the point where all the mass is concentrated 

- a freely suspended object will spin until its COM is directly below the point of suspension 

- this is because when the object is freely suspended, the objects weight acts as a distance from the pivot (point of suspension) so it creats a moment and swings until the COM is directly below the point of suspension so there is no distance and no moment 

symmetrical objects:

- you can find the COM by seeing where all lines of symmetry cross 

unsymmetrical objects:

-create a hole in the object and suspend the shape so its free to move. 

- also suspend a plumb line at the same point (string with a weight) and draw a line along the path of the plumb line on the shape

- then suspend the shape from another point with the plumbline 

-again draw along the path of the plumb line 

-where the 2 lines cross is the objects COM

-if the anticlockwise moments equal the clockwise moments, the object is balanced and it wont turn

-total anticlockwise moments = total clockwise moments 

-if the centre of mass isnt on the pivot, add the centre of mass in and the objects weight. then the distance from the pivot to the objects COM x the weight will give you one moment 

LEVERS:

-levers use the idea of balanced moments to make it easier for us to do work ie lift a load

- levers increase the distance from the force to the pivot so less force is needed to get the same moment 

- wheelbarrows, scissors, long bars

-if the total anticlockwise moment doesnt equal the total clockwise moment the object will turn

STABILITY:

- if an objects centre of mass moves beyond the edge of its base the object will topple over

- the most stable objects have a wide base and a low centre of mass. thats why some objects are weighted at the bottom to lower its COM to make it more stable

-its the line of action of weight that can be seen on diagrams 

- a pendulum is made by suspending a weight from a piece of string. if you pull the pendulum back and then let it go, it will swing back and forth

-the time taken for the pendulum to swing from one side to the other- AND BACK AGAIN is called the time period

- the time period for each swing is always the same - this is why pendulums are used for keeping the time in clocks 

- frequency- the number of complete swings (there and back) made by the pendulum per second 

TIME PERIOD (s)= 1 / FREQUENCY (Hz)

- the longer the pendulum, the longer its time period 

-examples- fair ground rides (pirate ship rides), clocks, swings 

- liquids are incompressable which means you cant squish them as their volume and density stay the same

- because they are incompressable and because they can flow, a force applied to a liquid at one point is transmitted to other points in the liquid 

-PRESSURE IN A LIQUID IS TRANSMITTED EQUALLY IN ALL DIRECTIONS 

- PRESSURE (Pa) = FORCE(N) / CROSS SECTIONAL AREA (m^2) 

hydraulic systems: used as force multipliers

- they have 2 pistons- one with a smaller cross sectional area than the other 

- so that at the first smaller piston, a small force is applied to exert a pressure

- because pressure is transmitted equally throughout the liquid, the pressure at the 2nd larger piston is the same. because f= p x area, the force exerted at this piston will be greater 

- this is used in braking systems in cars 

- velocity is both speed and direction 

- when an object is travelling in a circle, its direction is always changing. therefore its velocity is always changing. a change in velocity is accelerating- therefore the object is constantly accelerating

- there must be a resulatant force acting on the object to cause acceleration- this resultant force always acts towards the centre of the circle. 

- the force that keeps something moving in a circle and prevents it from moving of in a straight line is a centripetal force. it does this by constantly changing direction- and it can be 3 things:

• friction- a car going round a bend. the centripetal force of friction between the tyres and road acts towards the centre of the circle
• tension- a concer on a piece of string swinging in a circle. the tension on the string is the centripetal force- so if the string breaks the concer will fly off in a straight one 
• gravity- for planets orbiting a star, the centripetal force is gravity 

-depends on mass, speed and radius of the circle

SPEED- the faster the objects moving, the bigger the centripetal force needed to keep the object moving in a circle

MASS- the heavier the object, the bigger the centripetal force needed to keep the object moving in a circle

RADIUS- the smaller the radius, the larger the force needed to keep the object moving in a circle as its got more turning to do 

magnetic field: a region where magnetic materials 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 up of concentric circles with the wire in the centre. 

- arrows on field lines point from the north pole of the magnet to the south pole of the magnet

COIL OF WIRE:

- A magnetic field inside a coil of wire is very strong and uniform 

-outside the coil of wire the magnetic field is just like a regular magnetic field

- you can increase the strength of a magnetic field around the coil of wire by adding 'magnetically soft iron core' through the centre of the coil. 

- this makes an electromagnet

- a magnetically soft material magnetises and demagnetises very easily. so as soon as you turn off the current through the coil of wire, the magnetic field disappears. 

ELECTROMAGNETIC USES:

- they are useful as their magnetism can be switched on and off. 

example: CRAINS

- magnets can be used to attract and pick up magnetic materials eg in scrap yards

- and then you move it to where you want to drop it, and switch off the current so the objects are released

circuit breakers also work by electromagnet attraction which breaks the circuit 

-passing a current through a wire produces a magnetic field. if you then put this wire into another magnetic fields, there are 2 magnetic fields combining which produces a force

-the force experienced by the current carrying wire in a mag field is known as the motor effect

- the force gets bigger if either the current or magnetic field is made bigger

- the force that acts on the wire is at 90 degrees to both the wire and the magnetic field. to experience a full force, the wire must be at 90 degrees to the magnetic field. if the wire runs parallel to the mag field, no force will be felt

- if you change the direction of the current or magnetic field, the direction of the force will be reversed 

THUMB- force direction, 1- FIELD, 2- CURRENT

-electric motors use the motor effect to keep them moving by using  a coil of wire that is free to rotate between 2 magnetic poles

-when an electric current flows through the coil, it experiences a force (because of its mag field combining with magnets mag field). one force acts up and the other acts down either side of the magnet- which keeps the coil rotating. 

- the direction of the current must be reversed every half turn otherwise the coil comes to a halt. this is done by the split ring commutator which 'swaps the contacts every half turn to keep the motor rotating in the same direction' 

- it is direct current used and so swapping the polarity of the direct current or swapping the magnetic poles over changes the direction of the motor. 

- you use flemmings left hand rule on each side of the magnet to work out the direction of the force- so in theory whether the coil is turning clockwise or anticlockwise 

USES- if you link the coil to an axle (a pole) and attach something to the end of the axle, it will rotate eg fans or food mixers or cd players. you can increase the speed of a motor by increasing CURRENT or STRENGTH OF MAGNETIC FIELD. 

- the creation of a potential difference across a conductor that is experiencing a change in magnetic field

- created in the ends of a conductor - eg a wire. if the conductor is part of a complete circuit, current will start to flow.

- you do this by 'cutting' magnetic field lines by moving a magnet in a coil of wire or moving a wire in a magnetic field. 

-if you move the magnet in the other direction or if you change the polarity of the magnets- the direction of PD is reversed. 

-if you keep moving the magnet back and forth in the coil or the wire back and forth in the field, a PD is produced which keeps changing direction, so an AC current is produced. 

another way to produce a potential difference is if you constantly turn a magnet around in a coil of wire.

instead of cutting the magnetic field, the change in direction of the magnetic field produces a PD which can make a current flow through the wire. 

the direction of the PD will always be changing as the direction of the magnetic field reverses so a AC is supplied. 

EXAMPLE DYNAMOS:

- theyre used on bikes to power lights

- the cog wheel is attached to a wheel and so when the wheel turns the cog wheel turns

- a magnet is attached to the cog wheel and so the magnet turns round in a coil of wire, which produces a PD and and AC to power the lights 

-transformers use electromagnetic induction to change potential difference

-both step up and step down transformers have a primary and secondary coil, joined together by an iron core

-the primary coil is connected to an AC supply, so a magnetic field is produced which constantly changes direction. this magnetic field stays within the iron core

- the changing magnetic field is felt by the secondary coil, and an alternating potential difference  is induced by electromagnetic induction as the coil cuts through the magnetic field.  this then produces an alternating current 

- the relative number of turns on the secondary and primary coils determines whether the transformer is step up or step down.

-if there are more secondary coils than primary coils its a step up, and if there are more primary then secondary its a step down

- they dont work with DC as the magnetic field produced wont change direction so electromagnetic induction wont happen in the secondary coil 

no electricity flows round the iron core, it is purely for transferring the changing magnetic field from the primary to the secondary coil 

you can calculate the output or input voltage from a transformer if you know one of the voltages and the number of turns on each coil 

PD ACROSS PRIMARY COIL / PD ACROSS SECONDARY COIL = NO OF TURNS ON P / NO OF TURNS ON S 

- power = current x potential difference. because transformers are basically 100% efficient, power in= power out. so:

- Ip x Vp= Is x Vs 

SWITCH MODE TRANSFORMERS:

- transformers that operate at higher frequencies than traditional transformers. (50KHz- 200KHz)

- because they work at higher frequencies they can be much smaller and lighter- phone charger

- much more efficient as very little power used if no load is applied