AQA Physics Unit 3 (P3)

Properties 

• High frequency electromagnetic waves 
• Short wavelength (same size as the diameter of an atom) 
• Affects photographic film, just like light waves
• Easily absorbed by metal and bone
• Transmits (passes through) healthy tissue e.g. muscles
• Charged Coupled Device (millions of identical pixels) producing high res. images
• Highly ionising (damages molecules) that can kill healthy cells

Uses

• CT Scans
  - high resolution images of soft and hard tissue
  - x-ray tube fires rays that are detected by detectors
  - rotate, forming 2d slices of the body
  - eventually produced 3d images  
• Treating Cancer
  - wide beam focuses on tumour  
  Precautions - Radiographers wear lead aprons, leave the room and have shield areas 

Properties
- High frequency (>20,000Hz)
- Non-ionising radiation, longitudinal waves  

How it works
- waves are partially reflected at a boundary between material 
- when waves pass from one media to another, some is reflected and transmitted 
- allows us to point the pulse of ultrasound (where the boundary hits) 
- time taken for the reflection to reach the detector is how far the boundary is 
- this process is called ultrasound imaging 

Finding boundaries
- using oscilloscopes
- distance between interfaces in various media > S = V x T  

Uses
1. Breaking down kidney stones into sand like particles by concentrating high conc. waves
2. Prenatal scanning of a fetus when they read a boundary between diff media, some waves are reflected back. The time and distribution produces a video image. 

Refractive Index
- Refraction is when waves enter a different medium causing them to change speed
- Every transparent material has a refractive index
- Speed of light in a vaccum : Speed of light in that medium
        sin(i)
  n = -----
        sin(r)  
Convex Lenses (Converging) 
- Cause parallel rays of light to converge (move together) at the principal focus
- Used for magnifying glasses
- The distance the lens is from the object affects the image:
Further away = real, inverted, same size
Between = real, inverted, magnified 
Closer =  virtual, upright, magnified 
- Virtual images are diverging rays, so the light from the obj. appears to be coming from a diff place
- Real images is light from an object coming together to form an object on a screen 
Concave Lenses (Diverging) 
All images produced by concave lenses are virtual, upright and diminished 

Structure of the Eye
Retina (light energy -> electrical energy - impulses go to the brain) 
Lens (focuses light onto retina) 
Cornea (refracts the light) 
Pupil (the hole that lets lights enter in) 
Iris (controls the size of the pupil hole)
Cilary Muscle (controls the shape of the lens) 
Suspensory Ligaments (joins the muscle to the lens)
Accomodation (involving the cornea and lens) 
Cilary muscles relax, lens is thin, lens has low power
Cilary muscles contract, lens if fat, lens has high power 
Correcting Vision
Short Sight: Eyeball is too long, cornea+lens is too powerful (use concave) cant focus on distant objects, focuses too soon 
Long Sight: Eyeball is too short, cornea+lens is too weak (use convex) cant focus on near objects, focuses too slow 
Range of Vision
Far point: furthest distance the eye can focus on comfortably (infinity) 
Near point: closest distance the eye can focus on comfortably (25cm)  

Similarities with Cameras
1. image on the film is a real image
2. dimished image because the image is further away than the focal length of the lens
3. inverted image 
- the film in the camera act as the retina as the images produced are real & inverted 

Magnification and Power
- the focal length is related to the power of the lens 
- the more powerful a lens it, the more strongly it converges so the shorter the focal length
- the focal length is determined by 1. Refractive Index 2. Curvature of the lens 
- the more curved the surfaces, the more powerful the lens is 
- But, this means powerful lenses can be made thinner by usings mat. with high refractive     indexes 
- Converging lens = positive
- Diverging lens = negative  

Magnification = Image Height/Object Height

Power of Lens = 1 / Focal Length 

Total Internal Reflection 
- Visible light is sent down optical fibres using total internal relection
- The waves bounce off (reflect) in the inner core of glass and plastic
- But this can only work when waves travel from dense to less dense substances
- Total internal reflection can only happen when...
- The angle of incidence is BIGGER than the critical angle

Refractive Index = 1/SinC

Uses of Light
-Endoscope: use bundles of optical fibres carrying light back and forth
-Lasers: to cauterise, burn or cut objects  

Facts
- Centre of mass is the point where its mass is thought to be concentrated
- If freely suspended, the object will rest at its centre of mass directly below the point of suspension

How to find the centre of mass
Symmetrical objects: Follow the axis of symmetry of the object e.g. a square
Asymmetrical objects: hang the laminar and plumbline from a pivot point and draw all the lines from various points of the object to see where all the lines meet 

Pendulums 
- The longer the pendulum, the longer the time taken for one complete oscillation
- Time period = 1/Frequency  

Moments
- a moment is a turning effect of a force
- the size of a moment is measured by m=f x d (Nm, N, m)
- Principle of moments: an object at equilibrium (not moving)...  
- Sum of all the clockwise moments at any point = sum of all the anticlockwise moments    at any point
Balanced Moments & Levers 
- Levers use the idea of balanced moments
- Levers are force multipliers (they increase the distance which reduces the amount needed to get the same moment) 
- Levers = increase distance from pivot, when forces are applied, less force is needed     to get the same moment
- Force magnifiers: effort force moves further than the load force (effort can be smaller) 
- Distance magnifiers: load force moved further than the effort force (effort has to be     bigger)
Stability
- Stable objects = wide base, low centre of mass
- If the line of action of the weight lies outside the base, there will be a resulting moment   and the object will topple   

Liquids
- All liquids are virtually incompressible
- Pressure in a liquid is transmitted equally in all directions
- Force exerted at one point is transmitted to other parts of the liquid

Use of Hydraulics
- hydraulic systems are force multipliers
- car breaking systems and car jacks

Calculations
Pressure = Force/ Cross sectional Area
Pressure: Pa
Force: N
Cross sectional area: m2 

Circular Motion 
- velocity takes into account both speed and direction of an object
- circular motion is the constant change in direction causing the object to accelerate    towards the centre of the circle  
- the resultant force that keeps something moving in a circle is called the centripetal force

Types of Centripetal Force
1. Tension - bucket whirling round a rope 
2. Friction - cars going round bends
3. Gravity - moon and the earth 

Centripetal Force depends on...
1. Mass = larger mass, the bigger the centripetal force
2. Speed = the faster the object is moving, the bigger the centripetal force
3. Radius = a large force needs to keep something in small circles (smaller radius) 

Magnetic Field
A region where magnetic materials e.g. iron/steel and wire carrying currents experience forces acting on them
- the arrows on field lines always point from north to south
- magnetic field inside a coil of wire is strong and uniform
- magnetic field outside the coild is like a bar magnet (weak)
- increase the strength = add a magnetically soft iron core through the middle of the coil
 
Use of electromagnets
- Electromagnets can magnify and demagnify easily when they are turned on and off
- They are used in cranes (made of iron and steel lifters) at scrap yards/steel works

Motor Effect
-if the direction is reversed, the direction of the force is reversed too
-to experience the full force, the wire must be at 90º to the magnetic field
-if the wire is parallel to the magnetic field, there will be no experience of force 

Flemings Left Hand Rule (thumb= motion, first finger= field, middle finger= current) 
Right Hand Thumb Rule (thumb= current, fingers= magnetic field) 

Types of Transformers
- Use electromagnetic induction to change pd. They only work on AC. 
- Both have the primary and secondary, joined with an iron core
- Step-Up transformer: increase voltage, more turns on the secondary coil
- Step-Down transformer: decrease voltage, more turns on the primary coil
 
How they work
1. Primary coil is connected to an AC supply
2. AC passes through the primary coil wrapped around a soft iron core
3. The changing current produces a changing magnetic field
4. This induces (moves) the voltage to the secondary coil 
5. This induces the alternating current in the circuit connected to the secondary coil

Facts
- No electrical connection between primary and secondary coil 
- As the current in the primary increases/decreses steadily, there is a constant voltage induced in the secondary coil 
- As the voltage in the primary reaches its highest strength, the secondary reaches its weakest 
 

Caclulation

- We can calculate the output potential difference from a transformer if we know the input

Pd across primary coil              Number of turns on primary coil
---------------------------       =  -----------------------------------------
Pd across secondary coil          Number of turns on secondary coil 

Uses of Switch Mode Transformers

Switch mode transformers operate at higher frequencies than normal 
Operating between 50kHz - 200kHz
Can be made smaller and lighter, working on a high frequency supply
Useful as phone chargers, power supplies e.g. laptops
Efficient - use little power but no load is applied