P3 Physics AQA 2012

Expect a question on Centre of Mass as AQA usually put one on.

The Eye was new in 2012, and eye structure was on 2013 paper. This year long/short sightedness or near vision?

Refractive index was new in 2012 but was not examined in 2013.

There has traditionally always been a ray drawing question. Last year it was a converging lens with object beyond 2f. In June 2012, it was a diverging lens.

A question on explaining transformers or another electromagnetic device – possible long answer.

Expect long/difficult calculations as there are few equations in P3. Remember workings and units.

What is likely to come up?

Absorbed strongly by bone or dense tissue. Can penetrate easily through low density tissue.

CCD’s can digitise image. CT scans Wavelength ~ size of atom (1x10-10m). Ionising radiation – Risks to health X-Ray therapy

X-Rays

X-ray radiographs

CCD’s

CT Scanners

CT Scans

Above 20KHz. For imaging, typically 1-5MHz.

Transducer sends out short pulses and detects echoes.

Reflected at a density boundary. S=d/t applies to the echo. A-scans and B-scans. Ultrasound therapy/kidney stones/cleaners.

Ultrasound

Ultrasound Imaging•Ultrasound - Sound waves (longitudinal) with frequencies greater than 20KHz.•Typical frequencies used in imaging 1 – 5 MHz•The transducer produces short pulses and acts as a receiver in between pulses.

Ultrasound Pulses

Building the ImageA-scan. The peaks correspond to reflection from surfaces inside the body. Taller peak represent stronger reflections.

B-Scan. The transducer is moved across the body and the return signals are stored electronically.

The signal strength controls the brightness/colour of each part of the image displayed on a screen. Blood vessels inside the eye

How is light reflected from a surface?

Angles of incidence, reflection and refraction are always measured between the ray and the normal.

The normal at a point on a mirror is perpendicular to the mirror.

For a light ray reflected by a mirror: the angle of incidence = the angle of

reflection. Real images are formed where rays of light cross

so they can be produced on a screen. Virtual images cannot be produced on a screen –

they are where rays of light appear to have come from.

Image is virtual, same size as object, same distance behind mirror as object is in front,Upright and laterally inverted (back to front).

What is refraction?

Refraction of light is the change of direction of a light ray when it crosses a boundary between two transparent substances.

If the speed is reduced, refraction is towards the normal (e.g. air to glass).

If the speed is increased, reflection is away from the normal (e.g. glass to air). SPL: Alfred Pasieka

n = Refractive index Snells Law – n1 sinθ1 = n2 sinθ2 n for air/free space ~1

If the ray is travelling from air into another medium, n1 = 1 and the expression becomes n2 = sin θ1

sinθ2

Refraction

http://www.google.co.uk/url?sa=i&rct=j&q=refraction+of+light+waves+in+glass&source=images&cd=&cad=rja&docid=pNB8kMKM35rEVM&tbnid=Xs02NwKtSa8i-M:&ved=0CAUQjRw&url=http://www.opticampus.com/cecourse.php?url=nature_light/&ei=d56KUY2bHLSV0QXi3YDQAw&bvm=bv.46226182,d.d2k&psig=AFQjCNFzyrOMuxmfFr7iAFwmdNUE33WTPw&ust=1368125387471356

The angle of refraction depends of the angle of incidence and the refractive index of the two media.

Air n ~ 1 Glass n ~ 1.5 Water n ~1.33

If n1 > n2 (from a more to less dense material), then the critical angle sin c = n2/n1.

If the angle of incidence is greater that c, then total internal reflection will occur.

TIR used in endoscopes/reflectors/optical fibres.

Total Internal Reflection

EndoscopesUses Total Internal ReflectionA bundle of fibres send the light into the dark cavityA bundle of fibres receive the reflected light into a cameraGlass fibres are flexible and transmit/ receive images near the speed of light

Use curved surfaces to refract light. Can be converging(convex) or diverging(concave). Have a focal point. The focal length is measured

from the lens to the focal point. Power = 1/focal length (units D) +ve for converging lens, -ve for diverging lens. Parallel rays imply the object is at optical infinity. Image is produced where rays converge or appear

to be diverging from.

Lenses

What are converging and diverging lenses?

A converging lens makes parallel rays of lightconverge to a focus. The point where they arefocused is the principal focus of the lens.

A diverging lens makesparallel rays of light diverge(spread out). The pointwhere the rays appear tocome from is the principalfocus of the lens.

Corbis V257 (NT)

Principal rays In parallel emerges through/from focal

point. Through the centre undeviated. In through/towards focal point emerges

parallel. Image can be real or virtual. Image can be upright or inverted Image can be magnified or reduced. Magnification = Image size/Object size.

Image formation

What type of images do the lenses form?

A real image is formed by a converging lens if the object is between its principal focus and infinity (magnified if between F and 2F, minified if beyond 2F).

A virtual image is always formed by a diverging lens, and by a converging lens if the object is between the principal focus and the lens.

How is a converging lens used in a camera?

A camera contains a converging lens that is used to form a real image of an object.

Principal rays for a converging lens

For an object outside the focal point, a real inverted image will be formed.

                                                                   

    

How is a converging lens used in a magnifying glass?

A magnifying glass is a converging lens that is used to form a virtual image of an object.

Photo: S. Meltzer/Photolink/Photodisc 24 (NT)

                                                                                                              

     

For an object inside the focal point, a virtual erect image will be formed.

Principal rays for a diverging lens

Concave lenses are used to correct short sight. They always produce upright virtual images

Retina, Lens, Cornea, Pupil/Iris, Ciliary Muscle, Suspensory Ligaments.

The cornea and lens cause convergence of rays on the retina.

Accomodation – Lens becomes more curved to focus on near object.

Far point is infinity and near point is ~25cm for ‘normal’ eye.

The eye is similar to a camera with the film/CCD representing the Retina.

The eye

Gross structureSclera – Tough inelastic outer coating.Choroid – Vascular layer – Retina needs a rich blood supply.Retina – Photosensitive layer.Cornea – Main light-focussing structure. contributes ~2/3 refractive power of the eye.

Crystalline lens – Allows the eye to accommodate – adjust so that it is focussed on near objects.Pupil – Like the aperture of a camera. Allows light to enter the eye. The Iris can dilate/constrict the pupil to adjust the size of the pupil.Optic Nerve – The region where the optic nerve leaves the retina.Fovea – Very highly concentration of cone cells at the centre of our visual field

Long sight – The eye is too short or not powerful enough. Rays converge behind the retina. A Converging lens is used to correct.

Short sight – The eye is too long or too powerful. Rays converge in front of retina. A Diverging lens is used to correct.

The focal length (and therefore power) of a lens is determined by the refractive index and the curvature of its surfaces.

Vision defects

Refractive power The normal eye has a power of around 60

Dioptres (D). ~2/3 of this power is provided by the

Cornea. The rest is provided by the lens.

Simple Refractive errors

Hyperopia (long-sightedness)

Myopia (short-sightedness)

Moment = Force x Perpendicular distance between pivot and line of action of force.

For a system in equilibrium, the total clockwise moment must equal the total anticlockwise moment.

Levers. Small force x large distance to produce a large force at small distance.

Moments

What is a moment?

A moment is the turning effect of a force. Moment = force perpendicular

distance from the pivot to the line of action of the force = F d

F is the force in newtons. d is the perpendicular distance from the

pivot in metres. The unit of a moment is newton metres

(Nm).

What can you say about the moments of the forces acting on an object in equilibrium (it isn’t turning)?

The sum of the anticlockwise moments about any point

=the sum of the clockwise moments

about that same point.

Higher

Using levers

Calculating moments is important when you use levers.

The weight is called the load. The force you apply to the

crowbar is the effort. The point about which the

crowbar turns is the pivot. Levers enable you to lift

heavy loads with little effort.

What is the centre of mass of an object? The centre of mass of an object is the point

where its mass may be thought to be concentrated.

When a suspended object is in equilibrium, its centre of mass is directly beneath the point of suspension.

The centre of mass of a symmetrical object is along the axis of symmetry.

Tilt or topple?

An object will tend to topple over if the line of action of its weight is outside its base so …

… bodies with a low centre of mass and a broad base are more stable than bodies with a high centre of mass and a narrow base.

You can increase the stability of an object by making its base wider and its centre of mass as low as possible.

Higher

THE PENDULUMThe period of a pendulum is the time for one complete swing.The period of the pendulum T(sec) increases as the length increases. It is unaffected by the mass of the bobThe best way to determine the period is to time 10 complete swings and divide by10.The frequency f of the swings = 1/ T and T = 1/ f

Pressure = Force/Area Liquids transmit pressure in all directions as

they are virtually incompressible. Hydraulic systems use cylinders with

different cross-sectional areas to turn a small force into a large one (same pressure in each cylinder).

Hydraulics

HYDRAULIC PRESSURE

PRESSURE IN A FLUID IS THE SAME ALL THROUGH

P = F1 = F2 A1 A2SO A SMALL FORCE ON A SMALL AREA GIVES A BIG FORCE ON A BIG AREA

The small piston of this hydraulic lift has a cross sectional area of 3 cm2 and the large piston has an area of 200 cm2. What force F must be applied to the small piston to lift a load of 15,000 N on the large piston? Show your working.

Pressure at large piston due to load = 15000 200 = 75N/cm2

The pressure at the small piston must also = 75N/cm2

For small piston, P =F/A, then F = PxA = 75x3 = 225N

Acceleration towards the centre of curvature as velocity is dependent upon direction.

The resultant force causing acceleration is called a centripetal force.

Centripetal force acts towards the centre of curvature.

Centripetal force required increases as mass and speed increase and as radius decreases.

How can a body moving at a steady speed be accelerating?

When it’s moving in a circle at constant speed!

The object accelerates continuously towards the centre of the circle.

The centripetal force on it increases as:– the mass of the object increases,– the speed of the object increases,– the radius of the circle decreases.

Circular Motion

The car is travelling at a steady speed but the direction is always changing – so the velocity is changing. Acceleration is the change of velocity per sec - so the car is accelerating. The friction between the tyres and the road provide the centrepetal force to keep the car travelling in a circular path.

When a current flows through a wire, there is a magnetic field around the wire (right hand screw rule).

A solenoid will have a N and S pole (Right hand screw rule) and a magnetic field around it that grows and shrinks as the current in the coil changes.

The Motor Effect uses Flemmings left hand rule.

If current and field are parallel, there is no force.

Electromagnetism

USES OF ELECTROMAGNETISM

Flemmings left hand rule

First finger - Force

seCond finger - Current

thuMb - Movement

The three fingers are mutually perpendicular.Point the relevant fingers in the direction of the magnetic field (North – South)And the electric current (+ to -)

WHICH WAY WILL THE WIRE MOVE?

MAG FIELD ALWAYS N TO SCURRENT ALWAYS + TO -

Catapult Field

Electric motor

The Motor Effect - TheoryA wire carrying an electrical current that is

inside a magnetic field experiences a force.Force Factors: A stronger magnetic field or more current

results in more force. The direction of the force is reversed if the

direction of the current or the magnetic field reverses.

Zero force is found if the wire is parallel to the magnetic field lines and is greatest when the current is perpendicular to the field.

If an electrical conductor cuts across lines of magnetic flux, a P.D. Is induced across its ends. The same is true if the magnet moves with respect to the conductor.

If the conductor is part of a complete circuit, an induced current will flow.

This is used in the transformer.

Induction

How does electromagnetic induction work?

When a wire cuts the lines of a magnetic field, a potential difference (p.d.) is induced in a wire.

If the wire is part of a complete circuit, the induced p.d. causes a current in the circuit.

The current is increased if the wire moves faster or a stronger magnet is used.

a.c. current needed to create a fluctuating magnetic field around the primary coil.

The core links to the secondary side and so the magnetic field is transferred there.

A P.D. Is induced across the secondary coil. Vp/Vs = Np/Ns Transformers can be step up or step down. Switch mode tranformers – High frequency

50KHz-200KHz. Lighter as they have ferrite cores.

Transformers

What is the transformer equation?

p.d. across primary, VP number of turns on primary, NP

p.d. across secondary, VS number of turns on secondary, NS=

Higher

SPL: R. Maisonneuve, Publiphoto Diffusion

=

A transformer is used to step a p.d. of 230 V down to 10 V. The secondary coil has 60 turns. How many turns are there in the primary coil?

Use the transformer equation: S

P

S

P

NN

VV

6010NP=

230

10230 60

=NP = 1380 turns

VP = 230 V, VS = 10 V, NS = 60 turns

Assumed to be 100% for the purposes of calculations.

Pin = Pout IpVp = IsVs

Transformer Efficiency

1. X-rays and Ultrasound are part of the Electromagnetic spectrum.

2. Total internal reflection can occur in any material.

3. Transformers use an iron core because iron is a good conductor of electricity.

4. In circular motion, as the radius increases, the centripetal force required increases.

5. An object moving at constant speed could be accelerating.

True or False

6. As the focal length of a lens decreases, its power increases.

7. A converging lens can produce a real image or a virtual image.

8. Force = Pressure / Area9. If you use the same size force at a greater

distance from the pivot, the moment will always be bigger.

10. An object will be unstable unless its centre of mass lies outside its base.

11. Long-sightedness can be solved by using a diverging lens.