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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.
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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
CT Scanners
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Above 20KHz. For imaging, typically 15MHz.
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 - 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.
A-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
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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).
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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
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n = Refractive index
Snells Law – n1 sinθ1 =
n for air/free space ~1
n2 sinθ2
If the ray is travelling from air into another
medium, n1 = 1 and the expression becomes
n2 = sin θ1
sinθ2
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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
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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.
Uses Total Internal
Reflection
A bundle of fibres
send the light into
the dark cavity
A bundle of fibres
receive the reflected
light into a camera
Glass fibres are
flexible and
transmit/ receive
images near the
speed of light
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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.
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A converging lens makes parallel rays of light
converge to a focus. The point where they
are
focused is the principal focus of the lens.
A diverging lens makes
parallel rays of light diverge
(spread out). The point
where the rays appear to
come from is the principal
focus of the lens.
Corbis V257 (NT)
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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.
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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.
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A camera contains a
converging lens that
is used to form a real
image of an object.
For an object outside the focal point, a real inverted image will be
formed.
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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.
Concave lenses are used to correct short sight. They
always produce upright virtual images
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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.
Sclera – 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
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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.
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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
Myopia (shortsightedness)
Hyperopia
(longsightedness)
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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.
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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).
Higher
The sum of the anticlockwise
moments about any point
=
the sum of the clockwise moments
about that same point.
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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.
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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.
Higher
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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.
The 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 bob
The 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
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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).
PRESSURE IN A FLUID IS THE SAME ALL THROUGH
P = F1 = F2
A1
A2
SO A SMALL FORCE ON A SMALL AREA GIVES A BIG FORCE ON A BIG AREA
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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
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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.
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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.
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.
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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.
The three fingers are mutually perpendicular.
Point the relevant fingers in the direction of the magnetic field (North –
South)
And the electric current (+ to -)
First finger - Force
thuMb Movement
seCond finger Current
MAG FIELD ALWAYS
N TO S
CURRENT ALWAYS + TO -
A 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.
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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.
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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.
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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.
What is the
transformer equation?
Higher
p.d. across primary, VP
number of turns on primary, NP
p.d. across secondary, VS = number of turns on secondary, NS
=
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:
VP NP
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VS NS
VP = 230 V, VS = 10 V, NS = 60 turns
230
10
NP
=
=
NP
60
230  60
10
= 1380 turns
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Assumed to be 100% for the purposes of
calculations.
Pin = Pout
IpVp = IsVs
1.
2.
3.
4.
5.
X-rays and Ultrasound are part of the
Electromagnetic spectrum.
Total internal reflection can occur in any
material.
Transformers use an iron core because iron
is a good conductor of electricity.
In circular motion, as the radius increases,
the centripetal force required increases.
An object moving at constant speed could
be accelerating.
6.
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10.
11.
As the focal length of a lens decreases, its
power increases.
A converging lens can produce a real image
or a virtual image.
Force = Pressure / Area
If you use the same size force at a greater
distance from the pivot, the moment will
always be bigger.
An object will be unstable unless its centre
of mass lies outside its base.
Long-sightedness can be solved by using a
diverging lens.