Download P3 Revision 2016 File

Ultrasound and X rays- Imaging & Treating
Ultrasound – longitudinal sound wave
frequency above 20,000Hz . NON
IONISING
Ultrasound waves are partially
reflected when they meet a boundary
between two different media.
The time taken for the reflections to
reach a detector can be used to
determine how far away such a
boundary is.( Remember divide by 2 !)
d=sxt
Medical Applications
Pre – natal scanning,
removing kidney stones .
X ray – high frequency em wave with
a wavelength about same size as
diameter of atom. IONISING – this
can cause damage to DNA in cells
causing cancer/mutations
Absorbed by metal and bone but transmitted by
soft tissue, affect photographic film like light.
Operators protect themselves behind screens
that absorb x rays or limit exposure time.
CT Scans
Adv – high resolution images
image can be viewed from
any direction (3D)
Disad – mutate cells /kill cells
Dental X rays
Broken Bones – shadow pictures
Killing Cancer Cells
Film replaced by CCD’s means images
can be transmitted electronically
Light – Structure and Function of the Eye
Suspensory
ligament
Attach the cillary muscles to the lens
Retina
Light sensitive cells – light to electricity
Iris
Controls amount of light entering eye
Cillary
Muscles
Focus light at different distances by
changing the shape of the lens
Cornea
Focuses light
Lens
Focus light from Diff distances onto
retina
Accomodation is the ability of the lens
to change its shape to focus near and
distant objects.
The near point is approximately 25 cm
and the far point is infinity.
The film in a camera or the CCDs in a
digital camera is the equivalent of the
retina in the eye.
Lenses – The Refraction of Light
P= power of the lens measured in
dioptres (D)
f is the focal length in metres
A ray through the centre of the lens .
A ray through the focal point.
Put arrows on !
Is it real or virtual, magnified or smaller,
upright or inverted ?
Having materials with higher refractive
indices means that lenses can be
made thinner and lighter
Turning Forces – Balance and Stability
The centre of mass of an
object is that point at which
the mass of the object may
be thought to be
concentrated.
Wider base &
lower centre of
mass means more
stable
( less likely to
topple if pushed)
If freely suspended, an object will
come to rest with its centre of mass
directly below the point of
suspension.
If the line of action of the weight of an
object lies outside the base of the object
there will be a resultant moment and the
body will tend to topple.
Finding the Centre of Mass
Pierce the card in at least two places.
Suspend the card from one of these holes.
Hang a plumb-line from the point of
suspension.
Using the plumb-line as a reference draw
a vertical line on the card.
Repeat for the other hole(s).
The centre of gravity is where the lines
cross on the card.
T is in seconds
f is in Hertz
The time period just depends on the
length not the mass
Turning Forces – Moments
The turning effect of a force is called the moment.
M = moment in Nm or Ncm
F is force in Newtons
d is perpendicular distance in cm or m
If balanced the sum of the clockwise moments = sum of anticlockwise moments
Levers are force multipliers!
Forces – Hydraulics and Circular Motion
When an object moves in a circle it continuously
accelerates towards the centre of the circle.
This centripetal acceleration is because the
objects velocity is changing because its direction
is changing.
P = pressure in Nm-2 or Ncm-2 or Pa
Acceleration is the rate of change of velocity.
F is force in Newtons
A is X sectional area in cm-2 or m-2
Centripetal force
can be provided by:
Liquids are virtually incompressible,
and the pressure in a liquid is
transmitted equally in all directions.
•Friction
•Gravity
•Reaction force
•Tension
Pressure at A = 30/0.2 = 150 Nm-2
This is the same as the pressure at B
So F = P x A = 150 x 1 = 150N
It’s a force multiplier
The centripetal force ( resultant force)
needed to make an object perform circular
motion increases as:
■ the mass of the object increases
■ the speed of the object increases
■ the radius of the circle decreases.
Motor Effect Creating a Force
How it works
• Current through a wire
When current
changes
direction….
Applications
• Creates a magnetic field around it
• This interacts with the permanent
magnetic field
• Producing a force
A bigger force can be made by
having:
Stronger magnets
Larger current
• the current (in the coil) creates a
magnetic field (around the coil)
• so the magnetic field of the coil
interacts with the (permanent)
magnetic field of the magnets
Coils made from
copper because
low resistance
so does not get
as hot.
The Transformer Inducing a Voltage
Coils are insulated to
prevent short circuit.
Only works with ac
Soft iron
magnetises
and
demagnetises
easily
Step up
and step
down !
Switch mode use higher
frequency , less current
when no load
How it works
Calculations
• a.c current in primary coil
• Causes a changing magnetic
Put the numbers in first !
field in the core
• This induces an ac voltage in the
secondary
Power in = power out
Assuming 100% efficient
Electromagnetism Applications
The Relay – Used to switch on a more powerful circuit
•
•
•
Current flows in the electromagnet and creates a magnetic
field
The electromagnet attracts the iron armature
Which closes the switch gap
The Loudspeaker – A changing force to create sound
• the current flowing through the coil) creates a magnetic
field
• this magnetic field interacts with the perm mag field
• this produces a (resultant) force
• when the direction of the current changes, the direction of
the force changes
The Transformer– Changing voltage and current
• a.c current in primary coil
• Causes a changing magnetic field in the core
• This induces an ac voltage in the secondary
How Science Works
Conclusions
Write a statement that uses
what’s written on the graph axes or
At the top of a column in a table.
The higher the altitude the longer the
period of orbit.
Look to see if you can comment on
rate of change.
How could the data/Experiment be Improved ?
Increase range of ……..
Use a smaller interval in the ……..
Interpreting graphs
To describe the graph write a statement that
includes what is written on both the axes.
Include any comments about rate of change.
Identify any maximum or minimum and say
what value on the x axis they are .
If there are two lines then one mark will be
for comparing what they show.
How Science Works
A student used the apparatus
shown in the diagram to investigate
how the weight supported by an
electromagnet depends on:
•
the current, I, flowing through
the wire
•
the number of turns of wire, n,
wrapped around the iron core.
Conclusions
for all values of n weight supported
increases with increasing current
but not linearly
for a given current the weight
supported increases with increase in n
weight supported reaches a maximum
with n = 30 turns