Physics 3 Revision Notes

X-rays/Ultrasound:
X-rays are part of the electromagnetic spectrum
Ultrasound are sound waves
Ultrasound can shatter kidney stones
X-rays are ionising (knocks electrons out of orbit)
Ultrasound is any sound above 20,000 Hertz
Ultrasound works by timing reflection of the sound waves to work out
how far it has travelled
Muscle/skin will absorb ultrasound but will sill reflect some of it back
Ultrasound used instead of X-rays for baby looking because X-rays are damaging
X-rays used in CT scan to give 3d image
Nurses go behind lead to protect themselves from X-ray
Dentists may leave room during X-ray
X-rays need to have enough energy to pass through soft tissue but not bone
Focused X-rays can be used to kill cancer cells
Ultrasound can also be used to kill cancer cells by making them
vibrate so much they heat up and die. X-rays are better but are less
safe.
'Barium meal' are chemicals that you can swallow which will reflect
X-rays or ultrasound easier for soft tissue
Gel is put on body during an ultrasound because sounds travel easier
through solid/liquid than air

Lens diagrams:
• one line going through focal point (may be on other side but can be
duplicated the same distance)
• one line going through centre
Near point is closest point eye can focus on an object from, increases with age.
If two lines do not intercept it can be reversed
Virtual images if formed on left side
Real image if formed on right side
Diverging lens if there are arrows on diagram
Dotted line from focal point to point where other parallel line hits y
axis and then goes off in direction of arrow (typically top right)
Must convert focal length into metres
Focal length depends on:
Power of lens (curve)
Refractive index of the material
Medium that we put lens in (air,liquid)
Cornea: lets light in, protects it.
Laser surgery: modify cornea, reshapes it
Pupil: where light enters. Changes how much light is let in
Iris changes in size to control light
Ciliary muscles make lens thicker/thinner
Cameras/eyes have converging lenses, focus light onto a point, both
control how much light is let in
Refractive index: Sin i / Sin r
Lasers produce narrow highly focused beam of energy
Used for eye surgery, endoscope,

Centre of mass:
Hang it from two points (one at a time) and draw a line straight down,
meeting point is centre of mass

Pendulum:
Only length affects swing time
Timing 10 swings is better than 1 as reaction time impact is reduced
If line of action falls outside of the 'base' then there is a 'resultant moment' (it falls over)

Hydraulics:
Liquids used because virtually incompressible
Also because pressure in liquid is same everywhere
Digger/brake system uses hydraulic system
P=F/A (pressure force area)
F1/A1 = F2/A2
Force multipliers used

Transformers:
100% efficient (power in = power out)
Realistically are not 100% efficient, some energy lost as heat etc
Little transformers called switch mode transformer used in mobile phone chargers