Physics P3 revision check list
Mar2013
Page numbers refer to your text book. Look them up!
Section numbers refer to the AQA specification and do not match text book chapters.
P3.1 Medical applications of Physics
Physics has many applications in the field of medicine. These include the uses
of X-rays and ultrasound for both scanning and therapy, and of light for image
formation with lenses and endoscopes.
P3.1.1 X-rays
X-rays are part of the electromagnetic spectrum (P94,P96) They have a very
short wavelength and cause ionisation (P185).
They affect a photographic film in the same way as light.
They are absorbed by metal and bone.
They are transmitted by healthy tissue.
A contrast medium, such as barium compounds, may be used to make
particular soft tissue ‘visible’.
The wavelength of X-rays is of the same order of magnitude as the diameter of
an atom.
X-ray radiographs can be used to diagnose some medical conditions, for
example bone fractures and dental problems. Bones absorb X-rays, so appear
light on the negative image (X-rays make photographic film go dark).
Teeth absorb X-rays so appear light. Any cavities appear dark.
Lead plates are used to absorb X-rays to prevent them affecting other parts of
the body.
Film has been replaced in many modern applications by CCDs (chargecoupled devices) similar to those in digital cameras. These send electronic
signals to a computer which displays an image.
A CT scan (by computer tomography scanner) is a three-dimensional
computer generated image produced from multiple X-ray images.
Learn Fig 3 on P209. The X-ray tube moves around the patient. X-rays are
detected by a ring of detectors, which send electronic signals to a computer.
The detector signal depends on the different types of tissue along the X-ray
path, and on how the X-rays pass through each type of tissue.
The CT scanner can distinguish between different types of soft tissue, and can
produce a three-dimensional image, but it gives a much higher radiation dose to
the patient and is very expensive. P209 table 1.
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X-rays are used in radio-therapy to destroy cancerous tumours near the
surface of the body. This treatment uses X-rays of shorter wavelength than
those used for imaging.
X-rays are a dangerous form of ionising radiation. High doses kill living cells.
Low doses can cause cell mutation and cancer. There is no safe limit.
Workers who use X-ray equipment wear badges to tell how much radiation they
have been exposed to. They keep clear of the beam and may wear lead aprons
for protection.
P3.1.2 Ultrasound
The range of human hearing is about 20 Hz to 20,000 Hz.
Ultrasound waves have a frequency higher than the upper limit of hearing for
humans.
Ultrasound is used in prenatal scans of a baby in the womb. P210.
Ultrasound waves from a transducer at the body surface 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.
To calculate the distance between interfaces in various media use the equation
s= v x t
P211.
s is the distance travelled in metres, m
v is the wave speed in metres per second, m/s
t is the transit time in seconds, s
This equation will give twice the distance between a surface and an internal
boundary since the wave goes in and back to the transducer. So halve it to find
the depth.
Data may be taken from diagrams of oscilloscope traces, to find the time
taken. See P211 for examples, and oscilloscope time base P165.
Another example of an application is to find the length of an eyeball, called an
A-scan.
Ultrasound is not ionising, so it is safer than X-rays.
Ultrasound therapy can be used to remove kidney stones by breaking them up.
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P3.1.3 Lenses
Refraction is the change of direction of light as it passes from one medium to
another.
For a ray of light going from air into a medium, the angle of refraction is
always less than the angle of incidence (always measured from the normal)
refractive index, n = sin i / sin r
Snell’s Law P212
i is the angle of incidence
r is the angle of refraction
Make sure you can do sines on your calculator, which should be set to degrees
of angle.
When light goes from glass to air, the sine of the angle in air is n times the
sine of the angle in glass.
If the angle of incidence at the glass-air boundary is above the critical angle,
the ray is entirely reflected. This is called total internal reflection.
Refractive index ,n = 1 / sin c, critical angle
P214
This effect causes light to stay in optical fibres. A medical application is the
endoscope, for looking inside patients. One bundle of fibres takes light in for
illumination, another brings image light to an eyepiece or a digital camera.
Laser light may be sent down optical fibres to perform surgery, for example
cutting or cauterising. Also for eye surgery.
Lenses P216
A lens forms an image by refracting light.
In a convex or converging lens, parallel rays of light are brought to a focus at
the principal focus, marked F.
In a concave or diverging lens, parallel rays spread out. The point where they
appear to come from is the principal focus.
For both types of lens, the distance from the lens to the principal focus is called
the focal length, f
The nature of an image is defined by its size relative to the object (enlarged or
smaller) whether it is upright or inverted relative to the object and whether it is
real or virtual.
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A real image is formed if light from the object is focussed at an image. A virtual
image is constructed where diverging light appears to have come from.
The magnification produced by a lens is calculated using the equation:
magnification = image height / object height
You need to learn the nature of the image produced by a converging lens for an
object placed at different distances from the lens, ie for a camera, projector
and magnifying glass. P217...
Also the nature of the image produced by a concave or diverging lens. P219
Fig4. It is virtual.
You must be able to complete the construction of ray diagrams to show the
formation of images by converging and diverging lenses.
The power of a lens is given by:
P = 1/ f
P is power in dioptres, D
f is focal length in metres, m
The power of a converging lens is positive and the power of a diverging lens
is negative.
The focal length of a lens is determined by:
■ the refractive index of the material from which the lens is made, and
■ the curvature of the two surfaces of the lens.
For a given focal length, the greater the refractive index, the flatter the lens.
This means that the lens can be manufactured thinner.
P3.1.4 The Eye
You need to know the structure of the eye, and the function of each part. Learn
to label Fig1 on P220.
Light enters the eye through the cornea, which acts as a lens and a protection
layer.
The eye lens focuses light onto the retina.
The ciliary muscles change the thickness of the eye lens, and hence its focal
length, which allows the light to be focussed from varying distances. They are
attached to the lens by suspensory ligaments
The amount of light entering the eye is controlled by the iris, which changes the
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size of the pupil.
The normal human eye has a range of vision from a near point at 25cm to a
far point at infinity. Light from an object in this range can be focussed on the
retina.
Vision defects can be corrected using convex and concave lenses to produce
an image on the retina.
Short sight is when the eye focuses an image in front of the retina, called an
uncorrected image. It can be caused by the eyeball being too long, or the eye
lens being unable to focus.
Short sight is corrected using a diverging lens. P222 Fig1.
Long sight is when the uncorrected image is formed behind the retina. It may be
caused by the eyeball being too short, or the eye lens being unable to focus.
Long sight is corrected using a converging lens. P222 Fig2.
The eye may be compared to a camera. The film in a camera, or the CCD in a
digital camera, is the equivalent of the retina in the eye. P223.
P3.2 Using Physics to make things work
P3.2.1 Centre of mass
The centre of mass of an object is that point at which the mass of the object
may be thought to be concentrated.
If freely suspended, an object will come to rest with its centre of mass directly
below the point of suspension, for example plumb line or hanging basket.
You can find the centre of mass of a thin irregular sheet of material by hanging
it so that it is free to rotate, and drawing a vertical line using a plumb line.
Twice! P229 Fig.4
The centre of mass of a symmetrical object is along the axis of symmetry. P229
Fig.3.
Pendulum
For a simple pendulum, such as a plumb bob on a light line, its period of
oscillation increases with the length of line.
The time period for one complete oscillation is related to the frequency of
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oscillation by the equation:
T = 1/f
T is periodic time in seconds, s
f is frequency in hertz, Hz
Childrens’ playground swings and some fairground rides are pendulums.
P3.2.2 Moments
The turning effect of a force is called the moment.
The size of the moment is given by the equation:
M=Fxd
M is the moment of the force in newton-metres, Nm
F is the force in newtons, N
d is the perpendicular distance from the line of action of the force to the pivot
in metres, m
If an object is in equilibrium, ie not turning, the total clockwise moment must
be exactly balanced by the total anticlockwise moment about any pivot.
For a see-saw the forces are the weights of the persons. If in balance:
weight1 x distance1 = weight2 x distance2
(distances from the pivot)
A beam can be considered to have it’s centre of mass at it’s midpoint, and it’s
weight acts through that point. This can be used to find its mass. P231 Fig.3
A lever can be used to multiply a force. Examples P231 Fig.4
Stability
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.
If the line of action of the weight is inside the base, there will be a restoring
moment causing the disturbed object to fall back.
To make a system more stable, you should widen the base and lower the centre
of mass. Examples include the farm tractor and the child’s high chair.
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P3.2.3 Hydraulics
Pressure is defined as force per unit area, unit Pascal.
P = F/A
P is the pressure in pascals, Pa (N/m2)
F is the force in newtons, N
A is the area in metres squared, m2
Pressure of a load on the ground is reduced by a large footprint.
The same equation is used for liquids under pressure.
Liquids are virtually incompressible, and the hydraulic pressure in a liquid acts
equally in all directions.
A force exerted at one point on a liquid will be transmitted to other points in the
liquid.
The use of different cross-sectional areas on the effort and load side of a
hydraulic system enables the system to be used as a force multiplier.
For a hydraulic system with two pistons the pressure is the same for both so
Force1 / Area1 = Force2 / Area2
P235
A car jack uses a force applied to a small area piston to apply a large pressure
to a much bigger piston.
P3.2.4 Circular motion
When an object moves in a circle it continuously accelerates towards the
centre of the circle. This centripetal acceleration changes the direction of
motion of the body, not its speed.
The resultant force causing this acceleration is called the centripetal force
and is always directed towards the centre of the circle.
The centripetal force is provided by string tension for a ball on a string, by
gravity for a satellite in orbit around the Earth, and by friction on the road for a
car going round a corner.
At the top of a vertical fairground wheel the rider experiences a downward force
from gravity and from the wheel to keep him moving round in a circle.
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The centripetal force needed to make an object perform circular motion
increases as:
1. the mass of the object increases
2. the speed of the object increases
3. the radius of the circle decreases.
P3.3 Keeping things moving
Magnets
Magnets have two magnetic poles, North and South.
Like poles repel, unlike poles attract.
Lines of force or magnetic field lines go from North to South. They show the
direction of force on a North pole.
Electromagnets
An electromagnet is made of insulated wire on an iron core. Steel is
unsuitable for this as it keeps its magnetism when the current is turned off.
The strength of an electromagnet is increased by
1. increasing the current
2. increasing the number of turns
3. an iron core
Applications of electromagnets include their use on cranes for lifting iron and
steel. P242
You should learn the examples of electromagnets used in circuit breakers,
electric bells and relays. P243
P3.3.1 The Motor Effect
When a current flows through a wire a magnetic field is produced around the
wire.
If this wire is in a magnetic field there is a force produced which is at right
angles to the wire and to the field lines. P244 Fig.1
The size of the force can be increased by:
1. increasing the strength of the magnetic field
2. increasing the size of the current.
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The conductor will not experience a force if it is parallel to the magnetic field.
The direction of the force is reversed if either the direction of the current or the
direction of the magnetic field is reversed.
The direction of the force is found using Fleming’s left-hand rule. P244 Fig.2
First finger : magnetic Field
seCond finger : electric Current
thuMb : Motion of force
The motor effect is used to drive loudspeakers, and of course motors!
The DC (direct current) electric motor has a flat coil of wire mounted on a
spindle in a magnetic field. Because the current goes down one side of the coil
and back up the other, each side has an opposite direction of force. This
produces a moment which turns the motor.
A split-ring commutator is required to reverse the direction of flow through the
coil every half turn of the motor. P245 Fig.3
P3.3.2 Transformers
Electromagnetic induction
If an electrical conductor ‘cuts’ through a magnetic field, a potential
difference is induced across the ends of the conductor. If the wire is part of a
complete circuit then a current will flow. P246 Fig.2
If a magnet is moved into a coil of wire a potential difference is induced across
the ends of the coil. P246 Fig.3
In a transformer, an alternating current in the primary coil produces a
changing magnetic field in the iron core and hence in the secondary coil. This
induces an alternating potential difference across the ends of the secondary
coil. P248 Fig.2
In a step-up transformer the potential difference across the secondary coil is
greater than the potential difference across the primary coil.
In a step-down transformer the potential difference across the secondary coil
is less than the potential difference across the primary coil.
The potential differences across the primary and secondary coils of a
transformer are related by the transformer equation:
Vp / Vs = np / ns
P250
Vp is the potential difference across the primary coil in volts, V
Vs is the potential difference across the secondary coil in volts, V
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np is the number of turns on the primary coil
ns is the number of turns on the secondary coil
If transformers are assumed to be 100% efficient, the electrical power output
would equal the electrical power input.
Vp x Ip = Vs x Is
P251
Vp is the potential difference across the primary coil in volts, V
Ip is the current in the primary coil in amperes (amps), A
Vs is the potential difference across the secondary coil in volts, V
Is is the current in the secondary coil in amperes (amps), A
Because transformers are not 100% efficient, in reality if a given output power
and PD are required then the transformer will be built with more turns on the
secondary than given by the ideal equations.
In the National Grid for electrical power distribution, transformers are used to
change the PD to high voltages for transmission, typically 132,000V, and to
change it to 230 V for supply to homes.
You must be able to identify the transformers in P250 Fig.2
The greater the transmission PD the lower the current so the less energy lost in
heating the wires.
Switch mode transformer
Switch mode transformers are a recent innovation, commonly found in mobile
phone and laptop chargers. P249
They operate at a high frequency than 50Hz mains, often between 50 kHz and
200 kHz.
They have a ferrite core and are much lighter and smaller than traditional
transformers .
They use very little power when they are switched on but no load is applied (ie if
you leave them plugged in when not charging the phone).
The block diagram for their operation is on P249 Fig.4
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