Physics Module 3

A brief sumamry of all of Physics Module 3 using AQA Physics GCSE Revision Guide.

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Physics Module 3

Medical Applications of Physics - 1.1 X-Rays

X-Ray Properties - 

  • Affect photographic film in the same way as light
  • They are absorbed by metal and bone
  • They are transmitted by healthy tissue
  • Part of the electromagnetic specturm. High freq + short wl. 
  • Used to form images of bones on photographic paper in medicine and dentistry

CCDs and CT Scanners

Charge-coupled devices can be used to form electronic images of x-rays. CT Scanners use x-rays to produce digital images of a cross-section of the body. Some organs with osft tissue (intestine) can be filled with a contrast medium so that they show up on an x-ray. 


x-rays cause ionisation and damage living tissue. This means cautions must be taken (workers wear film badges, use lead screens for protection etc.). X-rays used can be used to treat cancer. Therefore not only can they cause Cancerous tumours they can cure them aswell. 

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Physics Module 3

Medical Uses of Physics - 1.2 Ultrasound

Ultrasound Waves - Waves of a higher frrequency than the human hearing capabilities (20-20,000                                Hz).

We can use electronic system to produce ultrasound waves to calculate how far away a boundary between two mediums is. The elecctronic system produces the ultrasound wave. When it hits the medium boundary part of teh wave is reflected back and eventually reaches a detector. The following calculation is used to calculate the distance. 

s = v x t

Distance (m) = Speed (m/s) x Time (s)

 Because it is a return ray, the ultrasound waves has travelled twice the distance. 

ltrasoudn is non-ionising and can be used in medicine for scanning. It can be used in pre-natal scanning, to shatter Kidney stones or to scan soft tissue (e.g. the eye). 

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Physics Module 3

Medical Uses of Physics - 1.3 Refractive Index

Refraction - The change of direction of light as it passes from one transparent medium to another. It occurs because the change in material causes a change in speed and therefore direction unless along a normal. 

Refractive Index - A measure of how much the substance can refract a light ray.


n = sin i 

     sin r

Refractive Index = Sine of Angle of Incidence

                               Sine of Angle of Refraction 

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Physics Module 3

Medical Uses of Physics - 1.4 The Endoscope

Critical Angle - Where in a lgiht ray passing from glass to air, the angle of incidence is so great                           that the refracted ray is along the boundary between the two mediums. 

Total Internal Refraction - Light passing from glass to air. Where the angle of incidence is so grerat                                      the whole ray is refracted back inside the glass rather than passing out                                        into the air. Greater angle of incidence than the critical angle. Here the                                        Angle of Incidence = Angle of Refraction. 

Refractive Index (n) =            1

                                                 sin critical angle

The Endoscope

  • Used to look inside a patients body without cutting it up. 
  • Contains optical fibres (thin glass fibres). Visible light can be passed down these fibres via total internal refraction. 
  • Laser light can be used to carry out operations in the body via an endoscope. (replaces light rays and can be used to cut, cauterise or burn). The colour of laser is matched to the tissues for maximum absorption. For example, laser surgery on the eye - passes through the cornea and absorbed on the retina. 
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Physics Module 3

Medical Uses of Physics - 1.5 Lenses Part

Converging (Convex) Lens

  • Parallel rays of light through this light are refracted so that they converge at a Principal Focus. Focal length is the distance form the lens to the Principal Focus. 
  • There is a Principal Focus on either side of the lens.  
  • If object is further than the P.F. then an inverted real image is formed. *
  • *Size of object depend on location of it to the eye. Nearer to lens, the larger it appears. 
  • If object is nearer than the P.F. then an upright, virtual image is formed. This image is magnified. Magnification can be calculated via

Magnification = Image Height

                       Object Height

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Physics Module 3

Medical Uses of Physics - 1.5 Lenses

Divergine (Concave) Lens

  • Light that passes through this lens is refracted away from a point - the Principal Focus. 
  • Focal length equals the length from the lens to the P.F
  • Principal Focus on either side.
  • The iamge produced by  a divergin (concave) lens is always virtual. 


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Physics Module 3

Medical Uses of Physics - 1.6 Using Lenses

We draw Ray diagrams to find the iamge that different lenses produces in different positions. 

You must - 

  • Include a Prinicpal Axis - This is a perpendicular line to the lens and it passes through the lens centre. 
  • Use three construction rays.

These construction rays must - 

  • One that travels parallel to the Principal Axis then to the principal focus through the lens
  • One that travels through the Principal Focus before travelling parallel to the Principal Axis
  • One that travels in one straight line through the point where the Prinicpal Axis and the lens cross. 

Where these all meet is where the object starts. 

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Physics Module 3

Medical Applications of Physics - 1.7 The Eye

  • Cornea - Transparent layer that helps to protect eye and to focus light onto the retina
  • Iris - Coloured ring of muscle that controls the amount of light entering the eye.
  • Pupil - The central hole formed by the iris. Here light enters the eye. 
  • Cilliary Muscles - Attatched to the lens by suspensory ligaments. The muscles change the thickness of the eye lens. 
  • Eye lens - Focuses light onto the retina
  • Retina - the light sensitive cells around the inside of the eye.
  • Blind spot - region where the retina is not sensitive to light (i.e. no light-sensitive cells present)
  • Optic Nerve - Carries nerve impulses from the retina to the brain. 

Human Eye = 'Near point' - 25cm and 'Far Point' - Infinity 


P (Power of lens in dioptres, d) = 1

                                                                          f (focal length in m)

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Physics Module 3

Medical Applications of Physics - 1.8 More About The Eye

Short Sight

  • Caused by  eyeball being too long or eye lens being too powerful.
  • Close objects clearly, distant objects blurred. 
  • Uncorrected image is formed in front of the retina.
  • Corrected using a diverging lens. 

Long Sight 

  • Caused by the eyeball being too small or the eye lens being too weak. 
  • Close objects blurry, distant objects clear
  • Uncorrected image is formed behind the retina
  • Corrected using a converging lens.

Focal length of a lens is determined by -

  • 1) The 'Refractive Index'  of the material from which it is made.
  • 2) The Curvature of the two surfaces of the lens.


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Physics Module 3

Using Physics to Make Things Work - 2.1 Moments

Turning Effect = Moment. They are exactly the same thing


M (Moment in Newton-metres, Nm) = F (Force in Newtons, N) x d (Perp. distance from the                                                                                                      line of action of the force to                                                                                          the pivot in metres, m)

How to Increase the Moment -

  • One must either increase the force
  • Or one must simply increase the distance to the pivot 


We use these to make life (no just jobs really) easier. When in use, the force we are trying to move is called the 'load' and the force applied to hte lever is the 'effort'. Trip down D.T. memory lane. A lever acts as a force multiplier so the effort we apply can be much less than the load. 

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Physics Module 3

Using Physics To Make Things Work - 2.2 Centre of Mass

Centre of Mass - The concentration of an objects mass on a single point. 

Equilibrium - When a freely suspended object comes to rest at its 'Centre of Mass' (COM). This                         is below the point of suspension. 

DIY - How to find out an objects COM (learn diagram on p85)-

  • Suspend the object (in this case a thin sheet) from a pin held in a clamp stand. Because it is freely suspended, it is able to turn.
  • When it comes to rest, hang a plumbline from the same pin. 
  • Mark the position of the plumbline against the sheet
  • Hang the sheet with the pin at another point and repeat the procedure
  • The Centre of Mass is where the lines the marked the position of the plumbline cross. 

COM position depends on the objects shape - sometimes its even outside of the object. For symmetrical objects its where the lines of symmetry cross. 

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Physics Module 3

Using Physics To Make Things Work - 2.3 Moments in Balance


This means we can take the Moments about any point and will find that the total clockwise moment and the total anticlockwise moment are equal e.g. see saws, balance scales. 

To calculate the force needed to stop an object turning we use the following equation - 

Sum of the Anticlockwise moments at any point = The sum of the the clockwise moments at that               point

Pretty much it. 

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Physics Module 3

Using Physics To Make Things Work - 2.4 Stability

   Line of Action of the weight of an object acts through its COM. If this ends up outside the object there will be a 'resultant moment' and the object will tend to topple over. Its basically a dotted line that goes from the COM perpendicularly down to the ground the object is on. So if the object (square in this example) was on one of its points when the LoA made it to the ground the square was on it would be outside the object which means the object would fall. If it were real. 

How to Increase Stability - 

  • Lower the COM of an object 
  • Wider the base of an object

An object will fall over if the resultant moment about its point of turning is not zero. 

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Physics Module 3

Using Physics To Make Things Work - 2.5 Hydraulics


P (Pressure in Pascals, Pa) = F (Force in Newtons, N)

           A (Cross-sectional area at right angles to the direction of the force in m)

Pressure in Liquids

  • Liquids are virtually incompressible
  • Pressure in a liquid is transmitted in all directions equally.
  • This means a force exerted in a liquid at a point will be transmitted to other points in the liquid. 

Hydraulic Pressure Systems

The force exerted by the system depends on - the force exerted on the system, the area of the cyclinder in which this force acts on, the area of the cylinder that exerts the force. Hydraulic systems can be used as a force multiplier due to different load and effort sized cylinders on either side of the system. So this means a small effort can be used to move a big load. 

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Physics Module 3

Using Physics To Make Things Work - 2.6 Circular Motion

When an object is travelling in a circular motion and constantly changing velocity it is accelerating. This acceleration is called 'Centripetal Acceleration'. 

The force that causes Centripetal Acceleration is called the 'Centripetal Force'. It always acts towards the centre of the circle. If this force stops acting the object will carry on in a straight line. 

How to Make The Centripetal Force Needed Increase - 

  • Increase the mass of teh object
  • Increase the speed of the object 
  • Decrease the radius of the circle. 

Centripetal Force is not a force in its own right; it is always provided by another force e.g gravitational force, electric force, tension. You may need to identify this other force in questions. 

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Physics Module 3

Using Physics To Make Things Work - 2.7 The Pendulum

Oscillating Motion - an object that moves back and forth along a line e.g. a pendulum.

Simple Pendulum - Consists of a mass on a string. When the mass is displaced to one side the                                pendulum oscillates through the equilibrium position.

Amplitude - The distance from the equilibrium to the highest position on either side. 

Time Period - The time for one complete oscillation cycle. It depends on the length of the pendulum. The frequency of the oscillations is the number of complete cycles of oscillations per second. They are related by the equation - 



Oscillations may come to an end due to transferrence of energy via friction at the fixed point of the string. 

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Physics Module 3

Using Magnetic Fields To Keep Things Moving - 3.1 Electromagnets


  • The ends are magnetic poles (North and South). The region surroundign the magnet  of which objects may be attracted is called the Magnetic Field. Field lines show the direction of the magnetism. 
  • Like poles attract. Opposites repel.


  •  When a wire is live there is a magnetic field produced around the wire. 
  • An electromagnet is made by wrapping insulating wire around a metal core - e.g. iron. When a current flows through the iron it becomes strongly magnetised. This is lost when the current switches off. This temporary magnetism makes electrommagnets very useful. 
  • They are ussed in scrapyard cranes, circuit breakers, electric bells and relays. 
  • See diagram (p91).
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Physics Module 3

Using Magnetic Fields To Keep Things Moving - 3.2 The Motor Effect

The Motor Effect - The force experienced when we palce a wire carrying a current in a magnetic                            field. The force is a maximum if the wire is at an angl of 90 degree angle to                             the magnetic field and zero if the wire is parallel to the magentic field. 

Flemings Left Hand Rule - look at diagram on p92.

 To increase size of force - Increase strength of magnetic field, Increase the size of the current.

The direction of the force on the wire is reversed if either the direction of teh current or the direction of the magnetic field is reversed. 

Electric Motor

The speed of the motor is increased by increasing the size of the current. The direction of the motor can be reversedby reversing the direction of the current. The coil spins (when a current goes through it) because a force acts on each side of the coil due to the motor effect and the force on one side of the coile is in the opposite direction tto the force on the other side. The split-ring commutator reverses the direction of the current around the coil every half-turn. Because teh sides swap over each ahlf-turn the coil is always pushed in the same direction. see p92 for diagram. 

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Physics Module 3

Using Magnetic Fields To Keep Things Moving - 3.3 Electromagnetic Induction

A potential difference is induced across the ends of an electric conductor if it cuts through magnetic field lines. 

Electromagnetic Induction - This occurs when a magnet is moved into a coil of wire and pd if induced across the ends of the coil. If the coil/wire is part of a complete circuit a current passes through it. 

If the direction or movement of the current is reversed or the polarity switched the direction of the induced pd is also reversed. A pd is only induce while there is movement. 

How to Increase the Size of the Induced PD - 

  • Increase the speed of movement
  • Increase the strength of the magnetic field
  • Increase the number of turns on the coil.

A pd is only induced when the wire or coild and the magnetic field move relative to each other. 

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Physics Module 3

Using Magnetic Fields to Keep Things Moving - 3.4 Transformers

Transformer - Consists of two coils of insulated wire (Primary and Secondary Coil). These are wound on the same iron core. When an alternating current passes through the Primary Coil it produces an alternating magnetic field. This continually expands and collapses. 

The alternating magnetic field lines pass through the Secondary Coil and induce an alternating potential difference across its ends. If the secondary coil is part of a complete circuit an alternating current is produced. 

The coils of wire are insulated so that current does not short cut across either the iron core or adjacent turns of wire but flows around the whole coil.

National Grid

  • A step-up transformer makes the pd across the secondary coil greater than across the primary coil. Secondary has more turns than Primary.
  • A step-down transformer makes the pd across the Secondary coil smaller than across the Primary coil. Primary has more turns than Secondary.

A switch mode has a ferrite core and it - operates a much higher pace, is lighter and smaller, uses very little power when there is no device connected across its output terminals,  

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Physics Module 3

Using Magnetic Fields To Keep Things Moving - 3.5 Transformers in Action

Uses transformers to step up and step down pd - a high pd and low ccurrent causes less loss of energy via heat loss. However the pd must go through a step down transformer to be safe to be used by consumers. 

The pd across and the number of turns on the Primary and Secondary coils aare related by the equation - 

V(p) = n(p)

V(s)    n(s)

Transformers are almost 100% efficient. For 100% efficiency they must V(p) x I(p) = V(s) x I(s)

  • A step-up transformer has more turns on the Primary coil
  • A step-down transformer has more turns on the Secondary coil. 
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Physics Module 3

Using Magnetic Fields To Keep Things Moving - 3.6 A Physics Case Study 

Applications of Physics are used in hospitals for both diagnosis and therapy - 

  • An ECG (Electrocardiogram) is used to measure the pd generated by the heart. 
  • Electronic devices are used to measure blood pressure. 
  • Digital thermometers are used to measure temperature. 
  • And endoscope is used to look inside the body without the use of large incisions. 
  • X-rays are used to take pictures of suspected broken bones
  • CT Scanners are used to build up digital pictures of a cross-section through the body 
  • MR Scanners use radio wavves to rpoduce detailed digital pictures of the body.

See  diagram on page 96.


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