# Physics P3

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## X-rays

• X-rays are part of the elecromagnetic spectrum, they have a high requency and short wave length
• Used as a medical application, to see images of bones
• Charged-coupled devises (CCDs) are used to form electronic images of X-rays
• CT scanners use X-rays to form images of a cross section of the body
• Soft tissue organs dont absorb X-rays, so an be filled with a constant medium that will absorb the rays
• X-rays are ionising so damage living cells, so are used in cancer treatment
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## Ultrasound

• Sound waves of frequency above 20,000 Hz
• A transducer transmitts and detects the ultrasound waves
• Each pulse, enters the body and once it hits the tissue it gets reflected back to the transducer as a sequence. Each pulse arrives back at different times, this is used to build up the image intended
• Distance travelled by the ultrasound pulse: s= 1/2 x v x t                                                       s=distance                                                                                                                         v=speed                                                                                                                             t=time
• They are non-ionising, so are safer than X-rays
• Used for scanning unborn babies and shattering kidney stones
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## Refractive Index

• Refraction, is the change of the direction of light as it passes from one transparent substance to another (air to glass)
• It takes place because waves change speed when they cross a boundary, the cahnge of speed causes the change of direction
• When a ray passes from air to glass it is refracted towards the normal
• Refractive index: n=sin i/sin r                                                                                               n=refractive index                                                                                                               i=angle of incidence                                                                                                           r=angle of refraction
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## Endoscopes

• The critical angle is the angle of incidence in a transparent substance the produces refraction along the boundary

• The critical angle is related to the refractive index by: n=1/sin c
• Endoscopes are bundles of optical fibres, they are very thin, flexible glass fibres
• Visible light can be sent along them for total internal reflection
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## Lenses - Converging

• Converging (convex) lens: - Parallel rays get focused (so that the converge) to the principle                                               focus

- The distance from the lens to the principle focus is called the focal                                              length

- There is a principle focus on either side of the lens as light can                                                    pass through in either direction

- If the object is further away from the lens than the principle focus                                                the image is real and inverted

- If the object is closer to the lens than the principle focus the image                                              is virtual, upright and magnified

• Magnification=image height/object height
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## Lenses - Diverging

• Diverging (concave) lens: - Parallel rays get refracted (so that they diverge) away from a point                                          called the principle focus

- The image produced is always virtual

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## The Eye

• Light enters the eye through the cornea
• The cornea and the eye lens focus the light onto the retina
• The iris adjusts the size of the pupil, controlling how much light enters the eye
• Ciliary muscles alter the thickness of the lens, to control focusing, they are attached to the lens by the suspensory ligaments
• The eye has a near point of 25cm and a far point of infinity
• Lens power: P=1/f                                                                                                               P=power in dioptres (D)                                                                                                       f=focal length in metres (m)
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• Short-sighted people: - See distant objects as blurred because the uncorrected image is                                            formed in front of the retina

-  It is caused my the eye ball being too long or the lens being too                                                 powerful

- It is corrected with a diverging lens

• Long-sighted people: - See close objects as blurred because the uncorrected image is formed                                  behind the retina

- It is caused by the eyeball being too short or the eye lens being too                                             weak

- It is corrected using a converging lens

• Focal length of the lens is determinded by the refractive index of the material from which the lens is made and the curvature of the two surfaces of the lens
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## Moments

• The turning effect of a force is called its moment
• M=Fxd                                                                                                                               M=moment in newton-metres (Nm)                                                                                       F=force in newtons (N)                                                                                                       d=the perpendicular distance from the line of action of the force to the pivot in metres (m)]
• To increase the moment: - Either the force must increase

- Or the distance to the pivot must increase

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## Centre of Mass

• Mass is concentrated to a single point, this point is the centre of mass
• Any object suspended freely will come to rest directly the point of suspension, this is know as equilibrium
• You can find the centre of mass of a object my suspending it freeling from a pin, while having a plumbline also suspended in front of it. Where the plumbline rests, a line is drawn. The process is repeated again but the object is suspended from a different point. Where the plumbline rests again another line is drawn. Where these lines cross is the centre of mass
• The centre of mass can sometimes lie outside the object
• Symmetrical objects has a centre of mass along its line of symmetry.
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## Moments in Balance

• For an object in equilibrium, the sum of the anticlockwise moments about any point=the sum of the clockwise moments about that point
• To calculate the force needed to stop an object turning we use the equation above
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## Stability

• The line of action of the weight of an object acts through iys centre of mass
• If the line of action of the weight lies outside the base of an object it will topple over
• Stability is increased by widening the base or lowering the centre of mass, because the wider the base and the lower the centre of mass the further it has to tilt before the line of action of the weight moves outside the base
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## Hydraulics

• Pressure: P=F/A                                                                                                                 P=pressure in pascals (Pa)                                                                                                 F=force in newtons (N)                                                                                                       A=cross-sectional area at right angles to the direction of the force in metres squared (m2)
• Pressure in a liquid is transmitted equally in all directions. A force excerted at one point will be transmitted to other parts in the liquid
• Hydraulic pressure systems use this, the amount of force depends on: - the force excerted on                                                                                                         the system

- the area of the                                                                                                                        cylinder that exerts                                                                                                                  the force

• The different cross-sectional areas on the effort and load sides of a hydraulic system means that the system can be used as a force multiplier
• A small effort can be used to move a large load
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## Circular Motion

• When as object moves in a circle it is continuously changing direction, so it is continuously changing velocity/speed
• It is accelerating, this acceleration is called: centripetal acceleration
• An object only accelerates when a resultant force is acting on it, this force is centripetal force and acts towards the centre
• If the centripetal force stops acting the object will move away in a straight line (tangent)
• Centripetal force increases when: - the mass of the object increases

- the speed of the object increases

- the radius of the circle decreases

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## The Pendulum

• A pendulum moves in an oscillating motion
• A simple pendulum oscillates through the equilibrium position (The position at which the pendulum comes to rest)
• The amlitude is the distance between the equilibrium position and the highest position of oscillation on either side
• Time period is fro left to right to left again and can be calculated by: T=1/f

T=time in seconds (s)

f=frequency for the                                                                                                                      number of complete                                                                                                                  oscillations per second                                                                                                            in hertz (Hz)

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## Electromagnets

• Like poles repel, opposite poles attract
• The region around the magnet is the magnetic field
• A magnetic field line is the line along which a plotting compass points
• An electromagnet is an iron core with a coil of insulated wire wrapped round
• When a current flows through the core it become strongly magnetised, but only temporarily. This temporary magnetism is very useful
• Elecromagnets are used in scrapyard cranes, circuit breakers, electric bells and relays
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## The Motor Effect

• When a current flows througha wire a magnetic field is produced around the wire
• A wire carrying an electric current is placed in a magnetic field experiences a force, this force is called the motor effect
• In the motor effect the force: - is increased if the current or the strength of the magnetic field is                                             increased

- is at right angles to both the direction of the magnetic field and                                                    to the wire

- is reversed if the direction of either the current or the magnetic                                                    field is reversed

• An electric motor has a coil which turns when a current is passed through it
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## Fleming's Left Hand Rule

This is used to work out the direction of the force (movement) on the wire

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## Electromagnetic Induction

• The process of creating a potential difference using a magnetic field
• If an electrical conductor ‘cuts’ through a magnetic field a potential difference is induced across the ends of the conductor.
• If a magnet is moved into a coil of wire a potential difference is induced across the ends of the coil
• The size of the indiced potential difference is increased by increasing: - the speed of the                                                                                                                  movement

- the strength of the                                                                                                                   magnetic field

- the number of turns on                                                                                                             the coil

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## Transformers

• Consists of 2 wires the primary coil and the secondary coil
• When an alturnating current passes through the primary coil it produces a magnetic field in the iron core, this field continually expands and collaspes
• The alturnating current in the primary induces an alturnating current in the secondary
• Alturnating current can only be used, because a continually changing magnetic field is needed to induce the secondry coil
• A transformer has an iron core
• A switch-mode transformer has a ferrite core
• A switch mode transformer: - opperates at a much higher frequency

- is lighter and smaller than a normal transformer

- uses very little power when no devise is plugged in

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## Transformers in Action

• Transformers are used to step potential difference up or down
• The transformer equation is:                                                                                                primary potential difference Vp           number of primary turns of wire Np

=

secondary potential difference Vs       number of seconary turns of wire Ns

• Step down transformer Ns is less than Np
• Step up transformer Ns is greater than Np
• For a 100% efficient transformer:                                                                                         Vp x Ip = Vs x Is

Vp = primary potential difference                                                                                              Vs = secondary potential difference                                                                                          Ip = primary current                                                                                                                Is = seconary current

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## Physics Case Study

• We use physics in hospitals whenever: - blood pressure or temperature is taken

- an ECG (electrocardiogram) is taken

- an endoscope is used

- a scanner is used

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