Triple & Further Science: Physics (III) Flashcards
What are X-rays used for and what properties do they have?
X rays are high frequency, short wavelength EM waves (wavelength roughly the same size diameter of an atom)
How do X-rays work?
X-rays are transmitted (pass through) healthy tissue and are absorbed by denser materials such as bone and metal
How are X-rays formed electronically?
Charge-coupled devices (CCDs) are silicon chips which detect X-rays
What are CT scans?
Computerised axial tomography (CT) scans use X-rays to produce high res. images of soft and hard tissue
How do CT scans form a 3D image?
X-rays are passed through the patient and many 2D images are linked forming a 3D image
How do X-rays treat cancer?
X-rays are carefully focused to ionise cancer cells without damaging too many normal cells
X-rays are focused on the tumour and the beam rotated minimizing healthy cell exposure
What precautions must be taken around X-rays and CT scanners?
X-ray dosage needs to be minimised by workers: lead aprons, stand behind lead screens or leave the room
Patients have lead shields across areas of the body which are not being scanned and exposure time is minimised
What is the human hearing range and what is ultrasound?
20Hz – 20’000Hz
A higher frequency of this upper limit is ultrasound
What happens to ultrasounds at the boundary between mediums?
The ultrasounds are partially reflected (some reflected and some transmitted and refracted)
The time it takes for reflections to reach the detector calculates the distance
What would the following ultrasound pulse reflect and refract like?
How can an oscilloscope trace be used to find boundaries?
If the speed of sound in the medium is known distance can be worked out using
s = v x t
- s = distance (m)
- v = speed (m/s)
- t = time (s)
What is ultrasound used for?
Breaking down kidney stones; pre-natal scanning; cleaning machinery
What are the pros / cons of X-rays, CT scanning and ultrasound?
X-rays and CT scans are ionizing (cause cancer if too high a dosage) and CT scans use many X-rays, whilst ultrasound are non-ionising and safer
Ultrasound images are fuzzier whilst X-rays are clear and CT scans are very detailed with high resolution.
What is refraction?
Refraction is the change of direction of a wave when entering a different medium (due to wave speed change)
Where is the likely path of the emergent ray?
What is the refractive index?
Refractive index of a medium is the ratio of the speed of light in a vacuum to the speed of light in that medium
What is the equation for reactive index?
Refractive index (n) = sin i ÷ sin r
What are the two main types of lens?
Converging lens – convex shape (bulges out)
Diverging lens – concave shape (caves inwards)
Draw a converging lens (convex)
Parallel rays of light converge (move together) at the principal focus
Draw a diverging lens (concave)
Parallel rays of light diverge (spread out)
What is the axis of a lens?
A line passing through the middle of the lens
What is the principal focus of a converging lens?
Where the rays hitting the lens parallel to the axis all meet
What is the principal focus of a diverging lens
Where the rays hitting the lens parallel to the axis appear to all come from
What is the focal length?
The distance from the centre of the lens to the principal focus (occurs on each side of the lens)
What are the 3 rules for refraction in a converging lens?
- An incident ray parallel to the axis refracts through the lens and passes through the principal focus on the other side
- An incident ray passing through the principal focus refracts through the lens and travels parallel to the axis
- An incident ray passing through the centre of the lens carries on in the same direction
What are the 3 rules for refraction in a diverging lens?
- An incident ray parallel to the axis refracts through the lens and travels in line with the principal focus
- An incident ray passing through the lens towards the principal focus refracts through the lens and travels parallel to the axis
- An incident ray passing through the centre of the lens carries on in the same direction
What is a real image?
A real image is where the light from an object comes together to form an image on a ‘screen’ (such as the retina of an eye / a camera film)
What is a virtual image?
A virtual image is when the rays are diverging so the light from an object appears to be coming from a different place
Draw a real image
Real image
Draw a virtual image
Virtual image
What is the symbol for a convex lens in a ray diagram?
Convex lens
What is the symbol for a concave lens in a ray diagram?
Concave lens
Complete the ray diagram for an image through a converging lens
Complete the ray diagram for an image through a diverging lens
What does a distance of 2F from the object to a lens do to the image?
An object at 2F will produce a real, inverted image the same size as the object and at 2F
What does a distance of between 2F and F from the object to a lens do to the image?
An object between F and 2F will make a real, inverted image bigger than the object and beyond 2F
What does a distance of less than F from the object to a lens do to the image?
An object nearer than F will produce a virtual image the correct way up bigger than the object on the same side as the lens
What does a diverging lens always produce?
A virtual image, the right way up, smaller than the object and on the same side of the lens as the object
What lenses are used in magnifying glasses and cameras?
Converging lenses
Where must an object being magnified be?
The object being magnified must be closer to the lens than the focal length
Draw a magnified image
Magnified virtual image
How can magnification be calculated?
Magnification = image height ÷ object height
How are focal length and power of a lens related?
P = 1 ÷ f
Power (D) = 1 ÷ Focal length (m)
What are the powers for converging and diverging lenses?
Converging lenses are positive, diverging lenses are negative
How is focal length of a lens determined?
Refractive index of the material and the curvature of the two surfaces of the lens
What is the basic structure of the eye?
What do the components of the eye do?
Cornea – transparent ‘window’ with a high refractive index for focusing light
Iris – coloured part of eye controlling light intensity entering
Lens – changes shape to focus light
Ciliary muscles and suspensory ligaments – change the shape of the eye (ciliary muscles contract = no tension; relax = flatter shape)
Retina – images form and sent via optical nerve to brain
What is the focal point of the eye?
25cm – infinity
How does a camera focus an image?
Light is refracted by a lens forming an image on the film (real image) but it is smaller than the object (object further than focal length of the lens) and is inverted
How is short sight corrected?
Using diverging lenses (helps focus distant objects)
How is long sight corrected?
Using converging lenses (helps focus near objects)
Using converging lenses (helps focus near objects)
A laser vaporises some of the cornea changing its shape which changes the focusing ability so it focuses on the retina correctly
How do optical fibres work?
Visible light is bounced off the sides of a thin inner core of glass using total internal reflection
The critical angle is very important (42o in glass)
What is an endoscope?
A tube containing optical fibres used for surgery (containing a light and a camera)
What is a moment and how is it calculated?
A turning force around a pivot
M = F x d
- M = moment (Nm)
- F = force (N)
- d = distance (m)
Where is the centre of mass?
Directly below the point of suspension
An object will not turn if
The anticlockwise moments = the clockwise moments
What are simple levers?
Simple levers are force multipliers – work is easier to do as the distance the force is applied is further from the pivot
What happens if the moments acting on an object are not equal?
The object will turn
What affects the time of a pendulum swing?
The length (longer = greater time to swing)
t = 1 ÷ f
Why are liquids used for hydraulics?
Liquids are virtually incompressible and can flow – a force can be applied to one point and be transmitted to all other points
How is pressure calculated?
P = F ÷ A
Pressure (Pa) = force (N) ÷ Cross-sectional area (m2)
What is pressure in liquids used for?
Hydraulic systems
What is circular motion?
Velocity which is constantly changing (accelerating towards the centre)
What does centripetal force depend on?
Mass, speed and radius
*Larger centripetal force needed for a fast, large vehicle going around a very tight bend
What is a magnetic field?
A region where magnetic materials and wires carrying currents experience a force acting on them
What does the right hand thumb rule show?
The way in which the magnetic field goes
What do the magnetic fields around a coil of wire look like and how can they be increased?
Magnetic fields can be increased using a magnetically soft iron core
What are electromagnets and how are they used?
Electromagnets are magnets which can be switched on / off depending upon current supply. Useful in cranes
What is the motor effect?
A force experienced by a current-carrying wire in a magnetic field
What is Fleming’s left hand rule?
Shows which way a force will act
Using Fleming’s left hand rule which direction is the force?
Force directed towards you!
How does a simple motor work?
Forces are on a spindle (one up, one down) with a split-ring commutator swapping the contacts every ½ turn
What is electromagnetic induction?
The creation of a potential difference across a conductor which is experiencing a change in magnetic field
How can a voltage be produced?
Moving a magnet inside a coil of wire (or vice-versa)
What are dynamos?
Dynamos use electromagnetic induction to generate a current (e.g. bike lights)
What are step-up transformers?
Step-up transformers step the voltage up (more turns on secondary coil)
What are step-down transformers?
Step-down transformers step the voltage down (less turns on secondary coil)
What current supply do transformers work in?
AC – they need the constantly changing magnetic field for induction
What is the transformer equation?
*Either way up
How is power worked out for a transformer (assuming it is 100% efficient)
P = I x V
VpIp = VsIs
What are switch mode transformers?
Switch mode transformers operate at a high frequency (50kHz – 200kHz)
What are switch mode transformers used for?
Mobile phone / laptop chargers as they are light and small (as they operate at a higher frequency)
Why are switch mode transformers more environmental?
They are more efficient using very little power when switched on without a load
