Physics Unit 3 Flashcards
Describe how ultrasound imaging works
Partial reflection means that when a wave passes from one medium to another, some is refracted and some is reflected
A pulse of ultrasound is pointed at an object and a detector is used to detect the reflected pulse (the time from when the ultrasound is emitted to when it is detected is measured)
Since we know the time and speed of the pulse of ultrasound, we can calculate the distance to the change in media
Describe CT scans
Computerised axial tomography uses X-rays to produce high resolution images of hard and soft tissue
The patient is put inside a cylindrical scanner and an X-ray is sent from an X-ray tube on one side and picked up by a detector on the other side
The tube and detector are rotated during the scan
A computer interprets the signals from the detectors to form an image of 2D slice through the body
Multiple scans can be done to compile a 3D image of the body
What 3 things do radiographers do to minimise exposure to X-rays?
Wear lead aprons
Stand behind a lead screen
Leave the room while scans are being done
X-ray radiation can’t penetrate lead
What are the 2 methods of X-ray imaging and how do they work?
When photographic film is hit by X-rays, it goes from clear to dark and cloudy until it turns black so if it is placed behind the patient, only the film behind the bones will stay colourless giving an image of the bone structure
The same can be done except charge couple devices (CCDs) which are tiny silicon chips are placed behind the patient. When hit by an X-ray, a CCD will send a signal to a computer so that a computerised image can be formed
Why can X-rays be used for treating cancer?
They cause ionisation meaning that they can kill living cells
What is ultrasound?
Sound with a higher frequency than humans can hear (above 20000Hz)
What are X-rays?
High frequency, short wavelength electromagnetic waves with a diameter the same size as an atom
Describe how X-rays are used to treat cancer
X-rays are focused on the tumour using a wide beam
The bean is rotated round the patient with the tumour at the centre
This minimises the exposure of normal cells to radiation reducing the chances of damaging the rest of the body
Describe how X-ray scans work
X-rays are transmitted by healthy tissue but are absorbed by denser material like bone and metal
This means that when X-rays pass through a person with an X-ray imaging method behind them, only the X-rays that have passed through healthy tissue will actually hit the imaging method
Compare the safety of the 3 methods of medical imaging
Ultrasound waves are non-ionising abs completely safe
X-ray scans do cause ionisation and can cause cancer if the dose is too high (can’t be used on developing babies)
CT scans use a lot more X-ray radiation than regular X-ray scans meaning there is more ionisation so use of this machine is minimised
Compare the image quality of the 3 methods of medical imaging
Ultrasound images are quite fuzzy so some conditions can’t be diagnosed using these images
X-rays produce clear images of bone and metal but not much else
CT scans produce very high quality images to diagnose complicated illnesses. Hard and soft tissue are differentiated. Multiple scans can be used to produce 3D images
What is the formula for calculating distance in an ultrasound scan?
Distance there and back (m) = wave speed (m/s) * time there and back (s)
Describe the main 2 uses of ultrasound
It can be used to destroy kidney stones which are hard masses that block the urinary tract. Ultrasound is directed at the kidney stone and it turns to sand-like particles that pass out of the body in urine
They can scan for a foetus because ultrasound is partially reflected by the boundary between the womb fluid and skin of the foetus
What are the 2 main methods of electromagnetic induction?
Moving a magnet through a coil of wire
Turning a magnet end to end in a coil of wire
What are switch mode transformers?
Transformers that operate at higher frequencies than traditional transformers meaning they are useful for chargers for home devices
What is electromagnetic induction?
The creation of a voltage across a conductor which is experiencing a change in magnetic field
Describe how moving a magnet through a coil of wire induces a voltage
You move a magnet through a coil of wire to induce a voltage and then move it through in the other direction to induce a reversed voltage to the one just induced
The constant change of direction of current causes an alternating current (AC)
Describe how transformers work
An iron core has 2 coils of wire around it called the primary and secondary coil
The primary coil produces a magnetic field that stays within the iron core and nearly all of it passes into the secondary coil
Because there is AC in the primary coil, the field in the iron core is constantly changing direction. This changing magnetic field causes an AC current in the secondary coil
No electricity flows round the iron core because it purely transfers the changing magnetic field
What are the 2 types of transformer and what is the difference between them?
Step-up transformers increase the voltage and decrease the current
Step-down transformers do the opposite
How is the voltage changed by a transformer?
If there are more coils on the secondary coil than the primary coil, the voltage is stepped up
If there are less on the secondary coil, the voltage is stepped down
Describe how turning a magnet end to end inside a coil of wire induces a voltage
As you turn the magnet, the magnetic field through the coil changes which induces a voltage in the coil of wire
After half a turn, the direction of the magnetic field in the coil reverses so the direction of the current reverses until the turn is completed
This creates an alternating current
What are transformers?
Transformers use electromagnetic induction to change voltage in an AC current
What is the transformer equation?
Voltage in primary coil / voltage in secondary coil = coils on primary coil / coils on secondary coil
Using the motor effect, what are the 2 ways that you can increase the force on the wire?
Increasing either the magnetic field or the current
What are the 2 ways that the direction of the motor can be reversed?
Swapping polarity of DC
Swapping magnet poles over
Describe the electromagnet
Physically, an electromagnet is a coil of wire with a current flowing through it
The magnetic field inside the coil of wire is strong and uniform
Outside the coil, the magnetic field is just like the one round a bar magnet
How do you use the Fleming’s left hand rule?
Put fingers on the left hand into practiced position
First finger points in direction of magnetic field
Second finger points in direction of current
Your thumb will point on the direction of the force
How do we get the highest amount of force on the wire if we can’t change the magnetic field and current?
Using the Fleming’s left hand rule to work out the direction of the force according the direction of the magnetic field and current
If it is not quite at the right angle, you will still get some force on the wire but less
What is a magnetic field?
A region where magnetic materials and also wires carrying currents experience a force acting on them
How do you draw a magnetic field for a wire with a current?
You use the right hand rule (stick up thumb and curl fingers)
The thumb represents the direction of the current and the fingers represent the magnetic field going round the wire and that direction
In a diagram, the current and magnetic field lines are drawn as lines with arrows on them
Describe how electric motors work
There are 2 side arms of a coil of wire between the north and south magnets
When the current flows through the coil and side arms the force on one side arm is up and one is down
The coil is on a spindle so this causes the coil to rotate
Every half turn, the split-ring commutator swaps round the contacts in order to keep the motor spinning in the same direction
What is the motor effect?
Passing an electric current through a wire produces a magnetic field around the wire
If you put that wire into a magnetic field, you have 2 magnetic fields combining which puts a force on the wire
How do you know the direction of the magnetic field?
The direction of the magnetic field is from the north magnet to the south magnet
How do you draw a magnetic field for a bar magnet?
You draw a metal bar with an N on one side and an S on the other
Draw magnetic field lines (with arrows) going from north to south over the top and under the bottom of the magnet and also spreading out from the north side and coming in from the south side
How can you make an electromagnet have a stronger field?
Add a magnetically soft iron core through the middle of the coil
A magnetically soft material is one that magnetises and demagnetises very easily
What is an electromagnet?
A magnet with a magnetic field can be turned on and off with an electric current
What causes the acceleration towards the centre of the circle?
Centripetal force
What is interesting about circular motion and why is this true?
The velocity is constantly changing because velocity is a vector and the direction of travel is constantly changing
This means that the object is accelerating towards the centre of the circle
What are the 3 most common types of centripetal force?
Tension
Gravity
Friction
What 3 factors affect centripetal force?
There has to be a larger centripetal force:
If the object is moving faster
If the object has a larger mass
If the circle is smaller
What is the formula for pressure?
Pressure = force / cross-sectional area
P = F / A
How do force multiplying hydraulic systems work?
A small force pushes a piston with a small cross-sectional area that puts pressure on a liquid that pushes another piston with a larger cross-sectional area
Since force = pressure * area and the pressure always stays constant, the force pushing the larger piston is larger than the force put into the first piston
What do hydraulic systems do?
They work as force multipliers mainly
They can also transfer force from one place to another but this is less important for GCSE
What is the unit of pressure?
Pascals (Pa)
Equal to N/m^2
Why is liquid useful for hydraulics?
It is imcompressible (volume and density always stay the same) which means that pressure can transmitted through a liquid
What is interesting about how pressure is transmitted through water?
Pressure in a liquid is transmitted equally in all directions
What does time period depend on?
The longer the pendulum, the greater the time period
What is a pendulum?
A simple pendulum is made by suspending a weight from a piece of string
When you pull back the pendulum and let it go, it will swing back and forth
How do you calculate time period?
T = 1 / f
Time period = 1 / frequency
What is the time period?
The time taken for the pendulum to swing from one side to the other and then back again
This is always the same even when the pendulum starts to slow
How do you calculate the moment?
M = F * d
Moment = force * perpendicular distance from line of action of the force to the pivot
What is the centre of mass?
The point of an object at which the whole mass is concentrated
When do objects tip over?
When the line of action of the weight of the object lies outside of the base of the object
What is balance in terms of moments?
When the clockwise moment and anti-clockwise moment are equal so that there is no resultant moment
What is the unit of a moment?
Newton-metres (Nm)
What is a moment?
The turning effect of a force
What 2 things make an object stable and why?
A wide base and a low down centre of mass
Both of these factors make the angle needed to make the line of action lie outside of the base of the object higher and therefore tip over the object
What is the line of action?
The direction of the force
What are levers and why do they work?
Levers use moments to make it easier for us to do work (like lifting an object)
They work by increasing the distance from the pivot at which the force is applied meaning that less force is needed to achieve the same moment
In short, they decrease the force needed by increasing the distance
How do you get the maximum moment?
The direction of the force has to be perpendicular to the object being pushed so that d in the formula (perpendicular distance from line of action of the force to the pivot) is as high as possible
Describe how you find the centre of mass of a flat shape
Suspend the shape and a plumb line from the same point and wait until they stop moving
Draw a line along the plumb line
Do the same thing again but suspend the shape from a different pivot point
The centre of mass is where the 2 lines cross
What is a stable object?
One that will not easily tip over
What happens when an object is swinging from a suspension and why?
The object will swing until its centre of mass is vertically below the point of suspension
Now there is no moment because the pivot is in line with the line of action of the force
What is the formula for critical angle?
Refractive index = 1 / sin c
c = critical angle
What is a laser?
A narrow, intense beam of light that produces light waves that always have the same wavelength
How does laser eye surgery work?
The laser vapourises some of the cornea to change its shape in order to change its focusing ability
How do you correct long sight and how does this work?
Put a converging lens in front of the eye so that near light rays are converged before they enter the eye so that the image forms on the retina
What are the 4 similarities between a camera and the eye and why?
In a camera, light is refracted by a lens forming an image on the film (or CCD). The eye does the same except the image forms on the retina
The images produced by a camera and the eye see both real -
- Are both inverted
- and are smaller than the object because the object is a lot further away than the focal length of the lens
What is short sight and what causes it?
Short sight is where people can’t focus on distant objects and is caused by the eyeball bring too long or the lens system being too powerful
Images of distant objects from too far in front of the retina
What effect does refractive index have on total internal reflection?
A high refractive index causes more internal reflection
What is total internal reflection?
When the light rays are only reflected by a material and not refracted through it at all
Why is laser surgery preferable to scalpel surgery?
Lasers burn and seal shut blood vessels as they cut through the tissue stopping blood loss and therefore infection
How do endoscopes work?
One optical fibres carries light to the area of interest and one carries an image back
The image can be viewed on an eyepiece or on an electronic screen if CCDs are used
How do you correct short sight and how does this work?
Put a diverging lens in front of the eye so that distant light rays are diverged before they enter the eye so that the image forms on the retina
What is an endoscope?
A thin tube containing optical fibres that allows surgeons examine the inside of the body
How do optical fibres work?
They carry visible light over long distances by bouncing light waves off the sides of a thin inner core until it reaches the end of the fibre
The waves bounce because of total internal reflection
When will total internal reflection occur?
When the angle of incidence is larger than the critical angle
What is far point?
The furthest distance that eye can focus comfortably (infinity for normally-sighted people)
What is long sight and what causes it?
Long sight is where people can’t focus on near objects and is caused by the eyeball bring too short or the lens system being too weak
Images of near objects form behind the retina
What are the 7 parts of the eye and what do they do?
The cornea is a transparent window with a convex shape and a high refractive index (it does all of the eye’s focusing)
The iris is the coloured part and is made up of muscles that control the size of the pupil to control the light intensity entering the eye
The lens changes shape to focus light from objects at varying distances. It is connected to the ciliary muscles by the suspensory ligaments
When the ciliary muscles contract, the lens takes on a flat spherical shape and when they relax, the lens takes on a thinner, flatter shape
Images are formed in the retina which is covered in light-sensitive cells that detect light and send signals to the brain
What is the advantage of endoscopes?
They allow detailed viewing of the inside of the body with only a tiny hole
This is called keyhole surgery
Describe how changing the angle of incidence affects reflection and refraction
If the angle of incidence is less than the critical angle, most of the light is refracted but a bit is internally reflected
If i = critical angle, the emerging ray comes out along the surface and there’s quite a lot of internal reflection
If i > critical angle, no light is refracted and total internal reflection occurs
What are the 2 main uses of laser surgery?
Treating skin conditions
Laser eye surgery
What do you have to remember when drawing optical fibres?
The angle of the reflection from the normal is the same as the angle of incidence for the light rays in a fibre
What is near point?
The closest distance the eye can focus on (usually 25cm)
How do you work out magnification?
Magnification = image height / object height
What is a converging lens?
A convex (bulges outwards) lens causes parallel rays of light to converge at the principal focus
What 2 factors decrease the focal length?
Increasing the curvature of the surfaces of the lens
Increasing the refractive index of the lens
What is the principal focus?
For a converging lens, the principal focus is where rays hitting the lens parallel to the axis all meet
For a diverging lens, the principal focus is where the rays hitting the lens parallel to the axis appear to all come from
There is a principal focus on both sides of the lens
What is the difference between a real image and a virtual image?
A real image is where the light from an object comes together to form an image on the display surface
A virtual image is where the rays are diverging so the light from the object appears to be coming from a completely different place
How does changing the lens type (diverging / converging) affect the lens power?
Diverging lenses have negative power and converging lenses have positive power
Describe how to draw a diagram for an image through a diverging lens
The diverging lens is shown as a vertical line with arrowheads pointing inwards
Draw a ray from the top of the object through the centre of the lens
Draw a ray that is parallel to the axis from the top of the object to the lens and refract it so that it appeared to have come from the principal focus on the side of the lens of the object
Draw a dotted line from the principal focus on the object side of the lens to where the top ray refracts in order to show where the ray appears to be coming from
The top of the image is where the dotted line ray meets the other ray
Describe the 3 scenarios where the distance from the lens affects the image
An object at 2F (2 * principal focus) will produce a real, inverted image the same size as the object and at 2F
An object between F and 2F will make a image that is real, inverted and bigger than the object and beyond 2F
An object nearer than F will make a virtual, upright image that is bigger than the object and on the same side of the lens as the object (because the refracted light rays are still diverging meaning there will be an virtual convergence on the object side of the lens)
How would you describe the image formed by a diverging lens?
The image is upright, smaller than the object, virtual and on the same side of the lens as the object
What are the 2 types of lens?
Converging lens
Diverging lens
How do magnifying glasses work?
The magnifying lens creating a magnified virtual image meaning that the object being magnified must be closer to the lens than the focal length
What is the unit of lens power?
Dioptres
What is the focal length?
The distance between the centre of the lens and the principal focus
How do lenses work?
They form images by refracting light and changing its direction
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 is very important when drawing ray diagrams?
To draw it very accurately because you might have to measure the heights of the object and image to work out magnification
Why is making a lens more powerful useful?
You can increase the power of the lens to make allowances for reducing the lens thickness
What are the 3 things you need to say to describe an image?
How big it is compared to the object
If it’s upright or inverted
If it’s real or virtual
What is a diverging lens?
A lens that is concave (caves inwards) and causes parallel light rays to diverge (spread out)
Why can’t magnifying lenses project images on to screens?
The light rays appear to be coming from a place that they are not coming from meaning the image is virtual
You can’t project a virtual image on to a screen
What type of lens is a magnifying lens?
Converging
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 so it appears to have come from 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 the axis of a lens?
A straight line passing through the middle of the lens
What is the formula that links the angle of incidence and angle of refraction?
Refractive index = sin i / sin r
Describe how you draw a ray diagram for an image through a converging lens
Draw a horizontal line to be the axis and draw a vertical line through it with arrows on the ends pointing outwards to be the converging lens
Draw the object on the axis and draw the principal focus marked by F on the other side of the lens
Draw a ray from the top of the object parallel to axis until it hits the lens and refracts down to go through the focal point
Draw a ray from the top of the object through the centre of the lens and don’t refract it
Where the refracted rays meet is below the axis but is the top of an inverted image of the object. Draw the image of the object
Describe how and why refraction happens
When waves slow down, they bend towards the normal and when they speed up, they bend away from the normal (if a wave is already travelling in the direction of the normal, it slows down but doesn’t bend)
The incident ray hits the boundary at the angle of incidence, the refracted ray travels through the new medium at the angle of refraction and then the emergent ray exits the new medium at the angle of incidence
How do you calculate the power of a lens?
Power = 1 / focal length
