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