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Flashcards in Physics 3 Deck (108):

Why do CT scans use high doses of x-rays?

To distinguish small variations in tissue density.


What does CT stand for?

Computerised axial tomography.


What does CCDs stand for?

Charge-coupled devices.


What are x-rays?

High frequency
Short wavelength
Electromagnetic waves


What is the wavelength of an x-ray the same size as?

The diameter of an atom.


How do CT scans work?

1) The patient is placed inside of a cylindrical scanner.
2) An x-ray beam is fired through the body and picked up by detectors.
3) Detectors and the X-ray tube is rotated during the scan.
4) Then a computer interprets the information from the detectors to form an image.
5) Many 2D images are put together to make a 3D image.


What sort of images do CT scans produce?

Detailed. This makes them good for diagnosing bone fractures.


What is a problem with CT scans?

The patient is exposed to a lot of x-rays.


How do CCDs work?

They are silicon chips which are divided up. They detect x-rays and produce electronic signals which can make high resolution images.


What are the safety measures taken when using x-rays?

Lead aprons.
Stand behind lead screens.
Limit the time you are exposed to x-rays.


What is ultrasound?

Sound with a higher frequency than the range of human hearing.


What is the range of human hearing?

20-20000 Hz


What is partial reflection?

When some wave reflects off a boundary between two media and some of the wave is transmitted. This occurs when a wave passes from one medium into another.


How can ultrasound be used for kidney stones?

An ultrasound beam can be used to concentrate high energy waves at the kidney stone which breaks it up into smaller particles.


How can you use ultrasound for pre-natal scanning?

When an ultrasound wave hits the boundary between the fluid in the womb and the skin of a fetus, some of the wave is rejected back and can be detected.


Out of the following three, which is the safest and which is the most dangerous?
CT scans

Safest= Ultrasound (as they are non ionising).
Most dangerous= CT scans.


Out of the following three, which produces the best quality image and which produces the worst?
CT scans

Best= CT scans.
Worst= Ultrasound.


Define refraction.

When a wave changes direction as it enters a different medium.


What causes refraction?

Change in density from one medium to another.


What happens when light enters a more dense medium?

It slows down and bends towards the normal.


What happens when light enters a less dense medium?

It speeds up and bends away from the normal.


What happens if a wave meets a boundary at 90 degrees?

It won't change direction.


What is refractive index?

The ratio of speed of light in a vacuum to the speed of light in a medium.


What are the two main types of lenses?

Converging (convex)
Diverging (concave)


What is the focal length?

Distance from the centre of the lens to the principal focus.


What are the two types of images?



What is a real image?

It can be captured on a screen.
The light rays pass through a point.


What are virtual images?

The light rays don't actually pass through a point. They only appear to have.


How can you describe an image?

How big it is compared to the object.
Whether it's real or virtual.
Whether it's upright or inverted.


What sort of image is produced beyond 2F?

The image will be between F and 2F on the far side of the lens.


What are CCDs used for?

Detecting x-rays and producing electronic signals which can be turned into a picture.


What image is formed at 2F?

Same size.
Sit at 2F.


What image is formed between F and 2F?

Real n
Sit beyond 2F.


What image will form between the lens and F?

Right way up.
On the same side as the object.


What image is formed by a diverging lens?

Right way up.
On the same side as the object.


What type of lens do magnifying glasses have?



How do magnifying glasses work?

They create a virtual, upright image which is larger and on the same side as the object. In order for this to occur, the object which is being magnified must be closer to the lens than the focal length.


What does the cornea do?

Does most of the eyes focussing.


What is the pupil?

Enables light to enter the eye.


What is the iris?

Made up of muscles that control the size of the pupil and therefore it controls the light intensity entering the eye.


What controls the size of the pupil?



What are the ciliary muscles connected to?

The lens.


How are the lens and ciliary muscles connected to each other?

By sensory ligaments.


What two things work together to change the shape of the lens?

Ciliary muscles and suspensory ligaments.


What is the retina?

Where images are formed. It is covered in cells which are sensitive to light so they detect the light and send signals to the brain which are then interpreted.


How does the eye focus light?

The lens and cornea focus the light. The cornea does most of it but the lens brings the light to focus on the retina. The power of the lens and the cornea is altered by the changing shape of the lens. When the ciliary muscles contract, there is tension in the suspensory ligaments which is released and the lens becomes a fat, spherical shape. This makes the lens powerful. If the muscles relax, the ligaments pull the lens into a flatter and thinner shape which makes the lens less powerful.


What is the far point?

Furthest distance in which the eye can see and focus on comfortably. For those with normal sight, there should not be a limit.


What is the near point?

Closest distance in which the eye can focus, this is normally about 25cm.


True or false.
A camera forms an image like an eye does.



How does a camera form an image?

Light travel to the camera and is then reflected by the lens, this forms an image on the film.


What sort of image does a camera form?

Smaller than the object.


What is the lens in a camera the equivalent of in the eye?

Lens and cornea.


What is the aperture like in the eye?

Pupil as they both effect the amount of light entering the eye.


What is the film of a camera like in the eye?



What is short sight caused by?

The eyeball is too long or the cornea is too and lens is too powerful. This results in the lens not being able to produce a image on the retina where it is supposed to be. Instead, images are in focus in front of the retina.


What can short sighted people not see?

They can't focus on distant objects.


What can long sighted people not see?

They cannot focus on near objects.


How can you correct short sight?

Diverging lens.


How can you correct long sight?

Converging lens.


What is long sight cause by?

The eyeball being took short or the the cornea and lens being too weak. This makes a near object being bought into focus behind the retina.


What is the focal length determined by?

Refractive index of the material used to make the lens- The greater the refractive index, the shorter the focal length.
The curvature of the two surfaces of the lens- The greater the curvature, the shorter the focal length.


True of false.
A converging and a diverging lens will have a negative power.

False. A converging lens will have a positive power where as a diverging lens will have a negative power.


How can lenses be made thinner?

By using materials with a high refractive index to make them.


What is total internal reflection?

When light passes through a boundary and all of it is reflected.


When can total internal reflection of light occur?

When light travels through a dense material towards a less dense substance.


What does total internal reflection depend on?

Th critical angle.


What is the critical angle?

The angle of incidence above which total internal reflection happens.


What happens if the angle of incidence is less than the critical angle?

Most light refracts but a little is internally reflected.


What happens if the angle of incidence is equal to the critical angle?

The emerging ray comes out along the boundary and there is quite a bit of internal reflection.


What happens if the angle of incidence is bigger than the critical angle?

Total internal reflection happens.


How can optical fibres carry visible light long distances?

As a result of total internal reflection.


How do optical fibres work?

By bouncing light off the sides of a thin inner core. The wave enters one end of the fibre and then undergoes total internal reflection repeatedly until it comes out the other end.


In optical fibres what must the inner core be to ensure that total internal reflection takes place?

The inner core must be denser than the outer layer.


How do endoscopes work?

Light is reflected down the first bundle and emerges in the patient which lights up an area. Some light is then reflected off the inside of the patient and enters the second bundle. The light reflects up the second bundle. The light inside the patient forms an image that can be seen through the eyepiece at the second bundle.


What is an endoscope?

A thin tube which contains optical fibres. It consists of two bundles of optical fibres.


What are the advantages of using endoscopes?

Keyhole surgery can now take place.


What is a laser?

Narrow and intense beam of light which all have the same wavelength.


What are the uses of lasers in medicine?

Treat skin conditions. They burn off the top layers of scarred skin which leaves behind the less scarred skin in the lower layers.
Laser eye surgery. A laser is used to vaporise some of the cornea in order to change its shape. This changes its focusing ability so that images can be formed on the retina.


What is the centre of mass?

The point at which all mass is concentrated.


True or false.
When an object is at rest, the centre of mass will always be vertically below the point of suspension.



What is the time period?

The time taken for a pendulum to swing from one side to another side and back again.


Pendulums and time period.

The longer the pendulum, the greater the time period.


What is the length of a pendulum?

Distance from its suspension point to the centre of mass.


What is a moment?

The turning effect of a force.


What is the line of action?

Straight line which passes through a point at which the force acts and in the same direction as the force.


Why are levers good?

Make work easier. They are also known as force multipliers which means that you can reduce the amount of force that's needed to get the same moment and you only have to increase the distance.


What happens if the total anti-clockwise moments on an object about a pivot are equal to the total clockwise moments?

The object will not turn.


What two things do most stable objects have?

Wide base.
Low centre of mass.


What are liquids?

Virtually incompressible.


Can pressure be transmitted throughout a liquid?

Yes, pressure in liquids is transmitted equally in all directions.


What are hydraulic systems?

Use a small force to get a bigger force.


What is centripetal force?

Keeps something moving in a circle.


What three things does the size of centripetal force depend on?

Radius of circle.


What is a magnetic field?

A region where magnetic materials and also wires carrying currents experience a force acting on them.


What is a electromagnet?

A magnet whose magnetic field can be turned off and on with an electric current.


How can you increase the strength of a magnetic field?

Adding a mechanically soft iron core through the middle of the coil. A magnetically soft iron core magnetises and demagnetises easily.


What is the motor effect?

When a current carrying wire experiences a force.


How can the force on a wire be increased?

Increasing the strength of a magnetic field.
Increasing the size of the current in the wire.


How can a wire experience a full force?

The wire must be 90 degrees to the magnetic field. If it is parallel then it won't experience any force.


Fleming's left hand rule.

Thumb- Motion/ Force
First finger- Field
Second finger- Current


How does the motor effect work?

1) The d.c. current flows through a loop and the two side arms which are both parallel to the field, experience a force as a result of the motor effect.
2) They experience the forces in opposite directions due to the fact that the current in each arm being opposite. The loop then begins to rotate around the axis as the forces act up and down.
3) When the wire loop comes to a vertical position the forces still act one up and one down on the same arms, so as a result the loop becomes stuck.
4) In order for the motor to keep on rotating in the same direction, the forces need to swap direction.
5) Reversing the direction of the current actually reverses the direction of the force and a split-ring commutator is used as it swaps the contacts of the loop every half turn, which reverses the current.
6) A split-ring commutator is a conducting ring with a gap between the two halves. As it rotates the part of the commutator that is in contact with each contact will change every half turn.
7) This changes the electrical contacts of the loop every half turn. This means that the force acting in each arm of the loop will also swap every half turn which allows rotation to continue in the same direction.


How do you increase the speed of a simple electric motor?

Increase the current.
Increase the strength of the magnetic field.


How can you change the direction of a simple electric motor?

Swapping magnetic poles over.
Swapping the polarity of the direct current supply.


What is electromagnetic induction?

The creation of a potential difference across a conductor which is experiencing a change in magnetic field.


How can potential different across the ends of a conductor be induced?

By moving the electrical conductor in a magnetic field.
By moving or changing a magnetic field relative to the electrical conductor.


What are transformers?

Devices which change the potential difference of an electrical supply.


What is the structure of a transformer made up of?

Primary and secondary coil. Iron core.


How does a transformer work?

1) An alternating current flows through the primary coil in a transformer and produces a magnetic field which magnetises the iron core.
2) As there is an alternating current in the primary coil, the magnetic field in the iron core is alternating as well.
3) This constantly changing magnetic filed cuts through the secondary coil.
4) The changing field induces an alternating potential difference across the secondary coil by electromagnetic induction.
5) This potential difference causes an alternating current to flow and it has the same frequency as the alternating current in the primary coil.