Physics Flashcards
What are X-rays part of
The electromagnetic spectrum
Properties of X-rays
Very short wavelength
Cause ionisation
They affect a photographic film in the same way as light
They are absorbed by metal and bone
They are transmitted by soft tissue
Their wavelength is of the same order of magnitude as the diameter of an atom
What can X-rays be used for
Diagnosis and treatment of some medical conditions e.g. CT scans, bone fractures, dental problems and killing cancer cells
What are CCDs
Charge-coupled devices
What does the use of CCD’s allow
Images to be formed electronically
What must be taken when X-rays and CT scanners are in use
Precautions such as standing behind lead or lead glass shields
What can electronic systems can be used to produce
Ultrasound waves, which have a frequency higher than the upper limit of hearing for humans
What is the range of human hearing
20Hz to 20,000 Hz
When are ultrasound waves partially reflected
When they meet a boundary between two different media
How can you determine how far away a boundary is
Using the time taken for the reflections to reach a detector
What can ultrasound waves be used in
Medicine e.g. removal of kidney stones, pre-natal scanning
What is refraction
The change of direction of light as it passes through one medium to another
How does a lens from an image
By refracting light
What happens at the principal focus
In a convex or converging lens, parallel rays of light are brought to a focus
How do you work out the refractive index
sin i/sin r
i- angle of incidence
r - angle of refraction
What is the nature of an image defined by
Its size relative to the object, (magnified/diminished)
Whether it is upright or inverted relative to the object
Whether it is real or virtual
What are converging lens used as
Magnifying glass
What is the equation for magnification
Image height/ object height
Key features of the eye
Retina Lens Cornea Pupil/ iris Ciliary muscle Suspensory ligaments
What is long sight caused by
The eyeball being too short or the eye lens being unable to focus
What is short sight caused by
The eyeball being too long or the eye lens being unable to focus
What is the power of a lens given by
P = 1/f
P - power in dioptres, D
f - focal length in metres, m
What is the power of a converging lens
Positive
What is the power of a diverging lens
Negative
What is our range of vision
The eye can focus on objects between the near point (approx. 25 cm) and the far point (infinity)
What is the film in a camera or the CCDs in a digital camera equivalent to
The retina in the eye
What is the focal length of a lens determined by
The refractive index of the material from which the lens is made
The curvature of the two surfaces of the lens
For a given focal length, the greater the refractive index …
… The flatter the lens. This means that the lens can be manufactured thinner
How do you work out the refractive index
Refractive index = 1/sin c
c - critical angle
How can visible light be sent
Along optical fibres e.g. the endoscope for internal imaging
What is the laser an energy source for
Cutting
Cauterising
Burning
e.g. eye surgery
What is the centre of mass of an object
The point at which the mass of the object may be thought to be concentrated
If freely suspended, where will an object come to rest
With its centre of mass directly below the point of suspension
What is the centre of mass of a symmetrical object along
The axis of symmetry
How do you calculate a time period of a simple pendulum
T=1/f
T= time period f = frequency in hertz, Hz
What does the time period depend on
The length of a pendulum
What is the moment
The turning effect of a force
How do you calculate the size of a moment
M= F * d
M = moment in Newton-metres, Nm F= force in newtons, N d = perpendicular distance from the line of action of the force to the pivot in metres, m
If an object is not turning, what must the total clockwise moment be balanced by
The total anti-clockwise moment about any pivot
What are levers
Force multipliers
What happens if the line of action of the weight of an object lies outside the base of the object
There will be a resultant moment and the body will tend to topple
What is virtually incompressible
A liquid
How is the pressure in a liquid transmitted
Equally in all directions
What enables hydraulic systems to be used as a force multiplier
The use of different cross-sectional area on the effort and load side of a hydraulic system
What is the pressure in different parts of a hydraulic system is given by
P= F/A
P - pressure in pascals, Pa
F - force in newtons, N
A - cross-sectional area in metres squared, m2
When an object moves in a circle, where does it continuously accelerate towards
The centre of the circle. This acceleration changes the direction of the body, not its speed
The centripetal force needed to make an object perform circular motion increases as:
The mass of the object increases
The speed of the object increases
The radius of the circle decreases
What happens when a current flows through a wire
A magnetic field is produced around the wire
The size of the force can be increased by:
Increasing the size of the magnetic field
Increasing the size of the current
When will the conductor not experience a force
If it is parallel to the magnetic field
When is the direction of the force reversed
If either the direction of the current or the direction of the magnetic field is reversed
If an electrical conductor ‘cuts’ through a magnetic field, what happens
A potential difference is induced across the ends of the conductor
What happens if a magnet is moved into a coil of wire
A potential difference is induced across the ends of the coil
In a step-up transformer the potential difference across the secondary coil is greater than …
… The potential difference across the primary coil
If transformers are assumed to be 100% efficient, what should the electrical power input equal
The electrical power output
What frequency do switch mode transformers operate at
A high frequency, often between 50kHz and 200kHz
Compared to traditional transformers working from a 50 Hz, switch mode transformers are :
Much lighter
Smaller
More efficient
When do switch mode transformers use very little power
When they are switched on but no load is applied
What is a time period in terms of pendulums
Time taken for a full swing
What is frequency in terms of pendulums
The number of swings in one second
What is a swing of a pendulum
A complete oscillation
The longer the pendulum …
The greater the time period
The lesser the frequency
(Time period and frequency have an inverse relationship)
The shorter the pendulum …
The lesser the time period
The greater the frequency
Where will force exerted on a liquid be transmitted to
Other points in the liquid
What happens to wires when a high current passes through them
Heats up and a strong magnetic field
If you reduce the current, what do you have to do to the voltage
Increase
What device can be used to increase or decrease the voltage of the ac
Transformers
Why is electricity generated as a.c. for the National Grid
Transformers don’t work with d.c.
How do you generate induced current
Move a magnet into a coil
What happens when you move a wire through a magnetic field
A current is induced
How could you get a simple electric motor to spin the coil faster
Use a stronger magnet
Use a coil with more turns
Increase the voltage
How could get a simple electric motor to make the coil spin in the opposite direction
Reverse the cell
Swap the magnets
Where does magnetic field come out of
The North
What is Fleming’s left-hand rule
Rule used to identify the direction of a force acting on a current-carrying wire in a magnetic field
What is the thumb representing in Fleming’s left-hand rule
Direction of the force / motion
What is the first finger representing in Fleming’s left-hand rule
Direction of the field
What is the second finger representing in Fleming’s left-hand rule
Direction of current
How to decrease the force acting on the wire
Use weaker magnets
Use less current
Rotation of magnets so the field is no longer perpendicular
When is the strongest force created in the motor effect
When the field and wire (current) are at 90 degrees
Why are transformers insulated
So that the current does not enter the core
How do transformers work
Alternating current in the primary coil produces a changing magnetic field in the core. This induces an alternating potential difference across the secondary coil
What is alternating current
One that reverses direction
5Hz of a.c. means that the direction is changed 5 times a second
What is the centripetal force provided by
Tension
Friction
Gravity
If the centre of mass is closer to the ground …
…The more stable it is
The more points of contact to a surface …
… The more stable an object is
In regular shapes, where is the centre of mass
Where the lines of symmetry cross. Therefore their line of action is inside the base each time. The shapes are in a state of balance or equilibrium
How to find the centre of mass of an irregular shape
Hang a plumb line
Suspend card on the plumb line and draw a straight line
Suspend on a different point of the card and draw another line
Find where it crosses
What is a plumb line
Mass on a string
What does the strength of reflection of ultrasound waves tell you
Whether the substance is hard or soft
What’s the speed of light
3*10^8
What doesn’t happen when a light travels at the normal
Refraction
What are converging lens
Convex lens
What are diverging lens
Concave lens
Focal point
Point at which light rays meet`
How to find the focal length
By focussing a distant object on a piece of paper through the lens. The focal length is the distance between the centre of the lens and the image
What lens is in our eye and drawn in ray diagrams
Converging
Where do rays parallel to principal axis always meet
At the focal point/ principal focus
When is the image real
When the rays meet at a point
When is the image virtual
When the rays don’t meet at a point
Centre axis
Line of symmetry through lens
What image is produced when the object is at more than 2F
Real
Inverted
Diminished
What image is produced when the object is at 2F
Real
Inverted
Same size
What image is produced when the object is between 2F and F
Real
Inverted
Magnified
What image is produced when the object is at F
No image is produced
What image is produced when the object is less than F
Virtual
Upright
Magnified
What image is produced with a diverging lens
Virtual
Diminished
Upright
Why does the eye have to work at focussing on near objects
At rest the eye naturally focuses on distant objects
What do the cornea and lens work together to do
Focus the image on the retina
Where does most the refraction occur in the eye
Cornea
What is ciliary muscles attached to
Suspensory ligaments
How does the lens provide fine focus
By changing shape to provide for more or less refraction
What s the lens made of
Elastic fibres and is attached to the suspensory ligaments
What happens when the ciliary muscles are relaxed
Suspensory ligaments pull tight
Lens pull tight (thin)
Light doesn’t bend
You can focus on distant objects
What happens when the ciliary muscles contract
Suspensory ligaments relax
Lens relax (fat)
Light bends
You can focus on near objects
How to correct long sightedness
Using a convex lens to focus light rays before they reach the lens
How to correct short sightedness
Using a concave lens
Similarities between the eye and cameras
Shutter is similar to iris
Film/CCD is similar to retina
Aperture is similar to pupil
Both have lens
When is the focal length shorter
When the lens has a higher refractive index
When the lens is more curved
Where is the critical angle
Where total internal reflection takes place
What does the ciliary muscles do
Pull the lens for focusing
What does the cornea do
Lets light into the eye and begins focusing (refracting)
What does the iris do
Control the amount of light entering the eye
What does the lens do
Focus light onto the retina
What does the optic nerve do
Send signals to the brain
What does the pupil do
Let light through to the lens
What does the retina do
Light sensitive layer - sends signals to the optic nerve
What does the suspensory ligament do
Hold the lens in place
What are CCD’s
Silicon chips about the size of a postage stamp, divided up into a grid of millions of identical pixels
They detect X-rays and produce electronic signals which are used to form high resolution images
How do CT scans work
A patent is put inside the cylindrical scanner and an X-ray beam is fired and picked up by detectors on the opposite side
X-ray tube and detectors are rotated throughout
A computer interprets signals to form 2D images
How are X-rays used to treat cancer
X-rays are focused on the tumour using a wide beam
Beam is rotated around patient with tumour at centre
This minimises the exposure of normal cells to radiation and reduces damage
What can you not do with a virtual image
Project it onto a screen
Examples of levers as force multipliers
Long sticks or bars
Wheelbarrows
Scissors
When is there a resultant moment
When the total anti-clockwise moments do not match the total clockwise moments
The object will then move
What is a magnetic field
A region where magnetic materials and wires carrying a current experience a force acting on them
Solenoid
Coil of wire
What is an electromagnet
A magnet whose magnetic field can be turned on and off with an electrical current
What is the function of a split-ring commutator
Swap the contacts every half turn to keep the motor rotating in the same direction
Electromagnetic induction
The creation of a potential difference across a conductor which is experiencing a change in magnetic field
Which coil has more turns in a step up transformer
Secondary
Which coil has more turns in a step down transformer
Primary
Where does current flow from
The positive to the negative
Where do electrons flow from
Negative to positive
In which materials do light travel fastest
Ones with lower refractive index
When is the only time total internal reflection can happen
When light is travelling from a more dense medium to a less dense one
