Physics paper 1 Flashcards
1
Q
what is a vector and name some examples
A
vector: a quantity with magnitude and direction
- force
- velocity
- displacement
- weight
- acceleration
- momentum
2
Q
what is a scalar and name some examples
A
scalar: a quantity with only magnitude
- speed
- distance
- mass
- energy
- temperature
- time
3
Q
define displacement
A
a vector quantity and it measures the distance and direction in a straight line from an object’s starting point to its finishing point
4
Q
what will the velocities of two bikes be that are going at the same speed in opposite directions
A
their speed is the same but their velocities will be different because they are going in different directions
5
Q
what is the difference between speed and velocity
A
speed is a measure of how fast something is going meaning you can have objects travelling at a constant speed with a changing velocity ( same speed whilst changing direction)
velocity is a speed in a given direction
6
Q
what is the average walking speed
A
1.4 m/s or 5km/h
7
Q
define acceleration
A
the change in velocity over time, deceleration is just negative acceleration
8
Q
what is the equation for acceleration
A
final velocity - initial velocity over time
9
Q
what is constant acceleration sometimes called
A
uniform acceleration
10
Q
what is acceleration due to gravity
A
uniform/constant for objects in free fall and it is roughly equal to 10m/s
11
Q
what is the equation for uniform acceleration
A
v squared - u squared = 2 x a x x
final velocity squared - initial velocity squared = 2 x acceleration x distance
12
Q
A van travelling at 23m/s starts decelerating uniformly at 2.0m/s squared as it heads towards a built up area 112m away. What will its speed be when it reaches the built up area
A
use the equation v squared - u squared = 2 x a x x
rearrange the equation so v squared is on its own because that is final velocity
v squared = u squared + ( 2 x a x x)
v squared = 23 squared + (2 x -2.0 x 112)
its negative because its deceleration
v squared = 81
square root of 81
v = 9m/s
13
Q
what do these mean on a distance time graph
- gradient
- flat section
- a steep line
- curve
- curve getting steeper
- a levelling off curve
A
- gradient = at any point it gives the speed of an object
- flat section = where the object is stationary
- a steep line = going faster
- curve = acceleration
- curve getting steeper = speeding up and increasing gradient
- a levelling off curve = slowing down and decreasing gradient
14
Q
how do you calculate speed on a distance time graph
A
gradient = change in the vertical over change in the horizontal
15
Q
how do you work out speed on a distance time graph if the line is curved
A
you draw a tangent to the curve and work out the gradient of the tangent
change in vertical over change in horizontal
16
Q
how do you find the average speed of a curved line on a distance time graph
A
use the equation speed = distance over time
17
Q
what do these mean on a velocity time graph
- gradient
- flat sections
- steeper sections
- uphill sections
- downhill sections
- a curve
A
- gradient = acceleration since it is change in velocity over time
- flat sections = a steady speed
- steeper sections = the greater the acceleration or deceleration
- uphill sections = acceleration
- downhill sections = deceleration
- a curve = changing acceleration
18
Q
how do you find the acceleration on a velocity time graph if the line is curved
A
use a tangent to the curve and find the acceleration of that point by finding the gradient of the tangent
19
Q
how do you find the distance travelled on a velocity time graph
A
the area under any section of the graph or all of it is equal to the distance travelled in that time interval
area under the curve
if the accelerations constant then you can split the area into triangles and rectangles to work it out
if the accelerations irregular you should count the number of squares beneath the line and multiply the number by the value of one square
20
Q
what is newtons first law
A
a resultant force is needed to make something start moving, speed up or slow down
if the resultant force on a stationary object is zero the object will remain stationary
if the resultant force on a moving object is zero it will just carry on moving at the same velocity (same speed and direction)
21
Q
when will the velocity change on a moving object
A
if the resultant forces isn’t zero, it will always produce acceleration or deceleration in the direction of the force
22
Q
a ball is dropped from a height, h, above ground. The speed of the ball just before it hits the ground is 5m/s. Calculate the height the ball is dropped from. Acceleration due to gravity is 10m/s squared?
A
this is uniform acceleration v squared - u squared = 2 x a x x u = 0 v = 5m/s a = g which is 10m/s rearrange the equation to get x on its own which is distance so height in this case x = (v squared - u squared) ÷ 2 x a (25-0) ÷ (2 x 10) = 1.25 m
23
Q
what are the forces on an object like if something is moving at a constant velocity
A
the forces are balanced
24
Q
what five forms can acceleration take
A
- starting
- stopping
- speeding up
- slowing down
- changing direction
25
describe newtons second law
directly proportional
the larger the force the more the object accelerates and this can be written as f ∝ a
the equation for this relationship is:
force = mass x acceleration
N = kg x m/s squared
acceleration is inversely proportional to the mass of an object because an object with a larger mass with accelerate less than an object with a smaller mass for a fixed resultant force
26
why are large decelerations dangerous
large decelerations of objects/people like in car crashed can cause injury
this is because a large deceleration requires a larger force to be stopped due to f = m x a ( force = mass x acceleration)
27
how can the force of large decelerations be reduced
by slowing the object down over a longer period of time and therefore decreasing its acceleration slower
we do this with safety features in cars such as seatbelts, air bags and crumple zones
28
how do seatbelts airbags and crumple zones increase collision time to reduce the force and dangers of large decelerations
seatbelts - stretch
airbag - slow you down gradually and act as a cushion that will take some of the force so you don't have to take it all
crumple zones - at the front and bag and crumple up easily in a collision increasing the time taken to stop and taking in some of the force from the collision
29
what can happen to the brakes of a car during large decelerations
the brakes of a vehicle do work on its wheels which transfers energy from the vehicles kinetic energy store to the thermal energy stores of the brakes. Very large decelerations may cause the brakes to overheat so they don't work as well which may cause the vehicle to skid and not be able to slow down enough to reduce force to a safe amount during a collision
30
what is mass
the amount of matter in an object and an object will have the same mass anywhere in the universe
31
how do you measure mass
mass balance
32
define weight
the force acting on an object due to gravity ( the pull of gravitational force on the object)
close to the earth this is caused by the gravitational field around the Earth
it is acting from a single point on the object called its centre of mass ( a point at which you assume the whole mass is concentrated)
measured in Newtons
33
how is weight measured
a calibrated spring balance called a newton meter
34
how do you calculate weight
```
weight = mass x gravitational field strength
N = Kg x N/Kg
```
35
what does gravitational field strength do with location
it varies
its stronger the closer you are to the mass causing the field and more massive objects to create stronger fields
this means that the weight of an object changes with location
36
a) what is the weight in newtons of a 2.0kg chicken on earth (10N/Kg)
b) the chicken has a weight of 16N on a mystery planet what is the gravitational filed strength of the planet
use the equation: w = m x g
a) 2.0kg x 10N/kg = 20 N
b) remember the mass of the chicken is the same on every planet it is only weight that changes
g = w ÷ m
16N ÷ 2.0kg
= 8.0N/Kg
37
does mass or weight of an object change between locations?
the weight changes the mass of an object is the same everywhere
38
what happens if an object is travelling in a circle at a constant speed
even if its speed is constant it is constantly changing direction so it is constantly changing velocity which means it is accelerating
this means there must be a resultant force acting on it
this force acts towards the centre of the circle
this force that keeps the object moving in a circle is called CENTRIPETAL FORCE
39
what is the core practical for investigating Newtons second law ( force = mass x acceleration )
motion of a trolley on a ramp:
- measure mass of trolley, masses and hanging hook and length of card to break the light gates
- adjust the height of the ramp so the trolley just starts to move to ensure that any other forces applied will be the main cause of the trolley accelerating as it travels down the ramp. The size of this acceleration depend on the mass of the trolley and the size of the accelerating force.
- mark a line before the first light gate so the trolley starts at the same point each time
- attach the trolley to mass hanger by string and hold trolley still
- let go and the time and speed will be recorded beneath each light gate
- the acceleration of the trolley can be used by acceleration = change in speed/time
40
during the trolley ramp experiment how do you change it to investigate:
- effect of trolley's mass
- effect of the accelerating force
mass:
- add masses one at a time to the trolley. keep the mass on the hook constant so the accelerating force is constant. Then carry out the experiment by letting the trolley roll down the ramp
accelerating force:
- start with all the masses loaded onto the trolley and transfer the masses to the hook one at a time and carry out the experiment each time you move a mass
- you transfer the masses because you need to keep the mass of the whole system (mass of trolley + mass on hook) the same because the accelerating force causes both the trolley and the hanging masses to accelerate
41
what should you find from the investigating motion (trolley on ramp) core practical
as the accelerating force increases the acceleration increases for a given trolley mass
as the mass of the trolley increases its acceleration decreases for a given force because mass and acceleration are inversely proportional
42
why are light gates the best option for short time intervals
they get rid of human error caused by reaction times
43
what equipment can you use to measure distance and time
- light gates
- for something large like a person's walking speed you can use a rolling tape measure and markers
- stopwatch
- record a video of the moving object and look at how far it travels each frame and if you know how ,any frames per second the camera records you can find the distance travelled by the object in a given number of frames and the time it takes to do so.
44
what is inertia
the tendency for motion to remain unchanged
until acted on by a resultant force, objects at rest will stay at rest and objects moving at a constant velocity will stay moving at that velocity (Newtons first law)
This tendency to keep moving with the same velocity is inertia
45
what is inertial mass
a measure of how difficult it is to change the velocity of an object
46
how can inertial mass be found
Inertial mass can be found using Newton's second law
F=m x a
you just rearrange it to give mass = force / acceleration so inertial mass is just the ratio of force over acceleration
47
what is Newton's third law
when two objects interact, the forces they exert on each other are equal and opposite
if you push something like a shopping trolley the trolley will push back just as hard and as soon as you stop pushing so does the trolley
48
If Newton's third law states that the forces are always equal, how does anything go anywhere?
because the two forces are acting on different objects
49
what happens when skater A (55kg) pushes against skater B (65kg)
when skater A pushes against skater B (the 'action' force ) she feels an equal and opposite force from skater B's hand - the normal contact force
both skaters feel the same sized force in opposite directions and so will accelerate away from each other
skater A will be accelerated more than skater B because she has a smaller mass (force = mass x acceleration)
these equally sized forces in opposite directions also explain the principle of the conservation of momentum
this is an example of forces not in equilibrium
50
what is the normal contact force
the equal and opposite force two objects exert and feel on one another
51
discuss Newton's third law for two objects in equilibrium e.g a book on a table
the weight of the book pulls it down and the normal reaction force from the table pushes it up. these forces are equal to each other because the book is in equilibrium and doesn't move
However this is NOT NEWTON'S THIRD LAW because the forces are different types and both acting on the book
the pairs of force DUE TO NEWTON'S THIRD LAW are:
- the book is pulled down by its weight due to gravity and the book also pulls back up on the earth
- the normal contact force from the table pushing up on the book and the normal contact from the book pushing down on the table
52
a full shopping trolley and an empty one are moving at the same speed. Explain why it is easier to stop the empty trolley that the full trolley over the same amount of time
an object with a smaller mass (in this case the empty trolley) will have a smaller inertial mass so less force is needed to stop it
53
what is momentum
- how much oomph something has
- it is the product of an objects mass and velocity
the greater the mass of an object or the greater its velocity the more momentum it has
- a vector quantity with size and direction
54
what is the equation for momentum
momentum = mass x velocity
p = m x v
kg m/s = kg x m/s
55
what is the conservation of momentum
in a closed system the total momentum before an event e.g a collision is the same as after the event
56
in snooker balls of the same size and mass collide with each other Each collision is an event where the momentum of each ball changes but the overall momentum stays the same (conservation of momentum)
explain why this happens
the red ball is stationary so it has zero momentum. the white ball is moving with velocity v so has a momentum of p = m x v
the white ball hits the red ball causing it to move and the red ball now has momentum. The white ball continues moving but at a much smaller velocity and so a much smaller momentum
the combined momentum of the red and white ball is equal to the original momentum of the white ball m x v
the white ball almost shares its velocity with the red ball
57
A 1500kg car travelling at 25m/s crashes into the back of a parked car. The parked car has a mass of 1000kg. The two cars lock together and continue moving in the same direction as the original moving car. Calculate the velocity that the two cars move with.
```
p = m x v
1500 x 25 = 37500
1500kg + 1000kg = 2500kg
v = p/m
375000 / 2500 = 15 m/s
```
58
what happens when a resultant force acts on an object for a certain amount of time
a change in momentum
a resultant force on an object causes it to accelerate
force = mass x acceleration
acceleration is just a change in velocity over time so
force = mass x change in velocity all over time
this means a force applied to an object over any time interval will change the objects velocity
mass x change in velocity is equal to momentum
so you end up with:
```
force = change in momentum over time
Newtons = kg m/s divided by sec
```
59
the faster a given change in momentum happens......
the bigger the force causing the change must be
| if time gets smaller, then force gets bigger
60
if someone's momentum changes very quickly...…..
like in a car crash the forces on the body will be very large and likely to cause injury
61
how can you show the conservation of momentum with snooker balls
before the collision the white ball has a momentum of 0.15kg x 4m/s = 0.6 kg m/s
the red ball has a momentum of zero so the momentum of the whole system is 0.6 kg m/s
when the balls collide the white ball exerts a force on the red ball causing it to start moving
but due to Newton's third law the red ball also exerts an equal and opposite reaction of the white ball causing the white ball to slow down
the collision lasts 0.1s. after the collision the white ball continues to move a 1m/s and the red ball begins moving at 3 m/s
the total momentum is
(0.15 x 1 ) + (0.15 x 3) = 0.6 kg m/s
so momentum is conserved
you can calculate the size of the force that caused this change in velocity and so change of momentum for each ball
the force exerted on the white ball by the red ball is equal and opposite to the force exerted on the red ball by the white ball. This shows Newton's third law
change in momentum over time
62
what is stopping distance and what is the equation
the distance covered between first spotting the hazard and the vehicle coming to a complete stop
stopping distance = thinking distance + braking distance
63
what is thinking distance and what affects it
the distance the car travels in the drivers reaction time
between noticing the hazard and applying the brakes
affected by:
- reaction time which is normally 0.25 seconds and affected by tiredness, alcohol, drugs and distractions
- your speed, the faster your going the further you'll travel during your reaction time
64
what is braking distance and what affects it
the distance taken to stop once the brakes have been applied
affected by:
- your speed
- the mass of the car
- state of the brakes
- friction between the tyres and road surface
- state of the road surface
65
in relation to thinking distance as speed increases …..
so does thinking distance at the same rate
66
in relation to braking distance and speed what is their relationship
braking distance and speed have a squared relationship
if speed doubles braking distance increases by a factor of four
if speed trebles braking distance increases by a scale factor of 9
67
what do brakes of a car do
they do work on the car's wheels
this transfers energy from the car's kinetic energy store to the thermal energy store of the brakes
to stop a car the brakes must transfer all of this energy so:
energy in the car's kinetic energy store = work done by the brakes
0.5 x m x v squared = f x d
68
what are the 6 stores of energy
```
kinetic
thermal
chemical
nuclear
elastic potential
gravitational potential
```
69
discuss kinetic energy store
when an object is moving it has energy in its kinetic energy store
energy is transferred to this store if an object speeds up and away from this store if an objects slows down
how much energy is in this store depends on mass and speed
the greater its mass and the faster its going the more energy it has in its kinetic energy store for example a high speed train will have a lot more energy in its kinetic energy store than you running
70
what is the kinetic energy equation
1/2 x mass x speed squared
71
what happens to kinetic energy store if you double the mass and double the speed
if you double the mass then the kinetic energy store doubles
if you double the speed the energy in the kinetic energy store quadruples because speed is squared
72
a car of mass 1450kg is travelling at 28m/s. Calculate the energy in its kinetic energy store giving your answer to 2s.f
0.5 x 1450 x 28 squared = 568400
| 570,000 J to 2s.f
73
when an object is at any height above the Earth's surface, it will have energy in its gravitational potential energy store
how do you calculate the change in energy in the gravitational potential store
ΔGPE = m x g x Δh
74
what is the conservation of energy
energy can be stored, transferred between stores and dissipated - but it can never be created or destroyed. The total energy of a closed system has no net change
75
what does it mean if you get a question where the energy of a system increases or decreases
it is not a closed system but you can make it into a closed system by increasing the number of things you treat as part of it
e.g a pan of water heating on a hob isn't a closed system but the pan, gas and the oxygen that burnt to heat it and there surroundings are a closed system
76
what are the four ways energy can be transferred
mechanically:
a force acting on an object and doing work e.g pushing or stretching
electrically:
a charge doing work against resistance e.g charges moving round a circuit
by heating:
energy transferred from a hotter object to a colder object e.g heating a pan on a hob
by radiation:
energy transferred by waves e.g energy from the sun reaching Earth by light
77
discuss the energy stores of a ball rolling up a slope
the ball does work against the gravitational force so energy is transferred mechanically from the kinetic energy store if the ball to its gravitational potential energy store
78
discuss the energy stores of a bat hitting a ball
the bat has energy in its kinetic energy store. Some of this is transferred mechanically to the thermal energy stores of the bat and the ball (and to the surroundings by heating). The rest is carried away by sound
79
discuss the energy stores of a rock dropped from a cliff
assuming there is no air resistance, gravity does work on the rock, so the rock constantly accelerates towards the ground. Energy is transferred mechanically from the rock's gravitational potential energy store to its kinetic energy store
80
discuss the energy stores of a car slowing down (without braking)
energy in the kinetic energy store of the car is transferred mechanically (due to friction between tires and the road) and then by heating to the thermal energy stores of the car and road
81
discuss the energy stores of a kettle boiling water
energy is transferred electrically from the mains to the heating element of the kettle and the by heating to the thermal energy store of the water
82
what does the conservation of energy principle mean regarding efficiency
total energy input = useful energy output + wasted energy
83
what can efficiency never be
100% or equal to 1
84
how can we increase efficiency
lubrication for friction or thermal insulation for heating
85
how does lubrication reduce energy transferred by friction
whenever something moves there is usually at least open frictional force acting against it
this transfers energy mechanically (work is done against friction) to the thermal energy store of the objects involved which is then dissipated by heating to the surroundings.
for example pushing a box along the ground causes energy to be transferred mechanically to the thermal energy stores of the box and the ground. This energy is then radiated away to the thermal energy store of the surroundings
for objects that are touching each other, lubricants can be used to reduce the friction between the object's surfaces when they move. Lubricants are usually liquids like oil so they can flow easily between objects and coat them.
86
how does insulation reduce the rate of energy transferred by heating
when one side of an object is heated the particles in the hotter part vibrate more and collide with each other. This transfers energy from their kinetic energy stores to other particles which then vibrate faster - this is called conduction and it transfers energy through the object
all materials have a thermal conductivity - it describes how well a material transfers energy by conduction
metals - high thermal conductivity
gases (like air) - low thermal conductivity
in a building the lower the thermal conductivity of its walls, the slower the rate of energy transfer through them meaning the building will cool more slowly
some houses have cavity walls made up of an inner and an outer wall with an air gap in the middle. the air gap reduces the amount of energy transferred by conduction because the air has a very low thermal conductivity
thicker walls help too - the thicker the wall, the slower the rate of energy transfer
87
discuss advantages and disadvantages of using fossil fuels
advantages;
- they are reliable and there is enough around to meet current demand
- cost to extract fossil fuels is low
- fossil fuel plants are relatively cheap to build and run
disadvantages:
- nuclear power plants are costly to build and to safely decommission
- fossil fuels are slowly running out
- fossil fuels release carbon dioxide into the atmosphere when they're burned which adds to the greenhouse effect and contributes to global warming
- burning coal and oil can release sulphur dioxide which causes acid rain. Acid rain can be reduced by taking the sulphur our before the coal is burned or cleaning up the emissions
- oil spillages affecting the wildlife
- nuclear power is clean but the nuclear waste is very dangerous to dispose of
88
what are the renewable energy resources
```
bio-fuel
wind
solar
hydro-electric
tidal
```
these will never run out
most of the damage the environment but not as much as non-renewable resources
they don't provide much energy
unreliable because they depend on the weather.
89
discuss bio-fuels
renewable resources made from plant products or animal dung
solid, liquid or gas and can be burnt to produce electricity or run car in the same way as fossil fuels
they are carbon neutral but only if your growing plants or raising animals at the same rate as your burning things
they are fairly reliable because crops take a short time to grow and different crops can be grown all year round
however they cannot immediately respond to energy demands so to combats this they are produced and stored for when they are needed
the cost to refine bio-fuels is very high and some worry that growing crops specifically for bio-fuels will mean there isn't enough space or water to meet the demands for crops that are grown for food
in some regions large areas of forest have been cleared to make room to grow bio-fuels resulting in lots of species losing their natural habitats. The decay or burning of this cleared vegetation also increases methane and CO2 emissions.
90
what do waves transfer
energy
91
what happens when a wave travels through a medium
the particles of the medium vibrate and transfer energy and information between each other. But overall the particles stay in the same place.
For example, if you drop a twig in a calm pool of water, ripples form on and move across the water's surfaces. The ripples don't carry the water (or the twig) away with them though
Similarly if you strum a guitar string and create a sound wave, the sound wave travels to your ear (so you can hear it) but it doesn't carry the air away from the guitar - if it did it would create a vacuum
92
what is the amplitude of a wave
the displacement from the rest position to a peak (crest) or trough
93
what is the wavelength of a wave
the length of a full cycle of the wave 9e.g crest to crest or trough to trough)
94
what is the frequency of a wave
the number of complete cycles of the wave passing a certain point per second
frequency is measured in Hz (hertz)
1 Hz is 1 wave per second
95
what is the period of a wave
the number of seconds it takes for one full cycle
| period = 1 / frequency
96
transverse waves
the vibrations are perpendicular to the direction the wave travels
- all EM waves
- s-waves
- ripples and waves in water
97
longitudinal waves
the vibrations are parallel to the direction the wave travels
- sound waves
- p-waves
longitudinal waves squash up and stretch out the arrangement of particles in the medium they pass through, making compressions (high pressure, lots of particles) and refractions (low pressure, fewer particles)
98
what are the two equations for wave speed
wave speed = distance/speed
| wave speed = frequency x wavelength
99
how can you measure the speed of sound
use an oscilloscope
by attaching a signal generator to a speaker you can generate sounds with a specific frequency. You can use two microphones and an oscilloscope to find the wavelength of the sound waves generated
1) set up the oscilloscope so the detected waves at each microphone are shows as separate waves
2) start with both microphones next to the speaker, then slowly move one away until the two waves are aligned on the display but have moved exactly one wavelength apart
3) measure the distance between the microphones to find one wavelength
4) You can then use the formula v = fλ to find the speed of the sound waves passing through the air - frequency is whatever you set the signal generator to in the first place
100
how can you measure the speed of water ripples
using a strobe light
1) using a signal generator attached to the dipper of a ripple tank you can create water waves to a set frequency
2) dim the lights and turn on the strobe light - you'll see a wave pattern made by the shadows of the wave crests on the screen below the tank
3) alter the frequency of the strobe light until the wave pattern on the screen appears to 'freeze' and stop moving. This happens when the frequency of the waves and the strobe light are equal - the waves appear not to move because they are being lit at the same point in their cycle each time.
4) The distance between each shadow line is equal to one wavelength. Measure the distance between lines that are 10 wavelengths apart, then find the average wavelengths
5) use v = fλ to calculate the speed of the wave
you can find the frequency by using a regular light so you can see the waves moving. Count how many waves pass a mark on the screen in a given time, then divide this by the time in seconds to find the frequency
101
how can you use peak frequency to find the speed of waves in solids
you can find the speed of waves in a solid by measuring the frequency of the sound waves produced when you hit the object e.g a rod with a hammer. Hitting the rod causes waves to be produced along the rod. These waves make the rod vibrate and produce sound waves in the air around the rod (this is how a percussion triangle works)
these sound waves have the same frequencies as the waves in the rod
1) measure and record the length of a metal rod
2) set up the apparatus shown in the diagram making sure to secure the rod at its centre
3) tap the rod with the hammer. write down the peak frequency displayed by the computer
4) repeat this three times to get an average peak frequency
5) calculate the speed of the wave using v = fλ which λ is equal to twice the length of the rod
102
what happens when a wave meets a boundary between two materials (a material interface)
three things can happen
ABSORBED - by the second material, the wave transfers energy to the material's energy stores. Often the energy is transferred to a thermal energy store, which leads to heating (how a microwave works)
TRANSMITTED - through the second material, the wave carries on traveling through the new material. This often leads to refraction which can be used in communications as well as in the lenses of glasses and cameras
REFLECTED - this is where the incoming ray is neither absorbed or transmitted but instead is 'sent back' away from the second material. This is how echoes are created
what happens depends on the wavelength of the wave and the properties of the materials involved
103
discuss refraction
- waves travel at different speeds in materials with different densities so when a wave crosses a boundary between materials it changes speed
- if the wave hits the boundary at an angle, this change of speed causes a change in direction - refraction
- if the wave is travelling along the normal it will change speed but its not refracted because it won't change direction
- the greater the change in speed, the more a wave bends
- the wave bends towards the normal if it slows down and away from the normal if it speeds up
- EM waves like light usually travel more slowly in denser materials
- how much an EM wave refracts can be affected by its wavelength - shorter wavelengths bend more. This can lead to the wavelengths spreading out (dispersion) e.g white light becoming a spectrum
- the frequency of a wave stays the same when it crosses a boundary as v = fλ this means that the change in speed is caused by a change in wavelength - the wavelength decreases if the wave slows down and increases as it speeds up
you might see refraction of light talked about in terms of optical density
104
what determines what frequencies of sound can be transmitted through an object
an object's size, shape and structure
105
how do we hear sound
when you eardrum vibrates
1) sound waves that reach your eardrum cause it to vibrate
2) these vibrations are passed on to tiny bones in your ear called ossicles, through the semi-circular canals and to the cochlea
3) the cochlea turns these vibrations into electrical signals which get sent to your brain
4) the brain interprets the signals as sounds of different pitches and volumes, depending on their frequency and intensity. A higher frequency sound wave has a higher pitch
5) human hearing is limited by the size and shape of our eardrum, and the structure of all the parts within the ear that vibrate to transmit the sound wave
106
what is ultrasound
frequencies higher than 20,000 Hz
| it is above the range of human hearing
107
discuss ultrasound waves getting partially reflected at boundaries
when a wave passes from one medium into another some of the wave is reflected off the boundary between the other two media and some is transmitted and refracted. this is partial reflection
what this means is that you can point a pulse (shrt urst
108
what is nuclear fission
a type of nuclear reaction that is used to release energy from uranium (or plutonium) atoms in a nuclear reactor
huge amounts of energy can be released this way by using a chain reaction
1) a slow moving neutron is fired at a large unstable nuclei often uranium-235. The neutron is absorbed by the nucleus - this makes the atom more unstable and causes it to split
2) when the U-235 atom splits it forms two new lighter elements ('daughter nuclei') and energy is released
3) there are lots of different pairs of atoms that uranium can split into e.g krypton-91 and barium-143 but all these nuclei are radioactive
4) each time a uranium atom spilts up it also spits out two or three neutrons which can hit other uranium nuclei causing them to split also and so on and so on
this is a chain reaction
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how are chain reactions carefully controlled
the neutrons released by fission reactions in a nuclear reactor have a lot of energy and these neutrons will only cause other nuclear fissions and cause a chain reaction if they are they are moving slow enough to be captured by the uranium nuclei in the fuel rods. These slow moving neutrons are called thermal neutrons
to do this the uranium fuel rods are placed in a moderator (for example, water) to slow down the fast-moving neutrons
control rods made of boron limit the rate of fission by absorbing excess neutrons. They are placed in-between the fuel rods and are raised and lowered into the reactor to control the chain reaction
this creates a steady rate of nuclear fission where one neutron produces another fission
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what happen if the chain reaction in nuclear fission isn't controlled
if the chain reaction in a nuclear reactor is left to continue unchecked large amounts of energy are released in a very short time. Many new fissions will follow each fission causing a runaway reaction that could lead to an explosion
111
how are nuclear power stations powered
by nuclear reactors that create controlled chain reactions
the energy released by fission is transferred to the thermal energy store of the moderator. This is then transferred to the thermal energy store of the water (coolant) in the boiler which causes the water to boil and energy to be transferred to the kinetic energy store of the steam
this energy is then transferred to the kinetic energy store of the turbine and then to the kinetic energy store of a generator. The energy is then transferred away from the generator electrically.
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what is nuclear fusion
two light nuclei collide at high speed and fuse to create a larger heavier nucleus. For example hydrogen nuclei can fuse to produce a helium nucleus
this heavier nucleus does not have as much mass as the two separate light nuclei did. Some of the mass of the lighter nuclei is converted to energy which is then released as radiation
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when does fusion happen
fusion only happens at really high pressures and temperatures (at about 10,000,000) this is because the positively charged nuclei have to get very close to fuse so the strong force of electrostatic repulsion has to be overcome
its really hard to create the right conditions for fusion because no material can withstand that kind of temperature
fusion reactors are really hard and expensive to build
there are a few experimental reactors around at the moment but none of them are generating electricity yet. It takes more power to get up to temperature than the reactor can produce
