Physics Flashcards
Conductors will only retain charge if
they are insulated from their surroundings
How can an objected be charged
- friction
- induction
If something is earthed
Can not become charged
Which way the electrons move?
Determined by whichever object has nuclei that attract the electrons less strongly (loses electrons)
Two factors which affect the electrostatic force
Larger the charges = larger the force
Larger distance = smaller the force
Sparking
Air between two objected becomes ionised by a large voltage and starts conducting Two charged objects that have air between them can discharge by a spark between them (when charge is large enough/distance is small enough)
How can risk of sparking be eliminated
By earthing; or if they are connected together by a wire then electrostatic charging cannot take place
Photocopying and printing
Charge being placed on the paper; exposed to toner powder which sticks to the paper at those locations as a result of electrostatic induction
Aircraft refuelling
Large volumes of fuel flow through the pipe - large amounts of friction - pipe is electrostatically charged - thus pipe is always earthed to prevent build up of charge
Wires crossing - NOT connected
Wires connected
Battery
Group of cells
Dc power supply symbol
Ac power supply symbol
The output from a power supply from mains electricity can be converted from ac to dc using
diodes as a ‘rectifier’.
A diode only allows current in one direction, in the direction of the arrow on the symbol.
Uk maims supply
50 Hz
(i.e. the current changes direction 100 times each second, producing 50 complete ‘to and fro’ cycles in one second).
Examples of good conductors:
- all metals, particularly copper, gold and silver
- carbon (in the form of graphite)
- ionic solutions.
Examples of good insulators (poor conductors):
- most non-metals, particularly plastics, rubber, dry wood, air, vacuum.
Water, unless extremely pure, is a conductor, so wet or damp materials are not good insulators.
all materials allow…
electric charge to move through them to some extent
Q
quantity’ of charge.
why metals are good conductors
ey contain free electrons that can move about through the metal and carry their (negative) charge with them
If a voltage is connected across a metal…
positive end of the metal attracts electrons and the negative end repels electrons.
In this way the electrons can move along the metal and cause a current (a flow of charge).
CURRENT DIRECTION
from the positive end of a conductor to the negative end
ELECTRON DIRECTION
Negative to positive
voltmeter is connected in
parallel
Why does a voltmeter need to have a HIGH resistance
otherwise it would tend to ‘short circuit’ the component across which it was connected
(because there would be a significant amount of current in the voltmeter instead of in the component).
Why does an ammeter need to have a low resistance
otherwise it would tend to reduce the amount of current that it was being used to measure.
Ohm’s law:
current is directly proportional to the voltage causing it at constant temp
resistance of the filament is not constant because
its temperature changes as the current in it changes
V-I graphs for fixed resistor + filament light bulb
thermistor
with a resistance that depends on its temperature.
Thermistor resistance
As its temperature increases, its resistance decreases.
Semi-conductor
LDR resistance
As the light intensity increases, the resistance of the LDR decreases.
Diode shows
Direction of current
Which one works
The temperature of the thermistor decreases.
What happens to the readings on meters 1 and 2?
1 = ammeter = decrease as more resistance from thermistors
2 = voltmeter = reduced current means reduced voltage in FIXED resistor as R is a constant in V=IR = causes voltage across thermistors to INCREASE, because total voltage supplied by battery has not changed
= voltmeter increases
Series
Current is same
Voltage adds up to total
Resistance adds up total
Parallel
Voltage is same
Current adds up in each branch
voltage
The difference in energy carried by each unit of charge either side of a circuit component (the energy lost or gained per unit charge)
How to find voltage across 2 resistors
Two resistors, of resistances 2Ω and 3Ω, are connected in series.
Combined resistance = 2 + 3 = 5Ω.
Note that 5Ω is greater than both 2Ω and 3Ω.
If these resistors are connected in series to a 10V supply, then the supply current = 10 / 5 = 2A.
The voltages across the two resistors are therefore 2 × 2 = 4V and 2 ×3 = 6V respectively.
Note that 4V and 6V add up to the supply voltage, 10V.
For two resistors of resistance R connected in series, the combined resistance is
R/2
Units of voltage
1V = 1JC-1
Two resistors, of resistances 3Ω and 6Ω, are connected in parallel. The parallel combination is connected in series with a third resistor, of resistance 4Ω, to a supply voltage of 18V. The 4Ω resistor dissipates a power of 36W.
How much energy is dissipated in the 6Ω resistor in 1 minute?
magnetic materials
iron, cobalt and nickel.
Magnetic field lines
NORTH TI SIUTH
Soft magnetic materials
easy to magnetise but also easily lose their magnetisation.
Iron
Hard magnetic materials
are difficult to magnetise but once they are magnetised, they are difficult to demagnetise.
Steel
Factors affecting the magnetic field created by an electric current
Reversing the direction of the current
Increasing the current increases
Proof to show electric current create magnetic field
Demonstrate by placing a small magnetic compass close to a current carrying conductor and then switching the current on and off ; compass needle will point north when the current is off and deflect from north when the current is on (current has created a magnetic field)
What is magnetic field actually created by
by these moving charges and not by the material through which they are moving (e.g. a copper conductor).
A beam of charged particles (e.g. electrons or ions) moving through a vacuum will also create a magnetic field, just like an electric current in a wire.
magnetic field pattern around a long, straight current-carrying wire:
consists of concentric circles,
that become farther apart at greater distance from the wire,
and have a direction given that can be predicted using a right-hand grip rule.
Right Hand rule
Magnetic field through solenoid
The strength of the magnetic field around a wire depends on:
the current in the wire: increasing current increases magnetic field strength
the distance from the wire: farther from the wire the field is weaker
the medium surrounding the wire: magnetic media such as iron can increase the field strength.
Iron
ferromagnetic material.
Each iron atom acts like a tiny bar magnet being north at one end and south at the other.
Differences between electromagnets and permanent magnets
How to increase strength if solenoid
- increase number of coils
- increase current
Why can’t increase current to crazy
Strong heating effect = melt insulation
How do strong electromagnets avoid this heat problem
by using superconducting coils, that is coils of zero resistance.
Disadvantage of superconducting coils
coils only become superconducting when cooled to extremely low temperatures using liquid helium.
A permanent magnet can be made by
placing a hard magnetic material in a strong external magnetic field, usually from an electromagnet.
How can hard / Permanent magnets suddenly lose their magnetism
Heated above curie temp = specific to each material
Maximum motor force
when the current and magnetic field are at right angles to one another.
Reversing the direction of the current OR the magnetic field..
Reversing the direction of the current OR the magnetic field
Reversing the directions of both the current and the magnetic field
results in no change in the direction of the motor effect force.
If the magnetic field and current are not at 90°
then the direction to use is the part (component) of the magnetic field that is perpendicular to the current, as shown in the diagram:
motor effect force is directly out of the page.
ANGLE
angle between magnetic field and current: force is greatest at 90° and zero at 0°.
Investigating the strength of the motor effect force
When there is a current in the wire there is an upward motor effect force on the wire.
By Newton’s third law, there is an equal downward force on the magnets.
The magnets press down on the top pan balance and the reading increases.
This change can be used to measure the motor effect force (using W = mg).
Tesla
1T = 1 Nm-1A-1
Angle in motor effect
when is force at maximum + minimum
It is at its maximum when the two forces are farthest apart (coil in plane of field)
zero when the two forces are in the same vertical plane (coil perpendicular to field).
graphite brushes in split
make a low friction sliding contact with the surface of the commutator but mai
The material used to make the brushes must be a
CONDUCTOR
Electromagnetic indication
ONLY HAPPENS IF THEIR IS A CHANGE = cutting field lines
Will Electromagnetic induction always induce voltage
Yes - but only current in a closed system
The magnitude of an induced voltage is directly proportional to:
the rate at which a wire cuts magnetic field lines
or
the rate at which the magnetic field through a conductor (e.g. a coil) changes.
The induced voltage will increase if:
- the magnet is moved faster – more field lines are cut per second – the rate of cutting field lines increases
- a stronger magnet is used – there is a higher density of field lines, so more field lines are cut per second than with a weaker magnet at the same speed – the rate of cutting field lines increases
the ac frequency in coil A is increased – the rate of change of the field through coil B increases
- the ac amplitude in coil A is increased – the rate of change of the field through coil B increases*.
Lenzes law
induced voltage is always in a direction that opposes the change that caused it.
The direction of an induced voltage reverses when:
the direction of the cutting of magnetic field lines reverses
an increasing magnetic field in a coil changes to one that is decreasing
a decreasing magnetic field in a coil changes to one that is increasing.
The amplitude of the output ac voltage increases if:
the coil is rotated more rapidly – greater rate of change of magnetic field through the coil (or of cutting magnetic field lines)
the magnetic field is stronger – greater rate of change of magnetic field through the coil (or of cutting magnetic field lines)
the coil has greater area –greater rate of change of magnetic field through the coil (or of cutting magnetic field lines)
there are more turns on the coil – each coil has the same induced voltage and these voltages add together because the turns are in series.
The frequency of the output ac voltage is equal to
the coil’s rotation frequency.
If the time for one rotation of the coil is doubled (generator)
the coil is rotating more slowly and it cuts the field at a lower rate inducing a smaller voltage
FREQUENCE HALFS USING f=1/time periods
Do the slip rings change polarity
Yes
For half of the rotation, one slip ring is positive and the other negative but in the other half of the rotation, the wires are moving through the field in the opposite direction, so the slip rings both change polarity, becoming negative and positive respectively.
Angle positions
Draw a graph of voltage output for one cycle
Increasing the frequency of rotation of the coil has two effects:
it increases the frequency of the output ac voltage because the direction of cutting of the field lines changes more rapidly
it increases the amplitude of the output ac voltage because the rate at which the field lines are cut increases.
What’s peak + what’s zero
Position 1 = peak
Position 2 = zero
Generators transfer
mechanical work (to rotate the generator) to electrical energy in the form of ac electricity
A step-up transformer
increases the voltage
Mains voltage
230V
Transformer equation
Vp = (ac) voltage across the primary coil
Vs = (ac) voltage across the secondary coil
np = number of turns on the primary coil
ns = number of turns on the secondary coil
Transform equation thing
only valid for an ideal transformer, i.e. one that is 100% efficient
Other transformer equation
the current ratio is the inverse of the voltage ratio
Put two transformer equations together
the current ratio is equal to the inverse ratio of the turns:
Why not 100% Effiecient
the resistance in the wires on the coils
heating effects in the core as it magnetises and demagnetises
currents induced in the core (eddy currents) by the changing magnetic field.
What is used in transmission lines
High voltage low current
- reduce losses due to heating of the cables.
The higher the voltage
the harder it is to insulate from other conductors.
Cause of weight
mass in a gravitational field
Cause of normal contact
two solid objects in contact with each other
Cause of drag
movement of an object through a fluid
Cause of friction
relative sliding motion between two solid surfaces
Magnetic
two magnets or a current in a magnetic field
Electrostatic
two charges or a charge in an electric field
upthrust
solid immersed in a fluid
thrust
driving force from an engine
lift
aerofoil (wing) moving through a fluid
non-contact forces
weight associated with a gravitational field,
magnetic force associated with a magnetic field
electrostatic force associated with an electric field
force-extension graph: steeper
steeper this graph, the more force is required to produce a given extension. = more rigid
The shallower this graph, the greater the extension for a given force.= less rigid
Elastic
if the spring / wire returns to its original length when the tension force is removed.
Hooke’s law.
Springs and wires that are being extended within their elastic limits experience an extension that is proportional to the tension force
F=kx
spring constant
force per unit extension
Spring constant is a measure of
Rigidity
Materials with high spring constants….
require large forces to produce small extensions
(Gradient)
Cross-sectional area + K
the greater the cross-sectional area, the greater the spring constant
Length + K
longer the wire, the smaller the spring constant
Combining springs - If two identical springs, each of spring constant k are connected together in series
same force will produce double the extension
so the spring constant is halved to 1/2K
Combing springs - two identical springs are connected in parallel
double the force will be needed to produce the same extension and so the spring constant is doubled to 2k
Force - extension = area under graph
work done by tension force / elastic potential = AS LONG AS ELASTIC LIMIT NOT EXCEEDED
Prove elastic limit formula
Mass + inertia
Mass is the property of an object that resists acceleration
Newton’s second law
resultant force is proportional to the rate of change of momentum
Two things about Newton’s third law that most people don’t know
the two forces in the pair are of the same type (e.g. both friction, or both normal contact, or both gravitational etc.)
the two forces in the pair act on different objects (one on body A and the other on body B).
the more massive the planet
the greater the gravitational field
gravitational field near the surface of the Moon
1.6
gravitational field near the surface of Jupiter
26
gravitational field near the surface of the Sun
280
free-fall acceleration
Only gravity
magnitude of the air resistance force
increases with increasing speed of motion
Turbulent vs streamlined air
Turbulent air flow gives rise to a larger air resistance force than streamlined air flow.
relationship between speed of motion and turbulence
greater the speed of movement of the object through the air, the more likely it is that the air flow will become turbulent
streamlined air flow
air resistance tends to be proportional to speed
turbulent air flow
, air resistance tends to be proportional to speed squared
Units of momentum
Ns
Force + momentum
force = rate of change of momentum
Change in momentum formula
momentum = external resultant force × time.
Work dome
Scalar
1 kWh is equivalent to
3 600 000 J.
Active forms of energy are:
- electrical energy (transferred by a current in a circuit)
- heat (thermal energy)
- light
- kinetic energy (energy of an object that is moving)
- sound.
Stored forms of energy are
- chemical potential energy (stored in a battery or cell)
- gravitational potential energy (stored due to height)
- strain potential (energy stored in a stretched spring)
Conduction
transfer of heat from one place to another through the passing on of kinetic energy between the particles of a substance
Where can conduction happen
Solid / liquid / gas if in contact + between states
How does conduction work
particles in the hotter region vibrate more energetically.
Over time, some of this energy is passed along to neighbouring particles, so that they also vibrate more energetically.
This process continues through the solid, so that the temperature rises
‘Good conductor’
heat transfers by conduction relatively quickly
What does conduction need
PARTICLES - CANT TRAVEL THROUGH VACUUM
Why are gases poor conductors
particles in a gas are far apart relative to their size.
Collisions are not frequent enough to transfer kinetic energy between particles as quickly as in liquids and solids.
why metals are particularly good thermal conductors.
Delocalised electrons - transfer energy much faster, by moving through the lattice and colliding with ions and with each other
Factor affecting rate of heat transfer by conduction
- heat gradient = higher temp difference faster conduction
- nature of substance = better thermal conductor
+ distance between the two objects = shorter distance faster rate
- surface area in CONTACT = larger area faster rate
When temp increases
Liquid expands + density decreases
When in a convection current - when does conduction come into play
As the warmer fluid rises, it gradually cools (by conduction of heat to the cooler fluid around it), becomes less dense, and tends to sink.
Conduction vs convection
Many houses have two layers of outer wall, with a cavity (empty space) between them. These cavities were originally designed to be filled with air, to reduce heat loss from the house in cold weather by conduction (since air is a good thermal insulator) = why bad
when heat from the house transfers through the inner wall to the air in the cavity, convection makes this air circulate.
This speeds up the transfer of heat to the outer wall and then out to the surroundings.
So fill with insulating material
During a sunny day, the surface of the land reaches a higher temperature than the surface of the sea. This causes a breeze to blow.
a) State and explain the direction of this breeze.
b) Explain why there is a breeze moving in the opposite direction at a higher altitude.
a) The air above the land and sea is warmed by conduction, but the air above the land becomes warmer because the land’s temperature is higher. This causes a convection current in which air above the land rises and is replaced by cooler air moving across from over the sea. So a breeze blows from the sea towards the land.
b) This is the uppermost part of the convection current, which flows horizontally from above the land to above the sea.
Thermal radiation
EM
What can transfer heat through vacuum
Thermal radiation
thermal energy is transferred by…
Infrared radiation
What emits thermal radiation
Any object or substance with a temperature above absolute zero
higher the temperature of an object
higher the rate at which it emits thermal radiation
if an object were to emit thermal radiation without any other energy transfers occurring….
, its temperature would decrease
All objects always emitting + absorbing thermal radiation = net goes from hot to cold
factors affecting rate of absorption and emission of thermal radiation
Absorbers + emitters
A good absorber of thermal radiation is a good emitter, and a poor absorber is a poor emitter
Why are survival blankets for cold people shiny
so that it is a poor absorber and emitter, and a good reflector, of thermal radiation.
It is better than, for example, a dull matt blanket at reflecting thermal radiation from the person back towards them, and it is poorer at emitting thermal radiation to the surroundings.
Can conduction, convection or radiation happen at same time
Yes
specific
per unit mass
Can you compress solid + liquid
Not really = both have fixed volume
When a substance is melting / boiling
absorbs thermal energy without increasing its temperature.
The absorbed energy is needed to increase the separations between the particles
Endo
When a substance is freezing / condensing
releases thermal energy without decreasing its temperature.
As the attractions between the particles increase and their separations decrease, energy is transferred from the sample to thermal energy of its surroundings
Exo
Things when finding volume of irregular solid
must be enough water initially in the cylinder to completely cover the sample
and the water must not overflow
sample does not react with or dissolve in the water
sample sinks in water
hydrostatic pressure
hydrostatic pressure = hρg
Wave definition
transfer of energy without net movement of matter
Transverse
vibration direction is perpendicular to the wave direction.
Longitudinal
vibration direction is parallel to the wave direction
Transverse waves
EM
waves in string
Seismic S- waves
Longitudinal
Sound waves
Ultrasound
Compression waves on a slinky / string
What is a mix of longitudinal and transverse
Water waves = elliptical motion
Features of longitudinal
Compression
Rarefaction
What are compression and rarefactions
Higher or lower pressure then atmospheric pressure
Mechanical waves
Vibrating particles - can’t go through vacuum
EM waves
Can go through vacuum
Travel at speed of light
Examples of mechanical waves
Sound
Ultrasound
Seismic waves
Water waves
Waves on a string / slinky
Examples of EM waves
RMIVUXG
How are EM waves formed
Charged particles such as electrons set up an electric field in the space around them.
When they are made to vibrate a magnetic field is also produced.
The pattern of electric field and magnetic field vibrations travels outwards as an electromagnetic wave.
Wavelength
distance between adjacent peaks (or troughs) in a transverse wave.
Amplitude
maximum displacement of a particle in the wave from its equilibrium position.
Period
time taken to complete one cycle of vibration (
oscillation). Measured in seconds.
1 Hz =
1 oscillation per second
incident angle
angle between the normal and the direction of the incident wave
reflected angle
angle between the normal and the direction of the reflected wave
law of reflection
incident angle = reflected angle
Smooth surface
all the normals are parallel to one another so all the waves are reflected in an orderly way = e.g. reflection from a mirror
Rough surfaces
normals at each point are in different directions so each ray is reflected in a random direction
(e.g. reflection from a white sheet of paper).
If a light ray slows down….
refracts toward the normal
If a light ray speeds up…
refracts away from the normal
If the waves are travelling along the normal…
they continue in the same direction (but still change speed).
What happens at air glass boundary
light ray slows down + refracts toward the normal
What happens at glass air boundary
light ray speeds up + refracts away from the normal
Wave crests on diagram
perpendicular to the direction of travel of the wave
If wave slows down
Peaks / crests closer together = because frequence never changes so wavelength must
When waves speed up
Peaks / crests further apart because frequency never changes so wavelength must
Energy is conserved at the boundary:
Incident energy = reflected energy + transmitted energy + absorbed energy
If the sides of the block are parallel
The refracted waves should be parallel
Why does the direction change for refraction
wave is travelling at a non-zero angle to the normal, different parts of the wave crest enter the second medium at different times
. In this case, the left hand end A) enters first and slows down first, moving a shorter distance than the right hand end B) of the same wave crest.
This causes the direction of the wave to change.
An astronomer is using a telescope to observe a distant star. Light from the star has to pass through the Earth’s atmosphere to reach the telescope and as it does so, it slows down. Which of the statements below, about the apparent position of the star is/are correct?
1) If the star is vertically above the telescope its apparent position in the sky will be the same as its actual position in the sky.
2) If the apparent position of the star is close to the observer’s horizon then its actual position is higher in the sky.
3) If the star can be seen when the telescope is at 45° to the horizontal then the actual position of the star in the sky must be at an altitude less than 45° to the horizontal.
What does water waves do in shallower water 💧
Slow down
Which of the following statements about the analogy between water waves and light is/are correct?
1) Water waves refract when they cross a boundary and their speed changes. Light waves refract when they move from water into glass so the speed of light in glass must differ from the speed of light in water.
2) When light waves strike a plane mirror the angle of reflection (measured from the normal) is equal to the angle of incidence (measured from the same normal). Therefore when water waves strike a plane barrier the angle of reflection (measured from the normal) is equal to the angle of incidence (measured from the same normal).
3) The wavelength of water waves is not changed by reflection. Therefore the wavelength of light waves will not be changed by reflection.
All correct
Doppler effect
relative motion between a source of waves and an observer, the wavelength and frequency of the waves detected by the observer is different from the wavelength and frequency of the waves received when there is no relative motion
Table for Doppler effect
Special Doppler
faster the observer approaches or recedes from the source, the greater the shift in frequency and wavelength.
Draw a mirror ray diagram
What do you need to make sure in mirror diagram
image in a plane mirror is formed at the same distance behind the mirror as the object is in front of it.
Draw a ray diagram for a corner reflector - add all angles
Why would emerging and incident rays be parallel
change of speed at both boundaries has the same ratio and the boundaries are parallel to one another
Do all EM travel at same speed not in a vacuum
No - Different wavelengths of visible light travel at different speeds in glass
Different wavelengths of visible light travel at different speeds in glass - what does this result in
deviated by different amounts from the normal + spectrum
What em slows down more
Shorter wavelengths (the blue end of the spectrum) slows down more than longer wavelengths so blue light is deviated more than red light.
Relationship between vibrations of the source and sound produced: (3)
The sound waves have the same frequency (or frequencies) as the vibrations of the source.
The amplitude of the sound waves depends on the amplitude of the vibrations of the source.
The speed of the sound waves is determined by the medium through which they travel and NOT by the source.
rarefies
creating a region of lower pressure
Sources of EM waves
Anything that causes electric charges to vibrate will emit EM waves.
E,g warm body contains vibrating atoms which contain charged particles, so all bodies emit a spectrum of EM radiation (because all bodies are above the absolute zero of temperature)
The hotter the body..
the more radiation and the more high frequency radiation.
How do antennas work
When the waves meet the rods they cause electrons in the rods to vibrate at the same frequency as the wave and this electrical signal is used to create the TV pictures.
How are x rays produced
when fast moving electrons crash into a metal target and sto
Applications of radio waves
Communications: radio and TV. Radar systems. Radio astronomy.
Hazards radio
Only hazardous if extremely intense.
Microwave applications
Satellite and space communications. Radar systems. Mobile phones. Wifi systems. Microwave cookers.
Microwave hazards
Tissues can be damaged if too much microwave radiation is absorbed by living tissues so protective (reflective) suits must be worn if working near a powerful transmitter. They can also cause cataracts in the eye.
Infrared applications
Radiant heaters. TV/DVD remote controls. Heat seeking missiles. Sensors on security lights. Optical fibre communications. Night sights. Thermal imaging. Weather satellites (IR photography).
Infrared hazards
Cell damage: burns.
Visible applications
Sight. Astronomical and terrestrial telescopes. Microscopes. Illumination. Optical fibre communications. LASERs
Visible hazards
Looking at an intense source of light can damage the retina of the eye. This is why you should not look directly at the Sun and never direct a telescope or binoculars towards it!
Uv applications
Causes some things to fluoresce – (e.g. washing powders in clothes so are often used at clubs and parties). Security marking. Can kill microbes so can be used to sterilise medical equipment. Insect control (UV attracts insects).
Uv hazards
Can damage the retina of the eye. Can cause sunburn and skin cancer.
X ray applications
X-ray images, CAT scans. Airport security. X-ray crystallography (investigating the structure of crystalline materials using X-rays). Detecting art forgeries. X-ray telescopes in astronomy.
X ray hazards
X-rays are a form of ionising radiation that can damage molecules, causing cell damage and various types of cancer
Gamma applications
Radiotherapy to kill cancer cells. Radioactive tracers. Food sterilisation. Locating cracks in pipes and turbines. Gamma-ray telescopes in astronomy.
Gamma hazards
Gamma-rays are a form of ionising radiation that can damage molecules causing cell damage and various types of cancer. They can also cause cells to mutate and if they affect sex cells or a developing embryo, the effects are seen in the next generation.
What can cause ionisation and heating
High frequency short wavelength EM waves such as X-rays and gamma-rays deliver energy in such a way that they can ionise atoms in the material that absorbs them.
Lower frequency, longer wavelength EM waves cannot cause ionisation but do cause heating, making particles in the absorbing material vibrate more strongly.
Reduce risk radio
Do not go too close to a powerful transmitter
Reduce risk microwaves
Limit direct exposure, especially to eyes. If working with strong transmitters, maintain a safe distance and wear a reflective suit. Limit the time for which you use a mobile phone.
Reduce risk infrared
Maintain a safe distance from intense sources. Wear reflective clothing.
Reduce risk visible
Do not look directly at bright sources. Never point a telescope directly at the Sun (unless it has a solar filter attached to the objective). Wear sunglasses. Place a shield in front of an intense source. Do not direct LASERs or LASER pens into the eye.
Reduce risk Uv
Do not look directly at a UV lamp. Limit skin exposure to UV – e.g. sunbeds. Use a protective sun cream if outside on a sunny day. Wear sunglasses that block UV.
Reduce risk x rays
Limit your exposure to X-rays, e.g. increase distance from source, wear protective clothing and/or stand behind a screen. Use lead shielding between an X-ray source and your body. Doctors should ensure that the potential benefit of X-ray treatment always outweighs the potential risk.
Reduce risk gamma
Maximise your distance from the source and minimise the time that you use the source. Use lead shielding to absorb some of the gamma radiation – e.g. by placing this between the source and your body. Never handle gamma-sources directly, always remotely.
Protons and neutrons are held together in the nucleus by
strong nuclear force = balances the electrostatic repulsion between the protons.
What’s a nuclide
any particular type (or ‘species’) of nucleus, characterised by the numbers of protons and neutrons it has.
Nuclides that decay are
Radioactive
What is alpha
2 protons + 2 neutrons
What is Gamma
Burst of em radiation
Speed of alpha
0.1 c
compared with speed of light,
c = 3 × 108 ms−1
Speed of beta
0.8c
compared with speed of light,
c = 3 × 108 ms−1
Speed of gamma
C
compared with speed of light,
c = 3 × 108 ms−1
Alpha origin
unstable nucleus emits two of its protons and two of its neutrons, bound together as a single particle.
Beta origin
One neutron of the unstable nucleus transforms into a proton (which remains in the nucleus) and an electron (which is emitted as the beta particle).
Gamma origin
Excess energy of the unstable nucleus is ejected in the form of gamma radiation
What’s alpha blocked by
Sheet of paper / human skin
How far can alpha travel
A few cm
What’s beta blocked by
Thin metal / alluminium
NOT BLOCKED by human skin
How far can beta go
several metres in air
What is gamma blocked by
Several cm of very dense metal = lead
How far can gamma go
hundreds of metres in air
Most variable penetrating ability
Beta - can be blocked by Thin metal OR low energy beta can not even penetrate skin
Count rate
number of radiation impacts detected by the counter per second
Least penetrating
Alpha
Most penetrating
Gamma
Most ionising
Alpha
Least ionising
Gamma
Why is alpha most ionising
although alpha particles travel at lower speeds than beta particles, alpha particles’ much greater mass means that they have more momentum
this, along with their double charge, gives them a strong tendency to interact with atoms and cause ionisation.
Why is more ionising less penetrating
Radiation that is very ionising quickly loses its kinetic energy as it travels through matter, and so it is not very penetrating.
What’s deflected by electric / magnetic fields
Alpha + beta
Factors that effect deflection in an electric field
Charge
Mass
Speed
Overall effect
Charge
Alpha particle has twice as much charge as beta, so experiences twice as much force in an electric field.
Mass
Alpha particle is about 8000 times heavier than beta, so acceleration a=Fm is about 28000 × or 14000× that of a beta particle in the same field.
Speed
Alpha particle typically has 10–20% of speed of beta – therefore it takes longer to move through the field so there is more time for the force to act.
Overall effect
The effect of the speed difference is less than the opposing effect of the charge and mass differences (with the mass difference being the dominant factor).
So an alpha particle is less deflected than a beta particle in the same electric field.
Natural sources of background radiation (typically over 80% of the total)
radon gas from the ground
rocks and buildings
cosmic rays
food and drink
Artificial sources of background radiation (typically under 20% of the total)
medical procedures (typically around 99% of the total radiation from artificial sources)
nuclear power and nuclear weapons testing
mean count rate from source =
mean count rate (measured with source present) − mean background count rate (measured with source absent)
State one reason why the background radiation rate may be higher in one region than in another.
different types of rock and soil emit radiation at different rates (because they contain different levels of radioactive isotopes).
What’s most dangerous in body
Alpha = highly ionising
What’s most damaging outside the body
Gamma = can easily penetrate skin + cause damage
What’s least damaging in the body
less ionising, and much of it will pass straight through cells without damaging them
What’s least dangerous outside body
alpha particles cannot penetrate the skin
WHAT DIRECTION DOES CURRRNT FLOW
FROM POSITIVE (LONG LINE) TO NEGATIVE (SHORT LINE)
Does kinetic energy change if velocity changes direction
NO BECAUSE V IS SQUARED
North
IS NOT UP ITS INFRONT OF YIU
