Stationary objects can accelerate , deccelerate or change direction when a reaultant force acts on it.
Without any resultant force, the object is either stationary or moving at constant velocity.
A resultant force is when all the forces acting on the object are added and it is the single force that has the effect as what all the force would do to the object
a=F/m is the equation that shows how acceleration is affected by the Force and mass.
It can also be written as F=ma
F-Force is measured in Newtons (N)
m-Mass is measured in kilograms (kg)
a-Acceleration is measured in metres per second squared (m/s^2)
Speed is different from velocity
Speed is how far an object travels in a given time
Velocity is the speed in a particular direction
Distance time graphs
Slope=constant speed
Flat= stationary
Slope downwards= constant speed back
Curves =Acceleration
Speed= distance/ time
(m/s ) (metres) (Seconds)
v=s/t
Velocity uses the same equation but distance can be negative and different yet travelling the same speed
Velocity time graphs
Slopes = constant acceleration
Flat line = constant speed
Downward slopes = constant deceleration
Curves = changing acceleration
Area of under the line of a velocity time graph represents the distance traveled.
Area of under the line of a velocity time graph represents the distance traveled.
a=(v-u)
____
t
a acceleration (m/s^2)
v-u difference in speed (m/s)
t time (s)
Forces and breaking
Thinking distance and braking distance affects the stopping distance of a car.
When a car travels are constant speed. the resultant force is zero as the force from the engine is balanced by the air resistance and friction,
When a car is traveling at constance speed, every force is balanced by an equal and opposite force.
Force of engine balanced by air resistance + friction
Weight balanced by reaction force of the road.
When a car is speeding up resultant force is not zero.
Force of engine is greater than the resistive forces therefore the car accelerates
When a car slows down the force from the engine is either reduced or the resistive forces has increased e.g. brakes.
Stopping distance = Thinking distance + braking distance
Thinking distance increases if the reaction time increases.
Factors that affect reaction time are:
Tiredness
Distractions
Influence of drugs.
Braking distance is the length a car goes before it halts when the brakes are applied.
It depends on the speed of the car
condition of tyres
road conditions.
Brakes causes a frictional force on the wheel. It reduces the kinetic energy of the vehicle and increases the temperature of the brakes.
Falling objects
Falling objects have different stages.
Object accelerate because of its weight when released, there is a resulting force which goes downwards.
The object keeps accelerating and gains speed but air resistance also increases
Eventually, the object's weight reaches a terminal velocity ( steady speed) cas the weight is balanced by the air resistance.
Lighter objects reaches terminal velocity faster than heavier objects as they have less weight.
On the moon there is no air so no air resistance, when different weight objects are dropped both drop at the same speed.
Velocity time graphs for falling objects
The slope shows initial acceleration
It curves off so it decelerates due to air resistance
It levels off and reaches a terminal velocity.
Weight and mass
W=m * g
W- weight is in Newtons N
m- mass is in kilograms kg
g- gravitational field strength measured in newtons per kilogram, N/kg
the g of the earth is around 10 N/kg
the moon has around 1.6 N/kg
Elasticity
Elastic objects store energy as elastic potential energy.
Work is done on the object to change its shape which stores elastic potential energy.
Hooke's Law
When an elastic object is stretched, the extension is directly proportional to the force applied to it.
F= k * e
F- Force in newtons ,N
k- Spring constant in newtons per metre,N/m
e is extension in metres ,m
In a graph of Force by Extension, the gradient is the spring constant.
There's a limit of portionality where when an object exceeds it, it will change the spring constant of the object permanently.
Kinetic Energy
Work Done
Work done and energy transferred are measured in Joules, J.
Is is when a force moves something.
W= F * d
Work done = Force * Distance
J - Joules N -Newtons m - metres
Power
How quickly work is being done
P = E / t
Power = work done / time
W -Watt J -Joules s -Seconds
Gravitational Potential Energy
Force of gravity pulling on us.
Ep = m * g * h
Gravitational potential energy = mass * gravitational field strength * height
J- Joules kg- Kilograms N/kg-Newtons per kilogram m- metres
Kinetic energy
Energy a moving object has
Ek = 0.5 * m * v^2
Kinetic energy = 0.5 * mass * speed^2
J- Joules kg- Kilogram m/s metres per second
Pendulum
Transfers gravitational potential to kinetic energy and back.
At highest point, no kinetic energy but maximum gravitational potential energy.
Gravitational potential energy transfers to kinetic energy as it swings down and it accelerates.
As it swings upwards kinetic energy is transferred to gravitational potential energy and decelerates
Energy is also transferred as heat to the surroundings so each swing is weaker.
Momentum
Momentum is the tendency of the object to keep moving in the same direction.
p = m * v
Momentum (kg m/s) = Mass (kg) * velocity (m/s)
Momentum also depend on the direction of travel.
it can change if the object change speed or change direction.
If there are no other external forces acting on the object, the total momentum stays the same in collisions and explosions.
Momentum is conserved
Always write momentum before = momentum after in a calculation.
Thinking distance and braking distance affects the stopping distance of a car.
When a car travels are constant speed. the resultant force is zero as the force from the engine is balanced by the air resistance and friction,
When a car is traveling at constance speed, every force is balanced by an equal and opposite force.
Force of engine balanced by air resistance + friction
Weight balanced by reaction force of the road.
When a car is speeding up resultant force is not zero.
Force of engine is greater than the resistive forces therefore the car accelerates
When a car slows down the force from the engine is either reduced or the resistive forces has increased e.g. brakes.
Stopping distance = Thinking distance + braking distance
Thinking distance increases if the reaction time increases.
Factors that affect reaction time are:
Tiredness
Distractions
Influence of drugs.
Braking distance is the length a car goes before it halts when the brakes are applied.
It depends on the speed of the car
condition of tyres
road conditions.
Brakes causes a frictional force on the wheel. It reduces the kinetic energy of the vehicle and increases the temperature of the brakes.
Falling objects
Falling objects have different stages.
Object accelerate because of its weight when released, there is a resulting force which goes downwards.
The object keeps accelerating and gains speed but air resistance also increases
Eventually, the object's weight reaches a terminal velocity ( steady speed) cas the weight is balanced by the air resistance.
Lighter objects reaches terminal velocity faster than heavier objects as they have less weight.
On the moon there is no air so no air resistance, when different weight objects are dropped both drop at the same speed.
Velocity time graphs for falling objects
The slope shows initial acceleration
It curves off so it decelerates due to air resistance
It levels off and reaches a terminal velocity.
Weight and mass
W=m * g
W- weight is in Newtons N
m- mass is in kilograms kg
g- gravitational field strength measured in newtons per kilogram, N/kg
the g of the earth is around 10 N/kg
the moon has around 1.6 N/kg
Elasticity
Elastic objects store energy as elastic potential energy.
Work is done on the object to change its shape which stores elastic potential energy.
Hooke's Law
When an elastic object is stretched, the extension is directly proportional to the force applied to it.
F= k * e
F- Force in newtons ,N
k- Spring constant in newtons per metre,N/m
e is extension in metres ,m
In a graph of Force by Extension, the gradient is the spring constant.
There's a limit of portionality where when an object exceeds it, it will change the spring constant of the object permanently.
Kinetic Energy
Work Done
Work done and energy transferred are measured in Joules, J.
Is is when a force moves something.
W= F * d
Work done = Force * Distance
J - Joules N -Newtons m - metres
Power
How quickly work is being done
P = E / t
Power = work done / time
W -Watt J -Joules s -Seconds
Gravitational Potential Energy
Force of gravity pulling on us.
Ep = m * g * h
Gravitational potential energy = mass * gravitational field strength * height
J- Joules kg- Kilograms N/kg-Newtons per kilogram m- metres
Kinetic energy
Energy a moving object has
Ek = 0.5 * m * v^2
Kinetic energy = 0.5 * mass * speed^2
J- Joules kg- Kilogram m/s metres per second
Pendulum
Transfers gravitational potential to kinetic energy and back.
At highest point, no kinetic energy but maximum gravitational potential energy.
Gravitational potential energy transfers to kinetic energy as it swings down and it accelerates.
As it swings upwards kinetic energy is transferred to gravitational potential energy and decelerates
Energy is also transferred as heat to the surroundings so each swing is weaker.
Momentum
Momentum is the tendency of the object to keep moving in the same direction.
p = m * v
Momentum (kg m/s) = Mass (kg) * velocity (m/s)
Momentum also depend on the direction of travel.
it can change if the object change speed or change direction.
If there are no other external forces acting on the object, the total momentum stays the same in collisions and explosions.
Momentum is conserved
Always write momentum before = momentum after in a calculation.
Electricity
Static electricity
Same charges repel and unlike charges attract
When rubbing two insulating material together they get electrically charged.
Only works for insulators as in metals the charge is earthed too easily.
Electrons move from one material to the other when rubbed against each other
The material that lost electrons become positively charged
The one that gained electrons become negatively charged.
Charge are equal but opposite.
Charged objects will repel or attract
Same charge repel
Different charge attract.
A charged object may also attract uncharged objects.
Circuit
Circuit diagrams must always be complete without any short circuits for it to work.
Wire should lead from battery to the battery.
Short circuit, electricity takes the path of least resistance, if there is a path without any components there is a short circuit.
Current
In a series circuit current is the same
In a parallel circuit, current is shared
Current is measured in Amps (A)
Measured using an ammeter in series.
I= Q / t
Current = Charge /time
Amps (A) Coulombs (C) Seconds (s)
Voltage
Potential Difference is needed to make a current flow through it.
Measured in Volts (V)
Measured using a voltmeter must be connected in parallel with the component.
V= W / Q
Voltage = Work Done / Charge
Volt (V) Joules (J) Coulombs (C)
As Voltage increases the Current increases as well.
Voltage is shared in a series circuit in each component
Resistance
Resistance increases as length of wire increases or thickness of wire decreases.
Electrons move through a conductor for an electric current.
Moving electrons collide with ions in the metal causing resistance.
With a longer wire there are more collisions so resistance increases.
Thinner Wires has fewer electrons to carry current.
V= I * R
R = V / I
Voltage = Current / Resistance
Volts (V) Amps (A) Ohms (Ω)
Current flowing through a resistor at a constant temperature is directly proportional to the voltage.
Filament lamps produce light as the filament (think coil of wire) heats up. The resistance of the lamp increases.
Direct and Alternating current
If current flows in one direction it is called direct current (DC).
Batteries and cells supply DC usually around 1.5 V
Alternating current constantly changes direction.
Mains electricity is AC supplying 230V in the UK with a frequency of 50Hertz (Hz)
Cables
A mains electricity cable has 2 or 3 inner wires with cores of copper as copper is a good conductor of electricity.
The outer layer is made from plastic as it is a good insulator.
There is a colour code
Blue -Neutral
Brown -Live
Green and yellow stripes -Earth
The Plug
The plug is made from tough plastic which is a good insulator.
The 3 pins are made from brass which are stronger than copper and a good conductor of electricity
There is a fuse between the live fuse and terminal which breaks if there is too much current going through it.
The Cable grip grips the cable to stop it moving around.
bLue goes Left
bRown goes Right
sTriped Goes Top.
Fuses
Used to protect appliances.
The fuse breaks if there is too much current flowing through it. This protects the appliance if something goes wrong.
It has a wire that melts easily and it heats up and breaks if the current is too great.
The fuse rating is slightly higher than the device's current need
Standard ratings are 3A 5A and 13A.
Circuit Breakers
Circuit breakers detect a difference in current, they act like fuses but can be used more than once and act a lot faster.
However, they are more expensive.
Earthing
The earth wire creates a safe route for current to flow if the live wire touches a metal casing.
The earth wire has a low resistance so it stops current from flowing through our body.
As the strong current goes to the earth wire, it breaks the fuse.
Double insulation
Plastic casings doesn't require a earth wire as it is an insulator
Oscilloscopes
Periods- Time taken for an AC supply for one complete oscillation. One peak to the next
Frequency
Number of oscillations per second.
Frequency = 1 / period
Power
P= E/ t
Power = Energy / time
Watt (W) Joules (J) Seconds (s)
In a circuit,
P= I*V
Power (W) = Current (A) * Voltage (V)
E =V*Q
Energy (J) = Voltage (V) * Charge (C)
Atoms and radiation
Static electricity
Same charges repel and unlike charges attract
When rubbing two insulating material together they get electrically charged.
Only works for insulators as in metals the charge is earthed too easily.
Electrons move from one material to the other when rubbed against each other
The material that lost electrons become positively charged
The one that gained electrons become negatively charged.
Charge are equal but opposite.
Charged objects will repel or attract
Same charge repel
Different charge attract.
A charged object may also attract uncharged objects.
Circuit
Circuit diagrams must always be complete without any short circuits for it to work.
Wire should lead from battery to the battery.
Short circuit, electricity takes the path of least resistance, if there is a path without any components there is a short circuit.
Current
In a series circuit current is the same
In a parallel circuit, current is shared
Current is measured in Amps (A)
Measured using an ammeter in series.
I= Q / t
Current = Charge /time
Amps (A) Coulombs (C) Seconds (s)
Voltage
Potential Difference is needed to make a current flow through it.
Measured in Volts (V)
Measured using a voltmeter must be connected in parallel with the component.
V= W / Q
Voltage = Work Done / Charge
Volt (V) Joules (J) Coulombs (C)
As Voltage increases the Current increases as well.
Voltage is shared in a series circuit in each component
Resistance
Resistance increases as length of wire increases or thickness of wire decreases.
Electrons move through a conductor for an electric current.
Moving electrons collide with ions in the metal causing resistance.
With a longer wire there are more collisions so resistance increases.
Thinner Wires has fewer electrons to carry current.
V= I * R
R = V / I
Voltage = Current / Resistance
Volts (V) Amps (A) Ohms (Ω)
Current flowing through a resistor at a constant temperature is directly proportional to the voltage.
Filament lamps produce light as the filament (think coil of wire) heats up. The resistance of the lamp increases.
LEDs
Diodes have a very high resistance on one side so current can only travel
the other way
LED produces light when a current travels through it, it is far more efficient than other light components it uses a much smaller current
thermistor is a variable resistor.
Its resistance decreases as its temperature increases.
LDRs are variable resistors that depends on the light intensity
The resistance increases as light intensity decreases.
Direct and Alternating current
If current flows in one direction it is called direct current (DC).
Batteries and cells supply DC usually around 1.5 V
Alternating current constantly changes direction.
Mains electricity is AC supplying 230V in the UK with a frequency of 50Hertz (Hz)
Cables
A mains electricity cable has 2 or 3 inner wires with cores of copper as copper is a good conductor of electricity.
The outer layer is made from plastic as it is a good insulator.
There is a colour code
Blue -Neutral
Brown -Live
Green and yellow stripes -Earth
The Plug
The plug is made from tough plastic which is a good insulator.
The 3 pins are made from brass which are stronger than copper and a good conductor of electricity
There is a fuse between the live fuse and terminal which breaks if there is too much current going through it.
The Cable grip grips the cable to stop it moving around.
bLue goes Left
bRown goes Right
sTriped Goes Top.
Fuses
Used to protect appliances.
The fuse breaks if there is too much current flowing through it. This protects the appliance if something goes wrong.
It has a wire that melts easily and it heats up and breaks if the current is too great.
The fuse rating is slightly higher than the device's current need
Standard ratings are 3A 5A and 13A.
Circuit Breakers
Circuit breakers detect a difference in current, they act like fuses but can be used more than once and act a lot faster.
However, they are more expensive.
Earthing
The earth wire creates a safe route for current to flow if the live wire touches a metal casing.
The earth wire has a low resistance so it stops current from flowing through our body.
As the strong current goes to the earth wire, it breaks the fuse.
Double insulation
Plastic casings doesn't require a earth wire as it is an insulator
Oscilloscopes
Periods- Time taken for an AC supply for one complete oscillation. One peak to the next
Frequency
Number of oscillations per second.
Frequency = 1 / period
Power
P= E/ t
Power = Energy / time
Watt (W) Joules (J) Seconds (s)
In a circuit,
P= I*V
Power (W) = Current (A) * Voltage (V)
E =V*Q
Energy (J) = Voltage (V) * Charge (C)
Atoms and radiation
An atom consist of a nucleus ,which have protons and neutrons, and electrons surrounding the nucleus.
Isotopes are atoms that have the same number of protons but different amount of neutrons.
Early model of the atom is the plum pudding model but was then disproved by Rutherford's nuclear model.
The nuclear model shows an atom having 3 subatomic particles.
It shows protons and neutrons at the centre in the nucleus which is very small. Electrons surround the nucleus in different energy levels.
Protons have +1 charge and 1 relative mass
Neutrons have 0 charge and 1 relative mass
Electrons have -1 charge and almost 0 mass.
The number of electrons in an atom is always the same as the number of protons.
Atoms are neutral but can gain or lose electrons.
They form ions which are charged particles
losing electrons makes it positive
gaining electrons makes it negative
The plum pudding model consists of a sphere of positively charged protons and electrons dotted around inside it.
Ernest Rutherford fired a beam of alpha particles at a very thin gold foil
the particles was repelled at different angles which meant that the positively charged particle was deflected by positively charge in the atom.
The atomic number of an element is the number of protons
Isotopes have the same number of protons but different atomic mass as they have different number of neutrons
Background radiation
It is radiation all around us. It comes from natural and artificial sources
Natural
Cosmic rays- radiation from space
Rocks- radioactive rocks that release radon gas 50%
Living things- plants absorb radioactive material and is passed in the food chain
Artificial around 15%
Radioactive waste
Nuclear fallout
X-ray machines
Photographic film
Goes darker when radiation absorbed
Contains different material for different radiation to penetrate them
Contains aluminium/ lead and an open area
Geiger muller tube
Detects radiation by absorbing radiation
Each time it absorbs radiation, it transmits an electrical pulse to a counting machine
It makes a clicking sound or display a count rate
The greater the radiation, the higher the count rate
Types of radiation
Alpha
Identical to a helium atom but does not have electrons
2 proton 2 neutrons
Beta
Electrons are emitted from the nucleus of a radioactive atoms when neutrons split forming protons and electrons
Gamma rays
These are electromagnetic radiation with a short wavelength.
Alpha radiation is the least penetration, it can be absorbed by just a piece of paper or skin
Beta radiation can penetrate air and paper but absorbed by a thin sheet of aluminium
Gamma radiation is the most penetrating and can penetrate air, paper and metal, it can only be stopped by centimetres of lead.
The thicker the substance the more radiation is absorbed, radiation becomes less intense the further the distance from the radioactive material.
Detecting radiation
Electric fields
Alpha particles are positively charged and beta particles are negatively charged.
Gamma is neutral.
Alpha particles are attracted to negatively charged plates (Cathode) and beta particles will be attracted to positively charged plates (anode)
The radiation are deflected by electric fields but gamma radiation is not deflected.
The radiation are also deflected by magnetic fields but gamma remain unaffected
Hazards
When radiation collide with living cells it damage them. If the nucleus is damaged, the DNA might alter and become cancerous. The cell would divide rapidly and cause health problems.
The greater the dose of radiation the more chance it will be that the cell will become cancerous.
With a very high dose it will kill the cell. This property is used to kill cancer cells and also microorganisms.
Inside body
Alpha particles are the most dangerous as they are easily absorbed by cells - the most ionising.
Beta and gamma radiation are not as dangerous as they are not as ionising and will usually pass through cells.
Outside body
Alpha particles are not as dangerous as it is unlikely to reach inside the body as the body is protected by skin which will absorb the alpha particle.
Beta and gamma radiation are the most dangerous as they can penetrate the skin and damage the cells inside.
Half Life
Radioactive atoms have unstable nuclei, they break down to change into a different type of atom.
It is not possible to predict when an individual atom decay but it is possible to measure how long it takes for the half of the nuclei of a piece of radioactive material to decay.
This is called the half life of the radioactive isotope.
It is the time it takes for the number of nuclei of the isotope in a sample to halve.
It is also the time it takes for the count rate to half.
Half life varies in different isotopes one can last millions of years but some can only last a few seconds.
Use
Radiation can be used in smoke detectors
sterilising medical instruments
killing cancer cells
dating rocks and materials
chemical tracers to help medical diagnosis
measuring thickness of material
Tracers
Radioactive chemicals are ingested into the body and it concentrate in damaged parts of the body.
Radiation detectors are placed outside the body and detect radiation emitted inside the body.
A computer build up an image of the inside.
It usually isn't harmful as the substance has a short half life so it cannot do much damage before it decays and it is not poisonous.
Beta and gamma radiation are used as they pass out of the body and are less likely be absorbed by sells than alpha particles.
Thickness of material
It is used to monitor and control the thickness of material such as paper, plastic and aluminium.
A thicker material absorbs more radiation so less radiation reaches the detector
the detector sends signal to the equipment which adjusts the thickness of the material
Smoke detectors
Smoke detectors contain an ionisation chamber which has a cathode and an anode which an alpha source emitter radioactive substance. The alpha particles ionise the air which produce the current as it causes a flow of charge. The substance has a long half life so it won't run out.
When there is smoke, the alpha particle is unable to ionise the smoke particles so the current drops. The electric circuit triggers the alarm.
Atomic mass = number of protons + neutrons
Atomic number = number of protons
Alpha decay
2 protons and 2 neutrons are lost when a nucleus emits an alpha particle
Atomic mass decreases by 4
atomic number decreases by 2.
Beta decay
A neutron changes into a proton and an electron.
The proton stays in the nucleus but the electron leaves as a beta particle
The atomic mass number stays the same
Atomic number increases by one
Nuclear fission
Nuclear fission means to split a nucleus.
Uranium -235 and plutonium-239 are most commonly used.
They must first absorb a neutron.
The nucleus splits into two smaller nuclei
The split releases 2 or 3 neutrons and energy is released
The neutrons released may be absorbed by other uranium or plutonium nuclei which causes them to split.
This is called a chain reaction.
Nuclear fusion
This is when two atomic nuclei join to make a large nucleus and this also releases energy.
Sun and starts use nuclear fusion.
Hydrogen nuclei join to form helium nuclei.
Hydrogen-1 join with hydrogen-1 to form helium-3
Stars
Stars form when dust and gas are pulled by gravity and clump together.
As they clump together, they get hot, they form a start when they are hot enough for nuclear fusion to start which releases energy and keep the star hot.
A star is stable because the forces are balanced. Outward pressure from expanding gasses are balanced by the force of the star's gravity.
When hydrogen has been used up it starts to fuse larger nuclei and it expands to become a red giant
When the nuclear reactions are over as the star can no longer fuse larger nuclei, they may begin to contract from the pull of gravity. It forms a white dwarf which fades and cools.
For a start with more mass, they will keep making nuclear reactions, expands and get hotter until it explodes as a supernova and throws dust + gases away. It collapses into a black hole or a neutron star.
Planets are formed when gravity pulls smaller amount of dust and gas together
Life cycles of star
Protostar > Main sequence star (Our sun)
Red giant > White dwarf > Black Dwarf (Path of Our sun)
Red super giant star> Supernova > Neutron star / Black hole.
Temperatures and pressures inside a star is great enough for nuclear fusion to happen.
Stars have enough hydrogen to maintain their energy output for million of years.
Hydrogen nuclei fuse together to form helium nuclei.
In a red giant star, the start fuse heavier elements from helium to iron.
Elements heavier than iron are formed in supernovas.
Heavy elements are found in suns and stars which suggest the solar system are formed from remains of supernovas
Alpha radiation is the least penetration, it can be absorbed by just a piece of paper or skin
Beta radiation can penetrate air and paper but absorbed by a thin sheet of aluminium
Gamma radiation is the most penetrating and can penetrate air, paper and metal, it can only be stopped by centimetres of lead.
The thicker the substance the more radiation is absorbed, radiation becomes less intense the further the distance from the radioactive material.
Detecting radiation
Electric fields
Alpha particles are positively charged and beta particles are negatively charged.
Gamma is neutral.
Alpha particles are attracted to negatively charged plates (Cathode) and beta particles will be attracted to positively charged plates (anode)
The radiation are deflected by electric fields but gamma radiation is not deflected.
The radiation are also deflected by magnetic fields but gamma remain unaffected
Hazards
When radiation collide with living cells it damage them. If the nucleus is damaged, the DNA might alter and become cancerous. The cell would divide rapidly and cause health problems.
The greater the dose of radiation the more chance it will be that the cell will become cancerous.
With a very high dose it will kill the cell. This property is used to kill cancer cells and also microorganisms.
Inside body
Alpha particles are the most dangerous as they are easily absorbed by cells - the most ionising.
Beta and gamma radiation are not as dangerous as they are not as ionising and will usually pass through cells.
Outside body
Alpha particles are not as dangerous as it is unlikely to reach inside the body as the body is protected by skin which will absorb the alpha particle.
Beta and gamma radiation are the most dangerous as they can penetrate the skin and damage the cells inside.
Half Life
Radioactive atoms have unstable nuclei, they break down to change into a different type of atom.
It is not possible to predict when an individual atom decay but it is possible to measure how long it takes for the half of the nuclei of a piece of radioactive material to decay.
This is called the half life of the radioactive isotope.
It is the time it takes for the number of nuclei of the isotope in a sample to halve.
It is also the time it takes for the count rate to half.
Half life varies in different isotopes one can last millions of years but some can only last a few seconds.
Use
Radiation can be used in smoke detectors
sterilising medical instruments
killing cancer cells
dating rocks and materials
chemical tracers to help medical diagnosis
measuring thickness of material
Tracers
Radioactive chemicals are ingested into the body and it concentrate in damaged parts of the body.
Radiation detectors are placed outside the body and detect radiation emitted inside the body.
A computer build up an image of the inside.
It usually isn't harmful as the substance has a short half life so it cannot do much damage before it decays and it is not poisonous.
Beta and gamma radiation are used as they pass out of the body and are less likely be absorbed by sells than alpha particles.
Thickness of material
It is used to monitor and control the thickness of material such as paper, plastic and aluminium.
A thicker material absorbs more radiation so less radiation reaches the detector
the detector sends signal to the equipment which adjusts the thickness of the material
Smoke detectors
Smoke detectors contain an ionisation chamber which has a cathode and an anode which an alpha source emitter radioactive substance. The alpha particles ionise the air which produce the current as it causes a flow of charge. The substance has a long half life so it won't run out.
When there is smoke, the alpha particle is unable to ionise the smoke particles so the current drops. The electric circuit triggers the alarm.
Atomic mass = number of protons + neutrons
Atomic number = number of protons
Alpha decay
2 protons and 2 neutrons are lost when a nucleus emits an alpha particle
Atomic mass decreases by 4
atomic number decreases by 2.
Beta decay
A neutron changes into a proton and an electron.
The proton stays in the nucleus but the electron leaves as a beta particle
The atomic mass number stays the same
Atomic number increases by one
Nuclear fission
Nuclear fission means to split a nucleus.
Uranium -235 and plutonium-239 are most commonly used.
They must first absorb a neutron.
The nucleus splits into two smaller nuclei
The split releases 2 or 3 neutrons and energy is released
The neutrons released may be absorbed by other uranium or plutonium nuclei which causes them to split.
This is called a chain reaction.
Nuclear fusion
This is when two atomic nuclei join to make a large nucleus and this also releases energy.
Sun and starts use nuclear fusion.
Hydrogen nuclei join to form helium nuclei.
Hydrogen-1 join with hydrogen-1 to form helium-3
Stars
Stars form when dust and gas are pulled by gravity and clump together.
As they clump together, they get hot, they form a start when they are hot enough for nuclear fusion to start which releases energy and keep the star hot.
A star is stable because the forces are balanced. Outward pressure from expanding gasses are balanced by the force of the star's gravity.
When hydrogen has been used up it starts to fuse larger nuclei and it expands to become a red giant
When the nuclear reactions are over as the star can no longer fuse larger nuclei, they may begin to contract from the pull of gravity. It forms a white dwarf which fades and cools.
For a start with more mass, they will keep making nuclear reactions, expands and get hotter until it explodes as a supernova and throws dust + gases away. It collapses into a black hole or a neutron star.
Planets are formed when gravity pulls smaller amount of dust and gas together
Life cycles of star
Protostar > Main sequence star (Our sun)
Red giant > White dwarf > Black Dwarf (Path of Our sun)
Red super giant star> Supernova > Neutron star / Black hole.
Temperatures and pressures inside a star is great enough for nuclear fusion to happen.
Stars have enough hydrogen to maintain their energy output for million of years.
Hydrogen nuclei fuse together to form helium nuclei.
In a red giant star, the start fuse heavier elements from helium to iron.
Elements heavier than iron are formed in supernovas.
Heavy elements are found in suns and stars which suggest the solar system are formed from remains of supernovas
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