Speed, Distance, Time (1)
The speed of a moving object can be calculated if the distance travelled and the time taken are known. The faster an object moves, the steeper is the line representing it on a distance-time graph. The velocity of an object is its speed in a particular direction. In velocity-time graphs sloping lines represent steadily increasing or decreasing velocities. Horizontal lines represent movement at constant velocities. When an object moves in a straight line at a steady speed, you can calculate itsaverage speed if you know how far it travels and how long it takes. This equation shows the relationship between speed, distance travelled and time taken:
For example, a car travels 300 m in 20 s. - Its speed is 300 ÷ 20 = 15 m/s.
The instantaneous speed of an object is the speed of an object at a particular instant. In practice it is the average speed over a very short period of time.
Forces occur when there is an interaction between two objects. These forces always happen in pairs – when one object exerts a force on another, it always experiences a force in return.
The green arrow shows the force on the weights as the weightlifter pushes upwards.
The red arrow shows the downwards force on the weightlifter's arm muscles.
These two forces are an interaction pair. They are equal in size, and opposite in direction.
You only have an interaction pair if the forces are caused by the interaction. In this case, the compression in the weightlifter's muscles is caused by the weight pushing down, and the upwards force on the weight is caused by the weightlifter's muscles.
Example of forces
Example: rocket engines
as the fuel burns, exhaust gases are produced. One common interaction pair of forces is found in a rocket or a jet engine:
- the rocket engine pushes these gases out backwards
- the gases push the space shuttle forwards, with the same size force in the opposite direction.
That's how rockets work.
One force causes another...
One force causes another
Sometimes a force is produced as a response to another force – these are not the same as interaction pairs.
A book on a table has a downwards force (its weight) due to gravity.
This downwards force, pushing on the table, produces an upwards force called reaction.
The weight and the reaction of the surface are the same size, and in opposite directions.
They are not an interaction pair, because the weight of the book is caused by the Earth's gravity, not by the table.
When two surfaces slide past each other, the interaction between them produces a force of friction.
When you push backwards on the floor with your foot, the friction between your foot and the floor exerts a backwards force on the floor. The other force of the interaction pair is the floor pushing your foot forwards.
The result is that you move forwards, but the floor stays still.
A common experiment is to show the change in friction with different surfaces. In this experiment, a block of wood is pulled along a surface with a force metre. The greater the forces needed to pull the block, the higher the friction.
Forces and motion
The momentum of an object is its mass multiplied by its velocity. The larger the mass and velocity the larger the momentum.
Forces change momentum - the larger the force the more quickly the momentum changes.
The resultant force is the overall result of all forces acting on an object.
- Gravity pulls down on the car
- The reaction force from the road pushes up on the wheels
- The driving force from the engine pushes the car along
- There is friction between the road and the tyres
- Air resistance acts on the front of the car
What resultant force does
The resultant force is the sum of all the different forces acting on the car.
You have to take account of the directions – the reaction forces on the wheels (blue arrows) add up to the same as the weight (green arrow), so these cancel out.
The driving force from the engine (yellow arrow) is in the opposite direction to the counter forces of friction (red arrows) and air resistance (purple arrow).
When the car is increasing its speed then all these forces add to give a single resultant force forwards.
Movement with balanced and unbalanced forces
A car or bicycle has a driving force pushing it forwards. There are alwayscounter forces of air resistance and friction pushing backwards.
You need to know how these forces compare if you are to predict what will happen to the speed of a moving object...
- If the driving force is greater than the counter forces, there is a resultant force forwards. This will make the car speed up
- If the driving force is less than the counter forces, there is a resultant force backwards. This will make the car slow down
If the driving force is the same as the counter forces, there is no resultant force, and so no change in velocity.
- If the car is already moving, it will carry on at a steady speed in a straight line
- If the car is not moving, it will stay still
If a ball is released from a height then forces begin to act on it. It will start to fall due to the force of gravity acting on it. The ball will begin to accelerate (its speed increases).
If an object is dropped that is light relative to its size, like a feather, it will speed up when released at first but then fall at a steady speed. This is due to air resistance.
The faster an object moves, the greater the force of air resistance on it becomes. The light object will reach a steady speed when the force of air resistance balances out the force of gravity.
Force and momentum change
The momentum of a moving object depends on its mass and its velocity:
Momentum (in kg m/s) = mass (in kg) × velocity (in m/s)
A resultant force acting on any object changes the momentum of that object.
The size of the change in momentum depends on the size of the resultant force and the time for which the force acts:
Change of momentum (in kg m/s) = resultant force (in newton, N) × time for which it acts (in s)
To give the same change of momentum, you can have:
- A larger force for a shorter time, or
- A smaller force for a longer time
Reducing forces in car crashes (1)
In a moving car the passengers and the driver all have momentum.
If the car crashes and comes to a sudden stop each of them will lose all their momentum. To make sure that the force on them is as small as possible, it is important that they stop gradually.
This is done with a seat belt, which stretches when the car stops moving, so that the person wearing the belt doesn't stop immediately.
Air bags have the same effect - they slow down the change in momentum, and so reduce the forces.
Reducing forces in car crashes (2)
In a crash the person's head hits the air bag instead of the windscreen. Because the air bag has a hole in it, the person's head pushes the air out of the bag. This makes their head come to a stop more slowly than if it had just hit the windscreen.
From the start of the crash to coming to a stop the people must lose all their momentum. The time of contact between the head and the air bag is much greater than it would be between the head and the windscreen. So the force is much less if there is an air bag because it takes longer.
Cars are fitted with front and rear crumple zones. As they gradually deform they increase the amount of time the person takes to come to a stop. This reduces the acceleration and force on the person, so reducing injury from impact.
Motion and energy changes
Work done and energy transferred are measured in joules (J). The work done on an object can be calculated if the force and distance moved are known.
Objects raised against the force of gravity increase gravitational potential energy.
The more mass an object has and the faster it moves, the more kinetic energy it has.
Work, force and distance
Work and energy are measured in the same unit, the joule (J).
Amount of energy transferred (joule, J) = work done (joule, J)
This equation shows the relationship between work done, force applied and distance moved:
Work done (joules, J) = force (newtons, N) x distance (metres, m)
The distance involved is the distance moved in the direction of the applied force.
Weight (newton, N) = mass (kilogram, kg) x gravitational field strength (N/kg)
The gravitational field strength on the Earth's surface is about 10 N/kg. This is quite handy because all you need to do to convert between weight and mass is to multiply the mass by 10.
For example, a 1kg bag of sugar weighs 1 × 10 = 10 N.
Gravitational potential energy
If you lift a book up onto a shelf you have to do work against the force of gravity. The book has gained gravitational potential energy. Any object that is raised against the force of gravity has an increase in its gravitational potential energy.
You should know and be able to use, the relationship between change in gravitational potential energy, weight and change in height.
This equation shows the relationship between gravitational potential energy (joule, J), weight (Newton, N) and change in height (metre, m):
Change in GPE = weight x change in height
For example, if a 1 N weight is raised by 5 m it gains 1 × 5 = 5 J of gravitational potential energy.
Every moving object has kinetic energy (KE). The more mass an object has and the faster it is moving, the more kinetic energy it has. So the bigger the object, the faster it will move.
This equation shows the relationship between kinetic energy (J), mass (kg) and speed (m/s):
Kinetic energy = 1/2 × mass × speed2
Conservation of energy (1)
Energy is always conserved – the total amount of energy present stays the same before and after any change.
Conservation of energy (2)
The pendulum shows the principal of conservation of energy in action. Gravitational potential energy is converted to kinetic energy and back, over and over again, as the pendulum swings.
The diagram shows a pendulum in three positions - the two ends of its swing and as it passes through the middle point.
When the pendulum bob is at the start of its swing it has no kinetic energy because it is not moving, but its gravitational potential energy (GPE) is at a maximum, because it is at the highest point.
As the bob swings downwards it loses height. So its gravitational potential energy (GPE) decreases. The work done on the bob by the gravitational force (weight) pulling it downwards increases its kinetic energy. The loss of GPE = the gain in KE.
At the bottom of its swing, the bob's kinetic energy is at a maximum and its gravitational potential energy is at a minimum - because it is at its lowest point.
Conservation of energy (3)
As the bob swings upwards it slows down. Its kinetic energy decreases as work is done against its weight. As it gains height the gravitational potential energy increases again.
At the very top of its swing it stops for a moment. It once again has no kinetic energy, but its gravitational potential energy is at a maximum.
At all points during the swing, the total (GPE + KE) is constant.
Note that in a real pendulum the bob's swing will become slightly lower with each swing, because some energy is lost (dissipated, 'wasted') through heating, due to air resistance.
Electric current is the flow of electric charge. Some insulating materials become electrically charged when they are rubbed together. A substance that gains electrons becomes negatively charged, while a substance that loses electrons becomes positively charged.
Charges that are the same (eg positive and positive) repel, while unlike charges (eg positive and negative) attract. Charged objects are able to attract small uncharged objects towards them, such as pieces of paper.
Charging an insulator
When you rub two different insulating materials against each other they become electrically charged. This only works for insulated objects - conductors direct the charge flow to earth.
When the materials are rubbed against each other:
- Negatively charged particles called electrons move from one material to the other
- The material that loses electrons becomes positively charged
- The material that gains electrons becomes negatively charged
- Both materials gain an equal amount of charge, but the charges areopposite.
Attraction and repulsion
If two charged objects are brought close together they are either attractedtowards each other, or they are repelled away from each other. If they have the same type of charge they will repel and push away from each other. Two negative charges placed near each other will repel each other, and so will two positive charges.
If they have opposite charges they will attract each other. When a positive charge and a negative charge are placed near each other they will attract each other.
Electrically charged objects can attract small uncharged objects that are close to them. For example, an electrically charged plastic comb can pick up small pieces of paper.
Metals are good conductors of electricity because metal atoms release some of their electrons & these electrons are free to move through the structure of the metal.
Insulators have no charges free to move, which is why they do not conduct electricity.
Atomic structure of a metal
In an electrical circuit, all the components have charges that are free to move. When a circuit is made, the battery causes these free charges to move. They move in a continuous loop around the circuit.
The strength of an electric current is measured in amperes, usually shortened to amps. Amps are a measure of how much electric charge is flowing round the circuit.
Size of a current
Resistance is measured in ohms. This can be calculated in different types of circuit. Resistance in the filaments of a lamp changes when filaments heat up. Diodes, temperature-dependent resistors (called thermistors) and light-dependent resistors (LDRs) are components which can alter the current.
The size of a current in a circuit depends on the voltage of the battery and theresistance of the components in the circuit.
Voltage can be thought of as the 'push' it exerts on charges in the circuit. A bigger voltage means a bigger 'push', resulting in a larger current.
Resistance & ohm's law (1)
An electric current flows when charged particles called electrons move through a conductor. The moving electrons can collide with the atoms of the conductor. This is called resistance and it makes it harder for current to flow. These collisions make the conductor hot. It is this that makes a lamp filament hot enough to glow.
Resistance is measured in ohms, Ω. The greater the number of ohms, the greater the resistance.
Resistance & ohm's law (2)
The equation below shows the relationship between resistance, voltage and current:
Resistance = voltage / current
Ohms (Ω) = volts (V) / amperes (A)
The current flowing through a resistor at a constant temperature is directly proportional to the voltage across the resistor. So, if you double the voltage, the current also doubles. This is called Ohm's Law. The graph shows what happens to the current and voltage when a resistor follows Ohm's Law.
Series circuits & variable resistors
Series circuits - When components are connected in series, their total resistance is the sum of their individual resistances. For example, if a 2 Ω resistor, a 1 Ω resistor and a 3 Ω resistor are connected side by side, their total resistance is 2 + 1 + 3 = 6 Ω.
If you increase the number of lamps in a series circuit, the total resistance will increase and less current will flow
Variable resistors - The resistance in a circuit can also be altered using variable resistors. For example, these components may be used in dimmer switches, or to control the volume of a CD player. Adjust the resistance in the simulation by clicking the + and - buttons to see the effect on the current.
Thermistors & LDRs
At low temperatures, the resistance of a thermistor is high and little current can flow through themThermistors are used as temperature sensors, for example, in fire alarms. Their resistance decreases as the temperature increases:
- At high temperatures, the resistance of a thermistor is low and more current can flow through them
LDRs (light-dependent resistors) are used to detect light levels, for example, in automatic security lights. Their resistance decreases as the light intensity increases:
- In the dark and at low light levels, the resistance of an LDR is high and little current can flow through it
- In bright light, the resistance of an LDR is low and more current can flow through it
Electric motors transfer electrical energy into kinetic energy, using magnets and coils of wire. These motors are used in devices including domestic appliances and DVD players.
A current-carrying wire or coil can exert a force on a permanent magnet. The wire could also exert a force on another nearby current-carrying wire or coil.
If the current-carrying wire is placed in a magnetic field (whose lines of force are at right angles to the wire) then it will experience a force at right angles to both the current direction and the magnetic field lines.
The electric motor
A simple electric motor can be built using a coil of wire that is free to rotate between two opposite magnetic poles. When an electric current flows through the coil, the coil experiences a force and moves.
The direction of the current must be reversed every half turn, otherwise the coil comes to a halt again. This is achieved using a conducting ring split in two, called a split ring or commutator. A coil of wire is used with lots of turns to increase the effect of the magnetic field.
A voltage is produced when a magnet moves into a coil of wire. This principle is used in generators to produce electricity - either a coil of wire rotates in a magnetic field, or a magnet rotates in a coil of wire. Transformers are used to increase or decrease the voltage of alternating current (AC) supplies.
A voltage is produced when a magnet is moving into a coil of wire. This process is called electromagnetic induction. The direction of the induced voltage is reversed when the magnet is moved out of the coil again. It can also be reversed if the other pole of the magnet is moved into the coil.
If the coil is part of a complete circuit then a current will be induced in the circuit.
Increasing the induced voltage
To increase the induced voltage:
- Move the magnet faster
- Use a stronger magnet
- Increase the number of turns on the coil
- Increase the area of the coil
It is not practical to generate large amounts of electricity by passing a magnet in and out of a coil of wire. Instead, generators induce a current by spinning a coil of wire inside a magnetic field, or by spinning a magnet inside a coil of wire. As this happens, a potential difference is produced between the ends of the coil, which causes a current to flow.
One simple example of a generator is the bicycle dynamo. The dynamo has a wheel that touches the back tyre. As the bicycle moves, the wheel turns a magnet inside a coil. This induces enough electricity to run the bicycle's lights.
The faster the bicycle moves, the greater the induced current and the brighter the lights.
Making AC electricity
When a wire is moved in the magnetic field of a generator, the movement, magnetic field and current are all at right angles to each other. If the wire is moved in the opposite direction, the induced current also moves in the opposite direction. Remember that one side of a coil in a generator moves up during one half turn, and then down during the next half turn. This means that as a coil is rotated in a magnetic field, the induced current reverses direction every half turn. This is called alternating current (AC). It is different from the direct current (DC) produced by a battery, which is always in the same direction. In practical generators, the coil is fixed, and mounted outside the magnet, and it is the magnet which moves.
The size of the induced voltage can be increased by:
- Rotating the coil or magnet faster
- Using a magnet with a stronger magnetic field
- Having more turns of wire in the coil
- Having an iron core inside the coil
The mains electricity is an AC supply. The voltage it supplies to our homes is 230V.
A transformer is an electrical device that changes the voltage of an AC supply. A transformer changes a high-voltage supply into a low-voltage one, or vice versa.
- A transformer that increases the voltage is called a step-up transformer.
- A transformer that decreases the voltage is called a step-down transformer.
- Step-down transformers are used in mains adapters and rechargers for mobile phones and CD players.
- Transformers do not work with DC supplies.
A transformer consists of a pair of coils wound on an iron core. The AC in one coil produces a changing magnetic field. This changing magnetic field induces a voltage in the other coil of the transformer.
How transformers work
A transformer needs an alternating current that will create a changing magnetic field. A changing magnetic field also induces a changing voltage in a coil. This is the basis of how a transformer works:
- The primary coil is connected to an AC supply
- An alternating current passes through a primary coil wrapped around a soft iron core
- The changing current produces a changing magnetic field
- This induces an alternating voltage in the secondary coil
- This induces an alternating current (AC) in the circuit connected to the secondary coil
It is important to know that
- There is no electrical connection between the primary and the secondary coils.
- Transformers only work if AC is supplied to the primary coil. If DC was supplied, there would be no current in the secondary coil.
- As the current in the primary coil increases steadily or decreases steadily, there is a constant voltage induced in the secondary coil.
- As the voltage in the primary coil reaches maximum strength the voltage induced in the secondary coil is at its weakest (zero volts).
The ratio between the voltages in the coils is the same as the ratio of the number of turns in the coils.
Primary voltage / secondary voltage = turns on primary / turns on secondary
This can also be written as:
Vp/Vs = Np/Ns
Step-up transformers have more turns on the secondary coil than they do on the primary coil.
Step-down transformers have fewer turns on the secondary coil than they do on the primary coil.
Parallel and series circuits
Electrical circuits can be represented by circuit diagrams. The various electrical components in the circuit are shown by using standard symbols. Components can be connected in series, or in parallel. The current and potential difference (voltage) are different in series and parallel circuits.
Circuit diagrams -
Two things are important for a circuit to work:
- There must be a complete circuit
- There must be no short circuits
To check for a complete circuit, follow a wire coming out of the battery with your finger. You should be able to go out of the battery, through the lamp and back to the battery.
To check for a short circuit, see if you can find a way past the lamp without going through any other component. If you can, then there is a short circuit and the lamp will not light.
Circuit symbols (1)
Circuit symbols (2)
Circuit symbols (3)
Circuit symbols (4)
Components that are connected one after another on the same loop of the circuit are connected in series. The current that flows in each component connected in series is the same.
The circuit diagram shows a circuit with two lamps connected in series. If one lamp breaks, the other lamp will not be lit.
The diagram shows a circuit with two lamps connected in parallel. If one lamp breaks, the other lamp will still be lit.
Because a parallel circuit has more paths for charges to flow along, the current is bigger, and the resistance of the whole circuit is smaller.
A current flows when an electric charge moves around a circuit. No charge can flow if the circuit is broken, for example, when a switch is open. Click on the animation to see what happens to the charge when the switch is opened or closed.
- Current is measured in amperes
- Amperes is often abbreviated to amps or A
- Current flowing in a component in a circuit is measured using an ammeter
- The ammeter must be connected in series with the component
A 'potential difference' across an electrical component is needed to make a current flow in it. Cells or batteries often provide the potential difference needed.
'Potential difference' is often called 'voltage'.
Measuring potential difference:
- Potential difference is measured in volts, V
- Potential difference across a component in a circuit is measured using avoltmeter
- The voltmeter must be connected in parallel with the component
Energy in circuits
The potential difference between any two points in a circuit is the energy transferred to, or from, a given amount of charge as it passes between those points. In the circuit above, the charges gain energy in the cell, and then transfer that same amount of energy into light and heat in the lamp. That is why the potential difference across the cell is the same as the potential difference across the lamp; it is the same amount of energy.
Cells and circuits - what happens?
Potential difference - A typical cell produces a potential difference of 1.5 V. When two or more cells are connected in series in a circuit, the total potential difference is the sum of their potential differences. For example, if two 1.5V cells are connected in series in the same direction, the total potential difference is 3.0 V. If two 1.5 V cells are connected in series, but in opposite directions, the total potential difference is 0 V, so no current will flow.
Current - When more cells are connected in series in a circuit, they produce a bigger potential difference across its components. More current flows through the components as a result.
Series circuits - characteristics
Current - When two or more components are connected in series, the same current flows in each component.
Potential difference - When two or more components are connected in series, the total potential difference of the supply is shared between them. This means that if you add together the voltages across each component connected in series, the total equals the voltage of the power supply.
Two identical resistors connected in series will share the potential difference. They will get half each. For example if two identical resistors are connected in series to a 3 V cell then the potential difference across each of them is 1.5 V. If resistors connected in series are not the same then the potential difference is larger across the larger of the resistors. For example if a 2 Ω and a 1 Ω resistor are connected in series to a cell of 3V, then the potential difference across the 2 Ω resistor will be 2 V, and across the 1Ω resistor will be 1 V. If there is a change in the resistance of one component then this will result in a change in the potential differences across all of the other components in the circuit.
Parallel circuits - characteristics
Current - When two or more components are connected in parallel, the total current flowing in the circuit is shared between the components. Check your understanding of this by answering the questions about the circuit seen here. Assume that both lamps are identical.
Potential difference - When two or more components are connected in parallel, the potential difference across them is the same. This means that if a voltage across a lamp is 12 V, the voltage across another lamp connected in parallel is also 12 V.
More on series circuits
The potential differences across resistors in series must add up to the battery voltage. This is because the total energy transferred by the battery must equal the amount of energy transferred to the other components in the circuit. Energy is always conserved.
The energy is transferred from the cell to the electric charge moving through the circuit. The charge then transfers energy to the components (bulbs, resistors, etc).
More energy is transferred by charge flowing through a larger resistance than through a smaller one.
This is why a large and a small resistor connected in series have different voltages across them. The large resistance has a larger voltage because more energy is being transferred as the charge flows through it.
More on parallel circuits
In parallel circuits, the voltage across each component is the same as the voltage of the battery. Each component in parallel has the same current as it would have if it were connected to the battery without the other components present.
This means that a higher resistance in parallel with a smaller resistance would have less current in it, as the same voltage will cause less current in a larger resistance than in a smaller one.
Why materials are radioactive
Radioactive elements give out radiation, and there are three different types. They penetrate materials in different ways, and take different amounts of time to decay.
Atomic structure and radiation
Atomic structure - Atoms contain three sub-atomic particles called protons, neutrons and electrons. The protons and neutrons are found in the nucleus at the centre of the atom. The protons and neutrons are held together in the nucleus by a strong force which balances the repulsive electrostatic force between the protons. The electrons are outside the nucleus.
Radiation - Radioactive elements give out ionising radiation from their nuclei. This happens all the time, whatever is done to the substance. For example, radiation is still given out if the substance is cooled in a freezer or involved in a chemical reaction.
Radioactive elements are found naturally in the environment. The radiation they give off is called background radiation.
Nuclear fusion & Einstein's equation
If hydrogen nuclei are brought close enough together then they have the ability to fuse. This process is called nuclear fusion. It takes a lot of work to force nuclei together but a lot of energy is released when they fuse.
The equation above is an example of nuclear fusion. Two isotopes of hydrogen have fused to create helium.
Isotopes are atoms of an element that have different mass numbers because they have different numbers of neutrons in the nucleus.
Einsteins equation - The following equation can be used to calculate the energy released during nuclear fusion (and fission) reactions:
E = mc2
Where: E = is the energy produced
- M = change in mass
- C = speed of light
Penetrating properties of radiation
Radiation can be absorbed by substances in its path. Alpha radiation travels only a few centimetres in air, beta travels tens of centimetres, while gamma radiation travels many metres. All types of radiation become less intense the further they are from the radioactive material, as the particles or rays become more spread out. The thicker the substance, the more the radiation is absorbed. But the three types of radiation penetrate materials in different ways.
Alpha radiation is the least penetrating. It can be stopped, or absorbed, by just a sheet of paper.
Beta radiation is able to penetrate air and paper. It can be stopped by a thin sheet of aluminium.
Gamma radiation is the most penetrating. Even small levels can penetrate air, paper or thin metal. Higher levels can only be stopped by many centimetres of lead, or many metres of concrete.
When an unstable nucleus decays, there are three ways that it can do so.
It may give out:
- An alpha particle
- A beta particle
- A gamma ray
Alpha particle - Two protons and two neutrons – the same as a helium nucleus
Beta particle - Fast moving electron
Gamma particle - High energy electromagnetic radiation
The nucleus of an atom can be represented as:
A = atomic mass (number of protons + neutrons)
Z = atomic number (number of protons)
X = chemical symbol (as shown on the periodic table)
When an alpha particle is emitted from the nucleus, the nucleus loses two protons and two neutrons. This means the atomic mass number decreases by four and the atomic number decreases by two. A new element is formed that is two places lower in the Periodic Table than the original element.
Radon decays into polonium when it emits an alpha particle. Here is the equation for that radioactive decay.
In Beta decay a neutron changes into a proton plus an electron. The proton stays in the nucleus and the electron leaves the atom with high energy, and we call it a beta particle.
When a beta particle is emitted from the nucleus, the nucleus has one more proton and one less neutron. This means the atomic mass number remains unchanged and the atomic number increases by 1.
Carbon-14 is a radioactive isotope of carbon. (It's a carbon atom with 8 neutrons instead of the usual 6.) Here is the equation for the beta decay of carbon-14 into nitrogen.
Half life - The nuclei of radioactive atoms are unstable. They break down and change into a completely different type of atom. This process is called radioactive decay. For example, carbon-14 decays to nitrogen-14 when it emits beta radiation. Over time, as the unstable atoms in a source of radiation change, the activity of the source goes down because there are fewer unstable atoms present to decay. It is not possible to predict when an individual atom might decay, but you can measure how long it takes for half the atoms to decay. This is called the half-lifeof this type of radioactive atom. There are two definitions of a half-life, but they mean essentially the same thing:
- The time it takes for the number of atoms in a sample to halve
- The time it takes for the activity of a source of radiation to fall to half its starting level
Different radioactive isotopes and types of radioactive material have different half-lives. For example, the half-life of carbon-14 is nearly 6000 years, but the half-life of radon-222 is under four days.
Isotopes and changing elements
All the atoms of a given element have the same number of protons. The number of neutrons can vary. Atoms of the same element that have different numbers of neutrons are called isotopes of that element.
When an atom emits alpha or beta radiation, its nucleus changes. It becomes the nucleus of a different element. This is because the number of protons in the nucleus determines which element the atom belongs to.
These are the changes that occur to the number of protons in an unstable nucleus when it emits a radioactive particle:
- Alpha particle - the number of protons goes down by two
- Beta particle - the number of protons increases by one
In each case, a different element is left behind.
Radiation is all around us. Radioactive materials do expose people to risk but they also offer benefits, and the arguments need to be weighed up in each case.
Ionising radiation and living cells - The radiations from radioactive materials –alpha, beta and gamma radiation – are all ionising radiations which can damage living cells.
This happens because ionising radiation can break molecules into bits called ions. These ions can then take part in other chemical reactions in the living cells.
This may result in the living cells dying, or becoming cancerous.
Sources of radiation
Radiation is all around us. It comes from radioactive substances including the ground, air, buldings materials and food. Radiation is also found in cosmic rays from space.
Cosmic rays - radiation that reaches the earth from outer space
Animals - animals emit natural levels of radiation
Rocks - some rocks give off radioactive radon gas
Soil and plants - radioactive materials from rocks in the ground are absorbed by the soil and passed onto plants
Hazards from radioactive materials
Radioactive materials in the environment, whether natural or artificial, do expose people to risks.
This can happen in two ways:
- The radiation from the material can damage the cells of the person directly. This is damage by irradiation.
- Some of the radioactive material can be swallowed or breathed in. While inside the body, the radiation it emits can produce damage. This is damage by contamination.
Length of time for radioactive materials to become safe - Radioactive materials, such as the radioactive waste from power stations, can be a real health hazard.
However, they do not stay radioactive forever. Each radioactive material has a half-life, and after this time it will have only half the activity it had before. Another half-life, and it's down to a quarter. Eventually, the activity will be similar to background radiation, and the material will be safe. For some sources, this could be thousands of years.
Uses of radiation (1)
Radiotherapy - Although ionising radiation can cause cancer, high doses can be directed at cancerous cells to kill them. This is called radiotherapy. About 40 per cent of people with cancer undergo radiotherapy as part of their treatment. It is administered in two main ways:
- From outside the body using x-rays or the radiation from radioactive cobalt
- From inside the body by putting radioactive materials into the tumour, or close to it
Some normal cells are also damaged by the radiation, but they can repair themselves better than the cancer cells are able to.
Radioactive tracers - Radioactive molecules of an atom can be used as radioactive tracers. These tracers emit gamma radiation that will almost all penetrate out of the body to be detected by a gamma camera. This camera traces where the radioactive molecule goes in the body. Images can then be formed to allow doctors to see areas of damage to assess what treatment is needed.
Uses of radiation (2)
Sterilising surgical instruments - Surgical instruments are sterilised using high doses of gamma radiation from radioactive cobalt. Food can also be sterilised by gamma radiation in the same way. The radiation kills microorganisms, preserving the food for longer. But in the UK only irradiated herbs, spices or vegetable seasonings, with the correct labels, are allowed. Major supermarkets say they will not stock irradiated foods because people are reluctant to buy them, even though the food itself is not radioactive.
The human senses cannot detect radiation, so we need equipment to do this. Photographic film goes darker when it absorbs radiation, like when it absorbs visible light. The more radiation the film absorbs, the darker it is when developed.
People who work with radiation wear film badges which are checked regularly to monitor the levels of radiation absorbed. There is a light-proof packet of photographic film inside the badge. The more radiation this absorbs, the darker it becomes when it is developed. To get an accurate measure of the dose received, the badge contains different materials that the radiation must penetrate to reach the film. These may include aluminium, copper, lead-tin alloy and plastic. There is also an open area at the centre of the badge.
Being exposed to radiation - People who may be exposed to radiation regularly include:
- Medical staff such as radiographers
- Workers at nuclear power stations
- Research scientists
Nuclear power stations
The main nuclear fuels are uranium and plutonium, both of which are radioactive metals. Nuclear fuels are not burned to release energy. Instead, heat is released from changes in the nucleus.
Just as with power stations burning fossil fuels, the heat energy is used to boil water. The kinetic energy in the expanding steam spins turbines, which drive generators to produce electricity.
Nuclear waste is given different categories.
Nuclear waste categories
Low level - Examples: Contaminated equipment, materials and protective clothing.
Disposal: They are put in drums and surrounded by concrete, and put into clay lined landfill sites.
Intermediate level - Examples: Components from nuclear reactors, radioactive sources used in medicine or research
Disposal: They are mixed with concrete, then put in a stainless steel drum in a purpose-built store.
High level - Examples - Used nuclear fuel and chemicals from reprocessing fuels
Disposal - They are stored underwater in large pools for 20 years, then placed in storage casks in purpose-built underground store where air can circulate to remove the heat produced. High level waste decays into intermediate level waste over many thousands of years.
Nuclear fission (1)
Nuclear power stations use the heat released by nuclear reactions to boil water to make steam.
The type of nuclear reaction used is called nuclear fission. In nuclear fission:
- A neutron collides with a uranium nucleus. A uranium nucleus is large and unstable
- The uranium nucleus splits into two similar-sized smaller nuclei
- More neutrons are released
- These neutrons can then collide with more uranium nuclei
These processes are repeated continuously, forming a chain reaction.
Rate of energy released - In a nuclear reactor, the reaction is controlled so that energy is released at a steady rate.
The energy released in nuclear fission is far greater than the energy released in a chemical reaction, such as burning fuel. This means that the power output of a nuclear power station is large. The lifetime of a nuclear power station is about 20 years.
Nuclear fission (2)
In considering the subject of nuclear power, it is important to weigh up the advantages and the disadvantages. These are some of the advantages:
- No carbon dioxide is produced when the station is operating
- There is a high power output
- A small amount of fuel is needed, when compared with coal or gas
These are some of the disadvantages:
- Hazardous radioactive waste is produced
- Building the power stations is quite expensive
- Decommissioning, ie taking apart, the power stations at the end of their lifetime is very costly
The nuclear fuel, usually uranium oxide, is held in metal containers called fuel rods. These are lowered into the reactor core. A coolant, usually water or carbon dioxide, is circulated through the reactor core to remove the heat. Control rods are also lowered into the core. These absorb neutrons and control the rate of the chain reaction. They are raised to speed it up, or lowered to slow it down.