- Created by: Nadine Norton
- Created on: 20-01-14 20:01
You can get an electrostatic shock if you are electrically 'charged' and you touch something that is earthed or if you're earthed and you touch something that is charged. For example, when you walk on a vinyl floor or one covered with a nylon carpet you 'charge up' because of friction. You can earth yourself, and so get an electrostatic shock by touching a metal door handle, water pipe or even another person.
Problems with static
Static is a nuisance when:
- Dust and dirt are attracted to insulators such as TV screens and computer monitors
- Clothes made from synthetic materials cling to each other and to the body, especially just after they've been in a tumble drier
Dangers of static
Static electricity can build up in clouds. This can cause a huge spark to form between the ground and the cloud. This causes lightning – a flow of charge through the atmosphere. Static is dangerous when:
- There are inflammable gases or vapours or a high concentration of oxygen. A spark could ignite the gases and cause an explosion.
- You touch something with a large electric charge on it. The charge will flow through your body causing an electric shock. This could cause burns or even stop your heart. A person could die from an electric shock.
The chance of receiving an electric shock can be reduced if:
- An object that might become charged is connected to the Earth by an earth wire, so any charge immediately flows down the earth wire.
- In a factory, machinery operators stand on insulating mats or wear shoes with insulating soles. This stops any charge flowing through them to the Earth.
- Lorries containing inflammable gases, liquids and powders are connected to the Earth by an earth wire before being unloaded. This means charge immediately flows down the earth wire preventing a spark that could cause an explosion.
- When an aircraft is refuelled, static can build up. This could cause sparks, which could ignite the fuel. A bonding line is used to earth the aircraft before it is refuelled.
Uses of electrostatics
Uses of electrostatics
Car manufacturers can save money by using charged paint spray guns. They work because like charges such as positive and positive repel, and unlike charges such as positive and negative attract.
The spray gun is charged positively, which causes every paint particle to become positively charged. Like charges repel and the paint particles spread out. The object to be painted is given a negative charge and so attracts the paint particles. The advantages of using this system are that less paint is wasted, the object receives an even coat and the paint covers awkward 'shadow' surfaces that the operator cannot see.
Insecticide sprays make use of static electricity. They can be sprayed from aircraft so that they cover a large area. With this method there is a risk that some of the spray will blow away or fall unevenly. To prevent this, the insecticide is given a static charge as it leaves the aircraft. The static drops spread evenly as they all have the same charge and are attracted to the earth.
An electric current is a flow of electric charge. The cost of electricity depends upon the amount of electrical power used, the amount of time it is used for, and the charge made for each unit of electricity. The more electrical energy used, the greater the cost. Electrical supplies can be direct current (DC) or alternating current (AC).
In order to flow, an electric current needs:
- A complete circuit
- Something to push the current around the circuit
An electric current is a flow of electric charge. Conventional current flows from the positive terminal of the power source to the negative terminal.
In wires, negatively charged electrons carry charge. These are free to move from atom to atom in conductors such as metals. They move in the opposite direction to the conventional current.
For a given amount of electrical charge that moves, the amount of energy transformed increases as the potential difference, known as voltage, increases.
Energy transformed (joule, J) = potential difference (volt, V) × charge (coulomb, C)
Charge, current and time
Electrical charge is measured in coulomb (C). The amount of electrical charge that moves in a circuit depends on the current flow and how long it flows for.
The equation below shows the relationship between charge, current and time:
charge (coulomb, C) = current (ampere, A) × time (second, s)
For example, if a current of 10 A flows for 30 s, then 10 x 30 = 300 coulombs of electrical charge moves.
Energy transferred, voltage and charge
For a given amount of electrical charge that moves, the amount of energy transferred increases as the voltage increases.
The equation below shows the relationship between energy transferred, voltage and charge:
energy transferred (joule, J) = potential difference (volt, V) × charge (coulomb, C)
For example, if the voltage is 120 V and the charge is 2 C, the energy transferred is 240 J (120 × 2).
Components that are connected one after another on the same loop of the circuit are connected in series. The current that flows across each component connected in series is the same.
Components that are connected on separate loops are connected in parallel. The current is shared between each component connected in parallel. The total amount of current flowing into the junction, or split, is equal to the total current flowing out. The current is described as being conserved.
A current flows when an electric charge moves around a circuit. No current 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
- The current flowing through a component in a circuit is measured using an ammeter
- The ammeter must be connected in series with the component
A potential difference, also called voltage, across an electrical component is needed to make a current flow through it. Cells or batteries often provide the potential difference needed.
Measuring potential difference:
- Potential difference is measured in volts, V
- Potential difference across a component in a circuit is measured using a voltmeter
- The voltmeter must be connected in parallel with the component
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 1.5 V 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.
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.
Resistance is a measure of how hard it is for a current to flow through a component in a circuit. Resistors are added into a circuit to reduce the amount of current flowing. The bigger the value of resistance, the lower the current.
Components such as bulbs have resistance. When more bulbs are added to a series circuit, resistance increases. This causes the current to decrease.
A variable resistor or rheostat is a device with variable resistance. It can be used to vary the amount of current in a circuit.
Current, potential difference and resistance
The size of the current flowing in a circuit depends on the potential difference (voltage) driving it and the amount of resistance it has to flow through.
For a fixed potential difference the amount of current decreases with increasing resistance.
For a fixed resistance the amount of current increases with increasing potential difference.
resistance (Ω) = voltage (V) / current (A)
This is summarised as V = I x R
Voltage = current x resistance
Resistor at constant temperature
The current flowing through a resistor at a constant temperature is directly proportional to the potential difference across it. A component that gives a graph like the one to the right is said to follow Ohm's Law.
The filament lamp
The filament lamp is a common type of light bulb. It contains a thin coil of wire called the filament. This heats up when an electric current passes through it and produces light as a result.
The filament lamp does not follow Ohm's Law. Its resistance increases as the temperature of its filament increases. So the current flowing through a filament lamp is not directly proportional to the voltage across it. This is the graph of current against voltage for a filament lamp. The diode
Diodes are electronic components that can be used to regulate the potential difference in circuits and to make logic gates. Light-emitting diodes (LEDs) give off light and are often used for indicator lights in electrical equipment such as computers and television sets.
The diode has a very high resistance in one direction. This means that current can only flow in the other direction. This is the graph of current against potential difference for a diode.
Thermistors and LDRs
Thermistors are used as temperature sensors - for example, in fire alarms. Their resistance decreases as the temperature increases:
- At low temperatures, the resistance of a thermistor is high and little current can flow through them.
- 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.
Energy transfer in a resistor
When electric current flows through a resistor some of the energy is transferred to heat energy. This results in the resistor heating up.
- Heating water in a kettle
- Electric fires
However this heating effect isn't always useful. When too much current flows through wires, they can become too hot and catch fire or burn the user.
Explaining the heat transfer
Metals have loose electrons in the outer shells which form a 'sea' of delocalised negative charge around the close-packed positive ions. As the electrons move through the lattice structure of the metal they collide with ions in the lattice. Each of these collisions causes energy to transfer to the lattice in the form of heat. As a result, the metal heats up.
Displacement, velocity, acceleration and force are all vector quantities. The speed of an object can be calculated from the slope on a distance-time graph.
The velocity of an object is its speed in a particular direction. The slope on a velocity-time graph represents the acceleration of an object. The distance travelled is equal to the area under a velocity-time graph.
Forces and motion
A vector quantity has a size and a direction. The following are all vector quantities:
Displacement is the distance travelled in a straight line. It has both a direction and a size.
The velocity of an object is its speed in one particular direction.
The acceleration of an object is calculated from its change in velocity and the time taken.
The force of an object is also a vector as it has a size (measured in Newtons) and a direction.
The velocity of an object is its speed in a particular direction. This means that two cars travelling at the same speed, but in opposite directions, have different velocities.
The vertical axis of a velocity-time graph is the velocity of the object. The horizontal axis is the time from the start.
Features of the graphs
When an object is moving with a constant velocity, the line on the graph is horizontal. When an object is moving with a constant acceleration, the line on the graph is straight, but sloped. The diagram shows some typical lines on a velocity-time graph.
The steeper the line, the greater the acceleration of the object. The blue line is steeper than the red line because it represents an object with a greater acceleration.
Notice that a line sloping downwards - with a negative gradient - represents an object with a constant deceleration - slowing down.
When an object moves in a straight line with a constant acceleration, you can calculate its acceleration if you know how much its velocity changes and how long this takes. This equation shows the relationship between acceleration, change in velocity and time taken:
- For example, a car accelerates in 5s from 25 m/s to 35 m/s.
- Its velocity changes by 35 - 25 = 10 m/s.
- So its acceleration is 10 ÷ 5 = 2 m/s2.
A stationary object remains stationary if the sum of the forces acting upon it - resultant force - is zero. A moving object with a zero resultant force keeps moving at the same speed and in the same direction.
Acceleration depends on the force applied to an object and the object's mass. Gravity is a force that attracts objects with mass towards each other. The weight of an object is the force acting on it due to gravity.
You should be able to use the idea of the resultant force on an object to determine its movement.
An object may have several different forces acting on it, which can have different strengths and directions. They can be added together to give the resultant force. This is a single force that has the same effect on the object as all the individual forces acting together.
When the resultant force is zero
When all the forces are balanced, the resultant force is zero. In this case:
- A stationary object remains stationary
- A moving object keeps on moving at the same speed in the same direction
The longer the arrow, the bigger the force. In this diagram, the arrows are the same length, so we know they are the same size.
When the resultant force is not zero
When all the forces are not balanced, the resultant force is not zero. In this case:
- A stationary object begins to move in the direction of the resultant force
- A moving object speeds up, slows down or changes direction depending on the direction of the resultant force
In this next diagram of the weightlifter, the resultant force on the bar is also not zero. This time, the upwards force is smaller than the downwards force. The resultant force acts in the downwards direction, so the bar moves downwards.
This can be shown with numbers in a calculation. If the upwards force was 3 N and the downward force 7 N then the resultant force would be 4 N (the difference between the two forces). It would act in a downwards direction.
Forces and acceleration calculations
Size of the force
An object will accelerate in the direction of the resultant force. The bigger the force, the greater the acceleration. Doubling the size of the (resultant) force doubles the acceleration.
An object will accelerate in the direction of the resultant force. A force on a large mass will accelerate it less than the same force on a smaller mass. Doubling the mass halves the acceleration.
Resultant force (newton, N) = mass (kg) × acceleration (m/s2).
You can see from this equation that 1 N is the force needed to give 1 kg an acceleration of 1 m/s2.
- For example, the force needed to accelerate a 10 kg mass by 5 m/s2 is:
- 10 x 5 = 50 N
- The same force could accelerate a 1 kg mass by 50 m/s2 or a 100 kg mass by 0.5 m/s2.
- Putting it simply, we can say that it takes more force to accelerate a larger mass.
Weight is not the same as mass. Mass is a measure of how much stuff is in an object. Weight is a force acting on that stuff.
You have to be careful. In physics, the term weight has a specific meaning, and is measured in newtons. Mass is measured in kilograms. The mass of a given object is the same everywhere, but its weight can change.
Gravitational field strength
Weight is the result of gravity. The gravitational field strength of the Earth is 10 N/kg (ten newtons per kilogram). This means an object with a mass of 1kg would be attracted towards the centre of the Earth by a force of 10 N. We feel forces like this as weight.
You would weigh less on the Moon because the gravitational field strength of the Moon is one-sixth of that of the Earth (1.6 N/kg). But note that your mass would stay the same.
On Earth, if you drop an object it accelerates towards the centre of the planet. You can calculate the weight of an object using this equation:
weight (N) = mass (kg) × gravitational field strength (N/kg)
When an object is dropped, we can identify three stages before it hits the ground:
- At the start, the object accelerates downwards because of its weight. There is no air resistance. There is a resultant force acting downwards.
- As it gains speed, the object's weight stays the same, but the air resistance on it increases. There is a resultant force acting downwards.
- Eventually, the object's weight is balanced by the air resistance. There is no resultant force and the object reaches a steady speed, called the terminal velocity.
What happens if you drop a feather and a coin together? The feather and the coin have roughly the same surface area, so when they begin to fall they have about the same air resistance.
As the feather falls, its air resistance increases until it soon balances the weight of the feather. The feather now falls at its terminal velocity. But the coin is much heavier, so it has to travel quite fast before air resistance is large enough to balance its weight. In fact, it probably hits the ground before it reaches its terminal velocity.
On the Moon
An astronaut on the Moon carried out a famous experiment. He dropped a hammer and a feather at the same time and found that they landed together. The Moon's gravity is too weak for it to hold onto an atmosphere, so there is no air resistance. When the hammer and feather were dropped, they fell together with the same acceleration.
The stopping distance depends on two factors:
- Thinking distance - It takes time for a driver to react to a situation. During this reaction time the car carries on moving. The thinking distance is the distance travelled in between the driver realising he needs to brake and actually braking.
- Braking distance - The braking distance is the distance taken to stop once the brakes are applied.
stopping distance = thinking distance + braking distance
Factors that might increase stopping distance
Thinking distance can be increased by:
- Greater speed
- Alcohol and drugs
Braking distance can be increased by:
- Greater speed
- Poor road conditions (icy, wet)
- Car conditions (bald tyres, poor brakes, full of people)
Another common force is friction.
When two surfaces slide past each other, the interaction between them produces a force of friction.
The blue and green arrows show the interaction pair of friction forces.
The book experiences a backwards force. This will tend to slow it down.
The table experiences a forwards force. This will tend to move it forwards with the book.
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.
A moving object has momentum. This is the tendency of the object to keep moving in the same direction. It is difficult to change the direction of movement of an object with a lot of momentum. Momentum is a vector quantity as it has both a force and a direction.
momentum (kg m/s) = mass (kg) × velocity (m/s)
Notice that momentum has:
- Magnitude - the amount of the object's mass
- Direction - because it depends on the velocity of the object
Conservation of momentum
So long as no external forces are acting on the objects involved, the total momentum stays the same in explosions and collisions. This is called conserved momentum. You can use this idea to work out the mass, velocity or momentum of an object in an explosion or collision.
The force is measured in newtons, N. The time is measured in seconds, s.
Safety features in vehicles
When there is a car crash, the car, its contents, and the passengers, decelerate rapidly. They experience great forces because of the change in momentum. This can cause injury. If the time taken for the change in momentum on the body is increased, the forces on the body are reduced too. Seat belts and crumple zones are designed to reduce the forces on the body if there is a collision.
Seat belts stop you moving around inside the car if there is a collision. However, they are designed to stretch slightly in a collision. This increases the time taken for the body's momentum to reach zero, so reduces the forces on it.
Air bags increase the time taken for the head's momentum to reach zero, so reduce the forces on it. They also act a soft cushion and prevent cuts.
Bubble wrap is used as a form of packaging in order to protect fragile contents. Small packets of air are sealed into the packaging which absorb the impact of any knocks.
Kinetic energy, work and power
Whenever 'work' is done energy is transferred from one place to another. The amount of work done is expressed in the equation: work done = force x distance. Power is a measure of how quickly work is being done. Power is expressed in the equation: power = work done / time taken.
Work and force
Work is done whenever a force moves something.
Work done (joules, J) = energy transferred (joules, J)
The amount of work done depends on:
- The size of the force on the object
- The distance the object moves
When a car brakes, its kinetic energy is changed into heat energy.
Work done by brakes = loss in kinetic energy
If the speed of the car doubles, the kinetic energy and braking distance quadruple.
kinetic energy = ½ x mass x speed2 This can be summarised as KE = ½mv2
Power is a measure of how quickly work is being done and so how quickly energy is being transferred.
More powerful engines in cars can do work quicker than less powerful ones. As a result they usually travel faster and cover the same distance in less time but also require more fuel.
Power is measured in Watts. One Watt is equal to one joule per second. It is calculated using the following equation.
Power (watts, W) = work done (joule, J) / time taken (seconds, s)
Work done = power x time taken
Time taken = work done / power
Gravitational Potential Energy
On Earth we always have the force of gravity acting on us. When we're above the Earth's surface we have potential (stored) energy. This is called gravitational potential energy. The amount of gravitational potential energy an object on Earth has depends on its:
- Height above the ground
The gravitational field at the Earth's surface produces a force of approximately 10 N (Newtons) on every mass of 1 kg. Gravitational field strength is symbolised by the letter 'g'. On larger planets, like Jupiter where the gravitational field strength is greater, the gravitational potential energy would also be greater.
GPE (J) = mass (kg) x gravitational field strength (or 'g') (N/kg) x height (m)
GPE = m x g x h
All moving objects have kinetic energy. The amount of kinetic energy they have depends on:
A person has more kinetic energy when running than walking. If a car and a lorry are driving at the same speed on the motorway the lorry has more kinetic energy than the car.
Kinetic energy = 1/2 x mass x speed2
Kinetic energy and braking distance
When a car brakes, its kinetic energy is changed into heat energy.
Work done by brakes = loss in kinetic energy
If the speed of the car doubles, the kinetic energy and braking distance quadruple.
Conservation of energy
Energy is always conserved – the total amount of energy present stays the same before and after any changes.
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. It shows the two ends of its swing and the position it will be in as it passes through the middle point.
Conservation of energy
How it works
- When the pendulum bob is at the start of its swing it has no kinetic energy because it is not moving. If however, its gravitational potential energy (GPE) is at a maximum, because it is at the highest point.
- As the bob swings downwards it loses height. 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.
- 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, or wasted, through heating. This is due to air resistance.
The idea of conservation of energy can be applied to a range of energy transfers, eg crash barriers or footballs being kicked. In an exam you may be expected to apply this knowledge to unfamiliar situations.
A diver who has a mass of 50 kg dives off a diving board 3.0 metres above the water level.
What is her kinetic energy when she reaches the water?
- Kinetic energy gained = gravitational potential energy lost
- Kinetic energy gained = weight × height
You must calculate her weight to use in this equation
- Weight = mass × gravitational field strength
- Weight = 50 kg × 10 N / kg
- Weight = 500 N
- Kinetic energy gained = weight × height
- Kinetic energy gained = 500 N × 3 m
- Kinetic energy gained = 1500 J
Nuclear fission and nuclear fusion
Atoms are made of three types of sub-atomic particle: neutrons and protons in the nucleus and electrons orbiting the nucleus. Some materials are radioactive because the nucleus of each atom is unstable and gives out nuclear radiation in the form of alpha particles, beta particles or gamma rays. The radiation can be detected using a Geiger counter. Nuclear reactors use a type of nuclear reaction called nuclear fission. Another type of nuclear reaction - nuclear fusion - happens in the Sun and other stars.
Atoms are made up of two main parts, the nucleus and orbiting electrons. Electrons can be lost or gained and this forms charged particles - ions.The nucleus contains two types of particles called neutrons and protons. As protons, neutrons and electrons are the building blocks of atoms they're called sub-atomic particles.
Some materials are radioactive because the nucleus of each atom is unstable and can decay, or split up, by giving out nuclear radiation in the form of alpha particles, beta particles or gamma rays. The nuclear radiation given off can be detected using a Geiger counter. The number of nuclei that decay and give off radiation every second is called the activity of the material and is measured in Becquerels (Bq).
If a radioactive material has an activity of 200 Bq, in 1 second 200 of its nuclei will decay and give off radiation. In 1 minute 12,000 (= 200 x 60) nuclei will decay. The activity of a radioactive material will decrease with time. This will be shown by a falling count rate, measured using the Geiger counter.
There are three ways an unstable nucleus can decay. It may give out:
- An alpha particle
- A beta particle
- A gamma ray
Types of radiation
Alpha Particle - Two protons and two neutrons - the same as a helium nucleus Beta Particle - Fast-moving electron Gamma ray - High energy electrromagnetic radiation
Ionising radiation is radiation that has enough energy to cause other atoms to lose electrons and form ions. The different forms of radiation have different levels of ionisation abilities:
- Alpha particles – very ionising
- Beta particles – moderately ionising
- Gamma rays – weakly ionising
When an alpha particle is emitted from a nucleus the nucleus loses two protons and two neutrons. This means the atomic mass number decreases by 4 and the atomic number decreases by 2. A new element is formed that is two places lower in the Periodic Table than the original element. Example: 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. Example: 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.
Nuclear power reactors use a reaction called nuclear fission. The fission is a source of energy for the generation of power. Two isotopes in common use as nuclear fuels are uranium-235 and plutonium-239.
Fission is another word for splitting. The process of splitting a nucleus is called nuclear fission. Uranium or plutonium isotopes are normally used as the fuel in nuclear reactors. Their atoms have relatively large nuclei that are easy to split, especially when hit by neutrons.
When a uranium-235 or plutonium-239 nucleus is hit by a neutron, the following happens:
- The nucleus splits into two smaller nuclei – daughter nuclei, which are radioactive
- Two or three more neutrons are released
- Some energy is released
The additional neutrons released may also hit other uranium or plutonium nuclei and cause them to split. Even more neutrons are then released, which in turn can split more nuclei. This is called a chain reaction. The chain reaction in nuclear reactors is controlled to stop it moving too quickly.
All nuclear reactors produce radioactive waste. At present the most dangerous waste is sealed in glass-like blocks which are buried deep within rocks. Careless disposal of waste in the past has led to pollution of land, rivers and the ocean.
As well as producing heat the nuclear reactor can be used to make other materials radioactive. The chain reaction inside the reactor releases neutrons. If a material is put into the reactor some of these neutrons may be absorbed by the nuclei of its atoms. This will make an atom's nucleus unstable which means it has become radioactive. These man-made radioisotopes are often then used as tracers in hospitals to diagnose and treat patients or in industry to detect leaks in pipes.
The nuclear reactor is designed to allow a controlled chain reaction to take place. Each time a uranium nucleus splits up it releases energy and three neutrons. If all the neutrons are allowed to be absorbed by other uranium nuclei the chain reaction will spiral out of control causing an explosion. To control the energy released in the reactor moveable control rods are placed between the fuel rods. These control rods are made of boron which absorbs some of the neutrons so fewer neutrons are available to split uranium nuclei. The control rods are raised to increase and lowered to decrease the number of free neutrons.
Nuclear fusion involves two atomic nuclei joining to make a large nucleus. Energy is released when this happens. Nuclear fusion can also be used as a source of energy.
The Sun and other stars use nuclear fusion to release energy. The sequence of nuclear fusion reactions in a star is complex, but overall hydrogen nuclei join to form helium nuclei. Here is one nuclear fusion reaction that takes place:
Nuclear fusion & cold fusion
Nuclear fusion involves a deuterium and a tritium nucleus colliding and being forced together. Both nuclei are positively charged and therefore will repel each other. This is known as electrostatic repulsion. The nuclei have to get very close in order to collide, which is approximately a million millionth of a millimetre. If the nuclei are moving very fast then they can overcome the electrostatic repulsion. The hotter a molecule is, the faster it will move and the more likely it is to collide.
For a nuclear fusion reactor to work, the temperature and pressure would each have to be very high. These extremely high temperatures and pressures are very difficult to reproduce and are very expensive. As a result, fusion as an energy source is a long way off.
In 1989 Martin Fleischmann and Stanley Pons claimed to have carried out nuclear fusion at 50°C. This became known as cold fusion. It seemed to offer the possibility of an energy source at a reasonable temperature.
The scientific community still rejects their results and their theory. Many attempts to reproduce the results have failed. This has prevented the theory from being validated. All new scientific theories are peer reviewed. This means that different scientists check the work by looking at the research and checking the results.
Background radiation is all around us and is mostly unavoidable. Most background radiation comes from natural sources, while most artificial radiation comes from medical examinations, such as x-ray photographs. Radiation has many uses. We measure the radioactivity of a source using half-lives.
Radiation is all around us. It comes from radioactive substances including the ground, the air, building materials and food. Radiation is also found in the cosmic rays from space.
Cosmic rays - Radiation that reaches the Earth from outer space Animals - All 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 hence passed on to plants
Uses of radiation
Nuclear radiation ionises materials. This changes atoms or molecules into charged particles.
Uses of alpha radiation
Ionisation is useful for smoke detectors. Radioactive americium releases alpha radiation, which ionises the air inside the detector. Smoke from a fire absorbs alpha radiation, altering the ionisation and triggering the alarm.
Uses of beta radiation
Beta radiation is used for tracers and monitoring the thickness of materials. Doctors may use radioactive chemicals called tracers for medical imaging. Certain chemicals concentrate in different damaged or diseased parts of the body, and the radiation concentrates with it. Radiation detectors placed outside the body detect the radiation emitted and, with the aid of computers, build up an image of the inside of the body. Radiation is used in industry in detectors that monitor and control the thickness of materials, eg paper, plastic and aluminium. The thicker the material, the more radiation is absorbed and the less radiation reaches the detector. It then sends signals to the equipment that adjusts the thickness of the material.
Gamma radiation is used in treatment of cancer and testing equipment. Medical equipment can be sterilised using gamma rays. This means that equipment made of plastic can be sterilised. The method using heat would destroyed it.
Half life and its uses
Radioactive decay is a random process. The radioactivity decreases over time. You cannot predict when one unstable nucleus is going to decay. However, you can make predictions about decay when you have large numbers of unstable nuclei. The activity of a radioactive substance is the number of nuclear decays per second. It is measured in Becquerels (Bq).The half-life of a radioactive material is the time it takes for half of the unstable nuclei to decay
The rate of decay of a radioactive material depends on:
- The type of material
- The number of undecayed nuclei present. The greater the number of nuclei present, the greater the rate of decay
Carbon dating - The amount of carbon-14 in the atmosphere has not changed in thousands of years. Even though it decays into nitrogen, new carbon-14 is always being formed when cosmic rays hit atoms high in the atmosphere. Plants absorb carbon dioxide from the atmosphere and animals eat plants. This means all living things have radioactive carbon-14 in them. When an organism, eg a tree, dies it stops taking in carbon dioxide. The amount of carbon-14 in the wood decreases with time as it decays into nitrogen with a half-life of about 5700 years. By comparing how much carbon-14 there is in the dead organism with the amount in a living one, the age of the dead organism can be estimated.
Dating rocks -The half-life of uranium-238 is 4500 million years. When it decays it forms thorium-234 which is also unstable. Finally, after a series of radioactive isotopes are formed it becomes lead-206, which is stable. The age of the rock can be calculated if the ratio of uranium to lead is known. As the rock gets older the proportion of lead increases. If half of the uranium-238 has turned into lead-206 the rock will be 4500 million years old.
Problems with radioactivity
Radioactivity has many uses but it can also cause problems. Ionising radiation can cause significant problems with damage to cell structures. Nuclear waste has also been a concern to scientists.
Ionising radiation and the body
The radiations from radioactive materials – alpha, beta and gamma radiation – are all ionising radiations which can damage living cells.
Ionising radiation can break molecules into smaller fragments. These charged particles are called ions. Ions can then take part in other chemical reactions in the living cells. As a result, ionising radiation damages substances and materials, including those in the cells of living things. The ions themselves can take part in chemical reactions, spreading the damage. This may result in the living cells dying or becoming cancerous. Radiation can also affect DNA, causing mutations.
Ionising radiation includes:
- Ultraviolet radiation, which is found in sunlight
- X-rays, which are used in medical imaging machines
- Gamma rays, which are produced by some radioactive materials
As a result, care needs to be taken when handling ionising radiation.
How ideas about radioactivity have changed
Ideas about the risks of radioactivity have changed over time. Steps to ensure safety include:
- Using tongs used to pick up sources
- Sources are kept in lead-lined containers
- Sources are never pointed at people
- Protective clothing worn by those who work with radioactivity
- Exposure times are limited
Ideas about science - risk
Scientific or technological developments often introduce new risks. Scientists have to consider these risks and often balance them against the potential benefits.
- The development of radioactive materials in the early 20th century led to the deaths of many workers. As the materials were new, no one realised they could be dangerous. Risk can sometimes be assessed by measuring its chance of occurring in a large sample.
- The safe dose that people may receive has been based on the rate of cancer in workers exposed to radiation over many years. It is important to be able to assess the size of risks in any activity. No activity is completely safe.
- The likelihood of dying from a nuclear accident has been calculated, and it is very low. Cycling is much more dangerous. For most activities that carry risks, there are benefits as well.
- A gamma scan gives doctors valuable information to help cure a patient. This benefit outweighs the slight risk from the gamma radiation itself.
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.
Unlike fossil fuels, nuclear fuels do not produce carbon dioxide.
Like fossil fuels, nuclear fuels are non-renewable energy resources. And if there is an accident, large amounts of radioactive material could be released into the environment. In addition, nuclear waste remains radioactive and is hazardous to health for thousands of years. It must be stored safely.
Nuclear waste is given different categories.
Category Examples Disposal
Low level Contaminated equipment, materials and protective clothing They are put in drums and surrounded by concrete, and put into clay lined landfill sites.
Intermediate level Components from nuclear reactors, radioactive sources used in medicine or research They are mixed with concrete, then put in a stainless steel drum in a purpose-built store.
High level Used nuclear fuel and chemicals from reprocessing fuels 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.