Revision help for the 9-1 OCR 21st Century Science GCSE PHYSICS B Exam Papers - learning objectives P1-P3
OCR Level 1/2 GCSE (Grade 9-1) in Physics B (Twenty First Century Science) (J259) - OCR 21st Century GCSE Grade 9-1 PHYSICS B Revision Summaries for physics Chapter P1, Chapter P2 and Chapter P3
For ALL other exam papers, use and bookmark the link below
These are my NEW revision summaries for Y10 starting in September 2016, first exams from May-June 2018 onwards. ALL my unofficial GCSE (Grade 9-1) revision help is based on the NEW 2016 official OCR 9-1 GCSE Twenty First Century Science B physics/science specifications. Make sure you know whether you are doing separate science OCR 9-1 GCSE Twenty First Century PHYSICS B OR OCR 9-1 GCSE Twenty First Century Combined Science B physics.
PAST PAPERS The Google [SEARCH] box at the bottom of the page should also prove useful
Be aware that both GCSE PHYSICS Papers 1 and Paper 2 assess the content from ALL 6 chapters !!!
(HT only) means higher tier only (NOT FT foundation tier) and (GCSE 9-1 physics only) means for the separate 21st Century GCSE science only, NOT for the Combined Science physics course
Syllabus-specification CONTENT INDEX of revision summary notes
Be aware that both Physics Paper 1 "Breadth in Physics" and Paper 2 "Depth in Physics" assess the content from ALL 6 chapters !!!
Revision SUMMARY Chapter P1: Radiation and waves (this page)
Revision SUMMARY Chapter P2: Sustainable energy (this page)
Revision SUMMARY Chapter P3: Electric circuits (this page)
Chapter P1: (For OCR GCSE (9–1) Twenty First Century Science PHYSICS B exam papers)
Chapter P1 Radiation and waves
Introduction (For OCR GCSE (9–1) Twenty First Century Science PHYSICS B exam papers)
In P1 you consider the uses of electromagnetic radiation and the possible health risks of radiation, both in nature and from technological devices, which are becoming of increasing concern.
Then you study a wave model for light and sound.
In Topic P1.1 you study the model of radiation, an important scientific model for explaining how one object can affect another at a distance, and links this to the idea that all parts of the electromagnetic spectrum behave in this way. It then goes on to use the radiation model to explain how electromagnetic radiation behaves and to consider the risks and benefits of the technologies that use electromagnetic radiation. In some cases, misunderstanding the term ‘radiation’ generates unnecessary alarm. Through considering the evidence concerning the possible harmful effects of low intensity microwave radiation from devices such as mobile phones, learners learn to evaluate reported health studies and interpret levels of risk.
In Topic P1.2 you are introduced to the idea that all bodies emit radiation to explain the greenhouse effect. Evidence for global warming is explored; scientific explanations for climate change draw on ideas about the way that radiation is emitted and absorbed by different materials. There is an opportunity to use both physical analogies and computer modelling to demonstrate the explanatory power of models in science.
Topic P1.3 looks at the properties of waves they have in common and how a wave model can be used to explain a great many phenomena, both natural and artificial. The reflection and refraction of waves on water provide evidence that light and sound can be modelled as waves.
In Topic P1.4 you consider the
behaviour of light and sound as they pass through a material interface,
including refraction of light in lenses and prisms and the use of sound and
ultrasound in imaging and detection.
What you should have learned and experienced from KS3 science about light, sound, and waves ...
Chapter P1.1 What are the risks and benefits of using radiations?
A model of radiation can be used to describe and predict the effects of some processes in which one object affects another some distance away. One object (a source) emits radiation (of some kind). This spreads out from the source and transfers energy to other object(s) some distance away. Light is one of a family of radiations, called the electromagnetic spectrum. All radiations in the electromagnetic spectrum travel at the same speed through space.
1. Be able to describe the main groupings of the electromagnetic spectrum – radio, microwave, infrared, visible (red to violet), ultraviolet, X-rays and gamma rays, that these range from long to short wavelengths, from low to high frequencies, and from low to high energies.
2. Be able to recall that our eyes can only detect a very limited range of frequencies in the electromagnetic spectrum.
3. Be able to recall that all electromagnetic radiation is transmitted through space with the same very high (but finite) speed.
4. Be able to explain, with examples, that electromagnetic radiation transfers energy from source to absorber
When radiation strikes an object, some may be transmitted (pass through it), or be reflected, or be absorbed. When radiation is absorbed it ceases to exist as radiation; usually it heats the absorber. Some types of electromagnetic radiation do not just cause heating when absorbed; X-rays, gamma rays and high energy ultraviolet radiation have enough energy to remove an electron from an atom or molecule (ionisation) which can then take part in other chemical reactions.
Exposure to large amounts of ionising radiation can cause damage to living cells; smaller amounts can causes changes to cells which may make them grow in an uncontrolled way, causing cancer. Oxygen is acted on by radiation to produce ozone in the upper atmosphere. This ozone absorbs ultraviolet radiation, and protects living organisms, especially animals, from its harmful effects.
Radio waves are produced when there is an oscillating current in an electrical circuit. Radio waves are detected when the waves cause an oscillating current in a conductor. Different parts of the electromagnetic spectrum are used for different purposes due to differences in the ways they are reflected, absorbed, or transmitted by different materials.
Developments in technology have made use of all parts of the electromagnetic spectrum; every development must be evaluated for the potential risks as well as the benefits. Data and scientific explanations of mechanisms, rather than opinion, should be used to justify decisions about new technologies.
5. Be able to recall that different substances may absorb, transmit, or reflect electromagnetic radiation in ways that depend on wavelength.
6. Be able to recall that in each atom its electrons are arranged at different distances from the nucleus, that such arrangements may change with absorption or emission of electromagnetic radiation, and that atoms can become ions by loss of outer electrons.
7. Be able to recall that changes in molecules, atoms and nuclei can generate and absorb radiations over a wide frequency range, including:
8. Be able to describe how ultra-violet radiation, X-rays and gamma rays can have hazardous effects, notably on human bodily tissues.
9. Be able to give examples of some practical uses of electromagnetic radiation in the radio, microwave, infrared, visible, ultraviolet, X-ray and gamma ray regions of the spectrum.
10. (HT only) Be able to recall that radio waves can be produced by, or can themselves induce, oscillations in electrical circuits.
Chapter P1.2 What is climate change and what is the evidence for it?
All objects emit electromagnetic radiation with a principal frequency that increases with temperature. The Earth is surrounded by an atmosphere which allows some of the electromagnetic radiation emitted by the Sun to pass through; this radiation warms the Earth’s surface when it is absorbed. The radiation emitted by the Earth, which has a lower principal frequency than that emitted by the Sun, is absorbed and re-emitted in all directions by some gases in the atmosphere; this keeps the Earth warmer than it would otherwise be and is called the greenhouse effect.
One of the main greenhouse gases in the Earth’s atmosphere is carbon dioxide, which is present in very small amounts; other greenhouse gases include methane, present in very small amounts, and water vapour. During the past two hundred years, the amount of carbon dioxide in the atmosphere has been steadily rising, largely the result of burning increased amounts of fossil fuels as an energy source and cutting down or burning forests to clear land.
Computer climate models provide evidence that human activities are causing global warming. As more data is collected using a range of technologies, the model can be refined further and better predictions made .
1. Be able to explain that all bodies emit radiation, and that the intensity and wavelength distribution of any emission depends on their temperatures.
2. (HT only) Be able to explain how the temperature of a body is related to the balance between incoming radiation, absorbed radiation and radiation emitted; illustrate this balance, using everyday examples including examples of factors which determine the temperature of the Earth.
Chapter P1.3 How do waves behave?
A wave is a regular disturbance that transfers energy in the direction that the wave travels, without transferring matter.
For some waves (such as waves along a rope), the disturbance of the medium as the wave passes is at right-angles to its direction of motion. This is called a transverse wave. For other waves (such as a series of compression pulses on a slinky spring), the disturbance of the medium as the wave passes is parallel to its direction of motion. This is called a longitudinal wave.
The speed of a wave depends on the medium it is travelling through. Its frequency is the number of waves each second that are made by the source. The wavelength of waves is the distance between the same points on two adjacent disturbances.
The ways in which light and sound waves reflect and refract when they meet at an interface between two materials can be modelled with water waves.
A wave model for light and sound can be used to describe and predict some behaviour of light and sound.
Refraction of light and sound can be explained by a change in speed of waves when they pass into a different medium; a change in the speed of a wave causes a change in wavelength since the frequency of the waves cannot change, and that this may cause a change in direction.
1. Be able to describe wave motion in terms of amplitude, wavelength, frequency and period.
2. Be able to describe evidence that for both ripples on water surfaces and sound waves it is the wave and not the water or air itself that travels.
3. Be able to describe the difference between transverse and longitudinal waves.
4. Be able to describe how waves on a rope are an example of transverse waves whilst sound waves in air are longitudinal waves.
5. Be able to define wavelength and frequency.
6. Be able to recall and apply the relationship between speed, frequency and wavelength to waves , including waves on water, sound waves and across the electromagnetic spectrum:
7. (a) Be able to describe how the speed of ripples on water surfaces and the speed of sound waves may be measured.
7. (b) Be able to describe how to use a ripple tank to measure the speed/frequency and wavelength of a wave.
8. (a) Be able to describe the effects of reflection and refraction of waves at material interfaces.
8. (b) Be able to describe how to measure the refraction of light through a prism.
8. (c) Be able to describe how to investigate the reflection of light off a plane mirror.
9. (HT only) Be able to recall that waves travel in different substances at different speeds and that these speeds may vary with wavelength.
10. (HT only) Be able to explain how refraction is related to differences in the speed of the waves in different substances.
11. Be able to recall that light is an electromagnetic wave.
12. Be able to recall that electromagnetic waves are transverse.
Chapter P1.4 What happens when light and sound meet different materials?
A beam of light is reflected from a smooth surface, such as a mirror, in a single beam which makes the same angle with the normal as the incident beam (specular reflection).
Light is scattered in all directions from an uneven surface.
Light is refracted at the boundary between glass (and water and Perspex) and air; this property is exploited in prisms and lenses.
When a beam of white light is passed through a prism, the emerging light beam is spread out showing the colours of the spectrum. This can be explained using the wave model, different colours have different wavelengths; different wavelengths travel at different speeds when passing through glass, water or Perspex.
What we perceive as white light is a mixture of different colours, ranging in wavelength from violet light (shortest visible wavelength) to red light (longest visible wavelength). A coloured filter works by allowing light of one or more wavelength through (transmission) and absorbing light of the other wavelengths.
An object appears white if it scatters all the colours of light that fall on it, and black if it scatters none (and absorbs all). It appears coloured if it scatters light of some colours and absorbs light of other colours. Its observed colour is that of the light it scatters.
(HT only) Sound travels better through solids and liquids than through air. The small bones in the middle ear transmit the sound waves from the air outside to the inner ear. The bones transmit frequencies most efficiently in the range 1 kHz and 3 kHz.
The ways in which sound waves are transmitted, reflected and refracted as they pass through liquids and solids are exploited in ultrasound imaging in medicine, in exploring the structure of the Earth and in using SONAR to explore under water.
1. Be able to construct and interpret two-dimensional ray diagrams to illustrate specular reflection by mirrors (qualitative only)
2. Be able to construct and interpret two-dimensional ray diagrams to illustrate refraction at a plane surface and dispersion by a prism (qualitative only).
3. Be able to use ray diagrams to illustrate the similarities and differences between convex and concave lenses (qualitative only).
4. Be able to describe the effects of transmission, and absorption of waves at material interfaces.
5. Be able to explain how colour is related to differential absorption, transmission, and scattering.
6. (HT only) Be able to describe, with examples, processes in which sound waves are transmitted though solids.
7. (HT only) Be able to explain that transmission of sound through the bones in the ear works over a limited frequency range, and the relevance of this to human hearing.
8. (HT only) Be able to explain, in qualitative terms, how the differences in velocity, absorption and reflection between different types of waves in solids and liquids can be used both for detection and for exploration of structures which are hidden from direct observation, notably ...
9. Be able to show how changes, in speed, frequency and wavelength, in transmission of sound waves from one medium to another, are inter-related.
Chapter P2: (For OCR GCSE (9–1) Twenty First Century Science PHYSICS B exam papers)
Chapter P2 Sustainable energy
Introduction (For OCR GCSE (9–1) Twenty First Century Science PHYSICS B exam papers)
Energy supply is one of the major issues that society must address in the immediate future. Citizens are faced with complex choices and a variety of messages from energy supply companies, environmental groups, the media, scientists and politicians. Some maintain that renewable resources are capable of meeting our future needs, some advocate nuclear power, and some argue that drastic lifestyle changes are required. Decisions about energy use, whether at a personal or a national level, need to be informed by a quantitative understanding of the situation, and this is an underlying theme of the chapter.
Topic P2.1 quantifies the energy used by electrical devices introduces calculations of efficiency and considers ways of reducing dissipation in a variety of contexts.
In Topic P2.2 you study national data on energy sources introduces a study of electricity generation and distribution; nuclear power generation, the burning of fossil fuels and renewable resources are compared and contrasted. Electrical safety in the home and a review of the energy choices available to individuals, organisations and society are also included.
What you should have learned and experienced from KS3 science about energy ...
Chapter P2.1 How much energy do we use?
Energy is considered as being stored in a limited number of ways: chemical, nuclear, kinetic, gravitational, elastic, thermal, electrostatic and electromagnetic and can be transferred from one to another by processes called working and heating.
Electricity is a convenient way to transfer energy from source to the consumer because it is easily transmitted over distances and can be used to do work in many ways, including heating and driving motors which make things move or to lift weights.
When energy is used to do work some energy is usually wasted in doing things other than the intended outcome, it is dissipated into the surroundings, ultimately into inaccessible thermal stores.
The power of an appliance or device is a measure of the amount of energy it transfers each second, i.e. the rate at which it transfers energy.
Sankey diagrams are used to show all the energy transfers in a system, including energy dissipated to the surroundings; the data can be used to calculate the efficiency of energy transfers.
1. Be able to describe how energy in chemical stores in batteries, or in fuels at the power station, is transferred by an electric current, doing work on domestic devices, such as motors or heaters.
2. Be able to explain, with reference to examples, the relationship between the power ratings for domestic electrical appliances, the time for which they are in use and the changes in stored energy when they are in use.
3. Be able to recall and apply the following equation in the context of energy transfers by electrical appliances :
4. Be able to describe, with examples, where there are energy transfers in a system, that there is no net change to the total energy of a closed system (qualitative only).
5. Be able to describe, with examples, system changes, where energy is dissipated, so that it is stored in less useful ways.
6. Be able to explain ways of reducing unwanted energy transfer e.g. through lubrication, thermal insulation.
7. Be able to describe the effects, on the rate of cooling of a building, of thickness and thermal conductivity of its walls (qualitative only).
8. Be able to recall and apply the equation ...
9. Be able to interpret and construct Sankey diagrams to show understanding that energy is conserved.
Chapter P2.2: How can electricity be generated?
The main energy resources that are available to humans are fossil fuels (oil, gas, coal), nuclear fuels, biofuels, wind, hydroelectric, tides and from the Sun.
In most power stations generators produce a voltage across a wire by spinning a magnet near the wire. Often an energy source is used to heat water; the steam produced drives a turbine which is coupled to an electrical generator. Other energy sources drive the generator directly.
The mains supply to our homes is an alternating voltage, at 50 Hz, 230 volts, but electricity is distributed through the National Grid at much higher voltages to reduce energy losses. Transformers are used to increase the voltage for transmission and then decrease the voltage for domestic use.
Most mains appliances are connected by a 3 core cable, containing live, neutral and earth wires.
The demand for energy is continually increasing and this raises issues about the availability and sustainability of energy sources and the environmental effects of using these sources. The introduction and development of new energy sources may provide new opportunities but also introduce technological and environmental challenges. The decisions about the energy sources that are used may be different for different people in different contexts.
1. Be able to describe the main energy resources available for use on Earth (including fossil fuels, nuclear fuel, biofuel, wind, hydroelectricity, the tides and the Sun)
2. Be able to explain the differences between renewable and non-renewable energy resources.
3. Be able to compare the ways in which the main energy resources are used to generate electricity.
4. Be able to recall that the domestic supply in the UK is a.c., at 50Hz and about 230 volts and explain the difference between direct and alternating voltage.
5. Be able to recall that, in the national grid, transformers are used to transfer electrical power at high voltages from power stations, to the network and then used again to transfer power at lower voltages in each locality for domestic use.
6. Be able to recall the differences in function between the live, neutral and earth mains wires, and the potential differences between these wires; hence explain that a live wire may be dangerous even when a switch in a mains circuit is open, and explain the dangers of providing any connection between the live wire and any earthed object.
7. Be able to explain patterns and trends in the use of energy resources in domestic contexts, workplace contexts, and national contexts.
Chapter P3: (For OCR GCSE (9–1) Twenty First Century Science PHYSICS B exam papers)
Chapter P3 Electric circuits
Introduction (For OCR GCSE (9–1) Twenty First Century Science PHYSICS B exam papers)
Known only by its effects, electricity provides an ideal vehicle to illustrate the use and power of scientific models. During the course of the 20th century, electrical engineers completely changed whole societies, by designing systems for electrical generation and distribution, and a whole range of electrical devices.
In chapter P3 you will learn how scientists visualise what is going on inside circuits and predict circuit behaviour.
Topic P3.1 introduces the idea of electric charge and electric fields
In Topic P3.1/P3.2 you study models of charge moving through circuits driven by a voltage and against a resistance are introduced.
A more general understanding of voltage as
potential difference is then developed in Topic P3.2/P3.3, which then continues
Topic P3.3/P3.4 concentrates on quantifying the energy transferred in electric circuits and how this is determined by both the potential difference and the current.
A reminder of earlier work on magnets and magnetic fields in Topic P3.4/P3.5 leads into an introduction to the electric motor.
In Topic P3.6 Applications of electromagnetism and in particular the electric motor have revolutionised people’s lives in so many ways – from very small motors used in medical contexts, to very large motors used to propel ships or pump water in pumped storage schemes.
In Topic P3.7 (GCSE Physics only) the process of electromagnetic induction is placed in the context of power generation and the use of transformers in power distribution.
What you should have learned and experienced from KS3 science about light, sound, and waves ...
Chapter P3.1 What is electric charge?
When two objects are rubbed together they become charged, because electrons are transferred from one object to the other. Electrons are negatively charged.
Objects with similar charges repel, and objects with opposite charges attract.
Around every electric charge there is an electric field; in this region of space the effects of charge can be felt; when another charge enters the field there is an interaction between them and both charges experience a force.
1. Be able to describe the production of static electricity, and sparking, by rubbing surfaces, and evidence that charged objects exert forces of attraction or repulsion on one another when not in contact.
2. Be able to explain how transfer of electrons between objects can explain the phenomenon of static electricity.
3. Be able to explain the concept of an electric field and how it helps to explain the phenomenon of static electricity.
Chapter P3.2 What determines the current in an electric circuit?
An electric current is the rate of flow of charge; in an electric circuit the metal conductors (the components and wires) contain many charges that are free to move. When a circuit is made, the battery causes these free charges to move, and these charges are not used up but flow in a continuous loop.
In a given circuit, the larger the potential difference across the power supply the bigger the current. Components (for example, resistors, lamps, motors) resist the flow of charge through them; the resistance of connecting wires is usually so small that it can be ignored. The larger the resistance in a given circuit, the smaller the current will be.
Representational models of electric circuits use physical analogies to help think about how an electric circuit works, and to predict what happens when a variable is changed.
1. Be able to recall that current is a rate of flow of charge, that for a charge to flow, a source of potential difference and a closed circuit are needed and that a current has the same value at any point in a single closed loop.
2. Be able to recall and use the relationship between quantity of charge, current and time:
3. Be able to recall that current (I) depends on both resistance (R) and potential difference (V) and the units in which these quantities are measured.
4. (a) Be able to recall and apply the relationship between I, R, and V, to calculate the currents, potential differences and resistances in d.c. series circuits using the formula
4. (b) Be able to describe an experiment to investigate the resistance of a wire and be able to draw the circuit diagram of the circuit used.
5. Be able to recall that for some components the value of R remains constant (fixed resistors) but that in others it can change as the current changes (e.g. heating elements, lamp filaments).
6. (a) Be able to use graphs to explore whether circuit elements are linear or non-linear and relate the curves produced to their function and properties.
6. (b) Be able to describe experiments to investigate the I-V characteristics of circuit elements. To include: lamps, diodes, LDRs and thermistors. Be able to draw circuit diagrams for the circuits used.
7. Be able to represent circuits with the conventions of positive and negative terminals, and the symbols that represent common circuit elements, including filament lamps, diodes, LDRs and thermistors.
Chapter P3.3 How do series and parallel circuits work?
When electric charge flows through a component (or device), work is done by the power supply and energy is transferred from it to the component and/or its surroundings. Potential difference measures the work done per unit charge.
In a series circuit the charge passes through all the components, so the current through each component is the same and the work done on each unit of charge by the battery must equal the total work done by the unit of charge on the components. The potential difference (p.d.) is largest across the component with the greatest resistance and a change in the resistance of one component will result in a change in the potential differences across all the components.
In a parallel circuit each charge passes through only one branch of the circuit, so the current through each branch is the same as if it were the only branch present and the work done by each unit of charge is the same for each branch and equal to the work done by the battery on each charge. The current is largest through the component with the smallest resistance, because the same battery p.d. causes a larger current to flow through a smaller resistance than through a bigger one.
When two or more resistors are placed in series the effective resistance of the combination (equivalent resistance) is equal to the sum of their resistances, because the battery has to move charges through all of them.
Two (or more) resistors in parallel provide more paths for charges to move along than either resistor on its own, so the effective resistance is less.
Some components are designed to change resistance in response to changes in the environment e.g. the resistance of an LDR varies with light intensity, the resistance of a thermistor varies with temperature; these properties used in sensing systems to monitor changes in the environment.
1. Be able to relate the potential difference between two points in the circuit to the work done on, or by, a given amount of charge as it moves between these points.
2. (a) Be able to describe the difference between series and parallel circuits: to include ideas about how the current through each component and the potential difference across each component is affected by a change in resistance of a component.
2. (b) Be able to describe how to practically investigate the brightness of bulbs in series and parallel circuits. Be able to draw circuit diagrams for the circuits used.
3. Be able to explain, why, if two resistors are in series the net resistance is increased, whereas with two in parallel the net resistance is decreased (qualitative explanation only)
4. Be able to solve problems for circuits which include resistors in series, using the concept of equivalent resistance.
5. Be able to explain the design and use of d.c. series circuits for measurement and testing purposes including exploring the effect of :
Chapter P3.4 What determines the rate of energy transfer in a circuit?
The energy transferred when electric charge flows through a component (or device), depends on the amount of charge that passes and the potential difference across the component.
The power rating (in watts, W) of an electrical device is a measure of the rate at which an electrical power supply transfers energy to the device and/or its surroundings. The rate of energy transfer depends on both the potential difference and the current. The greater the potential difference, the faster the charges move through the circuit, and the more energy each charge transfers.
The National Grid uses transformers to step down the current for power transmission. The power output from a transformer cannot be greater than the power input, therefore if the current increases, the potential difference must decrease. Transmitting power with a lower current through the cables results in less power being dissipated during transmission.
1. Be able to describe the energy transfers that take place when a system is changed by work done when a current flows through a component.
2. Be able to explain, with reference to examples, how the power transfer in any circuit device is related to the energy transferred from the power supply to the device and its surroundings over a given time using the formula
3. Be able to recall and use the relationship between the potential difference across the component and the total charge to calculate the energy transferred in an electric circuit when a current flows through a component
4. Be able to recall and apply the relationships between power transferred in any circuit device, the potential difference across it, the current through it, and its resistance
5. Be able to use the idea of conservation of energy to show that when a transformer steps up the voltage, the output current must decrease and vice versa
6. Be able to explain how transmitting power at higher voltages is more efficient way to transfer energy.
Chapter P3.5 What are magnetic fields?
Around any magnet there is a region, called the magnetic field, in which another magnet experiences a force. The magnetic effect is strongest at the poles. The field gets gradually weaker with distance from the magnet.
The direction and strength of a magnetic field can be represented by field lines. These show the direction of the force that would be experienced by the N pole of a small magnet, placed in the field.
The magnetic field around the Earth, with poles near the geographic north and south, provides evidence that the core of the Earth is magnetic. The N-pole of a magnetic compass will point towards the magnetic north pole.
Magnetic materials (such as iron and nickel) can be induced to become magnets by placing them in a magnetic field. When the field is removed permanent magnets retain their magnetisation whilst other materials lose their magnetisation.
When there is an electric current in a wire, there is a magnetic field around the wire; the field lines form concentric circles around the wire. Winding the wire into a coil (solenoid) makes the magnetic field stronger, as the fields of each turn add together. Winding the coil around an iron core makes a stronger magnetic field and an electromagnet that can be switched on and off.
In loudspeakers and headphones the magnetic field produced due to a current through a coil interacts with the field of a permanent magnet.
The 19th century discovery of this electromagnetic effect led quickly to the invention of a number of magnetic devices, including electromagnetic relays, which formed the basis of the telegraph system, leading to a communications revolution.
1. Be able to describe the attraction and repulsion between unlike and like poles for permanent magnets
2. Be able to describe the characteristics of the magnetic field of a magnet, showing how strength and direction change from one point to another.
3. Be able to explain how the behaviour of a magnetic compass is related to evidence that the core of the Earth must be magnetic.
4. Be able to describe the difference between permanent and induced magnets.
5. Be able to describe how to show that a current can create a magnetic effect.
6. Be able to describe the pattern and directions of the magnetic field around a conducting wire.
7. Be able to recall that the strength of the field depends on the current and the distance from the conductor.
8. Be able to explain how the magnetic effect of a solenoid can be increased.
Development of electromagnets have led to major changes in people’s lives, including applications in communications systems, MRI scanners and on cranes in scrapyards.
9. (GCSE Physics only) Be able to explain how a solenoid can be used to generate sound in loudspeakers and headphones
Chapter P3.6 How do electric motors work? (HT only)
(HT only) The magnetic fields of a current-carrying wire and a nearby permanent magnet will interact and the wire and magnet exert a force on each other. This is called the ‘motor effect’.
If the current-carrying wire is placed at right angles to the magnetic field lines, the force will be at right angles to both the current direction and the lines of force of the field. The direction of the force can be inferred using Fleming’s left-hand rule.
The size of the force is proportional to the length of wire in the field, the current and the strength of the field.
The motor effect can result in a turning force on a rectangular current-carrying coil placed in a uniform magnetic field; this is the principle behind all electric motors.
The invention and development of practical electric motors have made an impact on almost every aspect of daily life.
1. (HT only) Be able to describe the interaction forces between a magnet and a current-carrying conductor to including ideas about magnetic fields.
2. (HT only) Be able to show that Fleming’s left-hand rule represents the relative orientations of the force, the conductor and the magnetic field.
3. (HT only) Be able to select and apply the equation that links the force (F) on a conductor to the strength of the field (B), the size of the current (I) and the length of conductor(l) to calculate the forces involved:
4. (HT only) Be able to explain how the force on a conductor in a magnetic field is used to cause rotation in the rectangular coil of a simple electric motor
Chapter P3.7 What is the process inside an electric generator?
Mains electricity is produced using the process of electromagnetic induction.
When a magnet is moving into a coil of wire a potential difference is induced across the ends of the coil; if the magnet is moving out of the coil, or the other pole of the magnet is moving into it, there is a potential difference induced in the opposite direction. If the ends of the coil are connected to make a closed circuit, a current will flow round the circuit.
In a moving coil microphone sound waves cause a diaphragm to vibrate. The diaphragm is attached to a coil which is in the field of a permanent magnet. Sounds make the coil vibrate, inducing a changing potential difference across the ends of the coil. This potential difference drives a changing current in an electric circuit.
(HT only) In a generator, a magnet or electromagnet is rotated within a coil of wire to induce a voltage across the ends of the coil. The induced voltage across the coil of an alternating current (a.c.) generator (and hence the current in an external circuit) changes during each revolution of the magnet or electromagnet.
(HT only) To generate a d.c. split-ring commutator is used so that the current always passes from the same side of the generator.
(HT only) A changing magnetic field caused by changes in the current in one coil of wire can induce a voltage in a neighbouring coil.
(HT only) A simple transformer has two coils of wire wound on an iron core; a changing current in one coil of a transformer will cause a changing magnetic field in the iron core, which in turn will induce a changing potential difference across the other transformer coil.
(HT only) The discovery of electromagnetic induction and the subsequent development of power generators transformed the way we live, although with new developments in technology there are often unintended consequences.
1. (HT only) Be able to recall that a change in the magnetic field around a conductor can give rise to an induced potential difference across its ends, which could drive a current.
2. (HT only) Be able to explain the action of a moving coil microphone in converting the pressure variations in sound waves into variations in current in electrical circuits.
3. (HT only) Be able to recall that the direction of the induced potential difference drives a current which generates a second magnetic field that would oppose the original change in field.
4. (HT only) Be able to use ideas about electromagnetic induction to explain a potential difference / time graph showing the output from an alternator being used to generate a.c.
5. (HT only) Be able to explain how an alternator can be adapted to produce a dynamo to generate d.c., including explaining a potential difference time graph.
6. (HT only) Be able to explain how the effect of an alternating current in one circuit in inducing a current in another is used in transformers.
7. (HT only) Be able to describe how the ratio of the potential differences across the two circuits of a transformer depends on the ratio of the numbers of turns in each.
8. (HT only) Be able to apply the equations linking the potential differences and numbers of turns in the two coils of a transformer, to the currents and the power transfer involved and relate these to the advantages of power transmission at high voltages:
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