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Transcript
Electromagnetism (and a bit of energy and electricity) 6.2 understand that magnets repel and attract other magnets and attract magnetic substances 6.3 describe the properties of magnetically hard and soft materials 6.4 understand the term ‘magnetic field line’ 6.5 understand that magnetism is induced in some materials when they are placed in a magnetic field 6.6 describe experiments to investigate the magnetic field pattern for a permanent bar magnet and that between two bar magnets 6.7 describe how to use two permanent magnets to produce a uniform magnetic field pattern. 6.8 understand that an electric current in a conductor produces a magnetic field round it 6.9 describe the construction of electromagnets 6.10 sketch and recognise magnetic field patterns for a straight wire, a flat circular coil and a solenoid when each is carrying a current 6.11 understand that there is a force on a charged particle when it moves in a magnetic field as long as its motion is not parallel to the field 6.12 understand that a force is exerted on a current-carrying wire in a magnetic field, and how this effect is applied in simple d.c. electric motors and loudspeakers 6.13 use the left hand rule to predict the direction of the resulting force when a wire carries a current perpendicular to a magnetic field 6.14 describe how the force on a current-carrying conductor in a magnetic field increases with the strength of the field and with the current. 6.15 understand that a voltage is induced in a conductor or a coil when it moves through a magnetic field or when a magnetic field changes through it and describe the factors which affect the size of the induced voltage 6.16 describe the generation of electricity by the rotation of a magnet within a coil of wire and of a coil of wire within a magnetic field and describe the factors which affect the size of the induced voltage 6.17 describe the structure of a transformer, and understand that a transformer changes the size of an alternating voltage by having different numbers of turns on the input and output sides 6.18 explain the use of step-up and step-down transformers in the large scale generation and transmission of electrical energy 6.19 know and use the relationship between input (primary) and output (secondary) voltages and the turns ratio for a transformer: 6.20 know and use the relationship: input power = output power VP IP = VS IS for 100% efficiency 4.2 describe energy transfers involving the following forms of energy: thermal(heat), light, electrical, sound, kinetic, chemical, nuclear and potential(elastic and gravitational) 4.16 describe the energy transfers involved in generating electricity using: Wind, water, geothermal resources, solar heating systems, solar cells, fossil fuels, nuclear power 4.17 describe the advantages and disadvantages of methods of large scale electricity production from various renewable and nonrenewable resources. What is a magnet? • Princeton University definition: • Magnet: (physics) a device that attracts iron and produces a magnetic field • Magnetic: of or relating to or caused by magnetism; "magnetic forces" ;having the properties of a magnet; i.e. of attracting iron or steel • Magnetism: attraction for iron; associated with electric currents as well as magnets; characterized by fields of force Are we any wiser? 7 quick questions... 1. What do we call the region around a magnet where some materials experience a force? 2. Which materials experience a magnetic force? 3. What do we call the ends of a magnet? 4. Why? 5. How do you magnetise something? 6. What do you get if you cut a magnet in half? 7. What causes magnetism? What magnets do • Magnets exert a force on – Other magnets – Objects containing iron, cobalt and nickel (ferromagnetic materials) • Magnets always attract ferromagnetic materials • Magnets can attract or repel other magnets – Depends on polarity Bar Magnets • Bar magnets are permanent magnets made from magnetic materials. • 2 poles North (seeking) pole South (seeking) pole – A suspended magnet will align with the Earth’s magnetic field • Unlike poles attract, like poles repel. • How can you test if a piece of metal is a magnet? – You can only show that an object is a magnet if it repels a known magnet. Magnetic Fields • Magnets exert a force at a distance, no contact is needed. • The space around a magnet where a magnetic force is felt is called a magnetic field. • We represent a magnetic field with field lines. Magnetic field lines are not “real” (like contour lines on a map) • Field lines: – – – – go from North to South are closed loops never cross are most concentrated where the field is strongest Investigating Fields • A little magnet which is free to move will align itself with an applied magnetic field • We can use plotting compasses or iron filings (which will act like little magnets) to find the shape of field patterns Your turn… • Using plotting compasses or iron filings, draw the magnetic field patterns for: 1. A single bar magnet 2. Two magnets arranged as follows (a) (b) Ν S Ν S S N Ν S (c) (d) N S N N 2 magnets, opposite poles facing Single bar magnet 2 magnets, same poles facing Field lines are also called flux lines. Uniform Field • The magnetic field between a N and S pole is almost uniform. • We can get closer with slab magnets. • Can do even better with electromagnets N S Questions • Below is a bar magnet and a compass. Label the poles of the magnet and draw the field line on which the compass lies. Ν S • Identify the poles A-F below: N N S S N S Making magnets • A piece of magnetic material can be magnetised by placing it in a magnetic field (due to another magnet, or created electrically). • If it is a magnetically hard material (eg steel), it will retain its magnetism. • Permanent magnet • If it is a magnetically soft material (eg iron), it will demagnetise as soon as the field is removed. • Temporary magnet Earth’s Magnetic Field • The Earth has its own magnetic field, similar to that of a bar magnet. A magnet free to rotate will align with the Earth’s field Electricity and magnetism • Before the beginning of the 19th century, magnetism and electricity were thought to be separate phenomena. • In 1820 the Danish scientist Hans Christian Ørsted noticed that when a compass was placed near a wire carrying an electric current, the compass needle was deflected. Magnetic field around a wire carrying current • Direction of field given by Maxwell’s Right Hand grip rule (or think of a screw thread). Magnetic field due to a loop of wire • By applying RH grip rule around the wire we find the resulting magnetic field. Field due to a solenoid • A solenoid is an extended coil of wire. • The magnetic field pattern of a solenoid carrying current is very similar to that of a bar magnet. Electromagnets • A solenoid’s magnetic field can be made stronger by – Increasing the current – Increasing the number of turns of wire – Wrapping the turns around a magnetically soft core • Combining these can produce extremely powerful (and switchable) electromagnets – Video at bottom of page Uses of electromagnets • Smaller electromagnets are used in bells and switches Force on a current in a magnetic field – The Motor Effect • Why do we get movement? – Because a force is acting (called the Motor Effect). – NOT because of magnetic attraction (wire is nonmagnetic material) • Direction of force relative to: – Current? – Magnetic field? • Perpendicular to both The Motor Effect • We can increase the force produced by: – Increasing the magnetic field strength – Increasing the current in the wire – Increasing the number of turns of wire in the field (add coils) – Wrap the wire coils around a soft iron core • Only a wire crossing field lines experiences a force, not wires parallel to field lines – Greatest force when field and current are at 90 degrees • A conductor carrying a current in a magnetic field experiences a force perpendicular to both. • This is known as the MOTOR EFFECT. • The direction is given by Fleming’s left hand rule. Motion Fleming’s Left Hand Rule The Catapult Field • The force on the wire is due to the interaction of the fixed magnetic field and the field due to the current flowing in the wire. • Where the two fields are in the same direction they reinforce each other • Where they are in opposition they produce a weaker field. A simple electric motor • Rotation due to interaction of coil’s magnetic field and fixed magnet’s field Motor Effect Questions • A wire carries a current horizontally between the poles of a magnet, as shown below. The direction of the force on the wire is: • A from N to S • B from S to N • C opposite to the current direction • D In the direction of the current current • E vertically upwards? N S Motor Effect Questions • In the figure below, AB is a copper wire hanging from a pivot at A and dipping into mercury in a copper dish at B. It is suspended between the poles of a powerful magnet. • (a) Copy the diagram and add the magnetic field lines • (b) Mark in the direction of the conventional current • (c) What will happen when the switch is closed? A N S B Loudspeaker • The loudspeaker uses the motor effect to change electrical energy to sound. – The varying electrical signal changes the field due to the coil. – This interacts with the permanent magnet, moving the coil and attached cone. Electric motor operation Need a special electrical contact to reverse current direction every half turn: COMMUTATOR Commutator in action DC Electric Motor details • What you need to know... • The COMMUTATOR reverses the direction of the current in the coil every half turn, maintaining the direction of the couple and keeping the motor turning DC Electric Motor details • What you need to know... • The direction of rotation reverses if: – You reverse the field OR – You reverse the power supply • What happens if you reverse both? DC Electric Motor details • What you need to know... • The motor spins faster if you: – Make magnet stronger • Some real motors use electromagnets – Increase the coil current – Increase number of coils – Give coils an iron core Currents don’t need wires! • A beam of electrons in a vacuum tube is also a current • Such a beam of charged particles is deflected by a perpendicular magnetic field. – No effect if the field is parallel to direction of motion Electromagnetic induction • Any conductor experiencing a changing magnetic field (or moving across a steady magnetic field) will have a p.d. induced across it. • If a closed circuit is made, a current will flow. • This is the basis of almost all electricity generation. Inducing a bigger voltage • The induced voltage is bigger if we: – Move the wire quicker – Increase the magnetic field strength – Increase the length of wire in the field by wrapping it into a coil Electromagnetic induction Using a coil of wire increases the voltage produced Electromagnetic induction • The direction of the induced voltage can be reversed by: – Reversing the magnet – Moving the magnet in the opposite direction Electromagnetic induction • The size of the induced voltage can be increased by: – Moving the magnet faster – Increasing the number of turns on the coil – Using a stronger magnet – Larger area coil The faster magnetic field lines are cut by the wire, the bigger the induced voltage and current Generators / dynamos • To generate a continuous voltage we need a constantly changing magnetic field. • This is achieved by rotating a magnet in or near a coil of wire. • An ALTERNATING CURRENT is produced. • This is how mains electricity is generated Alternating output Magnet position • Output voltage is continually changing, and periodically changes direction (negative value on graph) Dynamos • Simplest to engineer with stationary coil and rotating magnet Alternating and direct current • A battery provides a direct current – Electrons always flow in one direction • A generator provides an alternating current – Direction of current changes over time, electrons flow back and forth • Both types of current are able to transfer electrical energy Why alternating current (AC)? • It is easy to generate • It is easy to transport over long distances with low power loss • Many applications (light, heating etc) work fine with AC • The mains supply to our homes is 230V AC • For applications which need DC, we can build electric circuits to convert the power supply. Or rotate the coil... • Wire just needs to cut field lines, it doesn’t matter which bit is moving • Slip rings are contacted to ends of the coil and rotate with it • Brushes slide against rotating rings and provide electrical contact • Output is AC voltage – Same machine as an AC motor • See here for an animated version Increasing generator output • In a real high power generator this is done by: – Using a stronger rotating electromagnet – Rotating the magnet faster • But mains electricity has a defined frequency (50 Hz) – Using many fixed coils with more turns – Putting an iron core inside the fixed coils Battersea Power Station, 1933 Rotating electromagnet Coils of wire Mutual induction • Remember electromagnets? – When a current flows in a coil of wire a magnetic field is produced • If an alternating current flows, then an alternating magnetic field is produced • If a second coil of wire experiences this changing field, a voltage is induced in it • Run this simulation and click in the transformer tab Induction cooker • AC in a coil induces a changing magnetic field • This in turn induces alternating currents in the metal pan • Resistive heating causes the pan to get hot, cooking the food Rechargeable toothbrush Transformers • A transformer consists of two coil mounted on a common iron core • An alternating current flowing in the primary coil produces a changing magnetic field • The iron core concentrates the field through the centre of the secondary coil • The alternating magnetic field induces an alternating current in the secondary coil • This happens even though there is no direct electrical connection between the two coils Transformers only work with AC. DC does not get through! Transformer action • A transformer can change the voltage. • The size of the voltage induced in the secondary coil depends on the number of turns in the primary and secondary coils and also the voltage applied to the primary coil. Voltage across secondary coil number of turns on secondary coil Voltage across primary coil number of turns on primary coil Vs Ns or Vp N p • If Vs>Vp: step-up transformer • If Vs<Vp: stepdown transformer Transformer example • A transformer has 100 turns on the primary coil and 300 turns on the secondary coil. If 20V AC is applied to the primary coil, what will be the voltage on the secondary coil? • A device is connected to the secondary coil which draws a current of 2 A. What is the current flowing in the primary coil? Vs N s Vp N p 300 Vs , 100 20 so Vs 60 V I pV p I sVs I p 20 2 60 Ip 6A Power in a transformer Power in Power out so I pV p I sVs • So if a transformer steps up the voltage, the current is stepped down. – You can’t get something for nothing! • This assumes an ideal transformer, where no energy is lost to heating Electricity transmission • When electricity is transmitted over power lines, some power is lost due to the resistance of the cables (as heat) • P = IV and V=IR so Power lost = I2R – So the higher the current, the more power we lose • A step-up transformer is used to convert electrical power to very high voltage (low current) for transmission over long distances to minimise this power loss • It is converted back to a more useful level at the other end by a step-down transformer Typical power transmission system What’s it all for? • Eg Maglev train (Shanghai) • 432 km/h top speed • No moving parts! (nearly) Energy Generation Electricity Generation • Can energy be generated? – No! It can only be transferred from one form to another • What we mean is electricity generation Industrial Electricity Generation • Most of the world’s electricity is produced in power stations: 1 2 3 4 Industrial Electricity Generation • What fuels can be used? • What are the relative advantages and disadvantages of using those fuels in power stations? • What other ways are there of generating electricity? How else can we turn turbines? • Are they better or worse than power stations? • How would you provide the electricity the world wants? Energy transfers • eg wind power: Kinetic energy of air Wind Rotational turbine kinetic energy generator Electrical energy • Draw energy transfer diagrams for: • Wave power, Fossil fuels, Tidal power, Hydroelectric power, Geothermal power, Nuclear power, Photovoltaic cells, Solar heating Renewable or non-renewable? • A non-renewable energy resource is one which cannot be replaced once it has been used. • A renewable energy resource is one which will not run out • Categorise the following energy sources: – Wind power, wave power, coal, tidal power, oil, hydroelectric power, gas, geothermal power, nuclear power, photovoltaic cells, biomass (wood), solar heating Industrial Electricity Generation • What are the advantages and disadvantages of: • Think about: • • • • • • • • Cost? Reliability? Continuity of supply? Easy to start and stop? Renewable/non-renewable? Access to fuel? Waste products and environmental impact? Location? • • • • • • • • • Wind power Wave power Fossil fuels Tidal power Hydroelectric power Geothermal power Nuclear power Photovoltaic cells Solar heating Energy resource Advantages Disadvantages 6.2 understand that magnets repel and attract other magnets and attract magnetic substances 6.3 describe the properties of magnetically hard and soft materials 6.4 understand the term ‘magnetic field line’ 6.5 understand that magnetism is induced in some materials when they are placed in a magnetic field 6.6 describe experiments to investigate the magnetic field pattern for a permanent bar magnet and that between two bar magnets 6.7 describe how to use two permanent magnets to produce a uniform magnetic field pattern. 6.8 understand that an electric current in a conductor produces a magnetic field round it 6.9 describe the construction of electromagnets 6.10 sketch and recognise magnetic field patterns for a straight wire, a flat circular coil and a solenoid when each is carrying a current 6.11 understand that there is a force on a charged particle when it moves in a magnetic field as long as its motion is not parallel to the field 6.12 understand that a force is exerted on a current-carrying wire in a magnetic field, and how this effect is applied in simple d.c. electric motors and loudspeakers 6.13 use the left hand rule to predict the direction of the resulting force when a wire carries a current perpendicular to a magnetic field 6.14 describe how the force on a current-carrying conductor in a magnetic field increases with the strength of the field and with the current. 6.15 understand that a voltage is induced in a conductor or a coil when it moves through a magnetic field or when a magnetic field changes through it and describe the factors which affect the size of the induced voltage 6.16 describe the generation of electricity by the rotation of a magnet within a coil of wire and of a coil of wire within a magnetic field and describe the factors which affect the size of the induced voltage 6.17 describe the structure of a transformer, and understand that a transformer changes the size of an alternating voltage by having different numbers of turns on the input and output sides 6.18 explain the use of step-up and step-down transformers in the large scale generation and transmission of electrical energy 6.19 know and use the relationship between input (primary) and output (secondary) voltages and the turns ratio for a transformer: 6.20 know and use the relationship: input power = output power VP IP = VS IS for 100% efficiency 4.2 describe energy transfers involving the following forms of energy: thermal(heat), light, electrical, sound, kinetic, chemical, nuclear and potential(elastic and gravitational) 4.16 describe the energy transfers involved in generating electricity using: Wind, , water, geothermal resources, solar heating systems, solar cells, fossil fuels, nuclear power 4.17 describe the advantages and disadvantages of methods of large scale electricity production from various renewable and nonrenewable resources.