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Transcript
Electromagnetic Induction Chapter 21 Power Plant Generating Electricity A motor is a device for transforming electrical energy into mechanical (kinetic) energy To generate electricity we need a device that will do the opposite It must transform mechanical energy into electrical energy Use an electric motor in reverse The current has been induced and the motor is acting like a generator Generating Electricity Many different generator designs, some produce direct current and some produce alternating current Some use permanent magnets and others use electromagnets A bicycle could be used as a dynamo Turbines – a device that is caused to turn by moving air, steam, or water, often used to generate electricity All generators have: A magnetic field (magnets or electromagnets) A coil or wire (fixed or moving) Movement (the coil and magnetic field move relative to one another) When the coil and the magnetic field move relative to one another a current flows in the coil if it is part of a complete circuit and this is know as induced current If the generator is not connected up to a circuit, there will be an induced e.m.f across its ends ready to make a current flow The principles of electromagnetic induction The process of generating electricity from motion is called electromagnetic induction Michael Faraday invented the idea of magnetic field lines and drew field lines to represent it Dynamo effect – electricity is generated when a coil moves near a magnet If the coil is part of a complete circuit the induced e.m.f. will make an induced current flow around the circuit The principles of electromagnetic induction Move one pole of the magnet downwards past the wire and a current flows Move the magnet back upwards and a current flows in the opposite direction Alternatively the magnet can be stationary and the wire can be moved up and down In figure b the same effect can happen moving the magnet in a coil Reverse the magnet to use the opposite pole and the current flows in the opposite direction Hold the magnet stationary next to the wire or coil and no current flows. They must move relative to each other Increasing the induced e.m.f. There are 3 ways to increase the e.m.f. induced in a coil or wire Use a stronger magnet Move the wire or coil more quickly relative to the magnet Use a coil with more turns of wire Generating a.c. Faraday’s discovery lead to the creation of bigger and bigger generators Alternating current – (a.c.) electric current that flows one way, then the other in a circuit The frequency of an a.c. supply is the number of cycles it produces each second Induction and field lines Picture the field lines coming out of each pole of the magnets shown in figure 21.5 If the magnet is stationary there is no cutting of field lines so no e.m.f. is induced As the magnet is moved, the field lines are cut by the wire, and it is this cutting of field lines that induces the current If the magnet is further from the wire the field lines are further apart and so fewer are cut giving a smaller e.m.f. This idea helps us to understand the factors that affect the magnitude and direction of the induced e.m.f If magnet is moved quickly the lines are cut more quickly and a bigger e.m.f is induced A coil gives a bigger effect than a single wire Fleming’s right-hand rule When a current flows in a magnetic field there is a force on it so that it moves. The directions of force, field and current were given by Fleming’s left-hand rule In electromagnetic induction the directions are given by Fleming's righthand rule An a.c. generator a.c. generator – a device such as a dynamo used to generate alternating current A d.c. motor working in reverse The other difference is the way the circuit is connected Commutator – a device used to allow current to flow to and from the coil of a d.c. motor or generator Slip rings – a device used to allow current to flow to and from the coil of an a.c. motor or generator An a.c. generator Why does this produce alternating current? As the coil rotates each side of the coil passes first the magnetic north pole and then the south pole. This means that the induced current flows first one way and then the other The current flows out through the slip rings. And each ring is connected to one end of the coil The alternating current flows out through the brushes which press against the rings There are four ways to increase voltage Turn the coil more rapidly Use a coil with more turns of wire Use a coil with a bigger area Use stronger magnets Spin the coil 50 times and the a.c. generator has a frequency of 50 Hz. Page 297 Direction of the induced e.m.f. How does an induced current know in which direction it must flow? The answer is that the current has a magnetic field around it This field pushes back against the field that is inducing the current Page 297 & figure 21.5b Power lines and transformers High voltages leave the power station and carried by power lines to customers Power lines – cables used to carry electricity from power stations to consumers National grid – the system of power lines, pylons and transformers used to carry electricity around a country When the power lines approach the area where the power is to be used they enter a local distribution center where the voltage is reduced Transformers typically reduce it to 230V Why use high voltages? Power is carried by cables both above and below ground. Underground cables are much more expensive High voltage means low current and this wastes less energy Energy carried is lost through heat because of resistance If current is reduced in half by doubling the voltage then the losses will be one quarter of their previous value. This is because power losses in cables is proportional to the square of the current flowing in the cables Double current gives 4 times the losses Three times the current gives 9 times the losses Transformers Transformers – a device used to change the voltage of an a.c. electricity supply They can be up to 99.9% efficient Passing through multiply transformers could cause significant loss if they were not as efficient Power stations typically generate electricity at 25kV. This has to be converted to the grid voltage of 400kV using a transformer. We say the voltage is stepped up by a factor of 16 Construction of transformers Primary coil – the incoming voltage 𝑉𝑝 is connected across this coil Secondary coil – this provides the voltage 𝑉𝑠 to the external circuit Iron core – this links the two coils There is not electrical connection between the two coils The voltages are both alternating and a transformer does not change a.c. to d.c. To step up the input voltage by a factor of 16 there must be 16 times as many turns on the secondary coil as the primary A step up increases voltage (more turns) A step down decreases voltage (less turns) If voltage is stepped up the current must be stepped down How transformers work The primary coil has alternating current flowing through it. It is thus an electromagnet and produces an alternating magnetic field The core transports this alternating field around the secondary coil Now the secondary coil is a conductor in a changing magnetic field. A current is induced in the coil. Secondary coil Few turns small e.m.f. Lots of turns large e.m.f. If a direct current is connected to a transformer there is no output voltage because the magnetic field produced by the primary coil is unchanging Page 300 Soft – describes a material that once magnetized can be easily demagnetised Calculating current & Energy saving Energy saving Electricity is transmitted in high voltages because the current can be small Calculating current 𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐𝑎𝑙 𝑝𝑜𝑤𝑒𝑟 𝑃 = 𝐼𝑉 Example 21.2 This allows for lower energy loss Page 301 The current flowing in the cables is a flow of coulombs of charge. At high voltage we have fewer coulombs flowing, but each coulomb carries more energy with it Thinking about power If a transformer is 100% efficient no power is lost in its coils or core Well designed transformers waste only about .1% of the power transferred through them