M. Ibbotson - Professional english in use engineering (794233), страница 26
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This limits the amount of current that can flow. Resistancealso depends on the materials used as conductors. For example, copper has a low resistanceand so is a good conductor.Materials with very high resistance, such as plastics, are called electrical insulators (orinsulators). Only very high voltages cause current to flow through them.
Materials that are goodinsulators are used to insulate conductors. An example is plastic insulation around electric wires.This stops people from touching the conductor and- if it is live (carrying current)- from gettinga dangerous electric shock.-Electrical powerThe text below, about electrical power, is from a home improvements magazine.The amount of current, in amps, required by an electrical appliance- such as a TV oran electric kettle -depends on the power of the appliance. This number- expressedin watts (W)- will be marked somewhere on the appliance. To calculate the requiredcurrent, simply take the wattage and divide it by the voltage of the electrical supply inyour home- around 230 volts in most of Europe.
Therefore, for an electric kettle with apower rating of 2,000 watts (as specified by the manufacturer), the current required is:2,000 watts230 volts92= 8 .7 ampsProfessional English in Use Engineering43.1Complete the word puzzle and find the word going downthe page. Look at A, B and C opposite page to help you.1 another term for amperage2 provided by a battery, for examplemeasured as a wattageallows current to flow through ithas very high electrical resistancecarried by moving electronsanother term for an electrical 'device'8 the consequence of a person touching a live conductor34567ELECTRIC2ELECTRIC345ELECTRICELECTRI cELECTRIC67ELECTRIC843.2IELECTRICIE LECTRICComplete the extract about current and power calculations using the words in the box.Look at A, B and C opposite to help you.ampscomponentsconductorcircuitcurrentohmsresistancesupplyvoltagevoltswattagewattsIn electrical calculations, electromotive force is expressed by the letter E, resistance bythe letter R, and current by the letter 1 (which comes from the word 'intensity').According to Ohm's Law: I = E/R.In other words, the (1) ................................
flowing through a (2) ................................ , measuredin (3) ................................ , equals the (4) ................................ of the electrical (5) ................................ ,measured in (6) ................................ ,divided by the total (7) ................................ ,measured in(8) ................................. To work out the value of R, it is necessary to calculate the totalresistance of all the (9) ................................
and connecting lengths of (1 0) ................................ thatmake up the circuit.Once both the voltage and amperage are known , it is possible to work out the power,measured in (11) ................................ , that will be consumed. Power (P) can be calculatedusing the equation P = EI. Therefore (12) ................................ equals voltage multiplied byamperage.Over 1-o t:fou~Say how much power is required by an electrical appliance you know about. and whatvoltage and current are used to power it.
Then use these values to calculate and state whatthe tota l resistance of the appliance is.Professional English in Use Engineering93W Electrical supply-Direct current and alternating currentThe current from a cell is direct current (DC) -a constant flow of electricity which travelsaround a circuit in one direction. The electricity supplied to homes and other buildings called mains electricity- is alternating current (AC). Unlike a DC supply, an AC supply flowsbackwards and forwards - its direction continually alternates.
The rate at which the currentalternates- called the frequency- is measured in hertz (Hz). For example, in the UK, ACsupply is 50 Hz - it alternates 50 times per second. On a graph, the AC supply of mainselectricity forms a sine wave.The current supplied to most homes is single-phase - it forms one sine wave. In factories andlarge buildings, which have powerful electrical equipment, the supply is often three-phase-effectively three currents, each with a different phase (timing). This provides a smoothersupply as it reduces the gaps between the voltage peaks.Note: The term mains electricity is not used in American English -terms like supply are used.~ AC generation and supplyMains electricity is generated (produced) at sites called power stations, which use largegenerators.
A generator converts mechanical energy to electrical energy. A generator rotates amagnet within an iron surround. The iron- called an armature- has coils of wire around it,called field coils (or field windings). As the magnet rotates, it causes current to flow throughthe field coils, due to electromagnetic induction.Current from the generators leaves the power station and enters the power grid (or grid)- thenetwork of power lines (cables) which transmit it around the country. At the point where itenters the grid, the electricity flows through transformers -specifically step-up transformers,which increase voltage and decrease amperage.
This reduces the energy lost from the powerlines over long distances, as high-voltage (HV) supplies flow more efficiently than low-voltage(LV) supplies. Before the supply is used by homes and other buildings, it passes through severalstep-down transformers, which reduce its voltage and increase its amperage.The supply may be stepped up to over 400,000 volts at the point where it enters the largetransmission lines (long-distance power lines) leaving the power station. It is normally thenstepped down in stages, first passing through a wider network of lower-voltage transmissionlines, and finally through the small distribution lines which supply streets and houses -inmany countries at around 230 volts.-DC generation and use+VThe extract below is from a consumer magazine.Photovoltaic cells (PVs) - or solar cells - are aneffective way of generating your own electricityfrom sunlight.
The current they produce can beused immediately, may be stored in rechargeablebatteries (like the ones in cars), or can be fedinto the power grid and sold to the electriccompany. But PVs produce direct current. This isfine for charging batteries, but is not suitable forpowering household appliances, which requirealternating current. For this, the DC supply fromPVs and batteries needs to go through an inverter-a device which converts DC to AC.-vA single-phase AC supply+V-vA three- phase AC supply94Professional English in Use Engineering44.1Complete the text about inverters using words from A opposite.
Look at A, B and C oppositeto help you.Inverters convert (1) ................................ ................................ to (2) ............................... .................................. If an inverter is used to supply electrical appliances in ahome, it must copy the supply of (3) ................................
electricity producedby the generators at power stations. Most inverters can produce a currentwhich alternates precisely at the required (4) ................................ -for example,50 (5) ................................ (SO cycles per second). However, not all types areable to produce a current which follows the pattern of a (6) ................................................................, like that of the (7) ................................-................................ AC supplyused in homes.
So-called 'square wave inverters' only produce a veryapproximate copy of this wave, which can affect the functioning of manyelectrical appliances.- - - mains AC supply- - - AC supply from a 'square wave inverter'+V-v44.2Choose the correct words from the brackets to complete the descriptions of different stagesof AC generation and supply (a-f). Then, put the stages in the correct order. Look at Bopposite to help you.abcdef44.3After the step-up transformer, the current enters a (distribution I transmission) line.Current is produced, by electromagnetic induction, in the (magnet I field coils) of a generator.The current goes from the last step-down transformer to a (distribution I transmission) line.The current leaves the power (grid I station) and enters the home.Amperage is reduced and voltage is increased by a (step-up I step-down) transformer.The current is stepped (up I down) from a higher voltage to a lower voltage, in stages.Decide whether the sentences below are true or false , and correct the false sentences.
Look atA, B and C opposite to help you.Photovoltaic cells produce direct current.2 The electricity supply from PVs can be used to charge rechargeable batteries.3 Rechargeable batteries supply electricity as alternating current.4 Inverters convert sunlight to alternating current.Ov~r -totjoa~Think of some large and small electrical appliances you're familiar with.
Explain theirelectrical supply requirements. What type of current is required, and how is it supplied and/or converted?Professional English in Use Engineering95II:J Circuits and components-Simple circuitsThe circuit diagrams below show lamps connected in a parallel circuit and in a series circuit.The supply has live and neutral conductors. On an alternating current (AC) supply, thedifference between live and neutral is that conductors on the neutral side of appliances areearthed- that is, connected to earth (the grOLmd).liveAC supply3 lamps in parallel-neutral3 lamps in seriesBrE: live; ArnE: phaseBrE: earth, earthed; ArnE: ground,groundedMains AC circuits and switchboardsWhere an AC supply enters a building, it is connected to a switchboard.
This has a number ofswitches to allow different circuits in the building to be switched on and off. Circuits include powercircuits. These supply the power sockets (or sockets) for the plugs on appliances. Usually, a circuitbreaker is fitted to each circuit. This is a safety switch that switches off automatically if there is aproblem. This may happen if a person touches a live conductor, or if there is a short circuit. A shortcircuit is when current flows directly from a live conductor to a neutral conductor -for example,due to damaged insulation.
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