Conservation of energy. Energy can neither be created nor destroyed.... but it can be converted from one form to another

Transcription

1 Using Energy

2 Types of energy There are two main categories: Kinetic (movement energy) Kinetic itself (ke) Light Heat (vibration of particles) Electrical (movement of charges) Potential (stored energy) Gravitational potential energy (gpe) Chemical potential energy Elastic/mechanical energy

3 Conservation of energy Energy can neither be created nor destroyed... but it can be converted from one form to another A transducer is any device that changes one form of energy into another form Electrical energy Light Heat

4 gravitational potential energy (gpe) falling BUNGIEEEE!! kinetic energy (ke) slowing rising kinetic energy (ke) rising mechanical potential energy/ elastic energy

5 The jumper eventually stops bouncing as their energy all gets converted to heat (mainly in the bungie rope) gravitational potential energy (gpe) rising kinetic energy (ke) friction with air friction with air falling Heat friction in rope rising friction in rope kinetic energy (ke) slowing mechanical potential energy/ elastic energy

6 Another example: Disk Brakes These convert kinetic energy to heat energy via the friction force Hydraulic (brake) fluid. Pushing down on the brake pedal compresses the brake fluid and this presses the pads against the disk

7 Pressing the brake pedal makes the brake pads press against the disk. The friction force converts the kinetic energy of the spinning wheel into heat energy, so the wheel slows down. kinetic energy heat energy brake pads disk axle hub

8 Kinetic energy friction force Thermal energy Light energy

9 Energy & Efficiency All machines and devices waste energy. Old style light bulbs waste lots. Efficiency is a measure of how well a device changes one form of energy into another form of energy 100J of electrical energy Total input energy 20J of light useful output energy energy 80J of heat energy Waste energy

10 100J of electrical energy Efficiency = the useful output energy the total input energy If you need a formula in the exam, it will be included in the question Calculate the efficiency of the bulb: Efficiency = useful output energy Total input energy Efficiency = 20J 100J = 0.2 Notice that efficiency has no units. This is because it is a ratio and the units cancel out 20J of light energy 80J of heat energy For every joule of electrical energy supplied to this bulb, 0.2J will be converted to light, and 0.8J will be wasted as heat

11 Sankey Diagrams These diagrams are good for showing energy transformations, and they show how much energy gets transformed. 100J of electrical energy 20J of light energy 100J of electrical energy 20J of light energy 80J of heat energy 80J of heat energy

12 Sankey diagrams are named after Irish Captain Matthew Henry Phineas Riall Sankey, who is considered to have been the first to use this type of diagram in1898 in a publication on the energy efficiency of a steam engine Provided for interest. You don't need to learn this

13 Example The diagrams show what happens to each 100 joules of energy from burning coal on an open fire and in a stove. 20 little squares = 100J 1 little square = 5J The examiners will expect you to work out the scale for yourself 15J 60J (a) Add the missing figures to the diagrams. 15J (b) Which is more efficient, the open fire or the stove? Give a reason for your answer. p.t.o. Hang on! 75J + 15J does not equal 100J! If this happens in a real exam just answer to the best of your ability and the Chief Examiner will make it come out fair

14 20 little squares = 100J 1 little square = 5J 15J 60J (a) Add the missing figures to the diagrams. 15J (b) Which is more efficient, the open fire or the stove? Give a reason for your answer. The stove. Heat is the useful output energy and the stove produces a much higher proportion of heat than the open fire, ie, the stove is more efficient.

15 Electrical Devices Electrical energy is very versatile. It can be transformed into most other kinds of energy: useful energy wasted energy Light bulb Electric heater Electric motor Hair dryer Light from glowing filament Thermal energy supplied to the surroundings Kinetic energy supplied to objects Kinetic and thermal energy given to the air Thermal energy supplied to the surroundings Light from the glowing heating element Thermal and sound energy supplied to the surroundings Sound energy released. Thermal energy in the dryer itself

16 Light bulb Electric heater Electric motor Hair dryer useful energy Light from glowing filament Thermal energy supplied to the surroundings Kinetic energy supplied to objects Kinetic and thermal energy given to the air wasted energy Thermal energy supplied to the surroundings light from the glowing heating element Thermal and sound energy supplied to the surroundings Sound energy released. Thermal energy in the dryer itself

17 supplied to wasted by The useful energy from a machine is always less than the total energy supplied to it Friction between the moving parts of a machine causes energy to be wasted by the machine Bcause energy is conserved, the energy supplied to a machine is the sum of the useful energy from the machine and the energy wasted by the machine

18 An electric motor is used to raise a weight. When you supply 60J of electrical energy to the motor, the weight gains 24J of gravitational potential energy (gpe). (1) How much of the 60J of energy is wasted? Total input = 60J Waste = 60J 24J = 36J Useful output = 24J (2) Calculate the efficiency of the motor Efficiency = the useful output energy the total input energy Notice that the efficiency is always less than 1.0 It has to be this is a universal law. = 24J 60J = 0.4 If you ever calculate an efficiency greater than 1.0, then you probably had the fraction upside down

19 Power Work is the act of changing one form of energy into another (eg, chemical potential to kinetic energy) Power is the rate at which a device transforms energy from one form to another (ie, the rate of doing work) A more powerful device transforms energy faster The unit of power is the watt (W), named for James Watt ( ) who notably significantly improved the design of the original steam engines, increasing their power.

20 One watt = one joule per second 1W = 1J/s...so a 100W lightbulb uses 100J of energy per second Human: Up to 75W of power = 75J of work done per second 1600x more powerful JCB excavator: Up to 120,000W of power =120kW = 120,000J of work done per second

21 The Power Formula Power (in watts, W) = amount of energy transferred (in joules, J) time taken (in seconds, s) All formulae are given in the question! An electric motor transfers 10,000J in 25s. What is its power? Power = amount of energy transferred time taken (in seconds, s) = 10,000J 25s = 400W

22 (1) Which is more powerful a 3kW kettle or a 10,000W cooker? 3000W (2)There are 20 million homes in Britain. If a 3kW kettle is switched on in 1 in 10 homes, how much power would need to be supplied? (3) A machine has a power rating of 100kW* If the machine runs for 2 minutes*, how much energy does it transfer? (higher tier) The cooker is more powerful 20million/10 = 2million. Total power = 2million x 3000W = 6,000,000,000W = 6GW 2 minutes = 120s 100kW = 100,000W Energy = Power x Time = 100,000W x 120s = 12,000,000J Power = energy time *traps for the unwary be careful

23 Paying for electrical energy The Electricity Board measures the energy it transfers to your house in "kilowatt hours" (kwh). Energy transferred = Power x time (in kwh) (in kilowatts, kw) (in hours, h) The proper unit of energy is joules!!!!!! The proper unit of power is watts!!!!!! The proper unit of time is seconds!!!!!! The Electricity Board has the power. They use whatever units they feel like!!

24 Energy transferred = Power x time (in kwh) (in kilowatts, kw) (in hours, h) How many kwh does a 1kW heater use in 2 hours? 1kWh x 2h = 2kWh How many kwh does a 120kW digger use in 30 minutes? 30 minutes = 0.5hours 120kWh x 0.5h = 60kWh How many kwh does a 100W bulb use in 1 day? 1 day = 24hours 0.1kWh x 24h = 2.4kWh 100W = 0.1kW The Electricity Board demands kilowatts and hours

25 The electricity board calls each kwh a "Unit" 1kWh = 1 "Unit" To get the cost we multiply the number of units by the cost per unit. eg, if one unit costs 3p, how much would 12kWh cost? 12kWh x 3p = 36p

26

27 A 4000W heater is left on for 3 hours. (1) How many kilowatt hours of energy does it use? 4000W = 4kW No. of kwh = 4kW x 3h = 12kWh (2) How many units of electricity does it use? 12kWh = 12 units of electricity (3) How much does it cost to run if one unit costs 3p 12 units x 3p = 36p Energy (kwh) = Power (kw) x time (h)

28 The National Grid This is the system of overhead high voltage cables that distribute electrical energy around the country. Power station 24kV 21kA *divided up between many houses and factories step up transformer 765kV 0.66kA 240V 2.1MA* step down transformer DO NOT learn any numbers step down transformer 69kV 7.3kA* Industry

29 When you pass a current through a wire the wire gets hot: electrical energy thermal energy This wastes electrical energy in the National Grid. We can reduce the energy loss by using devices called transformers which change the voltage and the current. A step up transformer increases the voltage, and decreases the current 24kV 21kA step up transformer 765kV 0.66kA A step down transformer decreases the voltage, and increases the current 765kV 0.66kA step down transformer 69kV 7.3kA

30 ...But Why? When we transform the electricity, the total amount of energy stays the same (trust me!) so we can send our electrical energy as: High voltage Low current The overhead cables stay fairly cool minimum wasted energy OR Low voltage High current The overhead cables get really hot maximum wasted energy So step up transformers decrease the current for efficient transmission. Finally, step down transformers are used to drop the voltage to values that are safe to use in the home.

31 Fuel for electricity conventional (fossil fuel) power stations In a conventional power station: Chemical potential energy in fuel Thermal energy in steam A convenional power station burns fossil fuels, ie, coal or oil or natural gas Kinetic energy in a turbine Electrical energy in a generator Kinetic energy in a generator

32 The fuel contains chemical potential energy which is released by burning the fuel. The fuel can be finely powdered coal, oil or natural gas. The heat is used to produce super heated steam. When steam is kept o under pressure, it can go above 100 C and therefore has much more energy.

33 The steam reaches 370 C and about 2000 atmospheres of pressure. Don't learn these numbers either The steam is then used to spin the blades of a turbine. The turbine is like a high tech windmill driven by steam. The turbine changes the thermal energy of the steam in to kinetic energy. o

34 The axle of the turbine goes into a generator. The generator produces electrical energy when it is turned by the turbine (more on this next year).

35 coal To the National Grid

36 Advantages: Very large amounts of electricity can be generated in one place using coal, fairly cheaply Conventional (fossil fuel) power plants Disadvantages: Burning any fossil fuel produces carbon dioxide, which contributes to the "greenhouse effect", warming the Earth Transporting oil and gas to the power stations is easy Gas fired power stations are very efficient A fossil fuelled power station can be built almost anywhere, so long as you can get large quantities of fuel to it Didcot power station, in Oxfordshire, has a dedicated rail link to supply the coal It also produces sulphur dioxide, a gas that contributes to acid rain. We can reduce this before releasing the waste gases into the atmosphere Mining coal can be difficult and dangerous. Strip mining destroys large areas of the landscape Coal fired power stations need huge amounts of fuel, which means train loads of coal almost constantly. In order to cope with changing demands for power, the station needs reserves. This means covering a large area of countryside next to the power station with piles of coal

37 Coal Greenhouse gases trap heat in the atmosphere, so adding more greenhouse gas leads to global warming and (almost certainly) to climate change increasing greenhouse gas emissions Oil Natural gas increasing sulfur content Sulfur in fuels combines with oxygen to produce sulfur dioxide gas which dissolves in rain clouds to produce acid rain

38 Supplying electricity nuclear power stations Most of a nuclear power plant is theb same as a conventional plant. The big difference is that the thermal energy used to make the steam comes from nuclear fuel. nuclear bit conventional bit A

39 Very similar to a conventional (fossil fuel) plant apart from this bit Nuclear energy in uranium fuel Thermal energy in steam Kinetic energy in a turbine Electrical energy in a generator Kinetic energy in a generator

40 Advantages: Nuclear power Disadvantages: Nuclear power costs about the same as coal, so it's not expensive to make Does not produce smoke or carbon dioxide, so it does not contribute to the greenhouse effect Produces huge amounts of energy from small amounts of fuel Produces small amounts of waste Nuclear power is reliable The N.I.M.B.Y. effect (not in my back yard) Produces nuclear waste which is highly radioactive, highly toxic and remains radioactive for thousands of years Parts of the plant become radioactive. The plant is very expensive to decommission There are hazards involved in moving nuclear fuel and nuclear wastes around No one wants a nuclear waste storage facility near them Produces materials that can be used to make nuclear weapons Takes a long time to start up/shut down

41 Meeting the demand Start up times quickest slowest } Natural gas Oil } Coal Nuclear These power stations can be started up quickly to meet extra demand, eg, when Eastenders finishes These power plants run all the time to provide the base load of power (the power which is always available on the National Grid)

42 Power supplied by the grid (in MW) Extra demand power is supplied by oil and natural gas power plants which can be started up and shut down quickly The base load is the power supplied to the grid at all times 41000MW (the base load) The base load power is provided by coal and nuclear plants that run all the time Excess power sold to France or Ireland via undersea power lines Time of day

43 Supplying electricity Alternative/Renewable energy These are increasingly important as they do not run out (renewable) and do not produce so much pollution as conventional power sources. The ultimate source of most renewable energy is the Sun which should last for at least another 5 billion years.

44 Solar Power (1) Solar cells Some materials can convert light energy into a voltage (and therefore into electrical energy) directly. Batteries store the energy. These two are very different solar cells Light (2) Solar water heaters Electrical The heat/infra red from the sun is trapped in water contained in black pipes. This is a way to get free hot water. Thermal/Infrared Thermal in water

45 (3) Solar furnace The world's largest solar furnace is in Odeillo, France. The mirrors focus heat (and light) onto a collection point o that can reach up to 3000 C. Odeillo, France The heat is used to produce steel from iron without the pollution caused by burning fossil fuels.

46 Advantages: Solar power Disadvantages: Solar energy is free it needs no fuel and produces no waste or pollution In sunny countries, solar power can be used where there is no easy way to get electricity to a remote place Handy for low power uses such as solar powered garden lights and battery chargers, or for helping your home energy bills Doesn't work at night Very expensive to build solar power stations. Solar cells cost a great deal compared to the amount of electricity they'll produce in their lifetime In the United Kingdom, solar power isn't much use for highpower applications, as you need a large area of solar panels to get a decent amount of power

47 Wind Power Wind is produced by the Sun heating the ground (see convection notes). The wind spins the wind turbine blades which spins a generator. There is a gear box in between the blades and the generator to get the generator spinning faster. kinetic energy (wind) kinetic energy (wind) electrical energy

48 Advantages: Wind power Disadvantages: Wind energy is plentiful and renewable. No greenhouse gas emissions or other pollutants Very good for people in remote areas who are not connected to an electrical grid Very few costs once installed Studies have shown that very few birds are killed by the turbines The area surrounding the turbines can still be used for farming Wind speed constantly changes throughout the day, and some days there is no wind Manufacturing wind turbines requires steel, concrete and aluminum. These materials require lots of electricity to produce, so some carbon dioxide (a greenhouse gas) will be released into the atmosphere (along with some other pollutants). It takes about 9 months of operation before the wind turbine becomes carbon neutral (saves the amount of greenhouse gases as is required to make it) They are very large and lots of people think that they are an eye sore. This is called visual pollution Generating electricity for turbines makes a lot of noise. This is called noise pollution Most windy areas are near coastlines, where land is the most valuable. Fairly expensive to build.

49 Wave power The wind pushes water to make waves. There are a number of ways to harvest the kinetic energy in waves: (1) Salter's ducks We'll find out how movement and magnetism combine to make electricity in Module 3, Year 11

50 (2) Oscillating water column plants These funnel the waves to increase their speed. The rushing water compresses the air in the funnel which spins the blades of a tubine which spins a generator to make electricity. The Scottish island of Aldernay gets 1/3rd of its power from the waves A Limpet 500 wave power generator The turbine also spins as the air is sucked out by the retreating waves. This maximises the power you can extract from the waves.

51 Each movement of the air spins the turbine which spins the generator Not required for the exam How to install a Limpet 500 wave power generator kinetic energy (generator) kinetic energy (wind) kinetic energy (waves) electrical energy

52 Advantages: Wave power Disadvantages: The energy is free no fuel needed, no waste produced Not expensive to operate and maintain Can produce a great deal of energy Depends on the waves sometimes you'll get loads of energy, sometimes almost nothing Needs a suitable site, where waves are consistently strong Some designs are noisy. This is noise poluution Must be able to withstand very rough weather

53 Hydroelectric power The Sun produces rain, which rushes down hill. The rushing water can spin turbines which spin generators to produce electrical energy. This water has gravitational potential energy because it is high up This water has kinetic energy gpe kinetic electrical

54 Advantages: Once the dam is built, the energy is virtually free No waste or pollution produced Hydroelectric Power Disadvantages: The dams are very expensive to build. However, many dams are also used for flood control or irrigation, so building costs can be shared Much more reliable than wind, solar or wave power Water can be stored above the dam ready to cope with peaks in demand Hydro electric power stations can increase to full power very quickly, unlike other power stations Electricity can be generated constantly Building a large dam will flood a very large area upstream, causing problems for animals that used to live there Finding a suitable site can be difficult. The impact on residents and the environment may be unacceptable (Like having your village flooded!) Water quality and quantity downstream can be affected, which can have an impact on plant life

55 Tidal Power This has nothing to do with the Sun (OK, sometimes the Sun works with the moon to produce extra high tides) High tide Low tide Low tide High tide The Moon's gravity pulls up the water to produce a high tide under the moon, and also on the other side of the Earth.

56 This form of wave power is the tidal barrage The rushing water spins a turbine which spins a generator gpe kinetic electrical

57 There have been plans for a "Severn Barrage" from Brean Down in Somerset to Lavernock Point in Wales. It could have more than 200 large turbines, and provide over 8,000 Megawatts of power (that's over 12 nuclear power station's worth). It would take 7 years to build, and could provide 7% of the energy needs for England and Wales. There would be a number of benefits, including protecting a large stretch of coastline against damage from high storm tides, and providing a ready made road bridge. However, the drastic changes to the currents in the estuary could have huge effects on the ecosystem. Don't learn this it is just an example Possible sites for a Severn barrage and their possible power outputs Severn estuary

58 Another way to harvest tidal energy would be to use submerged offshore turbines, that would work in much the same way as wind turbines. This has the advantage of being much cheaper to build than a tidal barrage, and does not have the environmental problems that a tidal barrage would bring. The system working in Strangford Lough, Northern Ireland

59 Advantages: Tidal Power Disadvantages: Once you've built it, tidal power is free. It produces no greenhouse gases or other waste. It needs no fuel. It produces electricity reliably and predictably. Not expensive to maintain. Tides are totally predictable. A barrage across an estuary is very expensive to build, and affects a very wide area. The environment is changed for many miles upstream and downstream. Many birds rely on the tide uncovering the mud flats so that they can feed. There are few suitable sites for tidal barrages. Only provides power for around 10 hours each day, when the tide is actually moving in or out. Offshore turbines are not ruinously expensive to build and do not have a large environmental impact.

60 Geothermal power This also has nothing to do with the Sun. Radioctive materials deep in the Earth produce heat. Thermal Kinetic (steam) Electrical The heat trapped in the water can be used to generate electricity. A typical power station operates at 25 35% efficiency, the Southampton Geothermal Heating & Cooling Plant operates at 70 85%

61 Advantages: Geothermal Power Disadvantages: Geothermal energy does not produce any pollution, and does not contribute to the greenhouse effect The power stations do not take up much room, so there is not much impact on the environment No fuel is needed Once you've built a geothermal power station, the energy is almost free. It may need a little energy to run a pump, but this can be taken from the energy being generated The big problem is that there are not many places where you can build a geothermal power station. You need hot rocks of a suitable type, at a depth where we can drill down to them. The type of rock above is also important, it must be of a type that we can easily drill through Sometimes a geothermal site may "run out of steam", perhaps for decades Hazardous gases and minerals may come up from underground, and can be difficult to safely dispose of