Download
1 / 37
380 likes | 600 Vues
Y10 Revision of Energy and Electricity. REVISION. How does heat travel through space? . The Earth is warmed by heat energy from the Sun. How does this heat energy travel from the Sun to the Earth?. ?. infrared waves.
E N D
How does heat travel through space? The Earth is warmed by heat energy from the Sun. How does this heat energy travel from the Sun to the Earth? ? infrared waves There are no particles between the Sun and the Earth, so the heat cannot travel by conduction or by convection. The heat travels to Earth by infrared waves. These are similar to light waves and are able to travel through empty space.
What are infrared waves? Infrared waves heat objects that absorbthem and are also known as thermal radiation. Heat can move by travelling as infrared waves. These are electromagnetic waves, like light waves, but with a longer wavelength. This means that infrared waves act like light waves: • They can travel through a vacuum. • They travel at the same speed as light – 300,000,000 m/s. • They can be reflected and absorbed.
worst emitter best emitter silver matt black white worst absorber best absorber Matt black surfaces are the best absorbers of radiation. Shiny surfaces are the worst emitters because they reflect most of the radiation away. Why are solar panels that are used for heating water covered in a black outer layer? Absorbing thermal radiation Infrared waves heat objects that absorb(take in) them. Certain surfaces are better at absorbing thermal radiation than others. Good emitters are also good absorbers.
HOTCOLD Why does heat transfer happen? Heat is a type of energy called thermal energy. Heat can be transferred (moved) by three main processes: 1. conduction 2. convection 3. radiation During heat transfer, thermal energy always moves in the same direction: Heat energy only flows when there is a temperature difference from a warmer area to a cooler area.
Why do objects get hotter or colder? Temperature is a measure of how hot an object is. Heat transfer only takes place when there is a temperature difference. The heat energy flows from a warmer area to a cooler area. Why does an ice lolly melt on a warm tongue? There is a temperature difference between the tongue and the lolly, so heat energy flows from the warm tongue into the cold ice lolly. This heat transfer means that the ice lolly melts as it gets warmer, and the warm part of the tongue touching it gets cooler. How might climate change cause the polar ice caps to melt?
What is conduction? Particles that are very close together can transfer heat energy as they vibrate. This type of heat transfer is called conduction. Conduction is the method of heat transfer in solids but not liquids and gases. Why? What type of solids are the best conductors? How are the particles arranged in a solid, a liquid and a gas? gas solid liquid
heat How do metals conduct heat? Metals are good conductors of heat. The outer electrons of metal atoms are not attached to any particular atom. They are free to move between the atoms. When a metal is heated, the free electrons gain kinetic energy. This means that the free electrons move faster and transfer the energy through the metal. This makes heat transfer in metals very efficient. Insulators do not have free electrons and so they do not conduct heat as well as metals.
Warmer regions of a fluid are less dense than cooler regions of the same fluid. The warmer regions will rise because they are less dense. The cooler regions will sink as they are more dense. This is how heat transfer takes place in fluids and is called convection. The steady flow between the warm and cool sections of a fluid, such as air or water, is called a convection current. What is convection?
What is payback time? cost of insulation saving each year payback time (in years) = payback time = 25 50 Payback time is the time it takes for the cost of installing insulation to be equalled by the savings made from reduced energy costs. Example: Adding silver reflectors behind radiators costs £25 and saves £50 per year. = 0.5 years (6 months)
Calculating payback time What is the payback time for these types of insulation? How heat escapes Cost of heat escaping per year Cost of insulation Payback time 3 years roof windows draughts walls £80 £40 £50 £100 £240 £3,200 £50 £500 80 years 1 year 5 years Why is double glazing popular if the payback time is so long?
Energy transfer in a television An energy transfer diagram shows the input and output energies for a device. This includes all the useful and wasted forms of energy. For example, in a television: light electrical sound heat
Energy cannot be created or destroyed, just changed in form. Law of conservation of energy Allenergytransfersfollowthelawofconservationofenergy: • This means that energy never just ‘disappears’. • The total amount of energy always stays the same, i.e. total input energy = total output energy. • In most energy transfers, the energy is transferred to several different forms, which may or may not be useful. • Energy that is transferred to unwanted forms of energy is wasted.
For example, the heat and light energy produced by this light bulb are too spread out to be reused. What happens to wasted energy? What happens to the wasted energy produced when energy is changed from one form to another? Remember that energy cannot be created or destroyed. Wasted energy spreads out into the surroundings – this is called dissipated energy. This dissipated energy is too spread out to do useful work and so cannot be reused.
useful output energy total input energy energy efficiency = How is energy efficiency calculated? The energy efficiency of a device can be calculated using this formula: • Useful energy is measured in joules (J). • Total energy is measured in joules (J). • Energy efficiency does not have any units. • It is a number between 0 and 1 which can be converted into a percentage by multiplying by 100.
Calculate the efficiency of a bulb This filament bulb is supplied with 100J of electrical energy, which it converts to 45J of light energy. • How much energy • is wasted? • In what form is the energy wasted? • What is the efficiency • of the bulb? Wasted energy = Total – Useful = 100J – 45J = 55J 55J Efficiency= Useful Total = 45 J 100 J = 0.45 or 45% heat 0.45 or 45%
What can be done to reduce the problems caused by burning fossil fuels? What waste do fossil fuels produce? Burning fossil fuels creates waste products that can act as pollutants and have harmful environmental effects. • Carbon dioxide – This greenhouse gas is the main waste product of burning fossil fuels. Increased levels of carbon dioxide due to human activities are thought to be connected with global warming. • Sulfur dioxide and nitric oxides – These gaseous pollutants contribute to the formation of smog and acid rain. • Ash– This waste solid is disposed of in landfill sites.
What is a solar power station? Solar power stations use the Sun’s energy to heat water and make steam, which then drives a turbine to produce electricity. Some solar power stations use a series of mirrors, called heliostats, to reflect light onto a boiler. This solar power station in California consists of about 1800 heliostats, with an electrical output of 10 megawatts. © Sandia National Laboratory/NREL
Why use solar cells to power satellites? Solar cells (or photocells) turn light energy from the Sun directly into direct current electricity. Manufacturing solar cells is very expensive and requires the use of highly toxic materials. However, once the solar cell is built it produces no pollution and requires little maintenance. Solar cells are ideal for use in remote locations where maintenance is difficult and other sources of electricity would be expensive. Satellites have been powered using solar cells since the 1950s.
What is biomass? Biomass, also known as biofuels or bioenergy, is material from living sources. The simplest biomass energy sources are plants, which can be burnt to produce steam to turn a turbine. Traditionally, wood is burnt to give heat but trees grow slowly and require a lot of land. Other materials, such as waste from poultry farms, can also be burnt. Biomass fuels are renewable as more plants can be grown, producing yet more biomass. The carbon dioxide released during burning biomass is absorbed by the replacement plants.Biomass is said to be ‘carbon neutral’ and does not add to global warming.
Sometimes, the hot water is trapped in natural underground reservoirs and does not reach the Earth’s surface. This water can be reached by drilling to depths of several kilometres. How can hot rocks generate electricity? A geothermal power plant can be built to utilize steam and hot water that rise naturally to the Earth’s surface. The steam and hot water provide a way of turning a generator, which then produces electricity. This is the largest producer of geothermal power in the world. It is in California and has an output of 750 MW of electricity. © David Parsons/NREL
However, some people consider this source of renewable energy to be noisy and an eyesore. There is also some concern that, if wind turbines are poorly located, they could kill migrating birds. How do wind turbines work? Wind turbines have large blades that capture the kinetic energy of the wind. This kinetic energy is used directly to turn the turbine and generate electricity. Wind turbines are a source of 'clean energy’ as they do not produce any polluting waste.
