Physics Revision http://www.bbc.co.uk/schools/gcsebitesize/science/aqa/
Which will melt the ice cube fastest? Why? Metal block Wood block
Which will melt the ice cube fastest? Why? The Metal block! Why? Metal block Wood block
Energy transfers • Conduction: heat energy transfer through solids e.g. metal rod (particles) • Convection: heat energy transfer through fluids e.g. liquids and gases (particles) • Radiation: heat energy transfer by waves through a vacuum and surface material
More energy = more vibrations! Kinetic theory • The kinetic particle theory explains the properties of the different states of matter. The particles in solids, liquids and gases have different amounts of energy. They are also arranged differently and move in different ways. Which state of matter has the most energy?
Just for metals… • The electrons in piece of metal can leave their atoms and move about in the metal as free electrons. • The parts of the metal atoms left behind are now charged metal ions. • The ions are packed closely together and they vibrate continually. • The hotter the metal, the more kinetic energy these vibrations have. • This kinetic energy is transferred from hot parts of the metal to cooler parts by the free electrons. • These move through the structure of the metal, colliding with ions as they go.
Heat vs temperature When materials change state (solid liquid) the temperature stays the same until all of the material has melted • Temperature and heat are not the same thing because: • Temperature is a measure of how hot something is • Heat is a measure of the thermal energy contained in an object. • Temperature is measured in oC and heat is measured in J. • Freezing point of water: 0oC
How does a hot air balloon work? • A burner at the base of the balloon warms the air inside giving the air molecules more kinetic energy. • As the air warms up, it moves upwards as it becomes less dense, then cools once away from the heat source, becomes more dense and moves round in a circular pattern known as a convection current. • When the balloon is full of hot air, it lifts off the ground because the hot air inside it is less dense, lighter and has more kinetic energy than the cold air outside it.
Infrared radiation • All objects emit (give out) and absorb (take in) thermal radiation, which is also called infrared radiation. The hotter an object is, the more infrared radiation it emits. • Infrared radiation is a type of electromagnetic radiation, which involves waves rather than particles. • This means that, unlike conduction and convection, radiation can even pass through the vacuum of space. This is why we can still feel the heat of the Sun, although it is 150 million km away from the Earth. Vacuum: Empty space which has NO particles!
worst emitter best emitter silver matt black white worst absorber best absorber Absorbing thermal radiation Certain surfaces are better at absorbing thermal radiation than others. Good emitters are also good absorbers. • Matt black surfaces are the best absorbers and emitters of radiation. • Shiny surfaces are the worst absorbers and emitters because they reflect most of the radiation.
U Values • U-values measure the effectiveness of a material as an insulator in buildings. • The lower the U-value is, the better the material is as a heat insulator. • For example, here are some typical U-values for building materials: • a cavity wall has a U-value of 1.6 W/m² • a solid brick wall has a U-value of 2.0 W/m² • a double glazed window has a U-value of 2.8 W/m².
Evaporation and condensation Evaporation and condensation are changes of state: • Evaporation involves a liquid changing to a gas • Condensation involves a gas changing to a liquid • Evaporation is the reason why damp clothes dry on a washing line. • Condensation is the reason why windows become foggy on a cold day.
Evaporation If a …………….. is at the……………., and moving ………. enough, it may ……… the liquid. This is called ………….. Evaporation Condensation Surface Slow Fast Molecule Escape Trap Freedom!
Evaporation If a molecule is at the surface, and moving fast enough, it may escape the liquid. This is called evaporation. Freedom!
4 factors that affect evaporation rate • Temperature • Surface area • Humidity • Air flow over the liquid surface Which ones also affect condensation? 2, 4
Questions: • Why are warm windy days the best days to hang out your washing? Why should washing be stretched out on the washing line? • You have spilled some water on your kitchen floor and you do not have any mops, clothes or similar material to soak up the water. What can you do to increase the rate of evaporation? • In the tropical rainforest it is difficult to get washing to dry (even if it is not raining!). Explain why.
Keeping warm or cold • The bigger the difference in temperature between an object and its surroundings, the greater the rate at which heat energy is transferred. • Other factors that affect the rate at which an object transfers energy by heatinginclude: • Surface area of object • Volume of the object • Material used to make the object • Nature of the surface that the object is touching
Animal Issues • Small animals like mice have a large surface area compared to their volume. They lose heat to their surroundings very quickly and must eat a lot of food to replace the energy lost. • Elephants have large ears with a large surface area compared to their volume. They lose heat to their surroundings more slowly and may even have difficulty avoiding overheating. Their ears allow heat to be transferred from the elephant to its surroundings, helping to keep the animal cool.
Human Issues • Engineers design heat transfer devices so that they gain or lose heat energy efficiently. For example, car radiators are flat, with many small fins to provide a large surface area. • Similarly, household radiators are thin and flat, and may have fins so that heat energy is transferred to the room quickly.
Types of energy and energy changes • Energy cannot be created or destroyed, it can only be changed from 1 form to another. • Nuclear • Chemical • Elastic • Gravitational • Kinetic • Thermal • Light • Sound • Electrical Wasted energy: energy transformed that isn’t required e.g. light bulbs Electrical light Electrical heat
Efficiency = Useful energy output x 100% Total energy input Efficiency Calculations
SPECIFIC HEAT CAPACITY At the end of a sunny day at the beach, you often notice that while the sand has become quite hot, the water has stayed cool.
SAME amount of HEAT ENERGY Small TEMPERATURE RISE Large TEMPERATURE RISE WATER SAND Putting the SAME AMOUNT OF HEAT into some materials gives a BIGGER TEMPERATURE RISE than in other materials
Specific Heat Capacity • The amount of energy needed to change the temperature of 1 kg of the substance by 1°C • Water has a particularly high specific heat capacity. This makes water useful for storing heat energy, and for transporting it around the home using central heating pipes.
An equation… Energy = mass x specific heat capacity x temperature change E = mc∆Ө Energy (E) Joules Mass (m) kilograms Temperature change (∆Ө) oC Specific heat capacity (c) J/kgoC Q: Calculate the energy needed to heat 1.5kg water from 20oC to 60oC. The specific heat capacity of water is 4200J/kgoC.
Specific latent heat • Fusion: The heat needed to change a mass of 1 kg the substance from a solid at its melting point into liquid at the same temperature. • Vaporisation: The heat needed to change the substance from a liquid at its boiling point into vapour at the same temperature. • An equation: • Energy (J) = Mass (kg) × Specific latent heat (J/kg) • E = ml
Energy calculations • The amount of electrical energy transferred to an appliance depends on its power and the length of time it is switched on. The amount of mains electrical energy transferred is measured in kilowatt-hours, kWh. One unit is 1 kWh. E = P × t E is the energy transferred in kilowatt-hours, kWh P is the power in kilowatts, kW T is the time in hours, h.
Different Units! Q: Change 2000W into kW and 7200s into hours… • On the last slide Power was measured in kilowatts instead of the more usual watts. • To convert from W to kW you must divide by 1,000. e.g. 2,000 W = 2,000 ÷ 1,000 = 2 kW. • Also time was measured in hours, instead of the more usual seconds. • To convert from seconds to hours you must divide by 3,600. e.g. 7,200 s = 7,200 ÷ 3,600 = 2 h.
Q: Double-glazing might cost £2,500 and save £100 a year. What is the payback time? Payback time = Initial cost Annual saving We can calculate the amount of electrical energy transferred by an appliance and how much it costs to run. This is useful for comparing the advantages and disadvantages of using different electrical appliances for a particular purpose and to save money by comparing costs. Total cost = number of units × cost per unit e.g. If 5 units of electricity are used at a cost of 8p per unit, the total cost will be 5 × 8 = 40p.
Energy Resources • Electricity is a very convenient form of energy that can be generated using different energy resources. Some of these resources are renewable and some are non-renewable. Each resource has advantages and disadvantages. The fossil fuels are coal, oil and natural gas. They were formed from the remains of living organisms millions of years ago and they release heat energy when they are burned. They are non-renewable.
Generating electricity Carbon capture • Carbon capture and storage is a way to prevent carbon dioxide building up in the atmosphere. It is a rapidly evolving technology that involves separating carbon dioxide from waste gases. The carbon dioxide is then stored underground, for example in old oil fields or gas fields such as those found under the North Sea.
Nuclear fuels • The main nuclear fuels are uranium and plutonium. These are radioactive metals. Nuclear fuels are not burnt to release energy. Instead, nuclear fission reactions (where the nuclei in atoms are split) in the fuels release heat energy. • The rest of the process of generating electricity is then identical to the process using fossil fuels. The heat energy is used to boil water. The kinetic energy in the expanding steam spins turbines, which then drive generators to produce electricity.
Power Stations • Power stations fuelled by fossil fuels or nuclear fuels are reliable sources of energy, meaning they can provide power whenever it is needed. However, their start-up times vary according to the type of fuel used. • Nuclear power stations and coal-fired power stations usually provide 'base load' electricity - they are run all the time because they take the longest time to start up. Oil-fired and gas-fired power stations are often used to provide extra electricity at peak times, because they take the least time to start up. • The fuel for nuclear power stations is relatively cheap, but the power stations themselves are expensive to build. It is also very expensive to dismantle old nuclear power stations and to store their radioactive waste, which is a dangerous health hazard.
National Grid and Transformers Power stations produce electricity at 25,000 V. Step-up transformers change the voltage to the very values needed to transmit electricity through the National Grid power lines. Electricity is sent through these at 400,000 V, 275,000 V or 132,000 V. This reduces energy losses during transmission but the voltages would be dangerous in homes. Step-down transformers are used locally to reduce the voltage to safe levels. The voltage of household electricity is about 230 V. • The National Grid • Electricity is distributed from power stations to consumers through the National Grid, which allows distant power stations to be used. It also allows a mix of different energy resources to be used efficiently to supply the country’s electricity, whatever the local demand. • Transformers • Electricity is transferred from power stations to consumers through the wires and cables of the National Grid. When a current flows through a wire some energy is lost as heat. The higher the current, the more heat is lost. To reduce these losses, the National Grid transmits electricity at a low current. This needs a high voltage. • Transformers are used in the National Grid. A transformer is an electrical device that changes the voltage of an alternating current (ac) supply, such as the mains electrical supply. A transformer that: • increases the voltage is called a step-up transformer • decreases the voltage is called a step-down transformer.
Electricity from a power station goes to: 1. Step-up transformers 2. High voltage transmission lines 3. Step-down transformers 4. Consumers, for example homes, factories and shops.
What are waves? • Waves are vibrations that transfer energy from place to place without matter (solid, liquid or gas) being transferred. Think of a Mexican wave in a football crowd: the wave moves around the stadium, while each spectator stays in their seat only moving up then down when it's their turn. • Some waves must travel through a substance. The substance is known as the medium and it can be solid, liquid or gas. E.g. sound • Other waves do not need to travel through a substance They may be able to travel through a medium, but they do not have to. Visible light, infrared rays, microwaves and other types of electromagnetic radiation are like this. They can travel through empty space. Electrical and magnetic fields vibrate as the waves travel.
Transverse and longitudinal • Light and other types of electromagnetic radiation are transverse waves. • In transverse waves the oscillations (vibrations) are at right angles to the direction of travel and energy transfer. • In longitudinal waves, the oscillations are along the same direction as the direction of travel and energy transfer.
Amplitude: Maximum disturbance from its undisturbed position. • Frequency:The number of waves produced by a source each second. It is also the number of waves that pass a certain point each second. • Time period: Time taken for one wave to pass. • Wavelength: Distance between a point on one wave and the same point on the next wave.
Sound Waves When an object or substance vibrates, it produces sound: • the greater the amplitude, the louder the sound • the greater the frequency, the higher the pitch. Sounds 1 and 2: the sound waves have the same frequency, so the sounds have the same pitch sound 2 has a greater amplitude than sound 1, so sound 2 is louder. Sounds 2 and 3: the sound waves have the same amplitude, so the sounds have the same loudness sound 3 has a greater frequency than sound 2, so sound 3 is higher pitched.
Calculating wave speed v = f × λ • v is the wave speed in metres per second, m/s • f is the frequency in hertz, Hz • λ (lambda) is the wavelength in metres, m.
Refraction • Sound waves and light waves change speed when they pass across the boundary between two substances with different densities, such as air and glass. This causes them to change direction and this effect is called refraction. • There is one special case you need to know. Refraction doesn't happen if the waves cross the boundary at an angle of 90° (called the normal) - in that case they carry straight on.
Diffraction • When waves meet a gap in a barrier, they carry on through the gap. However, the waves spread out to some extent into the area beyond the gap. This is called diffraction. • The extent of the spreading depends on how the width of the gap compares to the wavelength of the waves. Significant diffraction only happens when the wavelength is of the same order of magnitude as the gap. For example: • a gap similar to the wavelength causes a lot of spreading with no sharp shadow, eg sound through a doorway • a gap much larger than the wavelength causes little spreading and a sharp shadow, eg light through a doorway.