Energy

• 4.2 describe energy transfers involving energy stores:

  • energy stores: chemical, kinetic, gravitational, elastic, thermal, magnetic, electrostatic, nuclear

  • energy transfers: mechanically, electrically, by heating, by radiation (light and sound)

  Chemical: Anything that can release energy by a chemical reaction (eg; food, fuel)

  Kinetic: Anything moving has energy in its kinetic energy store.

  Gravitational: Anything that has the potential to fall has GPE.

  Elastic: Anything stretched (eg; springs, rubber bands)

  Thermal: Any object has thermal energy- the hotter it is, the more energy

  Magnetic: Energy stored when two magnets attract or repel each other.

  Electrostatic: Energy stored when charges repel or attract each other

  Nuclear: The energy stored within the nucleus of atoms.
• energy transfers

  Mechanically: A force moves an object through a distance

  Electrically: Charges move due to a potential difference

  By heating: Energy transferred from a hotter object to a colder object

  By radiation (light and sound): Energy is transferred as a wave
• 4.3 use the principle of conservation of energy


  The Principle of conservation of energy states that Energy is not created or destroyed in any process. It is just transferred from one store to another.
• 4.4 know and use the relationship between efficiency, useful energy output and total energy output:


  Efficiency = (useful energy output / total energy output) x 100

  
  

  Energy is only useful when it is transferred from one store to a useful store. The less energy that is wasted by normally heat or sound energy, the more efficient the device is.
• 4.5 describe a variety of everyday and scientific devices and situations, explaining the transfer of the input energy in terms of the above relationship, including their representation by Sankey diagrams
• Sankey diagram


  Sankey diagrams make it easy to see how much of the input energy is being usefully employed compared with how much is being wasted. The thicker the arrow, the more energy it represents. The energy flow is shown by arrows who's width is proportional to the amount of energy involved.

  
• 4.6, 4.9 describe how thermal energy transfer may take place by conduction

  Conduction:

  Thermal conduction is the transfer of thermal energy through a substance by the vibration of the atoms within the substance. It is the main form of energy transfer by heating in solids.

  
  

  To demonstrate conduction:

  1. Attach beads at regular intervals (every 5cm) to one half of a long metal bar (eg; copper).

  2. Hold the metal bar in a clamp stand, and, using a Bunsen burner, heat the side of the bar with no beads attached from the very end.

  3. As time goes on, energy is transferred along the bar by conduction and the temperature increases along the rod.

  4. The wax holding the beads will melt and the beads will fall as the temperature increases, starting with the beard closest to the point of heating.
• 4.6, 4.7, 4.9 convection

  Convection is the transfer of thermal energy through fluids by the upward movement of warmer, less dense regions of fluid.

  
  

  Immersion heaters in convector heaters work by:

  1. Energy is transferred from the heater coils to the thermal energy store of the water by conduction

  2. The particles near the coils get more energy so they start moving around faster, becoming less dense

  3. The reduction in density means that hotter water rises above the denser, cooler water

  4. As the hot water rises it displaces the colder water out of the way, making it sink towards the heater coils

  5. This cold water is then heated by the coils and rises and the process is repeated.

  
  

  To demonstrate convection:

  1. Place some purple potassium permanganate crystals in a beaker of cold water. Aim to put the crystals to one side of the beaker

  2. Using a bunsen burner, gently heat the side of the beaker with the crystals at the bottom

  3. As the temperature of the water around the potassium permanganate crystals increases, they begin to dissolve, arming a bright purple solution.

  4. This purple solution is carried through the water by convection, and so traces out the pathos the convection currents in the beaker
• 4.6 , 4.8, 4.9 Radiation


  Radiation is the transfer of energy by infrared waves. All objects are continually emitting and absorbing infrared radiation. An object that's hotter than its surroundings emits more radiation than it absorbs. An object that is cooler than its surroundings absorbs more radiation than it emits.

  
  

  To investigate thermal radiation:

  1. Place an empty Leslie cube on a heat-proof mat

  2. Boil water in a kettle and fill the Leslie cube with boiling water

  3. Wait a while for the cube to warm up, then hold a thermometer against each of the four vertical faces of the cube. They should all be the same temperature

  4. Hold an infrared detector 10cm away from one of the cube's vertical faces, and record the amount of IR radiation it detects

  5. Repeat this measurement for each of the vertical faces

  6. You will detect more infrared radiation from the black surface than the white one, and more from the matt surfaces than the shiny ones

  
  

  At a low temperature, most of the radiated energy is in the form of infrared waves. As the temperature of a metal object increases, it radiates in the visible spectrum as well. Things that do not burn will start to glow a dull red
• 4.10 explain ways of reducing unwanted energy transfer, such as insulation


  Insulation:

  Conduction is the main way thermal energy is transferred between the inside of a building and the outside, and occurs usually by the walls, windows and roof. To reduce heat loss you can:

  - use building materials that are good insulators but are also durable (brick)

  - Use layers of different materials. E.g. outer and inner layer of bricks, separated by an air cavity or gap.

  
  

  Then, to reduce convection currents within the air cavity, insulating panels made of glass fibre matting can be used to fill the gaps. The panels are usually surfaced with thin aluminium foil. The highly reflective surface reflects heat in the form of infrared radiation.

  
  

  Double-glazed windows are also used which trap a layer of air. Modern double glazing uses special glass to increase the greenhouse effect (heat radiation from the Sun can get in but radiation from inside the house is mainly reflected back again),

  
  

  Other:

  Thermostats may be used and the reduction of draughts from poorly fitting doors.

  
  

  In humans:

  When people are rescued from mountains suffering from the effects of cold they are usually wrapped in thin, highly reflective blankets. The interior reflective surface reflects heat back to their bodies while the outer reflective surface is a poor radiator of heat.

  
  

  In animals:

  Animals fluff up their feathers on cold days which increases the thickness of the trapped air layer around their bodies, reducing heat loss by conduction.

  
• 4.11 know and use the relationship between work done, force and distance moved in the direction of the force:


  Work done = force x distance moved in the direction of the force
• 4.12 know that work done is equal to energy transferred


  Work done = energy transferred
• 4.13 know and use the relationship between gravitational potential energy, mass, gravitational field strength and height:

  gravitational potential energy = mass × gravitational field strength × height GPE = m × g × h


  Gravitational potential energy = mass x gravitational field strength x height
• 4.14 know and use the relationship: kinetic energy = 12 × mass × speed2


  Kinetic energy = 1/2 x mass x speed^2
• 4.15 understand how conservation of energy produces a link between gravitational potential energy, kinetic energy and work


  he conservation of energy produces a link between gravitational potential energy, kinetic energy and work:

  
  

  Lifting an object in a gravitational field requires work. This causes a transfer of energy to the gravitational potential energy store of the raised object. The higher the object is lifted, the more energy is transferred to this store.

  The amount of energy in a g.p.e store depends on the object's mass, it's height and the strength of the gravitational field the object is in.

  
  

  When something falls, energy from its gravitational potential energy store is transferred to its kinetic energy store.

  For a falling object when there's no air resistance: Energy lost from the GPE store = energy gained in the kinetic energy store

  In real life, air resistance acts against all falling objects- it causes some energy to be transferred to other energy stores
• 4.16 describe power as the rate of transfer of energy or the rate of doing work


  Power is the rate of transfer of energy and the rate of doing work.
• 4.17 use the relationship between power, work done (energy transferred) and time taken:


  Power = work done (energy transferred) / time taken
• 4.18P, 4.19P describe the energy transfers involved in generating electricity using:

  • wind


  Wind turbines use energy from the kinetic energy store of moving air to generate electricity. Wind turns the blades, which turn a generator inside it.

  
  

  - Wind turbines are renewable and produces no greenhouse gases

  - But can only be harvested in regions where the wind blows with enough energy

  - Spoils the view, contributes to noise pollution, unreliable
• water


  Wave energy:

  As waves come into the shore they provide an up and down motion which can be used to drive a generator. Energy is transferred from the kinetic energy store of the waves to the kinetic energy store of the turbine, and used to generate electricity through a generator.

  
  

  - Renewable, produces no greenhouse gases

  - Spoils the view and is a hazard to boats

  - Semi unreliable as waves tend to die out when the wind drops

  
  

  Tidal power:

  As the tide comes in, the estuary is filled to a height of several metres. This water can then be allowed out through turbines at a controlled speed. The energy is transferred from the kinetic energy stores of the water to the kinetic energy store of the turbine, and used to generate electricity.

  
  

  - Renewable energy, no greenhouse gases

  - Prevents free access by boats, alters the habitat of the wildlife, not available continuously

  
  

  Hydroelectric power:

  Hydroelectric power often requires the flooding of a valley by building a big dam. Rainwater is caught and allowed out through turbines, transferring energy from the gravitational potential energy store of the water to kinetic energy stores as it falls, which is used to generate electricity.

  
  

  - Renewable, no greenhouse gases, can respond very quickly to changes in the demand for electricity

  - Loss of habitat due to the flooding of a valley, spoils the landscape, expensive to create
• Geothermal resources


  Water is pumped in pipes down to the hot rocks and forced back up due to pressure to turn a turbine which drives a generator. The energy is transferred from thermal energy stores to kinetic energy stores and used to generate electricity. In some places, geothermal energy is used to heat buildings directly.

  
  

  - Only possible in certain places where hot rocks lie quite near to the surface.

  - Free, renewable energy

  - Expensive to drill down and to build a power plant
• Solar heating


  Solar water heating panels are black water pipes inside a glass box. The glass lets energy from the Sun in, which is then absorbed by the black pipes and heats up the water.

  
  

  - Renewable and free after setting up

  - Only for small-scale energy production

  
  

  Curved mirrors are used to focus thermal radiation onto a boiler or pipes containing water to produce steam. The mirrors are controlled to select the Sun's heat onto the central tower throughout the day. The steam can be sued to drive turbines which be used to drive electricity generators.
• Solar cells


  Solar cells (photocells) use energy from the sun to directly generate electricity.

  
  

  - They are renewable and don't produce greenhouse gases

  - They need enough sunlight

  -Provides small scale electricity
• Fossil fuels


  The three fossil fuels are coal, oil, and natural gas. Coal produces the most CO2, and natural gas produces the least.

  
  

  1. As the fossil fuel burns (in oxygen) the energy in its chemical energy store is transferred to the thermal energy store of the water by heating

  2. The water boils to form steam, which turns a turbine, transferring energy mechanically to the kinetic energy store of the turbine.

  3. As the turbine revolves, so does the generator , which produces an electric current. The generator transfers the energy electrically away from the power station, via the national grid

  
  

  Disadvantages:

  - Fossil fuels release carbon dioxide into the atmosphere when burned in power stations. This CO2 contributes to global warming and climate change.

  - Burning coal and oil also releases sulphur dioxide into the atmosphere, which combines with water to form acid rain. This can damage plants, people, and buildings.

  - Fossil fuels are non- renewable.

  
  

  Advantages:

  - Burning fossil fuels releases a lot of energy, relatively cheaply

  - It is a reliable energy source
• Nuclear power


  Nuclear reactors use uranium to produce energy. The fission o uranium produces the heat to drive turbines. During the process, energy is transferred from nuclear energy stores to thermal energy stores by heating, then mechanically to kinetic energy stores, and finally transferred electrically through the national grid.

  
  

  - Non-Renewable, possibility of radioactive waste

  - Expensive to build and maintain

  - No greenhouse gases, produces a lot of energy