4.2 describe energy transfers involving the following forms of energy: thermal (heat), light, electrical, sound, kinetic, chemical, nuclear and potential (elastic and gravitational)
Thermal > Light = a very hot object
Thermal > Kinetic = steam engine
Light > Chemical = a tree
Electrical > Thermal = an electric fire
Electrical > Light = a light bulb
Electrical > Kinetic = an electric motor
Electrical > Sound = a loudspeaker
Sound > Thermal = a sound-absorbing cloth
Sound > Electrical = a microphone
Kinetic > Sound = hitting a drum
Kinetic > Thermal = friction
Kinetic > Electrical = a dynamo
Chemical > Light = a glow worm
Chemical > Thermal = a gas fire
Chemical > Electrical = a battery
Chemical > Kinetic = leg muscles
Chemical > Elastic = pulling a catapult
Nuclear > Light = an atom bomb
Nuclear > Kinetic = an atom bomb
Nuclear > Thermal = an atom bomb
Nuclear > Sound = an atom bomb
Elastic > Kinetic = releasing a catapult
Elastic > Gravitational = releasing a catapult
Gravitational > Kinetic = a falling object
4.3 understand that energy is conserved
When energy is transferred or transformed, none of it is lost. Some energy is always wasted, leaving in a different form (e.g. light bulbs heat up as well as lighting up)
4.4 know and use the relationship:
efficiency = useful energy output / total energy input
The relationship should be multiplied by 100 to give the percentage of efficiency so it can be compared to different devices.
4.5 describe a variety of everyday and scientific devices and situations, explaining the fate of the input energy in terms of the above relationship, including their representation by Sankey diagrams.
Most electrical devices lose energy as heat. For example in a light bulb, the useful energy is the light energy, and the waste is heat. A normal, inefficient light bulb would have a Sankey diagram that looked something like this:
The curved arrows are used to show waste energy, while the straight arrows show useful energy. You can see in this diagram that this light bulb is only 10% efficient, which is extremely wasteful. So, efficient, energy-saving light bulbs have been developed. Their Sankey diagrams look more like this:
You can see in this the straight arrow is much bigger than the curved arrow. This light bulb is 75% efficient, enormously better than the first bulb.
In drawing a Sankey diagram, it is important to keep the widths of the arrows proportional.
4.6 describe how energy transfer may take place by conduction, convection and radiation
Conduction is the transfer of energy through a substance without the substance itself moving. Metals are good conductors because of their close together ions and free electrons that can transfer energy. Gases are poor conductors because the particles are far apart and it takes longer for energy to travel through them. Heat is conducted more quickly if the conductor is shorter in length, bigger in cross-sectional area, and the temperature difference is greater.
Convection is when particles are heated, causing them to have more kinetic energy and thus move more and become less dense, causing the hot air to rise. As more hot air rises, it displaces the air that rose before it, forcing the air to move away, and as it does so, cool, condense, and sink. This is displaced by more sinking air, and forced back to the source of heat, where it will warm and rise again to repeat the cycle. This creates a convection current (convection only works in a fluid)
Radiation involves the emission of electromagnetic waves (like the
infrared waves in a toaster). All objects are continuously emitting and absorbing thermal radiation. If an object is hotter than its surroundings it will emit more, and if it is cooler it will absorb more; an icecube will melt and hot coffee will cool in the same temperature environment. This continues until the temperature is constant throughout all substances. This is the only form of heat transfer that doesn't involve particles and therefore can happen through a vacuum. Radiation is best both emitted and absorbed by matte black surfaces.
4.7 explain the role of convection in everyday phenomena
Convection is used in many everyday situations, e.g. heating a room with a radiator, in a kettle, boiling water on the stove, in insulating (by creating tiny pockets of air, each containing its own convection current, the energy is not easily transferred out), etc.
4.8 explain how insulation is used to reduce energy transfers from buildings and the human body.
In the home:
- Thick curtains: Trap a layer of air between windows and warm rooms, preventing hot air from reaching the glass through convection and conduction.
- Cavity wall insulation limits radiation through the walls, as well as conduction and convection.
- Double glazed windows have a layer of dry air between them that acts as an insulator between the cold, outside glass and the warm inside glass, limiting heat loss through conduction, convection and radiation.
- Carpets/rugs, especially with underlay, trap air and create a layer of insulation between the colder ground and the rest of the room
- Draught-proofing reduces heat loss through convection.
- Loft and roof insulation prevent heat loss through convection as the hot air rises.
Humans:
- Goosebumps in cold weather cause hairs to stand on end to trap air and provide a layer of insulation to the body
- More layers of clothing create layers of air between each item, which insulates the body.
- Fabric absorbs some of the radiation from the body, reducing heat loss.
