Physics - Energy Stores & Transfers (4th Form) Section 4
Stores of energy
Kinetic
Potential gravity
Chemical
Elastic
Thermal
Magnectic
Electrostatic
Nuclear
4 ways energy can be TRANSFERRED
Mechanically
Electrically
By heating
By radiation
Energy can be transferred mechanically through the movement of the parts in machines, and when the motion or position of an object changes. Sound waves and seismic waves (formed during earthquakes) are mechanical waves that transfer energy through materials and from place to place
Energy is transferred when an electrical circuit is complete. A simple circuit may consist of a battery, lamp and wires. Internal energy stored in the battery is transferred to moving charged particles in the wire. Some objects are hotter than others.
Some objects are hotter than others. Energy is transferred from the hotter object to the cooler one, and the difference in temperature between them decreases
Visible light, infrared light, microwaves and radio waves are forms of radiation. They are carried by waves (although unlike sound, these are not mechanical waves and can travel through empty space). Electric lamps and burning fuels transfer visible and infrared light to the surroundings.
IMPORTANT -Conservation of Energy
Energy can't be created or destroyed but can be transferred from one store to another.
Sankey Diagram
Sankey Diagram for TV
Sankey Diagram for Kettle
Conduction
The process by which heat or electricity is directly transmitted through a substance when there is a difference of temperature or of electrical potential between adjoining regions, without movement of the material.
Convection
The movement caused within a fluid by the tendency of hotter and therefore less dense material to rise, and colder, denser material to sink under the influence of gravity, which consequently results in transfer of heat:
Radiation
Radiation is the transfer of energy through waves (electromagnetic radiation) or fast travelling particles. Radiation can be in the form of heat, sound and light. Radiation can be felt or seen like light or detected through special instruments like X-ray. Radiation from a hot object is shorter and more intense than radiation from a cooler object.
Reducing Heat Loss
Silver foil
Vacuum
Trapped Air
Examples are loft insulation, hot water tank and pipe insulation,
double glazing and cavity wall insulation
How does cavity wall insulation work?
Many houses are built with cavity walls. There is an outside wall and an inside wall with an air gap between the two walls. This is a good way of preventing the inside wall of the house from becoming damp but the air gap can transfer heat by convection making the house colder.
Cavity wall insulation fills the air gap with a polymer foam. The foam is a type of plastic material that has air trapped inside it. The foam replaces the air in the gap between the bricks.
No heat can now be transferred by convection because the air has been replaced by foam. Conduction is very poor because both the polymer and the trapped air are good insulators. The room in the house stays warmer for longer.
Heat is conducted by air movement. As the hot air rises then the cold air above circulates to take it's place. Therefore the heat in a room with air heats up and circulates and then the hot air gives up it's heat to the colder surfaces of the windows etc so it is cooled and it then drops to take the place of the the now rising hot air. The same thing cannot happen in a vacuum as there is no air to heat up and therefore there is no circulation. This is why a double glazed window with a vacuum between surfaces would work better that a sealed unit of glass with a small amount of air trapped inside.
Wrapping an object in aluminum foil reduces the energy loss through infra-red radiation. Foil reflects radiatant energy in the same way that a mirror reflects light. This is very effective in keeping food hot after it has been cooked.