Physics - Making use of energy Flashcards
What is conduction?
The transfer of heat through a material by transferring kinetic energy from one particle to another.
What are 5 facts about conduction?
-Conduction occurs in solids.
-Metals are good conductors.
-Non-metals and gases are usually poor conductors.
-Poor conductors are called insulators.
-Heat energy is conducted from the hot end of an object to the cold end.
How does conduction happen in non-metals?
If one end of a solid is heated, the particles of the solid gain kinetic energy causing them to vibrate more. The atoms in a solid are close together and therefore the energy from the increased vibrations is passed on to the next atom, resulting in increased vibration for each subsequent atom. In non-metals, the process of conduction is slow.
How does conduction happen in metals?
The electrons in a piece of metal can leave their atoms and move about in the metal as free (or de-localised) electrons. The parts of the metal atoms left behind are now positively 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.
What is convection?
The transfer of heat energy through a moving liquid or gas.
How does convection occur?
-Cold air gets heated up
-Particles speed up as they have more energy
-Particles move further apart / volume increases
-Warm air rises as it is less dense than cold air
-As it moves away from the heat air cools down
-Cold air sinks as it is more dense
-This then repeats setting up a convection current.
How does heat transfer by radiation?
Heat can be transferred by infrared radiation. Unlike conduction and convection - which need particles - infrared radiation is a type of electromagnetic radiation that involves waves. Because no particles are involved, radiation can even work through the vacuum of space. This is why we can feel the heat of the sun.
How do different surfaces deal with infrared radiation?
-Dull, matt or rough, dark coloured - Good absorption and emission.
-Shiny, light coloured - Poor absorption and emission.
How do you measure the density of an object with an irregular shape?
If the object is an irregular shape, we need to place the solid into a cylinder of water and record the increase in the volume in order to calculate the volume of the object.
How is heat energy lost in houses?
-Heat energy is transferred from homes by conduction through the walls, floor, roof and windows.
-It is also transferred from homes by convection. For example, cold air can enter the house through gaps in doors and windows, and convection currents can transfer heat energy in the loft to the roof tiles.
-Heat energy also leaves the house by radiation through the walls, roof and windows.
What are some simple ways to reduce heat loss?
Simple ways to reduce heat loss include fitting carpets, curtains and draught excluders. It is even possible to fit reflective foil in the walls or on them.
How can double glazing prevent heat loss?
These special windows have air or a vacuum between two panes of glass. If the double glazing has a vacuum there will be no conduction or convection. Convection is minimised because there is little room for the air to move. Air is a poor conductor so there will be very little heat loss by conduction.
What is cavity wall insulation?
This involves blowing insulating material into the gap between the brick and the inside wall.
How can cavity wall insulation prevent heat loss?
The insulating material contains pockets of trapped air and therefore reduces conduction and convection. The fibres of the insulation prevent the trapped air from circulating and causing a convection current to be set up. Cavity wall insulation often has a layer of silver foil in order to reduce the heat lost by infrared radiation, by reflecting it back into the building.
How can loft insulation prevent heat loss?
The insulating material contains pockets of trapped air and therefore reduces conduction and convection. The fibres of the insulation prevent the trapped air from circulating and causing a convection current to be set up.
