Transmission of heat Flashcards
What is heat?
Heat refers to the amount of thermal energy that is being transferred from a hotter to a colder region.
What is thermal energy?
Calculated in Joules, thermal energy is the energy stored in a body due to its temperature.
What is temperature?
Calculated in Kelvin [K], it refers to how hot or cold an object is.
What is contraction? (Mass, volume, density change)
A process where a substance decreases in volume after it is cooled.
Mass: Remains the same
Volume: Decreases
Density: Increases
What is expansion? (Mass, volume, density change)
A process where a substance increases in volume after heat is applied to it.
Mass: remains the same
Volume: increases
Density: decreases
What is the rate of expansion of materials?
Brass > iron and copper > steel
What are bimetallic strips used in?
Thermostats to maintain a constant temperature in an electrical appliance.
What is the transfer of thermal energy?
Heat flows from a region of higher temperature to a region of lower temperature until the temperature until the temperature at both regions are in thermal equilibrium (same temperature).
What is conduction?
Conduction is the transfer of thermal energy through a medium without any flow of the medium.
Conduction in solids
As the object gains heat from the heat source, the particles at the heated end gain thermal energy and vibrate more vigorously about their fixed position. They start to collide with neighbouring particles, making them vibrate more vigorously as well. This process continues until the particles at the cooler end of the object are also set into vigorous vibration and the cooler end of the object becomes hot.
What is convection?
Convection is the transfer of thermal energy by means of convection currents in a fluid, due to the difference in density.
Convection CANNOT take place in a solid
Convection current in a liquid
When the water at the bottom of the flask is heated, it expands. The expanded water is less dense than the surrounding water, causing it to rise. Comparatively the cooler upper region of the water is denser and sinks. The difference in the densities of water at the different regions sets up a convection current.
Convection current in a gas
The flow of smoke represents a convection current. When the air above the candle is heated, it expands. As the air above the candle is less dense than the surrounding air, it rises out of the right chimney. The cooler surrounding air, being denser, sinks into the left chimney to replace the air that has escaped. The difference in the densities of air at the different regions sets up a convection current.
Would it be more efficient to place an air-con unit on the ground or on the ceiling?
It would be more efficient to place it on the ceiling.
When the air at the top region near the air-conditioning unit is cooled, it contracts. The cooled air is denser than the surrounding air and sinks. Since the lower region of the air is warmer, it is less dense and rises. The difference in the densities of air at the different regions sets up a convection current within the room.
What is radiation?
Radiation is the transfer of thermal energy in the form of electromagnetic waves such as infrared radiation without the aid of a medium.
*radiation can take place in a vaccum, unlike convection and conduction.
