P2.3 - transfer of thermal energy Flashcards
Thermal conductors vs insulators
Thermal conductors are materials that can transfer energy/heat quickly. E.g. iron, gold, graphite, steel, etc
Thermal insulators are materials used to reduce energy transfer. E.g air, glass, plastic, etc
What is conduction?
Unit for conductivity is w/m^2
When part of a solid material is heated, the atoms begin to vibrate even more rapidly around their fixed positions. The increase in vibration affects nearby atoms - the begin to vibrate more so the temperature in that area also increases. The process gradually leads to increased vibrations and temperatures throughout the material.
Conduction in metals vs non-metals
In non-metals, this is called lattice vibration, and it is fairly slow as the vibrations need to be passed along by interactions between each atom in turn. This is why non-metals are poor conductors.
Metals have a much faster process of conduction due to their delocalized electrons. These electrons are not fixed in place, so when a part of the metal is heated, the electrons in this area quickly spread out along the metal. As they travel, they collide with the atoms and cause increased vibration, raising the temperature.
Define and explain convection
When a fluid is heated, it becomes less dense than the fluid around it. Less dense materials float on top of more dense materials, so the warmer fluid floats upwards towards the surface. At the surface, the fluid cools down and becomes more dense, causing it to sink down. This causes a circulation of the fluid called a convection current - fluid rising up, spreading across the surface and sinking back down again.
Responsible for air currents.
Why are warmer fluids less dense?
Due to thermal expansion - particles in a fluid will move about quicker and take up a greater volume when the fluid is heated, which causes its density to decrease.
Define and explain infrared radiation
The emission of electromagnetic waves to transfer heat from hot objects
Hot objects emit infrared radiation, which is part of the electromagnetic spectrum. When an object emits this radiation, it cools down - the particles inside it slow slightly, so the temperature falls as the internal energy decreases. Absorbing radiation can warm objects up. Very hot objects will additionally emit visible light - this is how a light bulb works.
What happens to objects at room temperature in terms of radiation?
Objects at room temperature emit infrared radiation but they also absorb the sane amount, so their temperature stays the same because there is no overall change in their internal energy.
How does the Earth receive sunlight from the sun?
It receives sunlight because infrared radiation can travel through a vacuum. We see the visible light as well as feel the heating effect of the infrared radiation. This shows that radiation can transfer energy without the need for a medium.
Effect of surface color on radiation
Black surfaces are good absorbers and emitters of radiation. It cools and heats the fastest
Silver surfaces are poor absorbers; they reflect it instead. Cools down and heats up the slowest
White is in the middle
How does emission and absorption of radiation affect temperature?
If it emits more than it absorbs, it will cool
If it absorbs more than it emits it will warm up
If it emits and absorbs the same amount it will stay at a constant temperature
How does surface texture affect radiation?
Shiny surfaces are very smooth, which increases reflectiveness, meaning that they are not good at absorbing or emitting radiation
Rough and dull surfaces do both faster
Describe and explain 3 different experiments about radiation
- Demonstrating radiation - boiling water is poured into a Leslie’s cube which has different colored faces (black, white and silver). An infrared thermometer is held next to each of the faces to attempt to measure their ‘temperature’. It is not the actual temperature of the surface being measured but the infrared radiation emitted
- Demonstrating absorption of radiation - an electrical heater is placed between a black and silver metal plate. Pins are attached to both surfaces using petroleum jelly. The surface which heats up the fastest will have the pin fall first. CVs: both surfaces should be equal distance from infrared radiation; pins must be placed at equal heights
- Cooling by radiation (emission) - identically shaped containers (sa is control variable as larger sa allows it to cool faster) of different colors are placed on a heat resistant surface to decrease heat loss through conduction. Boiling water is poured into all the containers, and the initial temperature is recorded. Temperature is recorded every 30 seconds for 5 minutes.
Why does the rate of cooling decrease over time?
The amount of radiation emitted depends on the temperature difference
Explain how a vacuum flask prevents the transfer of thermal energy
- Stops a hot liquid from cooling or a cold liquid from heating. Its outer case is plastic/glass to limit conduction and convection from external environment, providing insulation;
- There is a vacuum layer between the inner and out walls, which prevents conduction and convection since both need particles to transfer energy;
- Inner wall has a silver, reflective surface (also glass) reduces emission or absorption of infrared radiation;
4, The stopper (lid) is filled with air and plastic foam, which reduces conduction.
How is energy loss minimized in buildings?
- Conduction is reduced by making the walls out of insulators such as brick and wood. They can also be made thicker, and to reduce conduction in the floor, thick carpets can be used to reduce it. Double glazing windows and filling the space between the two panes with low-pressure glass (since glass is very thin) can also held reduce heat loss through this transfer.
- Convection can be reduced by designing the top part of the building to restrict air flow (and therefore convection currents. Lofts are also insulated with materials like fiber glass.
- Energy loss by radiation can be minimized by painting the outside surface of the building with light colors (reduces the rate at which building heats up).
State and explain 2 examples where thermal energy transfer happens in everyday objects
- Kitchen pans - metals such as aluminium are used in kitchen pans to heat food because they are good thermal conductor. The handles are often made from insulators like wood to prevent burns.
- Heating a room - radiators heat the air close to them, which makes it rise as it becomes less dense when expanding. Colder air moves in to take it place and this air is then heated. This creates a convection current which transfers thermal energy throughout the room.
