P2 - Heating Flashcards
What is heating?
A process of transferring energy to the thermal energy store of an object from a hotter object/place to a cooler object/place.
How is energy transferred in a solid?
By the process of conduction when particles at a higher temperature vibrate and collide with other particles.
What happens when a hot and cold object are brought into contact?
The temperature difference between 2 objects is reduced until both reach thermal equilibrium. How quickly the energy transfers depends on the temperature difference.
How do we test different rods as conductors?
We had 4 metal rods made of aluminium, copper, iron and brass, each with a marble on the end stuck by wax. We used a bunsen burner on the other end of the rods (which were the same length and width) and measured how long the marbles took to fall. Aluminium was the best conductor and iron was the worst.
What is thermal conductivity?
The ability of a material to conduct thermal energy. The greater the thermal conductivity the more energy per second it transfers by conduction.
What does the energy transfer per second through an insulator depend on?
- Temperature difference through material
- Thickness of material
- Thermal conductivity of material
RP2 - Testing different insulators. What was the method?
We wrapped a beaker with a material and then added 100ml hot water to the beaker. When the temperature of the water reached 75℃ we started a timer and measured the temperature at 2 minute intervals for 8 minutes, recording the temperature in a results table. We then repeated it with different materials.
What is infrared (IR)?
A type of electromagnetic radiation that we can’t see but that we can detect as an increase in temperature when it is absorbed by an object.
Do objects emit or absorb infrared radiation?
All objects, no matter their temperature, emit and absorb infrared radiation.
What is intensity (of light)?
The amount of light recieved per second.
How do thermal imaging cameras work?
They convert the intensity of infrared radiation that they detect into visible light colours.
What happens as an object gets hotter?
The intensity of the radiation it emits increases at all wavelengths, it also emits more radiation at shorter wavelengths.
The peak of the graph shifts to shorter wavelengths at a higher temperature.
What determines the rate a surface absorbs and emits radiation?
The colour and how shiny a surface is. A light shiny surface emits and absorbs IR radiation at a much lower rate than a dark matt surface.
Why do emergency services use silver blankets?
Shiny silver is a poor absorber of infrared radiation so reflects the IR emitted by the person straight back to the person who will absorb it and stay warm.
What is happening with infrared radiation if an object has a constant temperature?
The object must be emitting IR at the same rate it absorbs it.
What is a black body?
An object that absorbs all electromagnetic radiation that hits it.
* A black body does not reflect or transmit any electromagnetic radiation (EM).
* A black body is also the best possible emitter of radiation, the radiation emitted by a black body is called black body radiation.
What is specific heat capacity and how is it calculated?
The specific heat capacity (c) of a substance is the energy needed to raise the temperature of 1kg of the substance by 1°C.
specific heat capacity = energy transferred / (mass x temperature change)
c = ΔE / mΔθ
Will an object with a lower or higher specific heat capacity experience a greater temperature change for the same mass and energy transfer?
The object with the lower specific heat capacity will experience a greater temperature change for the same mass and energy transfer.
RP1 - specific heat capacity of an aluminium block. What are the sources of error?
- parallax error (reading thermometer wrong) - read at eye level
- systematic error - ensure ammeter and voltmeter set to 0 at beginning
- random error (insulation not consistent) - use more consistent insulation around the block
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RP1 - specific heat capacity of an aluminium block. What did we do?
We placed insulation around the 1kg aluminium block and placed a heater and thermometer (to measure temperature change) in holes in the block. To measure the energy transfer we had to use a circuit as we did not have a joulemeter. In the circuit we had a battery, an ammeter and a voltmeter and used the equation ΔE = V x I x t to calculate the change in energy and then the specific heat capacity
What does the cavity in a cavity wall construction do?
Reduces heating of the surroundings by conduction as the air particles are spaced apart.
If this cavity is filled by an insulation layer with a very low thermal conductivity the energy transfer to the surroundings is further reduced.
What does a reflective foil between the radiator and outside wall do?
It reflects infrared radiation away from the wall and into the room and so reduces energy transfer by radiation to the surroundings.
What does fibreglass (in lofts) do?
The fibreglass or mineral wool has a very low thermal conductivity. Air pockets trapped inside the fibre/foam reduce the transfer of energy by convection so the rate of energy transfer through the roof will decrease.
What is the temperature of the Earth determined by?
The rate of incoming and emitted infrared radiation. Due to greenhouse gases produced by human activity the Earth’s temperature is increasing.
What is the greenhouse effect?
IR radiation from the sun is absorbed by the Earth’s surface and re-emited by the Earth as slightly longer wavelength IR. The shorter wavelength IR is emitted back into space but longer wavelength IR is absorbed by greenhouse gases and emitted back towards the Earth.
