# Physics: Unit Two

Physics: Unit Two
P2.1 - Energy transfer by conduction
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The energy transfer by conduction through a material depends on its thermal conductivity. The greater the thermal conductivity, the more energy energy per second it transfers by conduction.
Good insulators need to be materials that have low thermal conductivity, so energy transfer through them is as low as possible.
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The energy transfer per second through a layer of insulating material depends on: the temperature difference across the material, the thickness of the material, the thermal conductivity of the material.
To reduce the reduce the energy transfer as much as possible in any given situation: the thermal conductivity of the insulating material should be as low as possible, the thickness of the insulating layer should be as thick as is practically possible
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The Sun emits all types of electromagnetic radiation. Fortunately, the Earth's atmosphere blocks most of which are harmful to humans, apart from infrared radiation and light.
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The higher the temperature of an object, the more infrared radiation it emits in a given time.
A perfect black body is an object that absorbs all the radiation that hits it, which doesn't reflect or transmit any radiation.
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An object that has a constant temperature emits radiation across a continuous range of wavelengths.
If the temperature of the object is increased, the intensity of the radiation it emits it greater at every wavelength.
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The shorter the wavelength, the greater increase in intensity of that wavelength. Therefore, the peak intensity is at a shorter wavelength than it was at the lower temperature.
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When an object absorbs radiation faster than it emits radiation, its temperature increases.
The temperature of the Earth depends on a lot of factors, such as the rate that light and infrared radiation from the Sun are:
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reflected back into space or absorbed by the Earth's atmosphere or by the Earth's surface, emitted from the Earth's surface and from the Earth's atmosphere into space.
Some gases in the atmosphere (such as water vapour, methane, and carbon dioxide - greenhouse gases) absorb longer wavelength infrared radiation from Earth and prevent it from escaping into space.
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1) Infrared radiation from the Sun warms the ground. 2) Carbon dioxide molecules absorb and re-emit infrared radiation back to the ground.
P2.4 - Specific heat capacity
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When a substance is heated, its temperature rise depends on: the amount of energy supplied to it, the mass of the substance, and what the substance is.
Heating 0.1kg of water by 4'C requires an energy transfer of 1600J. Heating 0.2kg of water by 4'C required an energy transfer of 3200J.
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The specific heat capacity of a substance is the energy needed to raise the temperature of 1kg of the substance by 1'C.
energy transferred (J) = mass (kg) x specific heat capacity (J/kg'C) x temperature change ('C)
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A storage heater uses energy at night to heat special bricks/concrete blocks in the heater. They have a high specific heat capacity, so store a lot of energy, and warm up slowly when heater is on.
Water = 42,000 J/kg'C
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Oil = 21,000 J/kg'C
Aluminium = 900 J/kg'C
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Iron = 390 J/kg'C
Copper = 385 J/kg'C
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Concrete = 850 J/kg'C
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P2.5 - Heating and insulating buildings
Loft insulation eg. fibreglass (good insulator), reduces the rate of energy transfer through the roof. If there is a greater rate of layers of insulation, the rate of energy transfer through the roof will be less.
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Cavity wall insulation reduces the rate of energy transfer through the outer walls of the house. Cavity of an outer wall is the space between the two layers of brick that make up the wall, where insulation is pumped in. It traps air in small pockets.
Aluminium foil between a radiator panel and wall reflects radiation away from the wall and so reduces rate of energy transfer by radiation.
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Double-glazed windows have two glass panes with dry air or a vacuum between them. The thicker it is, the lower its thermal conductivity, meaning a slower rate of transfer of energy through conduction will occur.
If external wall of a warm building have thicker bricks and lower conductivity, transfer of energy will decrease and in turn, cost of heating will decrease.
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## Other cards in this set

### Card 2

#### Front

The energy transfer by conduction through a material depends on its thermal conductivity. The greater the thermal conductivity, the more energy energy per second it transfers by conduction.

#### Back

Good insulators need to be materials that have low thermal conductivity, so energy transfer through them is as low as possible.

### Card 3

#### Front

The energy transfer per second through a layer of insulating material depends on: the temperature difference across the material, the thickness of the material, the thermal conductivity of the material.

### Card 5

#### Front

The higher the temperature of an object, the more infrared radiation it emits in a given time.