- Thermal radiation is emitted by all objects as electromagnetic waves, and the hotter the object the more infrared radiation emitted.
- Matt, black surfaces absorb and emit more infrared radiation, but light, shiny surfaces are better reflectors.
- Radiation can happen even through a vacuum, unlike convection and conduction.
- The temperature of an object is determined by the average thermal energy of each of the particles in the object.
- When infrared radiation from the sun gets absorbed by the earth, some gets reflected, but the infrared light cannot then penetrate the atmosphere, because of greenhouse gases.
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- Conduction occurs in solids, where thermal energy provided to one part of an object is spread along the whole object until the whole object has the same temperature.
- Vibrations are passed on through the atoms, when particles with more kinetic energy pass on their kinetic energy to other particles.
- Temperature is proportional to the average vibrational energy of particles.
- Metals like copper are better conductors because they have delocalised electrons that can easily travel through the object and pass on their kinetic energy.
- Plastic and wood are good insulators, as well as fluids.
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- Convection is a heat transfer that occurs in liquids and gases.
- A convection current:
- When the fluid is heated, it is supplied with more thermal and kinetic energy, causing it to vibrate more and then spread out.
- This expansion means that the hotter fluid has a lower density.
- The number of particles or mass of particles does not change when the substance is heated, but the volume does.
- density = mass / volume shows how an increase in volume results in a decrease in density.
- The hotter fluid close to a heat source rises above the colder fluid, and colder fluid takes up the place of the hot fluid.
- Once the hotter fluid is far enough from the heat source, it becomes cool once again and sinks because of its high density.
- Convection results in the rising and falling of hot air on Earth, which causes tropical storms and hurricanes.
- Sea breezes are formed when cooler air from the sea takes the place of the hot air that has risen from the land during warm weather.
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Heat and Temperature
- There are differences between heat and temperature and the two should not be mixed up.
- Heat is thermal energy moving from one place to another, and is measured in Joules (J).
- Temperature is the average kinetic energy in the particles that make up an object, and can be measured in Degrees Celcius, Degrees Fahrenheit or Kelvin.
- One object has a higher temperature than another object with the same mass, we can say that it has more thermal energy.
- The net movement of thermal energy is always from a hot object to a cold object.
- If a beaker of water has the same thermal energy as another beaker of water, then both beakers of water poured into a larger beaker will have double the amount of energy as one beaker, but the temperature will stay the same because temperature tells us the average kinetic energy in the beaker.
- Objects at absolute zero have no thermal energy at all, so they are regarded as stationary.
- An object with a lower surface to volume ratio will lose heat energy at a slower rate.
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Evaporation and Condensation
- Evaporation occurs when molecules on the surface of a liquid are able to gain enough kinetic energy to escape the liquid and become a gas.
- Factors speeding up rate of evaporation include:
- Temperature difference between the liquid and surroundings.
- Surface area of the liquid.
- If there is a draught to take away molecules that have escaped.
- The effects of evaporation can be used to numb a person before they receive an injection:
- A liquid that easily evaporates, like an alcohol, is placed on the skin.
- In order to have enough energy to evaporate, it takes kinetic energy from the skin it is touching.
- The average thermal energy on the skin is decreased because of thermal energy being passed on to the fluid, so the skin is much colder and more numb.
- Condensation occurs when water molecules in a gas become cool enough to turn back into a liquid.
- This is what happens when some of the water molecules in the air hits a cold surface like a mirror in a steamy bathroom. A film of water covers the surface.
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Specific Heat Capacity
- The specific heat capacity of a substance tells us how much energy is needed to increase the temperature of 1kg of the substance by 1°C.
- For example, in order to increase 1kg of water by 1°C, 4200J of energy will be needed, but only 130J will be needed to increase 1kg of lead by 1°C.
- The equation for specific heat capacity is:
- E = m × c × θ
- E stands for energy in Joules (J)
- m stands for mass in kilograms (kg)
- c stands for specific heat capacity of the substance in J/kg°C
- θ stands for the temperature change in °C
- The equation is helpful in telling us how much energy will be needed to heat a specfic substance by a certain amount, or even what the temperature change is based on the energy needed, mass and specific heat capacity of the substance.
- Storage heaters use electricity at night to heat special bricks or concrete, which have a high specific heat capacity. This means that, during the day, they can slowly warm up the room, after having used electricity when it was cheaper.
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Heatng and Insulating Buildings
- There are many ways that buildings can be insulated, so that the least amount of heat is lost to surroundings, and less money is wasted on heating:
- Loft insulation uses fibreglass to reduce rate of heat transfer though the roof, while air between fibres helps reduce loss of heat through convection because the air is trapped and unable to move.
- Cavity wall insulation is when the space between the outside wall and inside wall is filled with a special insulator that traps air in small pockets to minimise convection currents.
- A draught excluder is fitted on doors so that air cannot escape through the doors easily because of convection.
- Double glazing is when two panes of glass are fitted on a window pane, with a small space for air or a vacuum, so that conduction is reduced, as well as convection if a vacuum is in place.
- The U-value tells us the energy per second passing through one square metre of a material, when the temperature difference is 1°C. A lower U-value means the material is a better insulator of heat.
- Payback time tells us how cost-effective a solution is, with a lower payback meaning more cost-effective:
- payback time (yrs) = cost of installation (£) ÷ savings per year in fuel costs (£)
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