- Energy can be transferred from one place to another by work or by heating processes.
- Heat energy can be transferred by radiation, conduction or convection.
- Heat radiation is the transfer of heat energy by infrared radiation.
- Conduction and convection involve the transfer of energy by particles.
- Conduction is the main form of heat transfer in solids.
- Convection is the main form of heat transfer in liquids and gases.
- Infrared radiation can be emitted by solids, liquids and gases.
- All objects continually emit and absorb infrared radiation. The bigger the temperature difference between an object and its surroundings the more radiation it radiates in a given time.
- Dark, matt surfaces are good absorbers and good emitters of infrared radiation.
- Light, shiny surfaces are poor absorbers and poor emitters of infrared radiation.
- Light, shiny surfaces are good reflectors of infrared radiation. Light, shiny surfaces are used in a vacuum flask to keep heat in or out, depending if its contents are hot or cold.
The three states of matter are solid, liquid and gas. the particles of a particular substance in each state are the same - only the arrangement and energy of the particles are different.
- Solids - strong forces of attraction hold the particles close together in a fixed, regular arrangement. The particles don't have much energy so they can only vibrate about their fixed position.
- Liquids - there are weaker forces of attraction between the particles. The particles are close together, but can move past each other, and from irregular arrangements. they have more energy than particles in a solid - they move in random directions at low speeds.
- Gases - there are almost no forces of attraction between the particles. The particles have more energy than those in liquids and solids - they are free to move, and travel in random directions at high speeds.
When you heat a substance you give its particles more kinetic energy - they vibrate faster. this causes solids to melt and liquids to boil.
Conduction of heat energy is the process where vibrating particles pass on their extra kinetic energy to neighbouring particles.
This process continues throughout the solid and gradually some of the extra kinetic energy is passed all the way through the solid, causing a rise in temperature at the other side of the solid. And hence an increase in heat radiating from its surface. Usually conduction is faster in denser solids, because the particles are closer together and so will collide more often and pass energy between them.
- Materials that have larger spaces between their particles conduct heat energy much more slowly, these are good insulators.
Metals conduct so well because the electrons are free to move inside the metal. At the hot end the electrons move faster and collide with other free electrons, transferring energy. These other electrons pass on their extra energy to other electrons. Because the electrons can move freely, this is obviously a much faster way of transferring the energy through the metal than slowly passing it between jostling neighbouring atoms. This is why heat energy travels so fast through metals.
Convection occurs when the more energetic particles move from the hotter region to the cooler region - and take their heat energy with them.
Convection is most efficient in round or square containers as the shape allows the convection currents to work best.
Convection currents happen when change in density occurs.
- Radiators heat the air around it. The heated, less dense air rises, warm air displaces the cool air. Cool denser air falls, Cool air flows to fill the gap left by rising, heated air.
Condensation & Evaporation
When a gas cools, the particles in the gas slow down and lose kinetic energy. The attractive forces between the particles pull them closer together. If the temperature gets cold enough and the gas particles get close enough together that condensation can take place, the gas becomes a liquid. Water vapour in the air condenses when it comes into contact with cold surfaces.
The rate of condensation will be faster if the:
- Temperature of the gas is lower
- The temperature of the surface the gas touches is lower
- Density is higher
- Airflow is less
Evaporation is when particles escape from a liquid. Particles can evaporate from a liquid at temperatures that are much lower than the liquids boiling point. Particles near the surface of a liquid can escape and become gas particles if:
- The particles are travelling in the right direction to escape the liquid.
- The particles have enough kinetic energy to overcome the attractive forces of the other particles in the liquid.
The particles with the most kinetic theory are more likely to evaporate from the liquid meaning the average speed and kinetic energy of remaining particles decrease. This decrease in average particle energy means the temperature of the remaining liquid falls. This cooling effect can be really useful. When you exercise you sweat, as the water from the sweat on your skin evaporates, it cools you down.
The rate of evaporation will be higher if the:
- Temperature is higher
- Density is lower
- Surface area is larger
- Airflow over the liquid is greater
Rate of Heat Transfer
Heat energy is radiated from the surface of an object. The bigger the surface area, the more infrared waves that can be emitted or absorbed from the surface, so the quicker the transfer of heat. For example radiators have large surface areas to maximise the amount of heat they transfer. This is why car and motorbike engines often have ‘fins’ which increase the surface area so heat is radiated away quicker. So the engine cools quicker.
Heat sinks are devices designed to transfer heat away from objects they’re in contact with for example computer components. If two objects at the same temperature have the same surface are but different volumes, the object with the smaller volume will cool more quickly, as a higher proportion of the object will be in contact with its surroundings.
Other factors, like the type of material, affect the rate too. Objects made from good conductors transfer heat away more quickly than insulating materials. It also matters what the material is in contact with, if an object is in contact with a conductor the heat will be conducted away much faster than if it is in contact with a good insulator.
The bigger the temperature difference between an object and its surroundings, the faster the rate at which energy is transferred by heating.
Rate of Heat Transfer 2
A flask is designed to limit heat transfer
- The bottles is double-walled with a vacuum between the two walls. This stops all conduction and convection through the sides.
- The walls either side of the vacuum are silvered to keep heat loss by radiation to a minimum.
- The bottle is supported using insulating foam. This minimises heat conduction to o from the outer bottle.
- The stopper is made of plastic and filled with cork or foam to reduce any heat conduction through it.
Humans and animals have ways of controlling heat transfer. In the cold, the hairs on your skin ‘stand up’ to trap a thicker layer of insulting air around the body. This limits the amount of heat loss by convection. Some animals do the same using fur. When you’re too warm, your body diverts more blood to flow near the surface of your skin so that more heat can be lost by radiation, that's why some people go pink when they’re hot. Generally, animals in warm climates have larger ears than those in cold climates to help control heat transfer.
Arctic foxes have evolved small eras, with a small surface area to minimise heat loss by radiation and conserve body heat.
Desert foxes on the other hand have huge ears with a large surface area to allow them to lose heat by radiation easily and keep cool.
Energy efficiency in the Home
There are lots of things you can do to a building tor educe the amount of heat energy that escapes. Some are more effective than others, and some are better for your pocket than others. The most obvious examples are in the home, but you could apply this to any situation where you’re trying to cut down energy loss.
The most effective methods of insulation are ones that give you the biggest annual saving. Eventually, the money you've saved on heating bills will equal the initial cost. This time is called payback time. The most cost-effective methods tend to be the cheapest. They are cost effective because they have a short payback time.
- Cavity wall insulation - Loft Insulation - reduces conduction and radiation in the roof space.
- Draught-proofing - reduce heat loss due to convection.
- Hot water tank jacket - reduces conduction and radiation
- Thick curtains - reduce heat loss by conduction and radiation.
- Cavity wall insulation - reduces convection and radiation across a gap.
U-values measure how effective a material is as an insulator. Heat transfers faster through materials with a higher U-values than through materials with low U-values. So the better the insulator the lower the U-value.
Specific Heat Capacity
The specific heat capacity of a substance is the amount of energy required to change the temperature of one kilogram of the substance by 1°c.
Energy transferred (J) = Mass (kg) x Specific heat capacity (J/kg°C) x Temperature change (°c)
Heaters have high heat capacities to store lots of energy.
The materials used in heaters usually have high specific heat capacities so that they can store large amounts of heat energy.
Water has a really high specific heat capacity. It's also a liquid, so it can be pumped around in pipes - ideal for central heating systems.
Electric storage heaters are designed to store heat energy at night, and then release it during the day. They store the heat using concrete or bricks, which have a high specific heat capacity.
Some heaters are filled with oil, which has a high heat capacity. Because it is lower than waters specific heat capacity, oil heating systems are often not as good as water-based systems. Oil does have a higher boiling point, which means oil-filled heaters can safely reach higher temperatures than water-based ones.
There are nine types of energy:
- Electrical Energy
- Light Energy
- Sound Energy
- Kinetic Energy
- Nuclear Energy
- Thermal Energy
- Gravitational Energy
- Elastic Potential Energy
- Chemical Energy
The last three above are forms of stored energy because the energy is not doing anything, it's waiting. Energy can be transferred usefully from one form to another, stored or dissipated - but it can never be created or destroyed. Energy is only useful when it can be converted from one form to another.
Efficiency of Machines
When energy is transferred only part of it may be usefully transferred, the rest is ‘wasted. Wasted energy is eventually transferred to the surroundings, which become warmer. The wasted energy becomes increasingly spread out and so becomes less useful.
Useful devices are only useful because they can transfer energy from one from to another. In doing so, some of this useful input energy is always wasted as heat. The less energy wasted the more efficient the device is said to be.
To calculate the efficiency of a device you can use:
Efficiency = Useful Energy/Power out/Total Energy/Power in.
No machine is 100% efficient as some energy is always lost as heat, the only exception to this is electric heaters as all the electricity is converted to useful energy.
Useful energy is concentrated energy, the entire energy output by a machine, both useful and wasted, eventually ends up as heat. This heat is transferred to cooler surroundings, which then become warmer. As the heat is transferred to cooler surroundings, the energy becomes less concentrated as it dissipates. The total amount of energy stays the same. The energy is still there, but as it becomes increasingly spread out, it can’t be easily used or collected back in again.
Sometimes ‘waste’ energy can actually be useful. Heat exchangers reduce the amount of heat energy that is ‘lost’. They do this by pumping a cool fluid through the escaping heat. The temperature of this fluid rises as it gains heat energy. The heat energy in the fluid can be converted into a form of energy that’s useful again - either in the original device, or for other useful functions. For example, some of the heat from a car’s engine can be transferred to the air that’s used to warm the passenger compartment.
The Cost of Electricity
Electrical appliances transfer electrical energy into other forms. The amount of energy that is transferred by an appliance depends on its power and the amount of time that the appliance is switched on.
- ENERGY = POWER x TIME
Energy is usually measured in joules - 1 j is the amount of energy transferred by a 1 W appliance in 1 s. Power is usually measured in watts or kilowatts. A 5 kW appliance transfers 5000 j in 1 s. When you're dealing with large amounts of electrical energy it’s easier to think of the power and time in kilowatts and hours - rather than I watts and seconds. So the standard units of electrical energy are kilowatt-hours.
- A KILOWATT HOUR is the amount of electrical energy used by a 1 kW appliance in an hour.
The cost of electricty can be calculated using 2 formulas:
- Units (kWh) = Power (kW) x Time (hours)
- Cost = Units x Price per unit
Energy Sources & Power Stations
There are 12 types of energy resources. They fit into renewable and non-renewable.
Non-renewable resources will run out:
- Coal, Oil, Gas, Uranium and Plutonium
- They all do damage to the environment, but are dependent sources of energy.
Renewable resources will never run out:
- Wind, Waves, Tides, Hydroelectric, Solar, Geothermal, Food, Biofuels
- They do little damage to the environment, but they provide little and unreliable energy.
Non-renewables are also linked to other environmental problems .
- All three fossil fuels release CO2 into the atmosphere when they're burned. This CO2 adds to the greenhouse effect, and contributes to global warming.
- Burning coal and oil releases sulfur dioxide, which causes acid rain. Acid rain can be reduced by removing the sulfur before the fuel is burned.
- Coal mining makes a mess of the landscape.
- Oil spillages cause serious environmental problems, affecting mammals and birds that live around the sea.
Electricity & The National Grid
The National Grid takes electricity from power stations to where it's needed in homes and industry. It enables power to be generated anywhere on the grid, and then be supplied anywhere else on the grid. To transmit this electricity you need either a high voltage or a high current. The problem with a high current is that you lose lots of energy through heat in cables. It's much cheaper to boost the voltage up really high and keep the current very low.
To get the voltage high requires transformers as well as a big pylon with huge insulators.
The transformers have to step up at one end, for efficient transmission, and then bring it back down to safe, usual levels at the other end.
The voltage is increased using a step-up transformer. It is then reduced again at the consumer end using a step-down transformer.
Waves transfer energy from one place to another.
The amplitude is the displacement from the rest positon to the crest. The wavelength is the length of a full cycle of the wave. Frquency is the number of complete waves passing a certain point per second.
Waves are either Transverse or Longitudinal.
Transverse waves have sideways vibrations that are perpendicular to the direction of energy transfer of the wave. Most waves are transverse:
- Electromagnetic, Ripples on water, Waves on string, a Slinky spring.
Longnitudal waves have vibations along the same line, the vibrations are parallel to the direction of energy transfer of the wae.
- Sound waves, Shock waves.
- Longnitudinal waves show areas of compression and rarefraction.
WAVE SPEED (m/s) = FREQUENCY (Hz) x WAVELENGTH (m)
When waves arrive at a new material, their direction of travel can be changed. This can happen by reflection, refraction or diffraction.
Relecion of light is what allows us to see an object. Light bounces of them into our eyes.
When light travelling in the same diraction reflects from an uneven surface, the lights relects off at different angles.
When light travelling in the same direction reflects from an even surface then it's all reflected at the same angle and you get a clear reflection.
ANGLE OF INCIDENCE = ANGLE OF REFLECTION
Diffraction is when waves 'bend around' obstacles, causing the waves to spread out
The amount of diffraction depends on the