Heat Transfer - Thermal Radiation
Thermal radiation is heat transfer by electromagnetic waves (infra red). All objects emit thermal radiation. The hotter an object is, the more thermal radiation it emits. Radiation does NOT involve particles, therefore it is the only heat transfer that can travel through a vacuum, like space, where there are no particles.
Dark, matt surfaces are better emitters of thermal radiation than shiny, white surfaces. They are also better absorbers of thermal radiation. This is why we wouldn't wear a black top on a hot day, as it would absorb radiation from the sun.
Central heating radiators are often shiny and white.... this is silly, as a matt, black one would radiate more thermal energy and keep us warmer!
Remember - a cold object can recieve heat energy and warm up
A hot object can emit heat energy and cool down.
You can transfer heat, but NOT TRANSFER 'COLD'
Conduction and Convection
Conduction - Mainly in solids. Liquids and gases are generally poor conductors. As you heat a solid, the particles gain energy and vibrate more. This energy passes to neighbouring particles and the heat is transferred throughout the solid. This happens a lot in metals, which are good conductors. Poor conductors are called insulators. Materials such as fibreglass are good insulators, because they contain pockets of trapped air. Fibreglass is used to insulate houses.
Convection - This only happens in liquids and gases. Heating a liquid or gas makes it less dense, so it rises, creating a space, which is filled by cooler stuff. This is then heated, creating a 'convection current'. These can be on a massive scale, ie onshore breezes.
Applying Heat Transfer to Design
Reduce heat loss by conduction - use insulators to trap in a layer of heat
Reduce heat loss by convection - reduce convection currents by trapping air in small pockets
Reduce heat loss by radiaton - use light shiny surfaces which are poor emitters
A radiator has a large surface area, so it can lose heat easily.
Small objects lose heat more easily than large objects.
Heat loss from a building can be minimised by putting aluminium foil behind radiators (to reflect heat back into the room), cavity wall insulation, double glazing and loft insulation.
A vacuum flask (like a thermos) does not allow heat to radiate out of the flask, so if you put tea in it, it will stay hot. Also, the flask will not allow heat to radiate in to the flask, so if you put ice in it, it will stay cold. (Common exam question, that!)
Forms of Energy
Energy exists in different forms
Energy cannot be created or destroyed, it can be transformed from one form to another.
Light - from the sun, or a lamp
Heat - Flows from a hot object to a cold object (never the other way!!)
Sound - From a loudspeaker or your voice
Kinetic - Anything moving
Nuclear - From nuclear reactions
Electric - Whenever an electric current flows
Gravitational - Stored in any object that can fall
Elastic - From any object that is stretched
Chemical - In foods, fuel, batteries etc
A machine is something that transfers energy from one place to another, or one form to another. The energy supplied to the device is called the input energy. The energy we get out of the device consists of useful energy and wasted energy.
Useful energy is energy in the place we want it and the form we need it. Wasted energy is energy which is not useful. Both wasted and useful energy end up being transferred to the surroundings, which become warmer. As energy spreads out, it becomes harder to 'catch' and use for further energy transfers.
Energy is measured in Joules. The efficiency of a device is calculated like this...
The efficiency can be left as a fraction, or x100 to give a percentage. No device is ever 100% efficient, except for an electric heater, which transforms all of the electrical energy into heat energy.
Electrical Devices and Electrical Power
In an electric device, eg a kettle, lamp, TV, heater etc, the energy comes from an electric current.
All electrical devices transform some of the energy into heat, but this may not be a useful energy transformation.
The unit of power is the Watt (W), equal to 1 joule/second
1 Kilowatt = 1000W
Power = energy transferred (Joules) / time taken (seconds)
The more powerful a device, the greater the rate at which it transforms energy.
When you get an electricity bill, it is measured in kWh (Kilowatt hours). This is a measure of the energy used. You can work it out by multiplying the power if a device in kW to the time it is used for in hours. To work out the cost, then multiply the number of kWh by the cost per unit of your electricity.
The National Grid
In Britain, electricity is distributed by the 'National Grid'. This is a network of cables and transformers.
We use 'step up' transformers to bring the voltage made in power stations up to the voltage carried by the cables in the grid.
We use 'step down' transformers to bring the voltage down for use in our homes.
A high 'grid voltage' reduces energy loss and makes the system more efficient.
Fuel to make Electricity
In most power stations, water is heated to produce steam. This steam drives a turbine which generates the electricity. The heat comes from burning fossil fuels, except in nuclear power stations, when the energy comes from nuclear 'fission'. More energy is released from 1kg of nuclear uranium undergoing fission than 1kg of fossil fuel being burnt.
Renewable energy -
Wind turbines - an electricity generator (windmill) on top of a pole!
Wave generator - a floating generator turned on by the waves
Hydroelectric - a generator powered by water flowing downhill (or stored in a reservoir and released)
Tidal - traps each high tide and uses it to generate electricity
Solar - Energy from the sun, trapped by solar panels
Geothermal - Energy released from radioactive substances deep inside the Earth
Energy and the Environment
Fossil fuels produce greenhouse gases, eg carbon dioxide
Nuclear fuels produce radioactive waster
Renewable energy resources can affect plant and animal life
Coal, Oil, Gas and Uranium are non renewable energy sources, and so will run out. Renewable resources will not run out. There are advantages and disadvantages to using each energy source.