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Physics Revision P2a - P2d

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A P

on 9 March 2013

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Transcript of Physics Revision P2a - P2d

Physics Revision P2b P2c P2d Physics Revision P2a - P2d P2a How does a photocell produce electricity? The sunlight is absorbed by the black photocells, because black is most absorbent.
The light provides enough heat for the electrons to be released or knocked out.
The free electrons mean a current has been created. What factors effect the amount of electricity produced? The area of the photocell affects how much electricity is produced because if there is a larger area, more is produced. Also, the intensity of sunlight reaching the photocell effects; if the photocell is closer to the source of light/heat, more can be absorbed and eventually there is more electricity produced. Advantages and Disadvantages of using photocells Advantages
Low maintenance
no need for power cables
no need for fuel
long life
renewable energy resource
no pollution waste Disadvantages
No power at night
No power during bad weather
Large area of cell required to power e.g. a house Passive solar heating Solar panels are an example of passive solar heating. The heat energy is absorbed from the sun, by the solar panel. This energy is used to heat things such as water, which can then be used for radiators, showers etc. The heat cannot escape because the wave length is changed. Wind Power The kinetic energy in wind is to turn a turbine. This rotates a magnet of coil around a magnet. The magnetism and movement of coil means a current/electricity is produced. The dynamo effect (electromagnetic induction) The dynamo effect is when electricity is generated by moving a magnet near a coil of wire. A simple generator of this type produces and alternating current/AC. A battery produces direct current/DC. The energy produced/current can be increased by increasing the speed of movement, adding more coils of wire or using a stronger magnet. Alternating Current Direct Current AC Generator Electricity is produced using magnetism and moving coils of wire. This is because the field lines are being cut. As the coil is moved one way, the current goes one way. When the coil is moved the other way or turned around, the current goes the other way. If the wire is moved slowly, less electricity is produced. Conventional power stations Fuel is heated to produce steam.
The steam is used to turn a turbine.
The turbine rotates a magnet inside a coil.
Magnetism and movement of coil means electricity is produced.
This is then put into the national grid.
The energy transfer is heat to kinetic to electric.
This electricity is measures in watts. Distribution of electricity across the country The electricity produced in power stations goes to step up transformers. This is so that the voltage is increased, meaning there is a lower current and less energy is wasted. Then high voltage power lines are used to transport the electricity to where it is needed. A step down transformer is then used to lower the voltage as a high voltage is not often required (unless in factories, for example). This way no energy is lost along the way. Wasted Energy A significant amount of energy is lost in conventional power stations. Quite a lot of this is through thermal energy as the fuel is heated yet it is not fully used, but is later cooled again and then reheated. Efficiency = Useful energy output x 100
Total energy input The green house effect Global warming is the increase in the temperature in the Earth's atmosphere and oceans. The green house gases causing this include carbon dioxide, carbon monoxide, methane, nitrous oxide, tetraflouromethane, hexafluoroethane and sulphur hexaflouride. Natural and man made effects, altering the climate Natural causes include the condensation in the water cycle, dust and gas from volcanic eruptions and CO2 waste from all living things. Man made effects include power stations, burning of fossil fuels and dust and gas from factories. Determining the cause of climate change It is difficult to determine the cause of climate change because the level of CO2 increases with the temperature of global warming. This is a man made cause. However, the most CO2 is produced by developing countries. This is a natural cause because there are less plants there, meaning that less CO2 is being used up. Common fuels used in power stations Nuclear fuel
Crude oil
Coal
Natural Gas
Fossil Fuels
Manure
Straw
Wood Advantages and Disadvantages (General/examples) Plenty of the energy source
No green house gases produced
Renewable - does not rely on fossil fuels
Advanced technology
High energy output
Low energy input
Cheap
Easy to maintain Non-renewable
Expensive
Radio-active emissions/waste
Waste products
Wasted energy
Hard to maintain
Expensive to shut down
High energy input
Low energy output Power and Energy usage Power (W) = Voltage (V) x Current (A)
Energy (J) = Power (W) x Time The unit of electrical energy supplied is the kilowatt hour. Cost of running an electrical device Cost per unit x Power (kW) x hours (kWh) = cost Transformers Step down transformers decrease the voltage or electricity. They have more coils on the primary side, and less on the right. Step up transformers increase the voltage and are the opposite. Isolating transformers keep the voltage the same. Relationship between the voltage and current As the voltage increases, the current decreases. When the current increases, the voltage decreases. Transmission at high voltages Energy is transmitted at high voltages so that the current is reduced, allowing less of this energy to be wasted through heat. It also reduces costs as less energy is required because less is wasted. Important equations Voltage in the primary coil = Number of turns in the primary coil
Voltage in secondary coil Number of turns in the secondary coil Primary voltage x Primary current = Secondary voltage x secondary current Vp = Np
Vs = Ns Vp x Ip = Vs x Is
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