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Revision as of 21:50, 9 May 2009
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==Important equations==
==Important equations==
There are two things you need to remember about transformers.
There are two things you need to remember about transformers.
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*The '''voltage''' in the secondary coil depends on the voltage in the primary coil ''and'' the ratio of the number of turns in the secondary and primary coils.
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:If we call:
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* The '''voltage''' in the secondary coil depends on the voltage in the primary coil ''and'' the ratio of the number of turns in the secondary and primary coils.
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: If we call:
** the voltage in and number of turns in the primary V<sub>p</sub\> and N<sub>p</sub>
** the voltage in and number of turns in the primary V<sub>p</sub\> and N<sub>p</sub>
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**the voltage in and number of turns in the secondary V<sub>s</sub\> and N<sub>s</sub>
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** the voltage in and number of turns in the secondary V<sub>s</sub\> and N<sub>s</sub>
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:then
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: then
:::::: V<sub>s</sub>/V<sub>p</sub>= N<sub>s</sub>/N<sub>p</sub>
:::::: V<sub>s</sub>/V<sub>p</sub>= N<sub>s</sub>/N<sub>p</sub>
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: or, rearranging:
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:or, rearranging:
:::::: V<sub>s</sub>= V<sub>p</sub> * N<sub>s</sub>/N<sub>p</sub>
:::::: V<sub>s</sub>= V<sub>p</sub> * N<sub>s</sub>/N<sub>p</sub>
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*Similarly, the '''current''' in the secondary coil depends on the current in the primary coil ''and'' the ratio of the number of turns in the secondary and primary coils. However, the current is inversely related to the number of turns.
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* Similarly, the '''current''' in the secondary coil depends on the current in the primary coil ''and'' the ratio of the number of turns in the secondary and primary coils. However, the current is inversely related to the number of turns.
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:If we call:
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: If we call:
** the current in and number of turns in the primary I<sub>p</sub\> and N<sub>p</sub>
** the current in and number of turns in the primary I<sub>p</sub\> and N<sub>p</sub>
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**the current in and number of turns in the secondary I<sub>s</sub\> and N<sub>s</sub>
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** the current in and number of turns in the secondary I<sub>s</sub\> and N<sub>s</sub>
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:then
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: then
:::::: I<sub>s</sub>/I<sub>p</sub>= N<sub>p</sub>/N<sub>s</sub>
:::::: I<sub>s</sub>/I<sub>p</sub>= N<sub>p</sub>/N<sub>s</sub>
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: or, rearranging:
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:or, rearranging:
:::::: I<sub>s</sub>= I<sub>p</sub> * N<sub>p</sub>/N<sub>s</sub>
:::::: I<sub>s</sub>= I<sub>p</sub> * N<sub>p</sub>/N<sub>s</sub>
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* The power in the primary is equal to the power in the secondary (assuming a '''perfect''' transformer). This is because energy is never created or destroyed.
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*The power in the primary is equal to the power in the secondary (assuming a '''perfect''' transformer). This is because energy is never created or destroyed.
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:::::: I<sub>p</sub>V<sub>p</sub> = I<sub>s</sub>V<sub>s</sub>
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::::::I<sub>p</sub>V<sub>p</sub> = I<sub>s</sub>V<sub>s</sub>
==Step up and Step down transformers==
==Step up and Step down transformers==
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A step down transformer is the opposite of a step up transformer. Let's say the primary has 900 turns and the secondary has 450 turns. This time the voltage will be half on the secondary as it is on the primary. So if the primary has 2V applied, the secondary will have only 1V across it.
A step down transformer is the opposite of a step up transformer. Let's say the primary has 900 turns and the secondary has 450 turns. This time the voltage will be half on the secondary as it is on the primary. So if the primary has 2V applied, the secondary will have only 1V across it.
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===Strategy for answering questions===
===Strategy for answering questions===
Questions on transformers will often be of the form "A coil has 3000 turns of wire on a soft iron. Another coil of 12000 turns is brought into contact with the first coil. This second coil has a 12V bulb attached to it completing a circuit.What voltage should applied to the first coil?"
Questions on transformers will often be of the form "A coil has 3000 turns of wire on a soft iron. Another coil of 12000 turns is brought into contact with the first coil. This second coil has a 12V bulb attached to it completing a circuit.What voltage should applied to the first coil?"
to answer a question like this:
to answer a question like this:
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#'''Decide which is the primary coil and which is the secondary'''-in this case the 3000 turn coil is the primary and the 12000 turn coil is the secondary
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#'''Decide if it is a step up or a step down transformer''' in this case it is a step up transformer because the secondary coil has more turns on it than the primary.
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#'''Work out a multiplication factor''' in this case 12000/3000 = 4
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#'''Use this multiplication factor to work out the required voltage''' - the secondary coil has to supply 12V so the primary has to be 12V/4 = 3V
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this website is good but as a note to the wikipedia/books administrators, could you PLEASE PLEASE PLEASE PLEASE PLEASE remove the edit feature on any item that is concerning GCSE/ A level so that wikipedia can finally be a reliable source, please do so because you will get many more 'hits' from students such as myself
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# '''Decide which is the primary coil and which is the secondary'''-in this case the 3000 turn coil is the primary and the 12000 turn coil is the secondary
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# '''Decide if it is a step up or a step down transformer''' in this case it is a step up transformer because the secondary coil has more turns on it than the primary.
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# '''Work out a multiplication factor''' in this case 12000/3000 = 4
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# '''Use this multiplication factor to work out the required voltage''' - the secondary coil has to supply 12V so the primary has to be 12V/4 = 3V
==Uses of transformers==
==Uses of transformers==
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The most important use of transformers is in the routing of mains electricity. The power disspated by a cable = I<sup>2</sup>R, where I is the current and R is the resistance. There is little that can be done to lower the resistance, so in order to lose as little energy as possible in heating up the cable, the current needs to be kept as low as possible. This is done using a step up transformer which increases the voltage to many thousands of volts. Because power is voltage times current, a high voltage means a low current for a given power value.(Notice that transormers do not obey Ohm's law). At the other end of the distribution network, the voltage is stepped down using another transformer so that the voltage going into homes and factories is not excessively high.
The most important use of transformers is in the routing of mains electricity. The power disspated by a cable = I<sup>2</sup>R, where I is the current and R is the resistance. There is little that can be done to lower the resistance, so in order to lose as little energy as possible in heating up the cable, the current needs to be kept as low as possible. This is done using a step up transformer which increases the voltage to many thousands of volts. Because power is voltage times current, a high voltage means a low current for a given power value.(Notice that transormers do not obey Ohm's law). At the other end of the distribution network, the voltage is stepped down using another transformer so that the voltage going into homes and factories is not excessively high.
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== further reading ==
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== Further reading ==
* [[Electronics/Transformers]]
* [[Electronics/Transformers]]
