Electricity Transmission And Distribution P
Visão Geral
The cables slung between pylons across open country are thinner than most people expect, and they are carrying the output of an entire power station. They manage it because the pd across them is enormous and the current in them is small, and that trade is the single decision on which the whole national supply rests. Get it wrong and a useful fraction of everything a station produces would be wasted warming up the countryside before it reached a single house.
This lesson follows the energy along that route. You will meet the device that makes the trade possible, a pair of coils sharing an iron core that raises a pd at one end of the country and lowers it again at the other, and you will find out why it flatly refuses to work on the steady output of a battery. Then you will do the arithmetic that shows just how much is saved, learn the two equations the paper hands you, and see why the charger on the end of your phone cable weighs almost nothing when the same job used to need a brick of iron.
Objetivos
- Electricity is distributed from power stations to consumers along transmission cables with transformers at both ends. Students should be able to identify and label the essential parts of an electric power transmission and distribution system.
- For a given power rating, a high distribution voltage reduces the current flowing, therefore reducing energy losses due to heating and making the system more efficient.
- A basic transformer consists of a primary coil and a secondary coil wound on a soft iron core. An alternating current in the primary coil of a transformer produces a changing magnetic field in the iron core and hence in the secondary coil. This induces a changing potential difference across the ends of the secondary coil and an alternating current flows. Students should be able to describe the basic structure and operation of a transformer. Knowledge of laminations and eddy currents in the core are not required.
- Step-up and step-down transformers are used to increase the voltage before the distribution lines and reduce it at the end to produce a safer voltage for local consumers.; In a step-up transformer the potential difference across the secondary coil is greater than the potential difference across the primary coil.; In a step-down transformer the potential difference across the secondary coil is less than the potential difference across the primary coil.
- The potential differences across the primary and secondary coils of a transformer, Vp and Vs , are related to the number of turns on the coils, np and ns , by: Vp / Vs = np / ns
- For a 100% efficient transformer, the electrical power output would equal the electrical power input. V p × I p = Vs × I s Where Vp and Ip are power input (primary coil) and Vs and Is are power output (secondary coil). Students should be aware that the turns ratio is selected to produce the required output from the input.
- Switch mode transformers are transformers that: operate at a high frequency, often between 50 kHz and 200 kHz; are much lighter and smaller than traditional transformers that work from a 50 Hz mains supply, making them useful for applications such as mobile phone chargers; use very little power when they are switched on but no load is applied.
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- Which transformer is placed between a power station generator and the transmission cables, and what does it do to the current in those cables? A. A step-up transformer, which increases the current B. A step-up transformer, which decreases the current C. A step-down transformer, which increases the current D. A step-down transformer, which decreases the current Answer: B
- A transformer has 2000 turns on its primary coil and 100 turns on its secondary coil. The primary is connected to a 230 V ac supply. What is the pd across the secondary coil? A. 4.6 V B. 11.5 V C. 46 V D. 4600 V Answer: B
- A fixed amount of electrical power is transmitted along an unchanged line. The transmission pd is doubled. What happens to the power wasted heating the cables? A. It doubles B. It halves C. It falls to one quarter D. It is unchanged Answer: C
- Why does a transformer produce no output pd when its primary coil is connected to a battery? A. The battery pd is too small B. The magnetic field in the core is steady, so nothing is induced C. The iron core blocks direct current D. The two coils are not connected to each other Answer: B
- At which frequency does a switch mode transformer usually operate? A. Between 50 Hz and 200 Hz B. Exactly 50 Hz C. Between 50 kHz and 200 kHz D. Between 50 MHz and 200 MHz Answer: C
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