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Capacitors

Capacitors. Separating charge and storing energy. In this section we will learn: What makes up a capacitor How it charges and discharges How much energy it can store What CAPACITANCE means and how it is measured. A useful analogy to model capacitance upon.

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Capacitors

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  1. Capacitors Separating charge and storing energy.

  2. In this section we will learn: • What makes up a capacitor • How it charges and discharges • How much energy it can store • What CAPACITANCE means and how it is measured. • A useful analogy to model capacitance upon.

  3. Capacitors STORE electrical energy. They SEPARATE electrical charge. At their simplest, they are two metal plates separated by an air gap.Sometimes they are rolled up like a roll of tape with an insulator between the plates. When they are charged up, electrons move round the circuit off one plate and onto another. Electrons never move across the gap.

  4. V - charge flows onto plate - charge flows off the plate +Q -Q Air, or another insulator

  5. As negative charge builds up on the plate, it becomes harder for more electrons to arrive, since the existing charge opposes it. Eventually the p.d. across the capacitor is the same as the cell, but it TAKES TIME for the charge to be deposited on the plates. Because the flow of charge is opposed, the CURRENT slows down, too. We’ve used this in GCSE as a time delay. Max V = same as cell V time

  6. Current And this curve looks very much like others that we’ve met…….. What do you think the AREA under this curve might represent? It’s the CHARGE separated by the plates. Time

  7. To DISCHARGE the capacitor, you just remove the cell, and attach a wire from one side of the capacitor to the other. The current then flows in reverse. Time Current flows in the opposite direction, but follows the same curve. Electrons flow in this direction V drops to zero Current

  8. ….and V “decays” in a familiar manner: Might this be exponential??? V time

  9. There’s a very good analogy for capacitors: Water Dam Pressure difference increases with water behind dam. -Q +Q Potential difference V increases as amount of charge separated increases. Charged plates

  10. So far, nothing about this component is CONSTANT. Can we find something that stays the same? How about the ABILITY of the capacitor to separate charge? At last… a constant. The gradient of the line is called CAPACITANCE. Charge Q, separated, in Coulombs V

  11. This is a very useful quantity. Since Q is PROPORTIONAL to V We can say that Q = constant x V Q = C V The units of Capacitance are FARADS, named after Michael Faraday, the “father” of electricity. A capacitance of 1 Farad will separate 1 coulomb of charge for every volt applied across the capacitor.

  12. Capacitance depends on: • Size of plates • Distance between plates • The insulator between the plates • There are lots of experiments you can do to check out the variables. • You can separate more charge if: • The plates are big • The plates are very close together • The insulator is very good.

  13. How much energy is stored in a capacitor? Total energy stored is area under the line p.d V V1 Work done to add Q is W = Q x V1 Q Charge separated Q in C

  14. If the total work done to charge up the capacitor is the area under the line, then this is the total energy stored by the capacitor. Area under the line = ½ Q V But we know that Q = C V So we can write Energy stored = ½ Q V = ½ C V V = ½ C V2

  15. Example. A capacitor is used to store energy in a camera flash unit. It has a capacitance of 470 mF and can be charged to 12 V. The Capacitor can discharge through the flash in 0.20 ms. How much energy is stored in the capacitor when it’s fully charged? Calculate the average power of the capacitor as it causes the flash. Why do we use the word “average”? Answers: Energy stored = ½ CV2 = 0.5 x 470 x 10-3 x 152 = 52.9 J Power =rate of energy transfer = 52.9/0.20 x 10-3 = 265 kW “Average” because the capacitor doesn’t discharge at a constant rate.

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