Chapter 6.1 - Capacitors Flashcards Preview

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Flashcards in Chapter 6.1 - Capacitors Deck (34)
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1

Capacitance

Charge stored per volt

2

Basic Equation for capacitance

C = Q/V

3

Structure of a capacitor

Two conductors separated by an insulator called a dieletric. Often rolled up to make it more compact.

4

Charging a capacitor (with reference to electrons)

When connected to a source of E.M.F. electrons will build up on the negative terminal of the capacitor. These electrons will then repel other electrons on the opposite plate, making it positively charged. As more electrons build up they repel further electrons from reaching the plate causing the current to decrease.

5

The P.D. over a fully charged capacitor

The same as that of the E.M.F

6

Discharging a capacitor (with reference to electrons)

Once the E.M.F is no longer being applied to the capacitor the electrons will flow from the negative plate through the wires.

7

How to experimentally measure capacitance

Set up a capacitance in series with an ammeter and a voltmeter around, along with a variable resistor. Turn on the power and adjust the variable resistor to keep the current the same (start it high). Regularly take readings for potential difference and current. Calculate charge at a given time by using Q = It. Plot a graph of Q against V and take the gradient of the line to be the capacitance. (Or you can use the final charger divided by the final voltage but this may be less reliable).

8

Measuring capacitance graphically

The gradient of a Q-V graph

9

Effective capacitance of multiple capacitors in parallel

C = C1 + C2 + C3...

10

Effective capacitance of multiple capacitors in series

1/C = 1/C1 + 1/C2 + 1/C3...

11

Energy stored by a capacitor from a graph

Area under a V-Q graph

12

Energy stored by a capacitor (basic equation)

E = (1/2)QV

13

Disadvantages of using capacitors as an energy store (2)

- Can't store much energy
- Energy slowly leaks out

14

Examples of capacitors used as energy storage (3)

- Flash devices for cameras. A capacitor stores energy which is then released very quickly, exciting a gas and causing a brief flash.
- Back up energy supply. Large capacitors can be used for a short term supply of back up energy (long enough to safely shut down computers and ensure data is saved in the event of a power cut).
- Pulsed power in nuclear fusion research. Large capacitors output power at extremely high values. The power output of the Z machine is greater than that of all power stations on earth combined!

15

At what point in charging a capacitor will there be the greatest current

At the beginning

16

Shape of a graph of Q against t for charging

1 - (exponential decay)

17

Shape of a graph of Q against t for discharging

exponential decay

18

Shape of graph of I against t

exponential decay for both charging and discharging

19

Effect of resistance on charging/discharging a capacitor

Takes longer to charge / discharge. Stored current is unchanged.

20

Factors affecting time taken to charge a capacitor

resistance and capacitance

21

τ (tau)

Time constant.
= CR

22

The main feature of exponential decay

rate of change of quantity is proportional to the magnitude of the quantity at that time

23

What is τ practically

The time taken for the charge on a capacitor to fall to 1/e (37%) of its original value

24

Equation for exponential decay

x = (x0)*e^(-kt)

25

Experiment to investigate charging/discharging a capacitor

Set up a circuit with a resistor, ammeter/datalogger and capacitor with a voltmeter around the capacitor. Charge the capacitor by connecting wires and record current and voltage regularly. Connect up the wires so that the charge can be discharged through a resistor and record current and voltage regularly.

26

Charge on left plate = 5C
Charge on right plate = -5C
Charge on capacitor = ?

5C

27

Determing τ from a log graph

plotting a graph of t against lnQ will give a straight line in the form
lnQ = ln(Q0) - (t/CR)
therefore the gradient will be -1/(CR) = -1/τ

28

Determing τ from a Q-t graph

Take a tangent and the gradient of the tangent is -Q/CR

29

dQ/dt

-Q/CR

30

Testing if a graph is exponential decay

Check if time to half is constant