21 - Capacitance

Question 1

capacitor

Answer 1

a component that stores charge, consisting of two plates separated by an insulator (dielectrics)

Question 2

capacitance

Answer 2

charge stored per unit potential difference across a capacitor

Question 3

farad

Answer 3

unit of capacitance, 1F = 1CV^-1

Question 4

supercapacitors

Answer 4

compact specialist capacitors with capacitance values in the thousands of farads

Question 5

total capacitance in series

Answer 5

store less charge for a given pd
1/C = 1/C1 + 1/C2 + …

Question 6

total capacitance in parallel

Answer 6

store more charge for a given pd
C = C1 + C2 + …

Question 7

how to read capacitance directly from piece of electrical equipment

Answer 7

use a multimeter set to capacitance or farads, place in parallel across whatever you want to measure capacitance of

Question 8

how energy is stored in a capacitor

Answer 8

external work provided by a power supply must be done to push electrons onto the negative plate, and to cause electrons the leave the positive plate - energy stored comes from energy of the power supply

Question 9

exponential decay

Answer 9

a constant ratio process in which a quantity decreases by the same factor in equal time intervals

Question 10

time constant (𝜏)

Answer 10

the product of capacitance and resistance, CR, for a capacitor-resistor circuit - equal to time taken for pd, current, or charge to decrease to e^-1 of its initial value when the capacitor discharges through a resistor

Question 11

discharging capacitor through a resistor

Answer 11

• charge is transferred from one capacitor plate to the other via current flowing through resistor
• charge stored by capacitor decreases with time, therefore pd across it decreases, therefore current in resistor decreases, eventually the pd, charge stored by capacitor, and current in resistor are all zero

Question 12

charging a capacitor

Answer 12

• when switch is closed there is briefly a maximum current in the circuit, and the capacitor starts to charge
• pd across capacitor increases from zero as charge is gathered
• since VR + VC = V0 (Kirchhoff 2), VR decreases as VC increases with time
• after a time dependant on CR, capacitor will be fully charged with pd of V0 - VR and current will be zero

Question 13

4 uses a capacitor is suited to because it can store energy and deliver it quickly

Answer 13

-defibrillator
-flash photography
-back-up power / UPS
-current smoothing

Question 14

why is a capacitor useful for a timing circuit

Answer 14

capacitor circuits can be slow and take time to charge