C21 (Capcitance)

Question 1

What’s a capacitor

Answer 1

Electrical compound in which change is separated (variety of shapes and sizes but basic construction same).

Question 2

What do capacitors consist off

Answer 2

2 metallic plates separated by an insulator (dielectric) such as air paper, ceramic, mica

Question 3

What’s EMF

Answer 3

Energy given per unit charge

Question 4

What’s the Def of the capacitance of a capacitor

Answer 4

The charges stored per unit p.d across it

Question 5

When is the current in a circuit =0

Answer 5

When the p.d across the plates is equal to the e.m.f of cell
The capacitor is then fully charged (net charge of zero)

Question 6

What does a capacitor do in terms of ectrical charge

Answer 6

Device separates electrical charge into -Q and +Q

Question 7

What is current directly proportional to

Answer 7

Voltage

Question 8

Equation for Capacitance

Answer 8

Q/V

Question 9

State the properties/ uses of capacitors

Answer 9

-Compact

-Can be charged easily to store energy

-Cannot store great deal energy in a small vol, however can release stored energy very quickly (generate high output power) camera flash.

-Used provide back up power for computers and emergency lighting when main supplies cut off briefly.

Question 10

Supercapacitor

Answer 10

Compact specialist capacitors
Capacitance value in the thousands of farads

Question 11

What are supercapacitor used as an alternative for

Answer 11

Battery packs
Memory backup devices
Emergency lighting

Question 12

What feature does supercapacitor have that rechargable batteries don’t

Answer 12

Rechargeable batteries degrade over time, supercapacitor can be used over and over again

Question 13

Capacitances value for capacitors in parallel

Answer 13

Together greater than individual (so they store more charge for given p.d)

Total Capacitance= C1 + C2 + …

Question 14

Charge across capacitors in parallel

Answer 14

Conserved
Therefore total charge stored = sum of the individual stored, Qt= Q1+Q2+…

Question 15

What do capacitors have the ability to do

Answer 15

Store energy

Question 16

Amount energy stored in capacitor depends on…

Answer 16

Value of a capacitance and the initial p.d across it

Question 17

What’s the equation for work done (capacitors)?

Answer 17

QxV
1/2 x Q x V
1/2 x V^2 x C
1/2 x Q^2/C

Question 18

The area underneath p.d and charge graph

Answer 18

Work done

Question 19

Where does the energy stored in capacitors come from

Answer 19

From energy of battery/ power supply

Question 20

A capacitor is discharged through a …

Answer 20

Resistor

Question 21

What’s towl equal to

Answer 21

RC

Question 22

For charging a capacitor what can we say about the p.d across different components in the circuit

Answer 22

Kirchoff’s second law

Question 23

What happens to the p.d across a resistor as a capacitor charges up and why

Answer 23

It drops exponentially, because as the capacitor charges up the current will be dropping exponentially as the capacitor fills with charge, to the point where the p.d across the capacitor is equal to the e.m.f of the power supply

Question 24

Features of capacitors

Answer 24

Compact

Can be charged easily, to store energy

Unlike chemical cells, they can’t store great deal energy in a small vol, however can release stored energy very quickly, generate high output power

Question 25

What are capacitors used for

Answer 25

Releasing stored energy very quickly, generates high output power (camera flash)

Used provide back up power for computers and emergency lighting when main supplies cut off briefly

Question 26

Equation for power output of a capacitor

Answer 26

E stored/ t of discharge

Question 27

How do camera flashes work

Answer 27

They have large power outputs (bright flashes) by discharging their capacitors in a very short period of time

Question 28

Smoothing capacitors

Answer 28

They smooth the waveform out
(Found in rectifier circuits, maintaining a fairly constant voltage value in circuit)