# capacitors

• Created by: argyrou
• Created on: 30-04-18 17:15
What is the function of a capacitor?
A device designed to store charge
1 of 26
Define capacitance
the charge stored per unit potential difference
2 of 26
That is the equation for capacitance
c=Q/V
3 of 26
what is the unit of capacitance?
Farrads F
4 of 26
What is the relationship of charge and potential difference when charging a capacitor?
they are proportional
5 of 26
what are the 2 equations to calculate energy stored in a capacitor?
E=CV²/2 = QV/2
6 of 26
what is the area under a pd against charge graph?
energy stored by the capacitor
7 of 26
what is the energy that is stored by the capacitor?
Electric potential energy
8 of 26
when discharging a capacitor, what happens to the current charge and pd and what is the term for this?
they all decrease when discharging and they decrease exponentially.
9 of 26
what is the percentage that the charge/current/pd drops down to after 1 time constant?
37%
10 of 26
What is the exponential equation to find charge when discharging a capacitor?
Q=Q e^(-t/RC)
11 of 26
what three quantities decrease exponentially when a capacitor is discharging in the form x=x e^(-t/RC)?
current, charge and pd
12 of 26
How many time constants does it take to discharge a capacitor? (pretty much)
5 (5RC)
13 of 26
What is the time constant?
RC - the time taken to discharge a capacitor to 0.37 of its initial value
14 of 26
what happens to voltage and what happens to current once a capacitor is fullt charged?
pd=source pd and current=0
15 of 26
What quantity has the same exponential graph when charging and discharging?
current
16 of 26
What does an exponential graph look like when discharging a capacitor?
Q against t graph:
17 of 26
what does an exponential graph when charging a capacitor look like (ignoring current)?
Q agaisnt t graph:
18 of 26
what is the equation for charge when charging a capacitor?
Q=Q (1-e^(-t/RC))
19 of 26
In the exponential equations, what is the value of e^-t/RC when t=0?
1
20 of 26
in the exponential equations, what is the value of e^-t/RC when t=infinity?
0
21 of 26
What is a dielectric?
electrically insulating materials that increase the ability of a parallel-plate capacitor to store charge.
22 of 26
What is relative permativity and what are the 3 different ways of calculating it?
Its the ratio of charge stored with the dielectric to the chaarge stored without the dielectric. =Q/Q =C/C =I/I
23 of 26
What is the capacitance of an 'empty' (no dielectric) capacitor c ?
(surface area of each plate x permitivity of free space)/spacing between the plates = A /d
24 of 26
What is the capacitance of a capacitor with a dielecrtic in it?
C=A /d
25 of 26
What are the 3 ways of getting a larger capacitance?
- make surface area of each plate as large as possible - making plate spacing as small as possible - filling space between plates with dielectric with largest possible relative permitivity
26 of 26

## Other cards in this set

### Card 2

#### Front

Define capacitance

#### Back

the charge stored per unit potential difference

### Card 3

#### Front

That is the equation for capacitance

#### Back ### Card 4

#### Front

what is the unit of capacitance?

#### Back ### Card 5

#### Front

What is the relationship of charge and potential difference when charging a capacitor?

#### Back ## Comments

No comments have yet been made

## Similar Physics resources:

See all Physics resources »See all Capacitors resources »