C21 (Capcitance) Flashcards
What’s a capacitor
Electrical compound in which change is separated (variety of shapes and sizes but basic construction same).
What do capacitors consist off
2 metallic plates separated by an insulator (dielectric) such as air paper, ceramic, mica
What’s EMF
Energy given per unit charge
What’s the Def of the capacitance of a capacitor
The charges stored per unit p.d across it
When is the current in a circuit =0
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)
What does a capacitor do in terms of ectrical charge
Device separates electrical charge into -Q and +Q
What is current directly proportional to
Voltage
Equation for Capacitance
Q/V
State the properties/ uses of capacitors
-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.
Supercapacitor
Compact specialist capacitors
Capacitance value in the thousands of farads
What are supercapacitor used as an alternative for
Battery packs
Memory backup devices
Emergency lighting
What feature does supercapacitor have that rechargable batteries don’t
Rechargeable batteries degrade over time, supercapacitor can be used over and over again
Capacitances value for capacitors in parallel
Together greater than individual (so they store more charge for given p.d)
Total Capacitance= C1 + C2 + …
Charge across capacitors in parallel
Conserved
Therefore total charge stored = sum of the individual stored, Qt= Q1+Q2+…
What do capacitors have the ability to do
Store energy
Amount energy stored in capacitor depends on…
Value of a capacitance and the initial p.d across it
What’s the equation for work done (capacitors)?
QxV
1/2 x Q x V
1/2 x V^2 x C
1/2 x Q^2/C
The area underneath p.d and charge graph
Work done
Where does the energy stored in capacitors come from
From energy of battery/ power supply
A capacitor is discharged through a …
Resistor
What’s towl equal to
RC
For charging a capacitor what can we say about the p.d across different components in the circuit
Kirchoff’s second law
What happens to the p.d across a resistor as a capacitor charges up and why
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
Features of capacitors
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
What are capacitors used for
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
Equation for power output of a capacitor
E stored/ t of discharge
How do camera flashes work
They have large power outputs (bright flashes) by discharging their capacitors in a very short period of time
Smoothing capacitors
They smooth the waveform out
(Found in rectifier circuits, maintaining a fairly constant voltage value in circuit)
