CAPACITORS Flashcards
What are capacitors made from
Two conducting plates separated by a gap or an insulating material.
Why is there a PD across capacitor
The +ve and -ve plates build up charge charge but this cannot transfer due to the gap and therefore a uniform magentic feild is created +ve to -ve which cuases a P.D
Capacitor equation
Q=CV. Charge = capacitance (farda) x voltage
Capacitance in series circuit
The series the total is 1/c = 1/c1+1/c2 +…. This is opposite to reistance
In parallel circuit for capacitance
You say Ctot = C1+C2+C3+….
Capacitor graph for charge and P.D
The gradient of the graph gives the total capacitance, if there is more then one line, the total off all gradients are the capacitance
For capacitor graph current against time
Area = It there fore the charge, so area under graph gives total charge stored by capacitor
How to investigate capacitors in series and parrallel
- set up circuit with power supply, variable resistor and ammeter
- adjust the veraible resistor to keep current constant
- data logger contected to voltmeter can see voltage change over time
- open switch and let discharge
- redo with different type of circuit
- for analysis do a graph for current against time and charge against p.d
Work done and capacitor and energy
Energy stored on a capacitor is equal to the work done to deposit charge on the plate
Graph energy on capacitors
P.d across capacitor is equal to proportional to the charge stored on it, so graph straight line through origin and therefore E=1/2 x QV
All equation for energy stored by capacitor
E=0.5(v)^2 x C. And E=0.5QV. And E = 1(Q)^2 / 2C
Uses of capacitors
Camera flash, can discharge very quickly
Back up power supply, can store charge to mitigate small power interruptions
Smoothing out P.D, smooth out peaks and troughs
What happens when charging a capacitor
When switch is closed charge starts to deposits on the negative terminal this then repels electrons off positive plate which causes an equal but opposite charge on positive plate, initially the current is high but once the capacitor is fully charged the current is 0
Discharging capacitors how to
Set up charged capacitor with a data logger which can see voltage and current over time and set up a resistor of a known resistance for the capacitor to discharge through +_Q will get closer to 0 over time as current decreases
Exponential decrease when discharging capacitor.
Q= Q0e^-t/CR. Q’s can be substituted to be I or V if looking for initial or end of either of the three.
When doing the maths divide the Q/Q0 before doing ant log removal of e
When charging a capacitor maths.
Q= Q0(1-e^-t/cr). Q can be replaced by V or I to get charging levels for the capacitor.
What affects time taken to charge or discharge a capacitor?
- the capacitance of the capacitor as affects amount of charge that can be transferred at a given voltage
- the resistance of the circuit as current is affected.
What is time constant
Time constant = CR this is then time taken from the charage, P.D or current to fall to 37% of its initial value.
Also time taken from the charage, P.D or current to rise by 63% of maximum value when charaging
How dose the discharge resistor affect time to discharge a capacitor?
The larger the resistor the in series with capacitor the longer it takes to discharge, the time for full charge of discharge is about 5CR
