Capacitsnce Flashcards
Remember to check if charging or discharging to use aproeptie equations
Pulling pushing what constsnt , pushing what decrease sand why
Charge and electric field
When pushing, use c = ea /d equation, e and a constant so d increases cappatance must decrease
- now you have this info use it in other equation tk see voltage must increase
- and if voltag and distance increase at same rate, ELECTRIC FIELD MUST DTAY THE SAME
When describing how charge is formed on both capacitor plates, should you use conventional or actual current
Use ACTUAL CURRENT flow, this time as we are talking about electrons
2 WAYS How to find time constanr experimentally using discharge setup
Which one to use?
1) Easy way is to time how long till 37%
2) long way is to use ln v / t and gradient which is better way
If 3 mark use easy if nit use long
3 marks why does capacitor come charged
Exact wording
1) electrons travel NON CONVENTIONAL CURRENT deposited at A
- this creates a build up of charge
- this repels electrons away from B and these eventually arrive at A, giving it a positbe charge
-
2) as same number of electrons arrive at A are repelled and leave B, charge formed is the SAME in magnitude
3) A positbe B negative
Remember when finding VOLUME if a wire what to do
Pi r 2 x the LENTGH IF WIRE like it’s a cylinder
How would you find AVERAGE CURRENT between discharges and charges
So need an equation relates charge to time
Current = charge / time
Can work out total charge calm
- and if you know the average time this runs for (so the time taken for it to COMPLETELY DISCHARGE)
- then even tho as it’s expoential decay, doing total charge / total time = average current
So total charge / total time (for discharge)= average current
How ti find average anything
No matter the weird relationship remember average will always be a simple x /y type thing
Total x /total y
Try find equation where y is time
So question about average current which ended up being charge / frequency
Where frequency 1/f represented time taken to close and open switch snd assumed before it was fully charged and discharged
However frequency doubled why doesn’t current double
Use previous parts of question and time constsnt
Now the time constsnt is COMPARABLE to time period
As a result if it’s comparable, it means the cappacitor won’t have enough time to DISCHARGE, the time period must be much bigger than time cinstsnt ti allow for this
Thus WHATS happening is it begins ti discharge and then charged again, begins and then charges again
- thus EQUATION for “ total charge being lost over time “ to give current wint work because it’s not fully discharging each time
And thus is because time constsnt is comparable to the time taken between restarts
So basically if time constsnt is comparable to the time taken before restarts, what does this mean for how much the cappacitor has discharged
ALWAYS GO BACK TO TIME CONSTANTS AS COMPARISON
Basically if they are comparable, then means not enough time for discharge to happen,
so can’t find the average current as not all charge has gone in time
If you just leave a cappacitor casual, what will happen to the charge and why? And so what happens to the electric field
Discharge, so electric field cut
Why is there not 0 current when discharging through a resistor in the resistor wben though capacitor fully charged
Explain the shape of graph
1) because the no current is in thr BRANCH connecting the capacitor wire to the rest, remember it’d parallel, this falls to 0, the residotr branch still has MAX current
But when discharge occurs it becomes series, with current dictated by pd of the capacitor, which as it falls current falls too
Finally if the discharge is cut off, or circuit cuts, the voltsge will remain at whatever it’s at, but the resistor won’t have any current bevaus circuit is cut off!
Why is current in reisdotr in parallel to fully chsrged cappaciotr max
Why does current in residotr drop to 0 at the end in this q
Because the current in cappaciotr branch goes to 0, but current in residotr branch max
When discharge happens, then current in resistor, which is now dependent on capacitor emf, decreases
Finally if I stop the discharging , the capacitor will stay at constant emf, but the current in the residotr will drop to 0, if the circuit disconnected
If don’t know how to solve a question, what should you try and do
Should work out something you can and that ight score a mark
Remember if the resistance decreases in a graph, yeah time period but what else?
(For discharging )
For thr same voltage, so will the CURRENT
Remember max current intial is = to V / total R!
EASY MARK
