Capacitors Flashcards
What direction is conventional current
Positive to negative (opposite to electron flow)
What is a capacitors do
An electrical component that stores charge on two separated metallic plates, an insulator is sometimes placed in between the plates to prevent charge from traveling accross the gap.
Describe the structure of a capacitor
They are 2 metal plates separated by an insulating layer, rolled up into a cylinder
What does the C stand for in capacitor equations and its unit
Capacitance
Units= farads
Define a Farad
When a capacitance of 1 Farad stores 1C of charge when 1V of p.d is applied across it
How is the negative and positive leg shown in the capacitor
The shorter leg is negative. The longer leg is positive
How can we increase the charge in a capacitor
Increase the area of the plates
Increase p.d
Place the metal plates closer together
Use an insulator with a greater permitivity
What happenes when a capacitor is connected to a DC supply
There id a brief current as the power supply draws electrons from one plate and deposits them on to the other. This leaves the first plate with +Q and the second with -Q, same magnitude of charge opposite charge therfore net charge+0 this is due to the conservation of charge . Current will flow in the circuit until the p.d between plates is equal to the emf of power supply
A student wishes to use 2 metal plates and a multimeter to measure the capacitance of the iron between the 2 metal sheets to determine a aloe for the permiativity of space by varying the separation between the 2 plates. How can this be carried out
How should they analyse the data
How can we determine the uncertainty
Method
Using a vernier calliper measure the distance between the 2 plates =d
Change the d by increments of 1 cm , from 1 cm to 10 cm
Then for each d measure the capacitance using multimeter , repeat 3 times for same d and find mean C
Use the calliper to measure width and length of plates to find area, area is kept constant
Analysis
Plot graph of C against 1/v so grad= permiativity X area
Draw a line of best fit should be straight line through origin
Uncertainty
Draw error bars using the 3 sets of data for capacitance and d
Then draw line of worst fit and then find the worst value for permiativity
Then find difference of the values
What is the equation for capacitors in parallel
C= C1+ C2+ etc
What is the equation for capacitors in series
1/C=1/C1 +1/C2 etc
Define exponential change
The rate of change of a quantity is proportional to the amount of that quantity, when energy is stored in a capacity is large the rate of flow of charge will also be large
Why is it that the rate of flow of charge is proportional to the quantity of charge
More charge = more electrons on -ve plate and therfore less on +ve plate due to repulsion, therefore p.d is greater therfore a larger rate of flow of charge, less electron = less p.d
Give the equation for the rate of change of voltage
Rate of change of V= -1/RC x V
What is the area under a charge against voltage graph?
Work done
What is the definition for time constant?
The time taken for the current charge of voltage to fall to one over E of its original value
Why isit important to use a voltmeter with a high resistance in a capacitor ircuit
So that the capacitor does not lose charge through the voltmeter
Explain an adv for using an analogue ammeter rather than a digital ammeter in a capacitor expe
Easer to read quickly
Explain why the current that charges the capacitor falls as the capacitor charges , there is a resistor connected in series to the capacitor
More charge leads to increased p.d accross the capacitor, this mean there is less of the p.d accross the resistor and therfore the current in circuit will also fall.
How do you find the time constant from a charging graph
63% of initial value, find corresponding time
Explain why a capacitor is unsuitable for a cordless telephone
Capacitor would need charging frequently, and can only power phone for short amount of time
The voltage accross capacitor woul fall continuously
electric powered vehicles both have ultracapacitors and batteries, ultra capacitors can store less charge than batteries however they can discharge very quickly, batteries can hold more charge but discharge at a slower rate, explain how the car uses this combination effectively in its journey
The ultracapacitor would be used for overtaking, acceleration, or going up a hill as it require large powers for a short period of time
The battery would be used for constant speed as smaller current is neede per second over a longer period of time
