6.1 Capacitors Flashcards
What’s a capacitor
An electrical component that stores charge on two separated metallic plates
Why is an insulator placed between the plates in a capacitor
To prevent the charge from travelling across the gap
How is capacitance defined
The charge stored per unit potential difference across the two plates
What’s the equation for capacitance
C=Q/V
Q= charge stored
V= potential difference across the two plates
What’s capacitance measured in
Farads F (CV^-1)
Why does the first plate in a capacitor have a charge of +Q and the second -Q
When a capacitor is connected to a DC power supply there is a brief current as the power supply draws electrons from one plate and deposits them on the other
Why are the charges on the plates of a capacitor equal and opposite
Due to the conservation of charge
How long will current flow in a circuit with a capacitor
Until the potential difference between the plates is equal to that of the e.m.f. of the power supply
What’s another purpose of a dielectric insulator in a capacitor
To increase the capacitance
How does a dielectric insulator increase the capacitance of a capacitor
By polarising in the electric field and effectively increasing the charge on the plates
What does the electric permittivity of a dielectric describe
The ability to polarise and strengthen the charge storage capability of the device
What’s the equation for the total capacitance of capacitors in series
CT=(1/C1+1/C2+1/C3+…+CN)^-1
How does Kirchhoff’s second law help derive the total capacitance in series
-V=V1+V2+V3+…+VN V=Q/C
-Q/CT= Q/C1+ Q/C2+ Q/C3+…+ Q/CN charge is constant
-1/CT= 1/C1+ 1/C2+ 1/C3+…+1/CN
-CT= (1/C1+1/C2+1/C3+…+CN)^-1
What’s the equation for total capacitance in parallel
CT=C1+C2+C3+…+CN
Why must work done be done by a power supply to deposit negatively charged electrons onto the negative plate of a capacitor
Because like charges repel according to Coulomb’s law
Why does work need to be done to remove electrons from the positive plate of a capacitor
Negative charges are attracted to positive regions
What are the equations for work done in charging up a capacitor
W= 1/2QV or 1/2V^2C or 1/2Q^2/C
What are capacitors used for
Storing and discharging large quantities of energy in a short time period
What are capacitors useful for
Short pulses of energy such as camera flashes and touch screens
