6.1 Capacitors Flashcards
What is a capacitor?
-A circuit component that stores energy by separating charges onto two electrical conductors
-The capacitance of a capacitor is the quantity of charge per unit potential difference across the capacitor
-The unit for capacitance is a farad which is one coulomb per volt
What is the structure of a capacitor?
-Two conductors separated from another by a gap with a dielectric material
-The width of the gap affects affect the amount of charge that can be transferred to each plate
What happens when a capacitor is connected to a source of e.m.f?
-Charge can’t flow through the plates of the capacitor
-Electrons will flow from the negative terminal through the circuit and onto one plate which becomes negatively charged and the negative terminal becomes positively charged
-This results in a potential difference and once the capacitor is fully charged it will be equal to the e.m.f of the cell
What is the equation for capacitance?
Capacitance=charge/potential difference
What is the total capacitance and charge of a circuit with capacitors in parallel equal to?
-The total charge is equal to the sum of the capacitors charge
-The total capacitance must also be equal to the sum of the capacitance of the capacitors as (Q=CV)
What is the total capacitance of capacitors in series?
-Applying Kirchhoff’s second law, the e.m.f of the battery will be equal to the sum of the potential differences of the individual capacitors (V=V1+V2+V3)
-As V=Q/C and charge cancels out the total charge of the circuit is (1/C=1/C1+1/C2+1/C3)
What are the uses of capacitors
-Capacitors can only be used to store small amounts of energy compared to a battery and they also slowly discharge due to charge leakage meaning they only really have uses in temporary energy storage
-E.g Flash devices for cameras or Back up energy supplies
How is a capacitor charged?
-When a capacitor is connected to a source of e.m.f electrons flow onto one plate and move off the other plate, until the potential difference across the plates are equal to the potential difference across the cell.
-When the movement of electrons stop the capacitor becomes fully charged
How does resistance affect the charging and discharging of capacitors?
-The greater the resistance the longer the capacitor will take to become charged or discharged
-The values for p.d charge and current however stay the same but the rate at which they accumulate or a released is affected.
What are two factors that affect the time taken for a capacitor to become fully charged or discharged?
-The resistance of the circuit will affect the rate at which the charge will flow
-The capacitance of the circuit is related to the amount of charge that can be stored across the plates of the capacitor
What are the equations for discharging a capacitor involving a time constant
Q=Q₀e^-t/RC
V=V₀e^-t/RC
I=I₀*e^-t/RC
How can you find the time constant of a discharging capacitor from a graph?
-Take natural logs and rearrange the formula
for e.g lnQ=LnQ₀*-t/CR
