6.1 Capacitors Flashcards
(17 cards)
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
Describe an investigation to measure the capacitance of a capacitor?
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
What is the apparatus for charging and discharging a capacitor and what happens if you add a resistor into the circuit?
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 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
Describe and draw the graphs for charging current, charge across capacitor and pd across the capacitor over time for a capacitor that is being charged?
Draw the graphs for discharging current, charge and p.d. of capacitor as it is discharging?
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
What is the work done/energy stored in a capacitor?
