Capacitance Flashcards
What are capacitors?
Electrical components in which charge is separated
what is capacitance? What is the unit?
The charge stored per unit p.d. across it. The unit is Farad.
Formula for capacitance?
C = Q/V
What can capacitors be used for?
alternative to battery packs, memory back up devices, and emergency lighting
What happens when capacitors are in parallel?
Together, their capacitance is greater than their individual capacitance.
Total charge: Q1 + Q2 + … = Qtotal
Total capacitance: C1 + C2 + … = Ctotal
What happens when capacitors are in series?
The individual capacitance value is greater then the total value of capacitance.
1/C = 1/C1 + 1/C2 + 1/C3 …
What is different with p.d. when the capacitors are in series?
According to Kirchhoff’s second law, the total p.d. V across the combination is the sum of the individual p.d.s across the capacitors,
V = V1 + V2 + …
What happens to charge when capacitors are in series?
The charge stored by each capacitor is the same
How do capacitors work?
Cells drive charge around a circuit. Capacitors will store some of this charge. They will fill with charge until the repulsion between negative electrons is as strong as the voltage from the cell trying to drive them onto the capacitor.
Why might you join a capacitors in parallel rather than series, in a circuit?
The total capacitance is greater then individual capacitance for parallel, so combination will store more charge for a given p.d. Whereas for series, it is smaller together.
Define discharging capacitors
A capacitor’s stored charge will be lost when it’s connected in a complete circuit.
What does the rate of discharge of a capacitor depend on?
- the amount of charge stored to begin with
- the circuit components through which the electrons need to pass in order to discharge
draw a circuit to demonstrate the discharge of capacitors through a resistor
look at physics tuition book
Describe the process of discharging a capacitor
the capacitor discharges through the voltmeter. Charge stored by capacitor decreases with time and hence the p.d.across it also decreases accordingly. Eventually, the p.d., charge stored by C, and current in resistor are all 0.
What does Io, Vo and Qo mean?
Io - max current at t=0
Vo - p.d at t=0
Qo - charge at t=0
how does the graph look for charge, current and p.d. when capacitor is discharging? What does that mean?
all of the graphs are decreasing exponentially. This means that the same period of time constantly reduces the charge, voltage and current by the same proportion. This is the constant-ratio property.
Define time constant and give formula. What is the time constant of capacitance?
Time constant - time taken for p.d./current/charge to decrease to e^-1 of its initial value
The time constant of C - resistant circuit is equal to the time taken for p.d. to decrease to e^-1 (about 37%) of initial value.
How does a charging circuit look like?
look on physics tuition book
How do we know the capacitance is charging from a graph?
the second graph in physics tuition book. The Vc is increasing on the graph so the cpacitance is rising.
What factors should be written down before showing the process of charging a capacitor? (4 factors)
- battery provides constant e.m.f (Vo)
- capacitance is C
- Resistor is R
- capacitor is initially uncharged and switch is open.
Describe the process of charging a capacitor when the switch is closed
- max current is in the circuit
- capacitor starts to charge up
- p.d. across capacitor (Vc) increases as it gathers charge
- Based on Kirchoff’s 2nd law, p.d. across resistor (Vr) and Vc must add up to Vo. Therefore, as Vc increases, Vr must fall
- Later on, depending on time constant (CR), capacitor will be fully charged
How do you know a capacitor is fully charged? (3 reasons)
- p.d. would equal to Vo (e.m.f)
- Vr will be zero
- current will be zero
1 Mega ohms to ohms?
1 Uc to C (capacitance)?
10^6
10^-6
define 1 farad
1 coloumb per unit volt
