10. Capacitors Flashcards
(24 cards)
What is capacitance?
The amount of charge it is able to store per unit potential difference across it.
Which equation links C, Q and V?
C = Q / V
What power source should be used with a capacitor?
D.C.
What does an I - t graph look like for a capacitor?
A straight line horizontally.
What does a Q - V graph look like for a charging capacitor?
A straight diagonal line through the origin.
What are some applications for capacitors?
- Camera flash
- Ultracapacitors as reliable backup power for short periods of time
- Smoothing out variations in D.C. voltage supplies, fills in troughs and smooths out peaks.
How is electric potential related to capacitors?
Because the plates of a capacitor are negatively and positively charged, the charges on the plates are being forced together which requires energy, which is supplied by the power source and stored as electric potential energy. When the charges are released, so is the electric potential energy.
What are the equations for energy stored in a capacitor?
E = 1/2 QV
E = 1/2 CV^2
E = 1/2 (Q^2 / C)
What is permittivity?
A measure of how difficult it is to generate an electric field in a medium. The higher the permittivity, the more charge that is needed.
What is the equation for relative permittivity/dielectric constant?
ε(r) = ε(1) / ε(0)
What are polar molecules?
Molecules with a positive and a negative end.
How does a charged capacitor affect a dielectric with polar molecules, and how does this in turn affect the capacitance?
Initially the molecules are aligned randomly, however after a charge is applied, an electric field is created between them so the polar molecules align with this field. Because the negative end is attracted to the positive plate and vice versa, and the molecules all have their own electric field, the electric field between the plated decreases as it is being opposed, so the potential difference needed to transfer a given charge to the capacitor decreases, so the capacitance increases.
What is the equation for capacitance?
C = (Aε(0)ε(r)) / d
Where A is the effective area of the plate, and d is the distance between the plates.
What does the I - t graph look like for a charging capacitor?
A bit like an inverse square graph, where it starts on the y-axis at I(0).
What does the V - t graph for a charging capacitor look like?
A bit like a square root graph, where it starts at the origin, rapidly increases, then levels off.
What does the Q - t graph for a charging capacitor look like?
A bit like a square root graph, where it starts at the origin, rapidly increases, then levels off.
What direction does the current flow in when a capacitor discharges?
The opposite direction to the charging current.
What does the I - t, V - t, and Q - t graph look like for a discharging capacitor?
All are like inverse square graphs, starting on the y - axis at I(0), V(0) and Q(0) respectively.
What does the gradient of the graph of ln(Q) against t equal?
- 1 / RC
What is RC?
The time constant.
What affects the time taken for a capacitor to charge and discharge?
- The capacitance of the capacitor
- The resistance of the circuit
What is the tine constant?
The time taken for the charge on a discharging capacitor to fall to about 37% (1/e) of Q(0). Also the time taken for it to rise to about 63% when charging.
In practice, approximately how long does a capacitor take to charge or discharge fully? in terms of the time constant.
5RC
What is the equation for the time to halve?
T(1/2) = ln(2)RC
