22 Questions
What is the main difference between resistors and capacitors?
Capacitors store energy, while resistors dissipate energy.
What is the purpose of the dielectric material in a capacitor?
To separate the two conducting plates.
What is the unit of capacitance?
Farad (F)
What is the symbol for a capacitor?
--| |--
What happens to the current flow when the capacitor is discharging?
It flows out of the positive terminal
How does the capacitance of a parallel-plate capacitor change when the distance between the plates is increased?
It decreases.
What is the relation between voltage and current in a linear capacitor?
i = Cdv/dt
What happens to the charge on a capacitor when the voltage across it is increased?
The charge increases.
What is the voltage across a capacitor dependent on?
The past history of the capacitor current
When is a capacitor being charged?
When current flows into the positive terminal.
What is the expression for the energy stored in a capacitor?
W = Cv^2/2
What is the definition of capacitance?
The ratio of the charge on one plate to the voltage difference between the plates.
What happens to the current through a capacitor when the voltage is not changing?
It becomes zero
Why does a capacitor resist an abrupt change in voltage?
Because it has memory
What is the power in a capacitor given by?
p = vi
What is the condition for a capacitor to be linear?
i = Cdv/dt
What is required for a discontinuous change in voltage?
An infinite current
What is a characteristic of an ideal capacitor?
It does not dissipate energy
What is typically included in a nonideal capacitor model?
A parallel-model resistance
What is the expression for the current through a capacitor?
i(t) = C dv/dt
What is the voltage across the 5µF capacitor in Example 1?
v(t) = 10 cos 6000t V
How much is the current through the capacitor in Example 1?
-0.3 sin 6000t A
Study Notes
Capacitors and Inductors
Capacitors
- Capacitors store energy in their electric field.
- A capacitor consists of two conducting plates separated by an insulator (dielectric).
- The charge stored is proportional to the applied voltage, q = Cv.
- Capacitance (C) is the ratio of the charge on one plate to the voltage difference between the plates.
- Unit of capacitance: farad (F).
- Capacitance depends on physical dimensions of the capacitor.
Capacitor Characteristics
- The current flows into the positive terminal when the capacitor is being charged.
- The current flows out of the positive terminal when the capacitor is discharging.
- Linear capacitors satisfy Equation 5.3: i = C dv/dt.
- Voltage-current relation: v = (1/C) ∫ i(t) dt + v(t0).
- Capacitor voltage depends on the past history of the capacitor current – has memory.
Power and Energy
- Instantaneous power: p = vi = Cv dv/dt.
- Energy stored: w = (1/2) Cv^2 or w = q^2/2C.
Important Points
- A capacitor is an open circuit to dc; the voltage across a capacitor must be continuous.
- The ideal capacitor does not dissipate energy.
- A real, nonideal capacitor has a parallel-model linkage resistance.
Examples
- Example 1: Calculate the current through a 5µF capacitor with a voltage v(t) = 10 cos 6000t V.
- Example 2: Calculate the voltage across an initially charged 1-mF capacitor with a given current waveform.
This quiz covers the fundamentals of capacitors and inductors, including their construction and energy storage. It explores the differences between resistors and storage elements, and provides an introduction to electric fields.
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