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Questions and Answers
What is the main difference between resistors and capacitors?
What is the main difference between resistors and capacitors?
- Resistors store energy, while capacitors dissipate energy.
- Resistors are active elements, while capacitors are passive elements.
- Capacitors are used in AC circuits, while resistors are used in DC circuits.
- Capacitors store energy, while resistors dissipate energy. (correct)
What is the purpose of the dielectric material in a capacitor?
What is the purpose of the dielectric material in a capacitor?
- To reduce the voltage across the capacitor.
- To separate the two conducting plates. (correct)
- To decrease the capacitance of the capacitor.
- To increase the capacitance of the capacitor.
What is the unit of capacitance?
What is the unit of capacitance?
- Hertz (Hz)
- Siemens (S)
- Farad (F) (correct)
- Ohm (Ω)
What is the symbol for a capacitor?
What is the symbol for a capacitor?
What happens to the current flow when the capacitor is discharging?
What happens to the current flow when the capacitor is discharging?
How does the capacitance of a parallel-plate capacitor change when the distance between the plates is increased?
How does the capacitance of a parallel-plate capacitor change when the distance between the plates is increased?
What is the relation between voltage and current in a linear capacitor?
What is the relation between voltage and current in a linear capacitor?
What happens to the charge on a capacitor when the voltage across it is increased?
What happens to the charge on a capacitor when the voltage across it is increased?
What is the voltage across a capacitor dependent on?
What is the voltage across a capacitor dependent on?
When is a capacitor being charged?
When is a capacitor being charged?
What is the expression for the energy stored in a capacitor?
What is the expression for the energy stored in a capacitor?
What is the definition of capacitance?
What is the definition of capacitance?
What happens to the current through a capacitor when the voltage is not changing?
What happens to the current through a capacitor when the voltage is not changing?
Why does a capacitor resist an abrupt change in voltage?
Why does a capacitor resist an abrupt change in voltage?
What is the power in a capacitor given by?
What is the power in a capacitor given by?
What is the condition for a capacitor to be linear?
What is the condition for a capacitor to be linear?
What is required for a discontinuous change in voltage?
What is required for a discontinuous change in voltage?
What is a characteristic of an ideal capacitor?
What is a characteristic of an ideal capacitor?
What is typically included in a nonideal capacitor model?
What is typically included in a nonideal capacitor model?
What is the expression for the current through a capacitor?
What is the expression for the current through a capacitor?
What is the voltage across the 5µF capacitor in Example 1?
What is the voltage across the 5µF capacitor in Example 1?
How much is the current through the capacitor in Example 1?
How much is the current through the capacitor in Example 1?
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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.
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