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ECE 201 Circuit Theory I1 Capacitance Capacitance occurs whenever electrical conductors are separated by a dielectric, or insulating material. Applying a voltage to the conductors can displace the charge within the dielectric. Current does not actually flow through the dielectric.

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ECE 201 Circuit Theory I2 Capacitance Capacitor –Structured as two parallel plates

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ECE 201 Circuit Theory I3 Capacitor Circuit Symbol (a) Component designation (C) Units – Farads –Usually μF or pF Reference directions for voltage and current (b)

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ECE 201 Circuit Theory I4 Voltage-Current Relationship

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ECE 201 Circuit Theory I5 Observations The voltage across a capacitor cannot change instantaneously (the current would be infinite). If the voltage across the terminals is constant, the current will be zero. (looks like an open circuit). Only a time-varying voltage can produce a displacement current.

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ECE 201 Circuit Theory I6 Express the voltage across the capacitor as a function of the current

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ECE 201 Circuit Theory I7 Power and Energy for the Capacitor

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ECE 201 Circuit Theory I8 Example 6.4 A voltage pulse described as follows is applied across the terminals of a 0.5μF capacitor:

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ECE 201 Circuit Theory I9 Derive the expressions for the capacitor current, power, and energy.

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ECE 201 Circuit Theory I10 Power

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ECE 201 Circuit Theory I11 Energy

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ECE 201 Circuit Theory I12 Energy is being stored whenever the power is positive. Energy is being delivered by the capacitor whenever the power is negative.

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