32.4
Capacitor em um circuito AC
Um capacitor é carregado passando uma corrente elétrica por ele, o que faz com que as placas comecem a acumular uma carga eletrostática. Como a intens…
Considere um capacitor conectado a uma fonte de tensão de corrente alternada. Lembrando a regra do loop de Kirchhoff, a tensão instantânea e a carga no capacitor podem ser determinadas.
A taxa na qual a carga entra ou sai do capacitor é equivalente à corrente que flui através do circuito, e a relação trigonométrica pode ser usada para determinar a corrente instantânea.
Quando a tensão e a corrente são plotadas juntas, a corrente através do capacitor conduz a tensão através do capacitor em um quarto de ciclo.
A relação entre corrente instantânea e tensão pode ser representada usando diagramas fasoriais, onde ambos os fasores giram na mesma frequência angular, com o fasor de corrente conduzindo o fasor de tensão em π por 2 radianos.
A relação entre a tensão de pico e a corrente de pico fornece a reatância capacitiva do capacitor, expressa em ohms.
A reatância capacitiva do capacitor depende inversamente da frequência da fonte de corrente alternada, onde uma alta frequência leva a uma baixa reatância capacitiva e vice-versa.
View the full transcript and gain access to JoVE Core videos
Q1: Why does current lead voltage in a capacitor connected to an AC circuit?
In an AC capacitor circuit, current leads voltage by 90 degrees because the charging current is maximum when the supply voltage is zero and changing most rapidly. As voltage reaches its peak, the rate of change becomes zero, so current drops to zero. This phase relationship occurs because current depends on the rate of voltage change, not the voltage magnitude itself.
Q2: What is capacitive reactance and how does it relate to frequency?
Capacitive reactance is the opposition to current flow in a purely capacitive circuit, measured in ohms and denoted by XC. It depends inversely on the frequency of the alternating current source: high frequency produces low capacitive reactance, while low frequency produces high capacitive reactance. The ratio of peak voltage to peak current determines the capacitive reactance value.
Q3: How can phasor diagrams represent the relationship between current and voltage in a capacitor?
Phasor diagrams represent instantaneous current and voltage as rotating vectors at the same angular frequency. The current phasor leads the voltage phasor by π/2 radians (90 degrees), visually showing their phase relationship. Both phasors rotate together, maintaining this constant 90-degree separation throughout the AC cycle.
Q4: At what points in the AC cycle does maximum charging current occur in a capacitor?
Maximum charging current occurs at 0 degrees and 180 degrees on the sinusoidal waveform, where the rate of change of supply voltage is greatest. At 0 degrees, voltage increases in the positive direction; at 180 degrees, it decreases most rapidly. Zero current flows at 90 degrees and 270 degrees, when voltage reaches its peak values and stops changing momentarily.
Q5: How does a capacitor respond to changes in AC supply voltage?
A capacitor charges and discharges continuously in response to changes in AC supply voltage. The rate of voltage change across the plates is directly proportional to the charging current. When voltage switches between positive and negative half cycles, the capacitor alternately charges and discharges, with current flow determined by how rapidly the voltage is changing.
Q6: What is the phase difference between voltage and current in a purely capacitive AC circuit?
The phase difference between voltage and current in a purely capacitive circuit is 90 degrees, or a quarter cycle. Current leads voltage by this amount, meaning current reaches its peak value one-quarter of a cycle before voltage does. This phase relationship is fundamental to understanding capacitor behavior in AC circuits and differs from resistor in an ac circuit components.
Q7: How can Kirchhoff's loop rule be applied to determine instantaneous voltage and charge on a capacitor?
Kirchhoff's loop rule states that the sum of voltages around a closed loop equals zero. Applied to a capacitor in an AC circuit, this rule allows determination of instantaneous voltage across the capacitor and the charge on its plates. Trigonometric relationships between voltage and current enable calculation of these values at any moment in the AC cycle.