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5.1 Introduction. What Is Power?
5.1 Introduction. What Is Power?
In Chap. 3, we defined power as the measure of the amount of effective work completed. We measure DC power in watts. In this chapter, we will answer to questions like how much energy does the coils and capacitors consume from the grid.
The coils and capacitors draw energy for only a quarter of a cycle before releasing it in the next cycle, resulting in an average zero power consumption. In AC we also measure power in watts. But,
Question What is the value of one watt? We know that the battery delivers power when the current I flows out of the positive voltage terminal of the battery. The power then is positive, and we have
WAB=UAB×q (5.1)
P=Wt=Uqt=UI (5.2)
Then, we see that one watt is equal to the product of one ampere and one volt.
练习题
A battery has a voltage of across its terminals. If a current of flows out of the positive terminal, what is the power delivered by the battery?
A.
B.
C.
D.
Which of the following correctly expresses the value of one watt?
A. One watt equals one volt divided by one ampere
B. One watt equals one ampere times one volt
C. One watt equals one ampere plus one volt
D. One watt equals one volt minus one ampere
In an AC circuit containing only coils and capacitors, why is the average power consumption zero?
A. Coils and capacitors do not allow current to flow through them
B. Coils and capacitors draw energy for a quarter cycle and release it in the next quarter cycle
C. Coils and capacitors convert all energy to heat immediately
D. Coils and capacitors have infinite resistance
In which types of electrical circuits is power measured in watts?
A. DC circuits
B. AC circuits
C. Only purely resistive circuits
D. Circuits containing reactive components like coils and capacitors
Power is defined as the measure of the amount of effective work completed.
A battery delivers positive power when current flows into the positive voltage terminal.
Coils and capacitors connected to an AC grid consume net energy from the grid over a complete cycle.
The work done when a charge moves across a potential difference is given by the formula ___.
Power can be expressed as the product of voltage and current using the formula ___.
Explain why inductors (coils) and capacitors have zero average power consumption in AC circuits, relating your answer to the phase relationship between voltage and current.
In an RLC series circuit, the inductor voltage leads the current by 90°. Based on this phase relationship and the power formula , which statement correctly explains why the average power consumed by a pure inductor is zero?
A. The voltage and current are in phase, so power oscillates symmetrically
B. The 90° phase difference causes power to alternate between positive and negative values equally over each cycle
C. The inductor has infinite impedance, preventing any current flow
D. The current through the inductor is always zero regardless of applied voltage
In an RLC series circuit, when the coil discharges stored energy, some of this energy charges the capacitor, and the capacitor returns energy to the coil during the following quarter cycle. This energy exchange between reactive components explains why coils and capacitors have zero average power consumption from the grid.
According to the power formula , one watt equals one ampere times one volt. In a purely resistive AC circuit, since the resistor voltage is ___ with the current, the instantaneous power is always positive, meaning the resistor continuously consumes energy.
Which of the following statements correctly describe the relationship between phase relationships and power consumption in AC circuits?
A. In a pure resistor, voltage and current are in phase, so instantaneous power is always positive
B. In a pure inductor, voltage leads current by 90°, resulting in zero average power consumption
C. In a pure capacitor, voltage lags current by 90°, resulting in zero average power consumption
D. All passive components (R, L, C) consume the same amount of average power from the source
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