# Voltage/Current phase diff and more

• martix
In summary, a capacitor or inductor does not just absorb energy, it also releases energy. It is the periodic absorption and release of energy that results in no net dissipation of energy.

#### martix

Voltage/Current phase difference and more...

Can someone please explain in physical terms what it means that the phase of the voltage lags/leads the current ...among other things...
Like:
1. Impedance - what's the difference between the real and the imag resistance and what's the physical significance of both.
2. The whole reactance idea(general and capacitive/inductive).

Last edited:

martix said:
Can someone please explain in physical terms what it means that the phase of the voltage lags/leads the current ...among other things...
Like:
1. Impedance - what's the difference between the real and the imag resistance and what's the physical significance of both.
2. The whole reactance idea(general and capacitive/inductive).
A phase difference between voltage and current simply means that points of maximum voltage do not occur at the same time as maximum current.

For a pure capacitor the greatest current occurs when the applied voltage begins to increase from 0 as there is not yet any charge on the capacitor. As the applied voltage increases the charge on the capacitor increases due to the current flow. At the same time, the capacitive reactance (ie. the voltage caused by the charge build up in the capacitor) increases and opposes the applied voltage, thereby limiting current. When the applied voltage is maximum, the applied voltage is the same as the opposing voltage from the build up of charge on the capacitor, so the current is zero. The applied voltage starts to decrease and the charge on the capacitor starts flowing out of the capacitor. The discharge current is maximum when the applied voltage is 0.

So you can see from that qualitative description of a capacitor that current is out of step with voltage. If you plot the sine curve for current and voltage, the current peak will occur 90 degrees (or 1/4 of a cycle) before the voltage peak.

The opposite occurs for an inductor. For an inductor, the voltage peak occurs 90 degrees before current peaks.

AM

Last edited:

martix said:
Can someone please explain in physical terms what it means that the phase of the voltage lags/leads the current ...among other things...
Like:
1. Impedance - what's the difference between the real and the imag resistance and what's the physical significance of both.
2. The whole reactance idea(general and capacitive/inductive).

And here's a thread from the EE forum where we discussed this also:

.

berkeman said:
And here's a thread from the EE forum where we discussed this also:

.
This is a useful thread. But with respect to this comment:

waht said:
Also note that power dissipated by an inductor or a capacitor is imaginary. That means instead of dissipating power as heat like a resistor, capacitor or an inductor is absorbing power (because of lead or lag).
The terminology might be misleading. In a circuit with a pure inductor or capacitor (R=0) to which an alternating voltage is applied, power is not dissipated. The energy consumed over any number of complete cycles is zero. That is:

$$\int_{0}^{nT} \vec{V}\cdot\vec{I}dt = 0$$

This, of course, ignores loss due to electromagnetic waves, which becomes significant at high frequencies.

A capacitor or inductor does not just absorb energy, it also releases energy. It is the periodic absorption and release of energy that results in no net dissipation of energy.

AM

A great explanation from Andrew Mason. Its amazing how the right formulation can put things into perspective. Had a little trouble figuring what happens to an inductor though. As someone mentioned in the other thread - "Most expositions explain the capacitive case but gloss over the inductive one."
As for the real/imag part - I found out about Euler's formula in the mean time. Amazing thing really. :)

Last edited:

## 1. What is voltage/current phase difference?

Voltage/current phase difference refers to the difference in timing between the voltage and current in an alternating current (AC) circuit. In an ideal AC circuit, the voltage and current are in phase, meaning they reach their maximum and minimum values at the same time. However, in real-world circuits, there may be a delay between the voltage and current, resulting in a phase difference.

## 2. How is voltage/current phase difference measured?

Voltage/current phase difference can be measured using an oscilloscope. The oscilloscope displays the voltage and current waveforms and allows for the calculation of the phase difference between them. Alternatively, a phase meter can be used to directly measure the phase difference between two signals.

## 3. What causes voltage/current phase difference?

Voltage/current phase difference can be caused by a variety of factors, including the inductance, capacitance, and resistance of the circuit components. These factors can affect the timing of the voltage and current, resulting in a phase difference.

## 4. How does voltage/current phase difference affect power in a circuit?

In an AC circuit, the power is calculated as the product of voltage and current, and the power factor is the cosine of the phase difference between them. Therefore, a larger phase difference results in a lower power factor, which means that more energy is wasted in the circuit. In practical terms, this means that a circuit with a high phase difference will have a lower efficiency.

## 5. How can voltage/current phase difference be corrected?

Voltage/current phase difference can be corrected by using power factor correction techniques, such as adding capacitors or inductors to the circuit. These components can help to balance out the reactive power and improve the power factor, reducing the phase difference and improving the efficiency of the circuit.