## Why Power is Zero (0) in Pure Inductive, Pure Capacitive or a Circuit in which Current and Voltage are 90° Out of Phase?

These types of questions are asked in electrical or electronics engineering interviews. So let’s back to the basic to clear the basic and important concepts of electrical engineering.

**1. Why Power in a circuit is Zero (0) in which Current and Voltage are 90° out of phase?**

If Current and Voltage are 90 degree out of phase, then the power (P) will be zero. The reason is as follow:

We know that power in single phase AC Circuits:

**P= V I Cos θ**

Where;

- P = Power in Watts
- V = Voltage in Volts
- I = Current in Amperes
- Cos θ = Power factor of the circuit i.e. phase difference between current and voltage waves.

If angle between current and Voltage are 90° (θ = 90), then

Power = P = V I Cos (90°) = 0 ….. → [Cos (90°) = 0]

So if we put Cos 90° = 0, then the overall power of the circuit will be zero (i.e. In pure Inductive circuit or capacitive circuit where voltage is leading by 90° from current and current is leading 90° from voltage respectively).

**2. Why Power in Pure Inductive Circuit is Zero (0).**

We know that in pure inductive circuit, current is lagging by 90 degree from voltage (in other words, Voltage is leading 90° from current) i.e the phase difference between current and voltage is 90 degrees.

As explained above, if current and voltage are 90° out of phase from each other like in pure inductive circuit, the total power of the circuit would be 0 as follow.

P= V I Cos θ

if angle between current and Voltage are 90° (θ = 90), then

Power P = V I Cos (90°) = 0

[ Note that Cos (90°) = 0]

This shows that **in case of pure inductive circuit, the total power of the circuit would have zero** as Cos 90° = 0.

**3. Why Power in Pure Capacitive Circuit is Zero (0)?**

We know that in pure capacitive circuit, current is leading by 90° from voltage (in other words, Voltage is lagging 90° from current) i.e the phase difference between current and voltage is 90 degree.

As mentioned above, if current and voltage are 90° out of phase from each other like in pure capacitive circuit, the total power of the circuit would be zero as follow.

Power in AC Circuit

P= V I Cos θ

if angle between current and Voltage are 90 ( θ = 90) Degree. then

Power P = V I Cos (90°) = 0

[ Note that Cos (90°) = 0]

This shows the same as like pure inductive circuit i.e. **in case of pure capacitive circuit, the total power of the circuit would be zero** as Cos 90° = 0.

In short, in a circuit where voltage or current leading or lagging 90° (phase difference = 90°) behind each other, the positive cycle cancel the negative which leads to the zero average power of the circuit i.e. the total power factor of the ac circuit is zero which leads to the zero average power.

## FAQ

### Why is power zero in a pure inductive circuit?

In this type of circuit, the current lags the voltage by an angle of 90 degrees. The Circuit containing pure inductance is shown below. Hence, the average power consumed in a purely inductive circuit is zero

### Why is the power factor for an inductor and capacitor zero?

For pure inductance and capacitor the voltage and current are always out of phase ,ie when current is high voltage is equal to zero and vice versa . So its product is always zero resulting in zero power being dissipated

### Why is power loss zero in inductor and capacitor?

If we consider power across capacitor and inductor then V and I are always out of phase by 90°. Thus cos90° = 0. That’s why average power across L and C is always 0. If we consider changing and discharging in symmetrical signal then is equal in magnitude and opposite in phase

### What is the power consumed in purely inductive and purely capacitive AC circuit?

Reason: No power is dissipated in a purely inductive or capacitive circuit even though a current is flowing in the circuit

### Why is power in a pure capacitive circuit zero?

Pure inductor and pure capacitor refers that it has no internal resistance. If internal resistance in inductor or capacitor is zero, so voltage and current will always be 90 degree out of phase with each other. and so, P=0

### Why is the power in a capacitor zero?

This is because while the voltage on the resistance is in phase with the current the voltage on the capacitor lags 90 degrees from the same current. Hence the internal resistance will be zero. If the inductor and capacitor are not ideal then they will consume power

### Why there is no power in inductor?

Because inductor stores energy when current increase and release energy when current is decrease. It is an inactive when there is no change in current. So average power in the inductor is zero

### Why power consumption is zero across a circuit containing inductance only?

During the first quarter cycle, the power supplied by the source is stored in the form of a magnetic field around the coil. In the next quarter cycle, the power is returned back to the source. This process continues in every cycle and due to which no power is consumed in the circuit