---
title: "Reactive Power | College Physics I"
description: "Reactive power is AC power that moves back and forth between source and load instead of doing net work, shaping power factor in College Physics I."
canonical: "https://fiveable.me/intro-college-physics/key-terms/reactive-power"
type: "key-term"
subject: "College Physics I – Introduction"
unit: "Unit 20"
---

# Reactive Power | College Physics I

## Definition

Reactive power is the part of AC electrical power that stores energy in a magnetic or electric field and then returns it to the source. In College Physics I, it shows up when you study phase shift, power factor, and AC devices like motors and transformers.

## What It Is

Reactive power is the part of power in an AC circuit that does not become permanent work, but instead sloshes back and forth between the source and the load. In College Physics I, you run into it when voltage and current are not perfectly in step with each other.

That out-of-step timing is called a phase angle. If current lags voltage in an inductive device, like a motor or transformer, energy is briefly stored in a magnetic field and then given back later in the cycle. If the load is capacitive, the same back-and-forth happens with an electric field instead.

The circuit still uses energy, but not in the same way as a resistive heater or light bulb. A toaster converts electrical energy mostly into heat, which is active power. A motor needs reactive power to keep its magnetic field going, even though that magnetic energy is not the final output you pay for as useful work.

This is why reactive power is tied to AC behavior rather than DC. In direct current, the voltage and current stay steady, so there is no repeating exchange caused by a phase shift. In AC, the current and voltage can peak at different times, and that mismatch creates reactive power.

You will usually see reactive power measured in VAR, or volt-amperes reactive. It is not measured in watts because it is not net energy per second doing work in the usual sense. Instead, it is part of the larger relationship between active power, apparent power, and power factor.

A simple way to picture it is to imagine pushing a swing. Some of your pushes add energy to the swing, while some of the motion gives energy back to you as the system reverses direction. Reactive power is the electrical version of that energy exchange, where the grid or circuit must keep supplying and receiving energy every cycle.

## Why It Matters

Reactive power shows up anywhere AC devices need fields to operate, especially motors, transformers, and some lighting systems. Without it, many of the machines used in labs, homes, and the power grid would not work the way they are supposed to.

It also changes how you interpret AC power problems. A circuit can have high current and still deliver less useful work than you expect because part of that current is tied to reactive effects, not active output. That is why power factor matters so much in AC analysis: it tells you how effectively the circuit is turning supplied power into real work.

In practical physics problems, reactive power helps explain why wires heat up, why voltage can sag along a line, and why engineers add capacitors for power factor correction. If you see a system with inductive loads, you should think about the phase relationship, not just voltage and current size. That shift in thinking is a big step in AC circuit reasoning.

## Connections

### Active Power

Active power is the part of AC power that actually becomes useful work, like heat, motion, or light. Reactive power is the back-and-forth exchange that supports fields in the circuit, while active power is the part that gets consumed. When you compare them, you can tell whether a device is doing real work or mostly shifting energy around.

### [Apparent Power](/intro-college-physics/key-terms/apparent-power)

Apparent power combines the voltage and current in an AC circuit without separating the work-producing part from the field-supporting part. Reactive power and active power are both components of apparent power. In problems, apparent power is the total size of the electrical demand, while reactive power tells you how much of that demand is tied to phase shift.

### [Power Factor](/intro-college-physics/key-terms/power-factor)

Power factor describes how well AC power is being converted into useful work, usually by comparing active power to apparent power. A lower power factor means more of the circuit’s current is involved in reactive exchange. That is why reactive power and power factor are usually discussed together in AC circuits.

### [phase angle](/intro-college-physics/key-terms/phase-angle)

The phase angle is the timing difference between voltage and current in an AC circuit. Reactive power appears when that angle is not zero, because energy is being stored and returned during the cycle. The bigger the phase shift, the more the circuit behaves like it has reactive components.

## On the AP Exam

A quiz or problem set may ask you to identify whether an AC device is mostly resistive, inductive, or capacitive by looking at the phase relationship between current and voltage. You might also be asked to explain why a motor draws reactive power even when it is not converting all of that energy into motion. In calculation problems, reactive power is often used with active power and apparent power to find power factor or to compare circuit behavior before and after adding a capacitor. If you see a circuit diagram or a waveform graph, look for the phase shift first, then decide whether the circuit is storing energy in magnetic or electric fields. The answer is usually about the direction of energy flow, not just the size of the current.

## Reactive Power vs Active Power

Reactive power is often mixed up with active power because both are part of AC power. Active power is the part that does net work, like heating a resistor or spinning a fan. Reactive power does not produce net work over a full cycle, it supports the fields that AC devices need in order to operate.

## Key Takeaways

- Reactive power is the part of AC power that is stored and returned each cycle instead of becoming net work.
- It appears when voltage and current are out of phase, especially in inductive and capacitive loads.
- Motors, transformers, and some lighting systems need reactive power to maintain their fields.
- Reactive power is measured in VAR, not watts, because it is not the same as useful work output.
- Power factor and reactive power are closely linked, so a low power factor usually means more reactive exchange in the circuit.

## FAQs

### What is reactive power in College Physics I?

Reactive power is the AC power that moves into and out of a circuit’s fields instead of being fully converted into work. It shows up when current and voltage are out of phase, which is common in inductors and capacitors. In physics problems, it helps explain why some devices draw current without turning all of it into heat or motion.

### How is reactive power different from active power?

Active power is the part of AC power that becomes useful output, like light, heat, or mechanical motion. Reactive power is the part that is temporarily stored in fields and then returned to the source. A circuit can have both at the same time, which is why AC analysis uses more than just voltage and current size.

### Why do motors need reactive power?

Motors need reactive power because they rely on magnetic fields to operate. The magnetic field has to build and collapse repeatedly in AC, and that energy exchange is the reactive part. Without that exchange, the motor would not maintain the field structure it needs for motion.

### How do I tell if a circuit has reactive power?

Look for a phase difference between current and voltage. If they are not in sync, the circuit is likely storing and returning energy each cycle, which means reactive power is present. Inductive loads usually make current lag, while capacitive loads make current lead.

## Related Study Guides

- [20.5 Alternating Current versus Direct Current](/intro-college-physics/unit-20/5-alternating-current-direct-current/study-guide/wStT2MhkFzgKB7dz)

## About This Document

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