---
title: "Secondary Coil | Principles of Physics II"
description: "Secondary coil in Principles of Physics II is the output coil of a transformer, where changing magnetic flux induces voltage according to the turns ratio."
canonical: "https://fiveable.me/principles-physics-ii/key-terms/secondary-coil"
type: "key-term"
subject: "Principles of Physics II"
unit: "Unit 8"
---

# Secondary Coil | Principles of Physics II

## Definition

The secondary coil is the output winding in a transformer. In Principles of Physics II, it is the coil where changing magnetic flux induces an emf, setting the output voltage by the turns ratio.

## What It Is

The secondary coil is the part of a transformer that receives energy by induction and delivers it to the output circuit. In Principles of Physics II, you treat it as the coil where a changing magnetic field creates an induced voltage, even though the secondary is not directly connected to the power source.

What makes it work is changing magnetic flux from the primary coil. When alternating current flows through the primary coil, it creates a changing magnetic field in the transformer core. That changing field links the secondary coil and induces an emf across its terminals. If the secondary circuit is closed, current can flow and power is transferred to the load.

The number of turns in the secondary coil controls how big the output voltage is compared with the primary. More turns means a larger induced voltage, fewer turns means a smaller one. That is why a transformer can be stepped up or stepped down depending on the turns ratio. The coil itself does not create energy, it only receives energy transferred through the magnetic field.

A good way to picture the secondary coil is as the side of the transformer that “picks up” the magnetic change. If the flux through the core changes faster or the secondary has more turns, the induced voltage is larger. If the flux does not change, there is no induction, so a steady DC current does not keep a transformer operating the way AC does.

The secondary coil also connects to the rest of the transformer story, especially losses and power transfer. In a real device, the output voltage is not perfect because some energy is lost as heat in the windings and through the core. But the ideal model focuses on the secondary as the place where Faraday’s law turns magnetic change into usable electrical output.

## Why It Matters

The secondary coil is the part of the transformer that turns the abstract idea of electromagnetic induction into a measurable output voltage. Once you know what the secondary is doing, you can explain how transformers make long-distance power transmission practical, why voltage can be increased or decreased without a direct electrical connection, and why the circuit on the output side behaves differently from the input side.

It also gives you a concrete way to use the turns ratio. If a problem tells you the number of turns on each coil, you can predict whether the transformer is step-up or step-down and calculate the output voltage from the input. That shows up a lot in problem sets because the secondary coil is where the result appears.

In lab work or diagrams, the secondary coil is usually the side connected to the load. That means it is the side you inspect when you are asked about the delivered voltage, current, or power. It also helps you reason about efficiency, since real transformers lose some energy before the output reaches the secondary.

## Connections

### primary coil

The primary coil is the input side of the transformer. Current in the primary creates the changing magnetic flux that induces voltage in the secondary coil. If you mix them up, the voltage relationship gets confusing, so it helps to remember that primary means input and secondary means output.

### turns ratio

The turns ratio compares the number of turns in the secondary coil to the number of turns in the primary coil. It tells you how the output voltage changes relative to the input. In most transformer problems, this ratio is the quickest path from the coil count to the answer.

### electromagnetic induction

Electromagnetic induction is the mechanism that creates the voltage in the secondary coil. A changing magnetic flux through the coil induces an emf, which is the whole reason transformers work. Without induction, the secondary would just be a loop of wire with no special output.

### [power transmission](/principles-physics-ii/key-terms/power-transmission)

Power transmission is where transformers matter most in real life. Utilities use step-up transformers to raise voltage for long-distance transmission, then step-down transformers to lower it again for homes and buildings. The secondary coil is the part that provides the new voltage level after the change.

## On the AP Exam

A quiz or problem-set question usually asks you to identify the secondary coil in a transformer diagram, decide whether it is the input or output side, or calculate the induced voltage from the turns ratio. You may also need to say whether the transformer is step-up or step-down based on which coil has more turns.

In circuit and induction questions, watch for the clue that the secondary only gets a voltage when the magnetic flux is changing. If the prompt mentions AC, that is your signal that the secondary can have an induced emf. If it mentions DC with steady current, the transformer idea usually breaks down except during switching or changing current.

For graphs or diagrams, you may be asked to trace how energy moves from the primary coil, through the core, and into the secondary coil and load. The safe move is to connect the coil count, the changing field, and the output voltage in one chain of reasoning.

## secondary coil vs primary coil

These get mixed up because both are transformer windings, but they do different jobs. The primary coil is where the input current creates the changing magnetic field. The secondary coil is where that changing field induces the output voltage. If you remember input versus output, the pair stays straight.

## Key Takeaways

- The secondary coil is the transformer winding where the output voltage is induced by a changing magnetic field.
- It does not create energy on its own, it receives energy transferred from the primary coil through electromagnetic induction.
- The number of turns in the secondary coil helps determine whether the transformer steps voltage up or down.
- A transformer only works properly when the magnetic flux through the secondary is changing, which is why AC matters.
- In problems, the secondary coil is usually the side connected to the load, so it is the side you use for output voltage and power reasoning.

## FAQs

### What is secondary coil in Principles of Physics II?

The secondary coil is the output coil of a transformer. Changing magnetic flux from the primary coil induces a voltage across it, which can then power a load. The size of that voltage depends on the turns ratio between the two coils.

### How does the secondary coil produce voltage?

It does not produce voltage by itself. The changing magnetic field created by the primary coil passes through the transformer core and induces an emf in the secondary coil according to electromagnetic induction. No changing flux, no induced voltage.

### Is the secondary coil the input or output?

It is the output side of the transformer. The primary coil is the input side that receives current from the source, while the secondary coil delivers the transformed voltage to the rest of the circuit.

### Why does the number of turns in the secondary coil matter?

The turns count changes how much emf is induced in the secondary coil. More turns generally mean a larger output voltage, while fewer turns mean a smaller output voltage. That is the basis of step-up and step-down transformers.

## Related Study Guides

- [8.4 Transformers](/principles-physics-ii/unit-8/transformers/study-guide/FJJmEsFKAaivlRdH)

## About This Document

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