---
title: "Critical Temperature | College Physics I"
description: "Critical temperature is the temperature below which a material becomes superconducting, with zero resistance and magnetic field expulsion in College Physics I."
canonical: "https://fiveable.me/intro-college-physics/key-terms/critical-temperature"
type: "key-term"
subject: "College Physics I – Introduction"
unit: "Unit 13"
---

# Critical Temperature | College Physics I

## Definition

Critical temperature is the temperature below which a material becomes superconducting, so it has zero electrical resistance and expels magnetic fields. In College Physics I, you meet it when studying superconductivity and cryogenic materials.

## What It Is

Critical temperature is the point where a material stops behaving like a normal resistor and enters the superconducting state. In College Physics I, that means you are looking at the temperature below which electrical resistance drops to zero and the material can also push magnetic fields out of its interior.

Above the critical temperature, electrons scatter off the lattice and the material acts like an ordinary conductor with some resistance. At or below that threshold, the microscopic behavior changes enough that current can flow without energy loss from resistance. That change is not gradual in the everyday sense, because the material crosses into a different physical state.

This is why critical temperature is tied to superconductivity rather than just to “better conduction.” A superconductor is not simply a very good conductor, and a critical temperature is not just the temperature where the resistance gets small. It is the boundary for the transition into the superconducting phase.

Many introductory physics examples focus on the cooling method as much as the temperature itself. Most superconductors need cryogenic cooling, and high-temperature superconductors are called “high-temperature” only because their critical temperatures are higher than older materials, not because they are warm. They may still need liquid nitrogen or similarly cold conditions.

The exact value of critical temperature depends on the material, its structure, and interactions inside the solid. In class, you might see it on a graph of resistance versus temperature, where the curve suddenly drops to zero, or in a discussion of why certain alloys, ceramics, or iron-based materials superconduct while others do not.

## Why It Matters

Critical temperature is the number that tells you whether a superconductor will actually work in the real world. If a material’s critical temperature is too low, you need expensive cooling to keep it superconducting, which changes whether it makes sense for a lab demo, a power cable, an MRI magnet, or a maglev system.

In College Physics I, it also gives you a clean example of a phase transition. You are not just memorizing a material property, you are tracking a change in behavior that shows up in resistance measurements, magnetic effects, and cooling methods.

The term also connects directly to how physicists compare different superconductors. A material with a higher critical temperature is easier to maintain in the superconducting state, so the value becomes a practical benchmark in materials science and applied physics.

When you see a problem or reading about superconductors, critical temperature tells you the cutoff for the effect. That makes it the line between “normal metal” behavior and the unusual properties that make superconductors useful.

## Connections

### [Superconductivity](/intro-college-physics/key-terms/superconductivity)

Critical temperature is the threshold for superconductivity. Below that temperature, the material enters the superconducting state and can carry current with zero resistance. If you understand superconductivity, you can treat critical temperature as the temperature boundary where that new state begins.

### [Meissner Effect](/intro-college-physics/key-terms/meissner-effect)

The Meissner Effect is the magnetic behavior that appears when a material becomes superconducting. Below the critical temperature, the material expels magnetic fields instead of letting them pass through normally. That is one of the clearest signs that the phase change has happened.

### [Cryogenic Cooling](/intro-college-physics/key-terms/cryogenic-cooling)

Cryogenic cooling is how many materials are kept below their critical temperature. In lab setups, the cooling method matters because crossing above the threshold ends superconductivity. This connection shows up in real equipment, where temperature control is just as important as the material itself.

### High-Temperature Superconductors

High-temperature superconductors are materials with unusually high critical temperatures compared with older superconductors. “High” still means very cold, but the higher threshold makes them more practical. This term is often discussed when comparing materials that work above liquid nitrogen temperatures to those that do not.

## On the AP Exam

A quiz or problem-set question may give you a material, a temperature, and a resistance graph, then ask whether the sample is superconducting. Your job is to spot the critical temperature as the point where resistance falls to zero and to say what happens above and below it. You may also be asked to connect that cutoff to magnetic field expulsion or to explain why a superconductor stops working when it warms up. In lab questions, look for the temperature where the measurement changes sharply rather than gradually.

## critical temperature vs Superconductivity

Critical temperature is the threshold where superconductivity begins, while superconductivity is the state itself. Think of critical temperature as the cutoff value and superconductivity as the behavior you get after crossing that cutoff.

## Key Takeaways

- Critical temperature is the temperature below which a material becomes superconducting.
- At and below this point, electrical resistance drops to zero and magnetic fields are expelled.
- It is a material-specific threshold, so different superconductors have different critical temperatures.
- A higher critical temperature usually means the material is easier to use in practical devices because it needs less extreme cooling.
- In physics questions, you often identify the critical temperature from a sudden change in resistance or from a description of the superconducting state.

## FAQs

### What is critical temperature in College Physics I?

It is the temperature at which a material transitions into the superconducting state. Below that point, resistance becomes zero and the material shows the magnetic behavior associated with superconductors.

### Is critical temperature the same as superconductivity?

No. Critical temperature is the threshold, and superconductivity is the state that appears below it. If a material is above its critical temperature, it may still conduct electricity, but it is not superconducting.

### What happens when a material goes below its critical temperature?

Its electrical resistance drops to zero, and it can expel magnetic fields through the Meissner Effect. That is the point where the material stops acting like a normal conductor and starts acting like a superconductor.

### How do you find critical temperature on a graph?

Look for the temperature where the resistance curve suddenly falls to zero or changes sharply. In many intro physics graphs, that point marks the transition into the superconducting state.

## Related Study Guides

- [13.5 Phase Changes](/intro-college-physics/unit-13/5-phase/study-guide/BlbIUmMNgB4v3I0s)
- [2.2 Vectors, Scalars, and Coordinate Systems](/intro-college-physics/unit-2/2-vectors-scalars-coordinate-systems/study-guide/G39DJEYkz0ssqMYu)
- [13.1 Temperature](/intro-college-physics/unit-13/1-temperature/study-guide/cTI2aeaCRtvMHj68)
- [13.3 The Ideal Gas Law](/intro-college-physics/unit-13/3-ideal-gas-law/study-guide/pk9BhBL1ouw34sZ7)
- [13.4 Kinetic Theory: Atomic and Molecular Explanation of Pressure and Temperature](/intro-college-physics/unit-13/4-kinetic-theory-atomic-molecular-explanation-pressure-temperature/study-guide/tkJCfyjd2F7TLAqq)

## About This Document

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- [llms.txt](https://fiveable.me/llms.txt): index of Fiveable's sections and URL patterns
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