---
title: "Thermal Reservoir | College Physics I"
description: "Thermal reservoir in College Physics I is a large heat source or sink that stays nearly constant in temperature while it absorbs or releases energy."
canonical: "https://fiveable.me/intro-college-physics/key-terms/thermal-reservoir"
type: "key-term"
subject: "College Physics I – Introduction"
unit: "Unit 15"
---

# Thermal Reservoir | College Physics I

## Definition

A thermal reservoir is a very large body that can give off or absorb heat with almost no temperature change. In College Physics I, it models the hot and cold sides of refrigerators, heat pumps, and other thermodynamic systems.

## What It Is

A thermal reservoir is the part of a thermodynamics problem that can absorb or supply heat without its temperature changing much. In College Physics I, you treat it as an enormous heat source or heat sink, so large that the heat transfer does not noticeably warm it up or cool it down.

That idea is a simplification, but it is a useful one. A real lake, a room of air, or the outdoor environment can act like a reservoir if the amount of heat exchanged is tiny compared with the amount of thermal energy already stored there. If you pull a small amount of heat from it, the temperature barely shifts, so the reservoir keeps behaving like the same hot or cold boundary.

This is why reservoirs show up in heat engines, refrigerators, and heat pumps. The device sits between two reservoirs, one hot and one cold, and energy moves between them. The hot reservoir can accept heat, the cold reservoir can supply heat, and the machine uses work to push heat in the direction it would not naturally go.

The phrase does not mean the reservoir is literally infinite. It means the reservoir is so large, compared with the system, that you can model its temperature as constant. That lets you focus on the energy flow of the device itself instead of tracking tiny temperature changes in the surroundings.

A common way to think about it is this: the reservoir sets the boundary conditions. If the cold side is your kitchen air and the hot side is the outdoors, then the refrigerator or heat pump exchanges heat with those environments while the temperatures of those environments stay approximately fixed during the process.

This is also where the second law of thermodynamics shows up in a very practical way. Heat does not move from cold to hot by itself, so the device needs work input. The reservoir model makes that direction of energy transfer easy to see, because you can label which side is supplying heat, which side is receiving it, and where the work is entering the cycle.

## Why It Matters

Thermal reservoirs are the setup that makes refrigerator and heat pump problems make sense in College Physics I. If you know what counts as the hot reservoir and the cold reservoir, you can track energy flow through the whole device instead of getting lost in the machinery.

This term also connects directly to coefficient of performance, or COP. The efficiency of a refrigerator or heat pump depends on the temperatures of the two reservoirs, so the reservoir temperatures are not just background details. They control how much work the device needs to move a given amount of heat.

Reservoir language also keeps you from mixing up the device with its surroundings. The refrigerator is not the reservoir. The room, freezer compartment, outside air, or another large body can be the reservoir, because its temperature stays nearly constant even when heat is exchanged.

When you read a diagram, the reservoir helps you identify the direction of heat flow and the sign of the energy terms. That makes it easier to solve problems about heat removed from a cold space, heat rejected to the surroundings, and work input to the cycle.

## Connections

### Heat pump

A heat pump is the device that moves heat between two reservoirs, usually from a colder place to a warmer one. The reservoirs are the boundaries that let you describe what the pump is doing. If you identify the hot and cold reservoirs correctly, you can tell whether the pump is heating a space or removing heat from it.

### Refrigeration cycle

The refrigeration cycle is the step-by-step process that makes heat flow from cold to hot. Thermal reservoirs show up at the ends of that cycle, where heat is absorbed from the cold side and released to the hot side. Without reservoirs, the cycle has no clear source and destination for thermal energy.

### [Thermodynamic equilibrium](/intro-college-physics/key-terms/thermodynamic-equilibrium)

A reservoir is often treated as staying in thermodynamic equilibrium even while it exchanges heat. That means its temperature, pressure, and other state variables are stable enough to model as constant. This is why you can use the reservoir temperature in formulas without constantly recalculating it after each bit of heat transfer.

### [Adiabatic compression](/intro-college-physics/key-terms/adiabatic-compression)

Adiabatic compression is a step in some cooling and heating cycles where a gas is compressed with little or no heat exchange. That works alongside reservoirs because the heat transfer happens later, when the working substance interacts with the hot or cold side. The reservoir is where heat ends up, not where the compression work happens.

## On the AP Exam

On a problem set or quiz, you may be asked to identify which objects in a refrigerator or heat pump diagram are the thermal reservoirs. Then you use that identification to label the direction of heat flow, decide where work is added, and interpret a COP calculation. A typical question might give you the temperatures of the hot and cold reservoirs and ask which setup is more efficient or how much heat the device can remove. In a lab or class discussion, you might also explain why the room, outdoor air, or a water bath can be modeled as a reservoir even though it is not truly infinite.

## Key Takeaways

- A thermal reservoir is a very large body that can absorb or release heat with almost no temperature change.
- In College Physics I, reservoirs are the hot and cold boundaries for heat pumps, refrigerators, and other thermodynamic devices.
- The reservoir model works because the system is much smaller than the surroundings, so the surroundings stay close to constant temperature.
- Reservoir temperatures matter because they help determine COP and the amount of work a device needs.
- If you can identify the reservoirs in a diagram, you can usually trace the heat flow correctly.

## FAQs

### What is a thermal reservoir in College Physics I?

It is a large body that can give off or absorb heat without a noticeable change in temperature. In thermodynamics problems, you use it to represent the hot and cold surroundings of a device like a refrigerator or heat pump.

### Is a thermal reservoir the same as a heat source?

Not always. A heat source is anything that provides heat, but a thermal reservoir is specifically large enough that its temperature stays nearly constant while heat is transferred. A reservoir can act as a source or a sink depending on the direction of energy flow.

### Why do refrigerators use thermal reservoirs?

Because the fridge has to move heat from the cold interior to the warmer room. The food compartment and the room outside are treated as reservoirs so you can track where the heat starts, where it ends up, and how much work the compressor must do.

### How do I identify a thermal reservoir on a physics diagram?

Look for the large environment on either side of the device, often labeled as hot or cold surroundings. If the object is so large that the problem treats its temperature as fixed, that is the reservoir. The working substance inside the machine is not the reservoir.

## Related Study Guides

- [15.5 Applications of Thermodynamics: Heat Pumps and Refrigerators](/intro-college-physics/unit-15/5-applications-thermodynamics-heat-pumps-refrigerators/study-guide/bjJgLzmhn1R9vSjb)

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