What Is the Zeroth Law of Thermodynamics?

The zeroth law of thermodynamics states that if two systems are both in thermal equilibrium with a third system, then the first two systems are also in thermal equilibrium with each other.

Key Takeaways: Zeroth Law of Thermodynamics

• The zeroth law of thermodynamics is one of the four laws of thermodynamics, which states that if two systems are in thermal equilibrium with a third system, then they are in thermal equilibrium with one another.
• Thermodynamics is the study of the relationship between heat, temperature, work, and energy.
• Most generally, equilibrium refers to a balanced state that does not change overall with time.
• Thermal equilibrium refers to the situation where two objects that can transfer heat to each other stay at a constant temperature over time.

Understanding Thermodynamics

Thermodynamics is the study of the relationship between heat, temperature, work—which is performed when a force applied to an object causes that object to move—and energy, which comes in many forms and is defined as the capacity to do work. The four laws of thermodynamics describe how the fundamental physical quantities of temperature, energy, and entropy change in various situations.

As an example of thermodynamics in action, placing a pot of water on a heated stove will cause the pot to heat up because heat is transferred to the pot from the stove. This in turn causes the molecules of water to bounce around in the pot. The faster movement of these molecules is observed as hotter water.

If the stove had not been hot, it would not have transferred any thermal energy to the pot; thus, the water molecules could not have begun moving faster and the pot of water would not have heated up.

Thermodynamics emerged in the 19th century, when scientists were building and improving steam engines, which use steam to help move an object such as a train.

Understanding Equilibrium

Most generally, equilibrium refers to a balanced state that does not change overall with time. This does not mean that nothing is happening; rather, that two influences or forces are balancing each other out.

Consider, for example, a weight hanging from a string attached to the ceiling. At first, the two are in equilibrium with one another and the string does not break. If more weight is attached to the string, however, the string will be tugged downward and may eventually break as the two are no longer in equilibrium.

Thermal Equilibrium

Thermal equilibrium refers to the situation where two objects that can transfer heat to each other stay at a constant temperature over time. Heat can be transferred several ways, including if the objects are in contact with one another or if heat is radiated from a source like a lamp or a sun. Two objects are not in thermal equilibrium if the overall temperature changes with time, but they can approach thermal equilibrium as the hotter object transfers heat to the colder one.

Consider, for example, a colder object touching a hotter object—like ice that has been dropped in a hot cup of coffee. After some time, the ice (later water) and the coffee will reach a certain temperature that is in between that of the ice and the coffee. Though the two objects were not in thermal equilibrium at the beginning, they approach—and eventually reach—thermal equilibrium, the temperature in between the hot and cold temperatures.

What Is the Zeroth Law of Thermodynamics?

The zeroth law of thermodynamics is one of the four laws of thermodynamics, which states that if two systems are in thermal equilibrium with a third system, then they are in thermal equilibrium with one another. As seen from the above section on thermal equilibrium, these three objects will approach the same temperature.

Applications of the Zeroth Law of Thermodynamics

The zeroth law of thermodynamics is seen in many everyday situations.

• The thermometer may be the most well-known example of the zeroth law in action. For example, say the thermostat in your bedroom reads 67 degrees Fahrenheit. This means that the thermostat is in thermal equilibrium with your bedroom. However, because of the zeroth law of the thermodynamics, you can assume that both the room and other objects in the room (say, a clock hanging in the wall) are also at 67 degrees Fahrenheit.
• Similar to the above example, if you take a glass of ice water and a glass of hot water and place them on the kitchen countertop for a few hours, they will eventually reach thermal equilibrium with the room, with all 3 reaching the same temperature.
• If you place a package of meat in your freezer and leave it overnight, you assume that the meat has reached the same temperature as the freezer and the other items in the freezer.
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