When two objects with different temperatures are placed in thermal contact, heat energy transfers between them. Over time, the net heat transfer becomes zero and both objects reach the exact same temperature. This state is known as thermal equilibrium, which is fundamental in understanding heating and cooling processes as well as the calibration and operation of liquid-in-glass thermometers.
See how heat transfers between a hot object P and a cold object Q until the net heat transfer becomes zero.
Object P is hotter than object Q. The rate of heat transfer from P to Q is higher than the rate from Q to P. There is a net transfer of heat from P to Q.
P cools down while Q warms up. The rate of heat transfer from P to Q decreases while the rate from Q to P increases.
Both objects reach the same temperature. Heat still transfers in both directions, but at exactly the same rate. Net heat transfer is zero.
Two objects are in thermal contact when heat energy can be transferred between them.
Thermal equilibrium is reached when two objects in thermal contact have the same temperature, equal rates of heat transfer, and zero net heat transfer.
The temperature of both objects is equal. The rate of heat transfer between the two objects is equal. The net rate of heat transfer between the two objects is zero.
Net heat transfer is the overall difference between heat transferred from one body to another and the heat received in return. At thermal equilibrium, net heat transfer is zero.
Oven baking cake batter: Hot air in the oven is in thermal contact with the cake batter. Heat flows from the hot air to the cake batter until it is baked.
Clinical thermometer: Placed under the tongue of a patient. Heat from the patient's body flows to the thermometer until thermal equilibrium is reached. The patient's body temperature can then be accurately measured.
Food in a refrigerator: Heat from warm food flows to the colder air in the refrigerator until thermal equilibrium is reached. The temperature of the food drops, keeping it fresh for a longer period.
Cooling drinks with ice cubes: Ice cubes absorb heat from the drink and melt. The melted ice water continues to absorb heat from the drink until thermal equilibrium is achieved.
A thermometric property is a physical property that can be measured, such as the length of a liquid column in glass, which changes continuously and linearly with temperature.
Temperature of melting pure ice, defined as \(0\text{ }^\circ\text{C}\). The length of the liquid column is marked as \(L_0\) (reference baseline).
Temperature of steam from boiling distilled water at standard atmospheric pressure, defined as \(100\text{ }^\circ\text{C}\). The length of the liquid column is marked as \(L_{100}\).
The process of marking a scale of reading on an uncalibrated thermometer uses two fixed reference points. The distance between \(0\text{ }^\circ\text{C}\) and \(100\text{ }^\circ\text{C}\) (\(L_{100}\)) is divided into 100 equal divisions, each representing \(1\text{ }^\circ\text{C}\).
Temperature determination formula
where \(\theta\) = unknown temperature of the liquid, \(L_\theta\) = length of the liquid column measured from the \(0\text{ }^\circ\text{C}\) mark to the \(\theta\text{ }^\circ\text{C}\) mark, and \(L_{100}\) = length of the liquid column measured from the \(0\text{ }^\circ\text{C}\) mark to the \(100\text{ }^\circ\text{C}\) mark.
The bulb of the thermometer must not touch the base or side walls of the beaker to ensure it only measures the temperature of the liquid. Wait until the liquid column stops expanding or contracting before marking the stem, ensuring thermal equilibrium has been established.
Answer in your mind, then press "Check Answer".
State two conditions that must be fulfilled when two bodies are in thermal equilibrium.
Why must a doctor wait for about one to two minutes after placing a clinical thermometer under a patient's tongue before reading the temperature?
In a calibration activity, the length of the mercury column is \(0\text{ cm}\) at the ice point (\(0\text{ }^\circ\text{C}\)) and \(12\text{ cm}\) at the steam point (\(100\text{ }^\circ\text{C}\)). When placed in an unknown liquid, the length of the column measured from the \(0\text{ }^\circ\text{C}\) mark is \(4\text{ cm}\). What is the temperature of the liquid?
"Cold flows from the cold object to the hot object."
Cold is not an entity that flows. Heat energy always has a net flow from the higher-temperature object to the lower-temperature object.
"Human body is always in thermal equilibrium with the surrounding air."
The human body is not in thermal equilibrium with the environment because the body constantly generates heat through internal metabolic processes and actively regulates its core temperature around \(37\text{ }^\circ\text{C}\).
"Thermal equilibrium means both objects have equal amounts of heat energy."
Thermal equilibrium means both objects have the same temperature, not the same internal heat energy.
Objective: Test your understanding of thermal equilibrium, thermometer calibration and daily life applications.
1 Two metal blocks, X and Y, are placed in thermal contact. Block X is initially at \(80\text{ }^\circ\text{C}\) and block Y is at \(30\text{ }^\circ\text{C}\). Which statement is correct when thermal equilibrium is reached?
2 Which physical property of liquid in a glass thermometer is used as a thermometric property during calibration?
3 When two objects are in thermal contact, heat flows until both objects reach the same , where net heat transfer becomes zero.
4 In calibrating a liquid-in-glass thermometer, the upper fixed point is the boiling point of distilled water, which is °C.
Match each thermal equilibrium situation or concept to its correct description. If using a phone, tap the answer first, then tap the matching box.
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