# Describe the relationship between two systems in thermal equilibrium

Q1:which of the following best describes the relationshipbetween two systems in thermal equilibrium? is it A. no net energyis exchanged? B. the volumes are. Two objects separately in thermodynamic equilibrium with a third object are in of physical systems, although we are most interested in the thermodynamics of thermal equilibrium, heat, which is a form of energy, is transferred between the. Thermal equilibrium is a relation between two bodies or closed systems, in which transfers are is no longer possible to describe the process by means of a.

Relation Between Internal Energy Work And Heat

The zeroth law says when two objects at thermal equilibrium are in contact, there is no net heat transfer between the objects; therefore, they are the same temperature. Another way to state the zeroth law is to say that if two objects are both separately in thermal equilibrium with a third object, then they are in thermal equilibrium with each other.

The zeroth law allows us to measure the temperature of objects. Any time we use a thermometer, we are using the zeroth law of thermodynamics. Let's say we are measuring the temperature of a water bath. In order to make sure the reading is accurate, we usually want to wait for the temperature reading to stay constant.

### Heat (Energy) Transfer and Thermal Equilibrium - Physics

We are waiting for the thermometer and the water to reach thermal equilibrium! At thermal equilibrium, the temperature of the thermometer bulb and the water bath will be the same, and there should be no net heat transfer from one object to the other assuming no other loss of heat to the surroundings. Converting between heat and change in temperature How can we measure heat? Here are some things we know about heat so far: When a system absorbs or loses heat, the average kinetic energy of the molecules will change.

Thus, heat transfer results in a change in the system's temperature as long as the system is not undergoing a phase change. The change in temperature resulting from heat transferred to or from a system depends on how many molecules are in the system. Such an adventure could be conducted in indefinitely many ways, with different fictive partitions. All of them will result in subsystems that could be shown to be in thermal equilibrium with each other, testing subsystems from different partitions.

For this reason, an isolated system, initially not its own state of internal thermal equilibrium, but left for a long time, practically always will reach a final state which may be regarded as one of internal thermal equilibrium. Such a final state is one of spatial uniformity or homogeneity of temperature. Thermal contact[ edit ] Heat can flow into or out of a closed system by way of thermal conduction or of thermal radiation to or from a thermal reservoir, and when this process is effecting net transfer of heat, the system is not in thermal equilibrium.

While the transfer of energy as heat continues, the system's temperature can be changing. Bodies prepared with separately uniform temperatures, then put into purely thermal communication with each other[ edit ] If bodies are prepared with separately microscopically stationary states, and are then put into purely thermal connection with each other, by conductive or radiative pathways, they will be in thermal equilibrium with each other just when the connection is followed by no change in either body.

But if initially they are not in a relation of thermal equilibrium, heat will flow from the hotter to the colder, by whatever pathway, conductive or radiative, is available, and this flow will continue until thermal equilibrium is reached and then they will have the same temperature.

One form of thermal equilibrium is radiative exchange equilibrium. In this situation, Kirchhoff's law of equality of radiative emissivity and absorptivity and the Helmholtz reciprocity principle are in play.

## Thermal equilibrium

Change of internal state of an isolated system[ edit ] If an initially isolated physical system, without internal walls that establish adiabatically isolated subsystems, is left long enough, it will usually reach a state of thermal equilibrium in itself, in which its temperature will be uniform throughout, but not necessarily a state of thermodynamic equilibrium, if there is some structural barrier that can prevent some possible processes in the system from reaching equilibrium; glass is an example.

Classical thermodynamics in general considers idealized systems that have reached internal equilibrium, and idealized transfers of matter and energy between them.

An isolated physical system may be inhomogeneous, or may be composed of several subsystems separated from each other by walls. If an initially inhomogeneous physical system, without internal walls, is isolated by a thermodynamic operation, it will in general over time change its internal state. Or if it is composed of several subsystems separated from each other by walls, it may change its state after a thermodynamic operation that changes its walls.

Such changes may include change of temperature or spatial distribution of temperature, by changing the state of constituent materials.

• Heat and temperature

A rod of iron, initially prepared to be hot at one end and cold at the other, when isolated, will change so that its temperature becomes uniform all along its length; during the process, the rod is not in thermal equilibrium until its temperature is uniform. In a system prepared as a block of ice floating in a bath of hot water, and then isolated, the ice can melt; during the melting, the system is not in thermal equilibrium; but eventually its temperature will become uniform; the block of ice will not re-form.

### Heat and temperature (article) | Khan Academy

A system prepared as a mixture of petrol vapour and air can be ignited by a spark and produce carbon dioxide and water; if this happens in an isolated system, it will increase the temperature of the system, and during the increase, the system is not in thermal equilibrium; but eventually the system will settle to a uniform temperature.

Such changes in isolated systems are irreversible in the sense that while such a change will occur spontanteously whenever the system is prepared in the same way, the reverse change will practically never occur spontanteously within the isolated system; this is a large part of the content of the second law of thermodynamics.

Truly perfectly isolated systems do not occur in nature, and always are artificially prepared. In a gravitational field[ edit ] One may consider a system contained in a very tall adiabatically isolating vessel with rigid walls initially containing a thermally heterogeneous distribution of material, left for a long time under the influence of a steady gravitational field, along its tall dimension, due to an outside body such as the earth.

It will settle to a state of uniform temperature throughout, though not of uniform pressure or density, and perhaps containing several phases. It is then in internal thermal equilibrium and even in thermodynamic equilibrium.