Temperature

Unit of Temperature in Physics

The Kelvin is the SI unit and is always used in scientific work, as many other properties are related to temperature in units of Kelvin. The lowest temperature possible is called absolute zero and corresponds to \(0\text{ K}\). At absolute zero no more heat can be removed from the system. Classically, this is the temperature when all molecular movement stops. While temperature has a lower limit, note that there is no upper limit.

In addition to the Kelvin scale, there are two other main units of measuring temperature, the Celsius scale and the Fahrenheit scale. However, you have to keep in mind that these two scales, as they are much commonly used in the everyday life, the scale used in physical environments is the Kelvin scale, as it is the SI unit.

Temperature Symbol

As we mentioned earlier, the SI unit of temperature is Kelvin and its symbol is \(\text{K}\). Room temperature is equal to around \(290\text{ K}\), so the Kelvin scale is not very useful in everyday situations and different scales have to be used instead.

In everyday life, you will see temperatures in terms of either the Fahrenheit scale \(ºF\) or the Celsius scale \(ºC\). These are the scales used on thermometers in school laboratories. The majority of countries in the world use the Celsius scale. It is defined so that water has a freezing point of \(0ºC\) and a boiling point of \(100ºC\), at atmospheric pressure. \(0\text{ K}\) is approximately equal to \(-273ºC\) (it is \(-273.15ºC\) to be exact).

Measuring Temperature

Temperature can be measured using a thermometer. You may have an idea in your head of what a normal thermometer looks like but in fact, they can come in many different forms. This is because lots of different physical properties can be used as the basis for thermometers, as long as they depend on temperature. For most substances, the increase in volume is directly proportional to the increase in temperature (if the pressure is held constant) and this is used for most thermometers. Other properties can also be used such as electrical resistance (as resistance increases with temperature for metal conductors), thermal expansion of a solid, and pressure (if the volume is held constant).

A mercury thermometer is most commonly used in school laboratories. It consists of a thin glass tube with liquid mercury inside. The level of mercury tells you the temperature of the surroundings. As the temperature of the thermometer increases, the mercury expands and the level of the liquid rises. Mercury has many properties that make it the right substance to use in thermometers in laboratories. It is a good conductor of heat, it remains as a liquid over a wide range of temperatures (\(-39ºC\) to \(357ºC\)) and it is a good conductor of heat (this means that it has a fast response to changes in temperature).

A mercury thermometer consists of a glass tube with a thinner tube containing liquid mercury inside, pixabay.

Temperature Formula

The temperature in Celsius can be found from the temperature in Kelvin by the following formula:

\[T_C=T_K-273\]

where \(T_C\) is the temperature in Celsius and \(T_K\) is the temperature in Kelvin.

On the Fahrenheit scale, the freezing point of water is \(32ºF\) and the boiling point is \(212ºF\) (again both at atmospheric pressure). One degree Fahrenheit is less than one degree Celsius. The two scales are related by the formula:

\[T_F=\dfrac{9}{5}T_C+32\]

in which \(T_F\) is the temperature in Fahrenheit.

Difference Between Heat and Temperature

The concepts of heat and temperature can often get confused to mean the same thing when they are actually different, although they are very closely related. As explained above, temperature is a measurable quantity and it describes the average kinetic energy of the molecules in a system. Heat is the transfer of thermal energy due to a difference in temperature between molecules. Heat is a measure of how energy flows through an object. Heat is measured in Joules, \(\text{J}\), the same as the unit of energy.

A situation that demonstrates the difference between heat and temperature is when you make a hot cup of coffee and leave it out. The coffee has a much higher temperature than its surroundings and so its molecules have a greater kinetic energy than those in the surroundings. The energetic molecules in the coffee will slowly transfer thermal energy to the surroundings and will cool down themselves – heat is transferred to the surroundings. Once the coffee has cooled down to the same temperature as the room it is in, no more heat will be transferred. This is because the average kinetic energy of the molecules in the coffee and in the surroundings is the same, so no energy is transferred as a result.

Heat is transferred from a cup of coffee to the surroundings due to the difference in temperature, toppr

When no energy is transferred between the coffee and the surroundings, they are said to be in thermal equilibrium. If there is another object that is at the same temperature as the room, it will also be in thermal equilibrium as the cup of coffee. This seems self-explanatory but it is a key point in thermodynamics and is called the zeroth law of thermodynamics.

The zeroth law of thermodynamics can be easily visualised by considering three bodies in thermal equilibrium, clubtechnical.

The zeroth law relates back to temperature, the main message of the law is that all bodies have a property called temperature and when two bodies are in thermal equilibrium, their temperatures are equal. This may seem like an obvious statement, but it is still an important one and we use the zeroth law all of the time. For instance, if you wanted to verify that two objects are at the same temperature, you can simply measure both of their temperatures with a thermometer. The objects do not need to be brought together to see if there is heat transfer when they are in contact.