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Temperature

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Temperature

Temperature is a very well-known concept in everyday life: we understand not to touch a hot stove because it will give off heat and burn our hands as it has a high temperature. We wear gloves when it is snowing outside because we want to retain heat and will get cold otherwise as the temperature is very low. Why do we feel the sensation of heat? How is this related to the temperature of an object? What actually is temperature? These important questions will be covered in this article.

Temperature Definition

Temperature is a key concept in physics and particularly thermodynamics, which is the study and application of thermal energy in different systems.

Temperature is a measure of the average kinetic energy per molecule in a substance.

The SI unit of temperature is Kelvin, . The lowest temperature possible is called absolute zero and corresponds to . 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.

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 ( to ) and it is a good conductor of heat (this means that it has a fast response to changes in temperature).

Temperature Mercury thermometer image StudySmarterA mercury thermometer consists of a glass tube with a thinner tube containing liquid mercury inside, pixabay.

Unit of Temperature

In physics the unit of temperature used is kelvin, . 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. Room temperature is equal to aroundso 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 or the Celsius scale. 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 ofand a boiling point of, at atmospheric pressure. is approximately equal to (it is to be exact).

Temperature Formula

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

,

whereis the temperature in Celsius and is the temperature in Kelvin.

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

,

in which is the temperature in Fahrenheit.

The body temperature of humans is normally around. Work out the values for body temperature on both the Kelvin scale and the Celsius scale.

It is easiest to first work out the equivalent temperature on the Celsius scale. Recall the equation to convert Celsius to Fahrenheit:

This can be rearranged to give the temperature in Celsius in terms of the Fahrenheit scale:

The value for body temperature

in Fahrenheit can then be plugged in to find the result in Celsius:

,

to the nearest degree Celsius.

Instead of converting from Fahrenheit straight to Kelvin, we can use our previous answer and use the simple equation relating Celsius and Kelvin:

This gives the value of body temperature in kelvin as:

.

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, , 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.

Temperature Coffee heat transfer StudySmarter

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 states that if bodies A and B are in thermal equilibrium with a third body C, bodies A and B are also in thermal equilibrium with each other.

Temperature zeroth law of thermodynamics StudySmarterThe 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.

Temperature - Key takeaways

  • Temperature is a measure of the average kinetic energy per molecule in a substance.

  • Temperature is measured in Kelvin (K) in physics. The lowest possible temperature is 0 K, this is called absolute zero.

  • The Fahrenheit scale and the Celsius scale are two other scales used for measuring temperature. They are more suitable for temperatures recorded in everyday life.

  • A thermometer is used for measuring temperature. There are many different types of thermometers that use how different physical properties change with temperature such as volume, pressure and resistance.

  • The thermometer used in school laboratories is usually a mercury thermometer, which consists of liquid mercury in a glass tube.

  • Heat is different from temperature; it is the transfer of thermal energy due to a difference in temperature.

  • If two objects are at the same temperature, no heat is transferred between them and they are in thermal equilibrium.

  • The zeroth law of thermodynamics states that if bodies A and B are in thermal equilibrium with a third body C, bodies A and B are also in thermal equilibrium with each other.

Frequently Asked Questions about Temperature

Temperature is a measure of the average kinetic energy of the molecules in a substance.

Many physical properties depend on temperature such as pressure, volume and resistance. This can be used as the basis for different types of thermometer.

The three scales used to measure temperature are the Kelvin, Celsius and Fahrenheit scales.

Heat is the transfer of thermal energy between molecules due to a difference in temperature between them.

Final Temperature Quiz

Question

Which temperature scale is more useful in everyday life?

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Answer

Celsius.

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Question

Which is greater, one unit in degrees Celsius or one Kelvin?



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Answer

They are the same.

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Question

What is the type of thermometer most commonly used in school laboratories?

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Answer

A mercury thermometer.

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Question

What causes heat transfer?

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Answer

A difference in temperature

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Question

Why is mercury a good substance to use in a glass thermometer?

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Answer

It is a liquid over a wide range of temperatures (-39°C to 357°C).

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Question

In a mercury glass thermometer, what physical property of the mercury depends on its temperature and is hence used to measure temperature?

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Answer

The volume of the mercury.

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Question

What is the value of absolute zero in degrees Celsius?

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Answer


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Question

What is the lowest possible temperature in Kelvin and what is the name given to this temperature?

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Answer

0 K - this is called absolute zero.

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Question

What units is heat measured in?

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Answer

Joules, .

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Question

What is the zeroth law of thermodynamics?

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Answer

The zeroth law of thermodynamics states that if bodies A and B are in thermal equilibrium with a third body C, the bodies A and B are in thermal equilibrium with each other.

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Question

​​​​​If two bodies with the same temperature are in contact, there will be no heat transfer. Is this true or false?


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Answer

True.

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Question

A hot cup of coffee is placed on a table in a room, is heat transferred to the room or to the coffee?

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Answer

To the room.

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