A room-temperature metal spoon is put into a cup of hot tea. What happens?

The tea is hotter than the spoon. Heat moves from hot to cold. It keeps moving until the temperatures match. The spoon warms, the tea cools slightly, and the two reach thermal equilibrium

What is 25 degrees Celsius in kelvin?

To convert, add 273. 25 plus 273. 298 kelvin

Why does a cup of tea cool faster than a pot of soup at the same temperature?

Both are at the same temperature, so the average particle speed is the same. The pot holds far more substance, so far more thermal energy. Giving the same amount of heat to the room drops the cup by many more degrees. Because the cup stores much less thermal energy despite the equal temperature

Energy and Heat · Grade 8

How does a cup of boiling water differ from a bath at forty degrees?

In temperature the cup wins easily, but in thermal energy the bath holds far more. Temperature measures the average speed of the particles; thermal energy depends on how many particles there are as well. The bath contains thousands of times more molecules, so it can deliver much more heat despite being merely warm.

Learning objectives

Explain why a cup of boiling water and a full bath at forty degrees hold different amounts of thermal energy
Predict the direction in which heat will flow between two bodies from their temperatures alone
Describe what thermal equilibrium is and how you would know that it has been reached
Convert a temperature between degrees Celsius and kelvin

Two different quantities

Temperature measures the average kinetic energy of a single particle in a substance. It does not depend on how much substance there is: a cup of water at ninety degrees and a swimming pool at ninety degrees are at exactly the same temperature.

Thermal energy is the sum of the kinetic energies of all the particles together. It depends both on temperature and on quantity, which is why the pool holds perhaps a million times more of it than the cup.

Heat is the name for energy moving between bodies because of a temperature difference. It is not a property a body has, but something that happens between two of them — which is why "this object contains a lot of heat" is not quite right. It contains thermal energy.

Which way heat travels

Heat always moves from the hotter body to the colder one, never the other way round by itself. You do not need to know how big the bodies are or what they are made of; the two temperatures alone settle the direction.

The transfer continues until the temperatures match. That state is called thermal equilibrium, and in it heat is still crossing in both directions — but at exactly equal rates, so there is no net change.

This is why ice does not "put cold into" a drink. There is no particle of cold. The drink gives energy to the ice, and because the drink loses energy, its temperature falls.

Celsius and kelvin

The Celsius scale is defined by water: zero at freezing, one hundred at boiling under ordinary atmospheric pressure. That is convenient for daily life but physically arbitrary.

The kelvin scale starts at absolute zero, the temperature at which particle motion is minimal and below which nothing can be cooled. That point sits 273 degrees below zero on the Celsius scale.

Converting is simple: add 273 to a Celsius reading to get kelvin, subtract 273 to go back. One degree is the same size on both scales, so a difference of ten degrees Celsius is also a difference of ten kelvin.

Worked examples

A room-temperature metal spoon is put into a cup of hot tea. What happens?
1. The tea is hotter than the spoon
2. Heat moves from hot to cold
3. It keeps moving until the temperatures match
Answer: The spoon warms, the tea cools slightly, and the two reach thermal equilibrium
What is 25 degrees Celsius in kelvin?
1. To convert, add 273
2. 25 plus 273
Answer: 298 kelvin
Why does a cup of tea cool faster than a pot of soup at the same temperature?
1. Both are at the same temperature, so the average particle speed is the same
2. The pot holds far more substance, so far more thermal energy
3. Giving the same amount of heat to the room drops the cup by many more degrees
Answer: Because the cup stores much less thermal energy despite the equal temperature

Common mistakes

Equating a high temperature with a lot of energy: Temperature is an average per particle. A spark from a firework can be at a thousand degrees and still not burn you, because there are very few particles in it.
Saying that ice adds cold: Cold is not a thing that travels. The ice takes energy from the drink, and the drink cools because it is losing energy — the direction is always heat leaving, never cold arriving.
Thinking thermal equilibrium means nothing is moving: Heat keeps crossing in both directions, at equal rates. What stops is the net change in temperature, not the transfer itself.

What to remember

Temperature is an average; thermal energy is a total.
Heat flows hot to cold, always.
Equilibrium: equal temperature, no net change.
Kelvin = Celsius + 273.