Temperature and Matter
5. Temperature and Heat
Learning outcomes
- I can distinguish between temperature and heat.
- I can explain that heat is energy transferred because of a temperature difference.
- I can describe the concept of thermal equilibrium.
- I can explain how internal energy is related to the motion and arrangement of particles.
- I can identify situations in which heat flows from a warmer object to a cooler object until equilibrium is reached.
Temperature and Heat Are Not the Same Thing
In everyday language, we often use the words heat and temperature as though they mean the same thing. In physics, however, they describe different ideas.
Temperature is a measure related to the average kinetic energy of the particles in a substance.
Heat is energy transferred from one object or region to another because there is a temperature difference.
A useful distinction is:
Temperature describes the thermal state of an object.
Heat describes energy being transferred.
For example, suppose a hot metal spoon is placed into cool water.
- The spoon initially has a higher temperature.
- The water initially has a lower temperature.
- Energy transfers from the spoon to the water.
- This energy transfer is called heat.
Heat is therefore not something that simply "sits inside" an object. When discussing energy stored within an object, we use the concept of internal energy.
Temperature
Temperature tells us about the thermal condition of an object.
At the particle level, temperature is related to the average kinetic energy of the particles.
When temperature increases, particles generally have greater average kinetic energy.
Depending on the state of matter, this can mean:
- particles vibrate more energetically
- particles move faster
- collisions become more energetic
When temperature decreases, the average kinetic energy of the particles decreases.
Common temperature scales include:
- degrees Celsius, °C
- kelvin, K
- degrees Fahrenheit, °F
In science, Celsius and kelvin are particularly common.
The SI unit of temperature is the kelvin (K).
Temperature and Particle Motion
Imagine two containers containing the same substance.
Container A is at 20°C.
Container B is at 80°C.
The particles in both containers are moving, but the particles in Container B have a greater average kinetic energy.
This does not mean that every individual particle in Container B is moving faster than every particle in Container A. Particle speeds vary.
Temperature is related to the average kinetic energy of the particles.
This word — average — is important.
What Is Heat?
Heat is the transfer of energy caused by a temperature difference.
Heat naturally transfers from a region of higher temperature to a region of lower temperature.
We can represent this as:
warmer object → energy transfer → cooler object
The transfer continues while a temperature difference exists.
For example:
Hot coffee → surrounding air
Warm hand → ice cube
Hot pan → cooler food
Warm room → cold window
In each case, energy is transferred from the warmer region toward the cooler region.
Worked Example: An Ice Cube in Water
Suppose an ice cube at 0°C is placed in water at 25°C.
Which direction does heat flow?
The water has the higher temperature.
Therefore:
water → ice
Energy is transferred from the warmer water to the colder ice.
It is common to say that the ice "makes the water cold," but this can hide what is physically happening.
The important energy transfer is:
The warmer water loses energy while the colder ice gains energy.
The water therefore cools while the ice gains energy and may melt.
Heat Is Energy Transfer
Heat is measured in joules (J) because heat is a transfer of energy.
This is another important difference between heat and temperature.
| Quantity | What it describes | Common unit |
|---|---|---|
| Temperature | Thermal state related to average particle kinetic energy | °C or K |
| Heat | Energy transferred because of a temperature difference | J |
| Internal energy | Energy associated with particles within a system | J |
A temperature of 50°C is therefore very different from a heat transfer of 50 J.
They are different physical quantities and have different units.
Internal Energy
The internal energy of a system is the energy associated with the microscopic particles that make up the system.
At this level, internal energy includes contributions from:
- the kinetic energy of particles
- potential energy associated with interactions and arrangements of particles
The kinetic part is connected to particle motion.
Particles may:
- move from place to place
- rotate
- vibrate
The potential-energy part depends on particle arrangement and the forces between particles.
This becomes especially important during changes of state.
Internal Energy and Heating
When energy is transferred to an object by heating, its internal energy can increase.
Sometimes this causes the temperature to rise.
For example:
A metal block is heated.
Energy is transferred to the metal.
Its particles vibrate more energetically.
Their average kinetic energy increases.
The temperature rises.
We can describe the process as:
energy transferred by heating → internal energy increases → temperature rises
However, an increase in internal energy does not always produce a temperature increase.
Internal Energy During Changes of State
Consider ice melting at its melting point.
Energy continues to be transferred to the ice, but during the phase change its temperature can remain approximately constant.
Where does the energy go?
It contributes to changing the arrangement and interactions of the particles, increasing the potential-energy component of the system's internal energy rather than increasing their average kinetic energy.
This allows the solid structure to break down and the substance to change into a liquid.
Therefore:
Internal energy can change even when temperature does not.
This is one reason temperature and internal energy should not be treated as the same thing.
Thermal Equilibrium
Suppose a hot metal block is placed in contact with a colder metal block.
Initially:
Block A: 80°C
Block B: 20°C
Energy transfers from the hotter block to the colder block.
As this happens:
- Block A cools.
- Block B warms.
- Their temperature difference becomes smaller.
Eventually, both objects reach the same temperature.
At this point they are in thermal equilibrium.
Thermal equilibrium occurs when objects in thermal contact have the same temperature and there is no net heat transfer between them.
Importantly, particles are still moving and energy is still being exchanged microscopically. There is simply no net transfer of energy from one object to the other due to a temperature difference.
Worked Example: Hot Metal in Cool Water
A metal block at 90°C is placed into water at 20°C in an insulated container.
Initially:
Metal: 90°C
Water: 20°C
Therefore:
metal → water
Energy transfers from the metal to the water.
The metal cools.
The water warms.
Suppose they eventually both reach 30°C.
At this point:
Metal: 30°C
Water: 30°C
The two substances have reached thermal equilibrium.
There is no longer a temperature difference driving a net heat transfer between them.
Predicting the Direction of Heat Flow
The direction of heat transfer can usually be predicted simply by comparing temperatures.
Situation A
Object A = 70°C
Object B = 30°C
A → B
Situation B
Object A = 10°C
Object B = 40°C
B → A
Situation C
Object A = 25°C
Object B = 25°C
The objects are at the same temperature.
There is no net heat transfer due to a temperature difference.
A Cold Object Does Not "Send Cold"
This is a very common misconception.
Suppose you hold an ice cube.
Your hand feels cold.
It may seem as though "cold" is moving from the ice into your hand.
That is not what is happening.
Your hand is warmer than the ice.
Therefore, energy transfers:
hand → ice
Your hand loses internal energy, so its temperature decreases.
You experience this as a feeling of cold.
There is no substance called "cold" flowing into your hand.
Temperature Does Not Tell Us the Total Internal Energy
Consider:
- a small cup of water at 90°C
- a large swimming pool at 30°C
The cup has the higher temperature.
However, the swimming pool contains vastly more water and therefore vastly more particles. Its total internal energy can be much greater.
Temperature does not simply tell us how much total energy an object contains.
This is another reason we must distinguish between:
- temperature
- heat
- internal energy
Worked Example: Cup vs. Bathtub
Consider two samples of water:
Cup: 200 mL at 60°C
Bathtub: 100 L at 40°C
Which has the higher temperature?
The cup.
Which contains more particles?
The bathtub.
Which is likely to have greater total internal energy?
The bathtub, because it contains far more water.
This shows why a higher temperature does not necessarily mean greater total internal energy.
How Heat Can Be Transferred
Energy can be transferred because of temperature differences in several ways.
The three major mechanisms are:
- conduction
- convection
- radiation
Conduction
Energy is transferred through interactions between particles, particularly in solids.
Example:
A metal spoon becomes hot when left in a pot of hot soup.
Convection
Energy is transferred through the bulk movement of fluids such as liquids and gases.
Example:
Warm water rises while cooler water sinks in a heated pot.
Radiation
Energy is transferred by electromagnetic waves.
Radiation does not require matter.
Example:
Energy from the Sun reaches Earth through space.
These mechanisms will often operate together in real situations.
Everyday Example: A Hot Drink Cooling
Imagine a cup of tea at 85°C in a room at 22°C.
The tea is warmer than its surroundings.
Energy therefore transfers from the tea to:
- the cup
- the surrounding air
- the table
- nearby objects and surroundings
As the tea loses energy, its temperature falls.
Over time, the tea approaches the temperature of its surroundings.
Eventually, if conditions remain stable long enough:
tea temperature ≈ room temperature
The tea and its surroundings approach thermal equilibrium.
The tea does not normally continue cooling indefinitely below room temperature simply because it started out hot.
Everyday Example: A Cold Drink Warming Up
Now consider a drink at 5°C placed in a room at 22°C.
This time, the room is warmer than the drink.
Energy transfers:
surroundings → drink
The drink's internal energy increases and its temperature rises.
Eventually:
drink temperature ≈ room temperature
Again, the system approaches thermal equilibrium.
Notice that the direction of heat transfer has reversed compared with the hot drink.
The rule remains the same:
Heat transfers from higher temperature toward lower temperature.
Thermal Equilibrium and Thermometers
The idea of thermal equilibrium explains how a thermometer measures temperature.
When a thermometer is placed in contact with an object:
- the thermometer and object may initially have different temperatures
- energy transfers between them
- the thermometer's temperature changes
- eventually they approach thermal equilibrium
The thermometer reading then represents the temperature of the object being measured, within the limits of the instrument.
This is why you usually need to wait for a thermometer reading to stabilize.
Worked Example: Using a Thermometer
A thermometer initially reads 20°C.
It is placed into warm water at 60°C.
Initially:
water = warmer
thermometer = cooler
Therefore:
water → thermometer
Energy transfers to the thermometer.
Its temperature rises.
After sufficient time, the thermometer approaches the same temperature as the water.
The reading stabilizes.
This is an example of thermal equilibrium being used to make a measurement.
Internal Energy at the Particle Level
It is useful to think about three related ideas separately.
Temperature
Related to the average kinetic energy of particles.
Internal Energy
Associated with the microscopic kinetic and potential energies of the particles in the system.
Heat
Energy transferred because of a temperature difference.
A useful summary is:
Temperature difference → heat transfer → change in internal energy
That change in internal energy may cause:
- a temperature change
- a change of state
- or both, depending on the situation.
Real-World Thermal Equilibrium
Thermal equilibrium occurs or is approached in many everyday situations.
Food in a Refrigerator
Warm food placed in a refrigerator transfers energy to the colder surroundings.
The food cools until it approaches the refrigerator temperature.
Ice in a Drink
The warmer drink transfers energy to the colder ice.
The drink cools while the ice gains energy and melts.
A Metal Spoon in Soup
The hot soup transfers energy to the cooler spoon.
The spoon becomes warmer.
A Person Entering Cold Water
The human body is warmer than the water.
Energy transfers from the body to the water.
A Cold Phone Brought Indoors
A cold phone brought into a warm room gains energy from its warmer surroundings until it approaches room temperature.
Common Mistakes
"Heat and temperature are the same."
Incorrect.
Temperature describes a thermal state related to average particle kinetic energy.
Heat is energy transferred because of a temperature difference.
"An object contains heat."
This wording can be misleading.
An object has internal energy. Heat refers to energy in transfer because of a temperature difference.
"Cold flows from one object to another."
Incorrect.
Energy transfers from the warmer object to the cooler object.
"A higher temperature means more total internal energy."
Not necessarily.
The amount of material also matters, as do its state and microscopic properties.
"At thermal equilibrium, particles stop moving."
Incorrect.
Particles continue their microscopic motion. There is simply no net heat transfer caused by a temperature difference between systems at the same temperature.
"Adding energy always increases temperature."
Not always.
During a change of state, energy can increase internal energy while the temperature remains approximately constant.
Check Your Understanding
1. What is the difference between temperature and heat?
2. Why is heat measured in joules?
3. An object at 80°C touches an object at 25°C. In which direction does energy transfer?
4. What happens to the temperature difference as the two objects approach thermal equilibrium?
5. Define thermal equilibrium.
6. Does heat continue to flow in one net direction between two objects once they are at the same temperature?
7. Explain why an ice cube feels cold when you hold it.
8. What does temperature tell us about particles?
9. What two microscopic forms of energy contribute to internal energy?
10. Why can internal energy increase without temperature increasing during melting?
11. A cup of water and a swimming pool have the same temperature. Must they have the same total internal energy? Explain.
12. A cold drink is placed in a warm room. Predict the direction of heat transfer and describe what eventually happens.
13. Why should you wait for a thermometer reading to stabilize?
14. Explain why saying "cold moves from the ice into my hand" is scientifically incorrect.
Key Terms
- Temperature – a measure related to the average kinetic energy of particles.
- Heat – energy transferred because of a temperature difference.
- Thermal equilibrium – a condition in which systems in thermal contact have the same temperature and there is no net heat transfer between them.
- Internal energy – energy associated with the microscopic kinetic and potential energies of particles in a system.
- Kinetic energy – energy associated with motion.
- Potential energy – stored energy associated with particle positions, arrangements, and interactions.
- Conduction – energy transfer through interactions between particles, especially important in solids.
- Convection – energy transfer involving the movement of a fluid.
- Radiation – energy transfer by electromagnetic waves.
- Temperature difference – the difference in temperature between two objects or regions that can drive heat transfer.
Key Takeaways
- Temperature and heat are different physical quantities.
- Temperature is related to the average kinetic energy of particles.
- Heat is energy transferred because of a temperature difference.
- Heat naturally transfers from a warmer object to a cooler object.
- An object's internal energy includes microscopic kinetic and potential energy.
- Heating can increase an object's internal energy.
- Increasing internal energy does not always increase temperature because energy can also change the arrangement of particles, such as during melting.
- Thermal equilibrium occurs when objects in thermal contact reach the same temperature and there is no net heat transfer between them.
- Cold does not flow from one object to another; energy transfers from the warmer object toward the cooler one.
- Temperature does not tell us the total internal energy of an object because the amount and type of material also matter.