Alignment
Learning Intentions
By the end of the lesson, students will be able to:
- Explain that temperature changes when energy is added to or removed from a system.
- Describe temperature change using the kinetic particle model.
- Distinguish between energy changes that change temperature and energy changes that cause phase change.
- Identify the system and surroundings in heating and cooling examples.
Success Criteria
By the end of the lesson, students have successfully:
- Explained that heating a system increases the average kinetic energy of its particles when there is no phase change.
- Explained that cooling a system decreases the average kinetic energy of its particles when there is no phase change.
- Used particle-level reasoning to explain everyday heating and cooling phenomena.
- Recognised that during a phase change, added or removed energy does not change temperature.
- Interpreted simple temperature-time graphs for heating and cooling without phase change.
Syllabus Reference
- Unit 1: Thermal, Nuclear and Electrical Physics
- Topic 1: Heating Processes
- Science Understanding: Kinetic Particle Model and Specific Heat Capacity
- Explain that a change in temperature is due to the addition or removal of energy from a system, without phase change.
Phenomenon
A metal spoon left in a cup of hot tea becomes hotter, while a warm drink placed in a fridge becomes colder.
In both cases, the temperature of the object changes because energy is transferred between the system and its surroundings.
For the spoon, energy is added to the spoon from the hot tea.
For the drink in the fridge, energy is removed from the drink and transferred to the colder surroundings.
The key question is:
Why does adding or removing energy change temperature when no phase change occurs?
Key Idea
When energy is added to a system without causing a phase change, the particles in the system gain kinetic energy and move faster on average. This causes the temperature of the system to increase.
When energy is removed from a system without causing a phase change, the particles lose kinetic energy and move slower on average. This causes the temperature of the system to decrease.
A change in temperature shows that the average kinetic energy of the particles has changed.
Concept
The concept and thought that best describes the cause of the phenomenon is below.
Temperature is a measure of the average kinetic energy of the particles in a substance.
If a system gains energy by heating and there is no phase change, that added energy increases the average kinetic energy of the particles. The particles move faster, so the temperature increases.
If a system loses energy by cooling and there is no phase change, the average kinetic energy of the particles decreases. The particles move more slowly, so the temperature decreases.
This can be described qualitatively as:
- energy added to system → particle kinetic energy increases → temperature increases
- energy removed from system → particle kinetic energy decreases → temperature decreases
This lesson focuses on situations where there is no phase change. That means the substance remains in the same state, such as solid, liquid or gas, while its temperature changes.
Convention
The key conventions associated with the concept and in the branch of established knowledge is below.
- The system is the object or substance being studied.
- The surroundings are everything outside the system.
- Heating means energy is transferred into a system because of a temperature difference.
- Cooling means energy is transferred out of a system because of a temperature difference.
- Heat flows naturally from a hotter object to a colder object.
- Temperature is measured in degrees Celsius,
, or kelvin, . - A positive temperature change means the system has warmed:
- A negative temperature change means the system has cooled:
- Temperature change is calculated using:
- Without phase change:
- adding energy usually increases temperature
- removing energy usually decreases temperature
- During a phase change:
- energy changes the arrangement or separation of particles
- temperature remains constant until the phase change is complete
Misconceptions
Common misconceptions students have regarding the concept when applying to various situations and solving problems. It could be a conceptual, mathematical or logical misconception.
- Students may think that temperature and heat are the same thing. Heat is energy transferred because of a temperature difference; temperature is related to average particle kinetic energy.
- Students may think that coldness is transferred. In physics, energy is transferred from the warmer object to the cooler object.
- Students may think that adding energy always increases temperature. During a phase change, energy can be added while temperature stays constant.
- Students may think that all particles in a substance move at the same speed. Temperature describes the average kinetic energy, not the kinetic energy of every individual particle.
- Students may think that a large object at low temperature has less thermal energy than a small object at high temperature. Total thermal energy depends on mass, particle number, material and temperature.
Further Reading
- QCAA Physics Unit 1: Heating Processes
- Textbook section on kinetic particle model and thermal energy
- Textbook section on heating curves and cooling curves
- PhET simulation: States of Matter
- PhET simulation: Energy Forms and Changes
Explicit Instruction
Begin with the cup of tea example.
A spoon placed in hot tea becomes warmer because energy is transferred from the hotter tea to the cooler spoon. The spoon is the system if we are studying the spoon. The tea is part of the surroundings.
At the particle level, the particles in the spoon vibrate more strongly as energy is transferred into the spoon. Their average kinetic energy increases, so the temperature of the spoon increases.
Now consider a warm drink placed in a fridge. If the drink is the system, energy is transferred out of the drink to the colder surroundings. The particles in the drink move more slowly on average. Their average kinetic energy decreases, so the temperature of the drink decreases.
The relationship can be summarised as:
| Energy transfer | Particle motion | Temperature change |
|---|---|---|
| Energy added to the system | Average kinetic energy increases | Temperature increases |
| Energy removed from the system | Average kinetic energy decreases | Temperature decreases |
| Energy added or removed during phase change | Particle arrangement changes | Temperature remains constant |
This lesson is restricted to cases where there is no phase change.
For example:
- warming water from
to is a temperature change without phase change - cooling copper from
to is a temperature change without phase change - melting ice at
is not a temperature change without phase change - boiling water at
is not a temperature change without phase change
Temperature change is calculated using:
where:
is the change in temperature is the final temperature is the initial temperature
A positive value of
A negative value of
Worked Examples
Worked Example 1
A beaker of water is heated from
Step 1: Identify the system
The system is the water.
Step 2: Calculate the temperature change
Step 3: Interpret the sign
The temperature change is positive, so the water has warmed.
Step 4: Explain using particles
Energy has been added to the water. Since there is no phase change, this added energy increases the average kinetic energy of the water particles. The particles move faster on average, so the temperature increases.
Answer
The water gained energy. The average kinetic energy of its particles increased, causing the temperature to increase by
Worked Example 2
A metal block cools from
Step 1: Identify the system
The system is the metal block.
Step 2: Calculate the temperature change
Step 3: Interpret the sign
The temperature change is negative, so the block has cooled.
Step 4: Explain using particles
Energy has been removed from the metal block. Since the block remains solid, there is no phase change. The average kinetic energy of the particles decreases, so the temperature decreases.
Answer
The metal block lost energy to its surroundings. The particles vibrated less strongly on average, causing the temperature to decrease by
Worked Example 3
A student heats ice at
Step 1: Identify the situation
The ice is melting. Melting is a phase change.
Step 2: Apply the key idea
During a phase change, added energy does not increase temperature. Instead, the energy changes the arrangement and separation of particles.
Step 3: Explain using particles
The added energy allows particles to overcome some of the forces holding them in fixed positions in the solid structure. The substance changes from solid ice to liquid water, but the temperature remains
Answer
The student is incorrect because the energy added during melting is used to change the state of the substance, not to increase the average kinetic energy of the particles. Therefore, the temperature remains constant during the phase change.
Check for Understanding
Check 1
A cup of soup cools from
- Identify the system.
- Calculate
. - State whether energy was added to or removed from the system.
- Explain the temperature change using the kinetic particle model.
Expected response
- The system is the soup.
- Energy was removed from the soup.
- The average kinetic energy of the soup particles decreased, so the temperature decreased.
Check 2
A student says, “When an object cools down, cold enters the object.”
Explain why this statement is incorrect.
Expected response
Cold does not enter the object. Instead, energy is transferred out of the warmer object to cooler surroundings. As the object loses energy, the average kinetic energy of its particles decreases, so its temperature decreases.
Check 3
A beaker of water is heated from
Which part involves a temperature change without phase change?
A. Heating from
B. Boiling at
C. Both A and B
D. Neither A nor B
Expected response
A. Heating from
Investigation (Alternative to Explicit)
Hypothesis
If energy is added to water using a heater and no phase change occurs, then the temperature of the water will increase because the average kinetic energy of the water particles increases.
Data Collection
Equipment
- beaker
- water
- thermometer or temperature probe
- hot plate or immersion heater
- stopwatch
- retort stand and clamp
- safety glasses
- heatproof mat
Method
- Measure a fixed volume of water into a beaker.
- Record the initial temperature of the water.
- Heat the water gently using a hot plate or immersion heater.
- Record the temperature every
seconds. - Stop heating before the water reaches boiling point.
- Plot temperature against time.
Safety
- Wear safety glasses.
- Avoid touching hot glassware.
- Keep electrical equipment away from spilled water.
- Do not heat the water to boiling for this investigation.
- Use tongs or heatproof gloves when handling hot equipment.
Analysis
Students should:
- Calculate temperature changes using
. - Identify whether energy was added or removed.
- Describe the relationship between heating time and temperature.
- Explain the trend using the kinetic particle model.
- Identify whether a phase change occurred.
Expected graph:
- The temperature should increase over time.
- If heating is approximately constant, the graph should show an approximately linear increase before boiling.
- The graph should not include a flat section, because the investigation avoids phase change.
Evaluation
Students should consider:
- Was energy transferred evenly to the water?
- Was energy lost to the beaker and surroundings?
- Was the temperature probe placed consistently?
- Was the water stirred to maintain uniform temperature?
- Was the same mass of water used in each trial?
- Did the water remain in the liquid state for the entire investigation?
Limitations:
- Some energy heats the beaker instead of the water.
- Some energy is lost to the air.
- The hot plate may not provide perfectly constant heating.
- The thermometer may have measurement uncertainty.
Improvements:
- Use insulation around the beaker.
- Use a lid to reduce energy loss.
- Stir the water gently before each reading.
- Use a digital temperature probe.
- Repeat the investigation and average results.
Problems
The following problems are designed to develop conceptual and mathematical understanding of temperature change due to energy transfer without phase change.
-
A copper block is heated from
to . - a. Calculate
. - b. State whether energy was added or removed.
- c. Explain the change using the kinetic particle model.
- a. Calculate
-
A glass of water cools from
to in a refrigerator. - a. Identify the system.
- b. Identify the surroundings.
- c. Calculate
. - d. Explain why the temperature decreases.
-
A student heats a metal rod but it does not melt.
- a. What happens to the average kinetic energy of the particles?
- b. What happens to the temperature?
- c. Why is the phrase “without phase change” important?
-
A substance is heated and its temperature remains constant.
- a. Is the substance definitely undergoing a temperature change?
- b. What process might be occurring?
- c. Where is the added energy going?
-
Explain why a swimming pool and a cup of tea can have different total thermal energies even if the cup of tea has a higher temperature.
-
A beaker of ethanol warms from
to . - a. Calculate
. - b. Explain what happens to the particles.
- c. State whether this is an example of energy addition or energy removal.
- a. Calculate
-
A metal spoon is placed in hot soup.
- a. Identify the direction of energy transfer.
- b. Explain why the spoon’s temperature increases.
- c. Explain what eventually happens if the spoon and soup are left together for a long time.
-
A hot rock is placed into cool water.
- a. Which object loses energy?
- b. Which object gains energy?
- c. What happens to the temperature of each object before thermal equilibrium is reached?
-
A student says, “If the particles in a substance move faster, the substance must be changing state.” Explain why this is incorrect.
-
Sketch a temperature-time graph for water being warmed from
to without boiling. Label the section where energy is being added and temperature is increasing.
Followup
Self-check
Students should be able to answer the following questions:
- Can I identify the system and surroundings in a heating or cooling example?
- Can I calculate temperature change using
? - Can I explain temperature increase in terms of increased average kinetic energy?
- Can I explain temperature decrease in terms of decreased average kinetic energy?
- Can I explain why temperature does not change during phase change?
- Can I distinguish between heat, temperature and thermal energy?
Next Topic
The next topic is specific heat capacity.
Students will learn that different materials require different amounts of energy to produce the same temperature change. This relationship is described using:
where:
is the energy transferred is the mass of the substance is the specific heat capacity is the change in temperature