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Heat Transfer

サイト: Young Education
コース: Thermal Energy and Gas Laws
ブック: Heat Transfer
印刷者: Guest user
日付: 2026年 09月 25日(金曜日) 03:22

目次

1. Thermal Equilibrium and Heat Flow

Learning outcomes

  • I can explain that heat flows naturally from warmer objects to cooler objects.

  • I can define thermal equilibrium and describe when it occurs.
  • I can predict the direction of heat transfer between two objects.
  • I can distinguish between heat transfer and temperature change.
  • I can identify examples of thermal equilibrium in everyday life.

When solids, liquids, and gases are heated at constant pressure, they expand because the particles that make up these substances gain kinetic energy and move more vigorously. In a solid, the particles are tightly packed in fixed positions and can only vibrate. When heated, these vibrations become stronger, causing the particles to push slightly farther apart from each other. This results in a small increase in size, or expansion. Liquids expand more than solids because their particles are not fixed in place and can move past one another more freely. As temperature increases, the particles move faster and spread out slightly, increasing the volume of the liquid. Gases expand the most when heated because their particles are already far apart and free to move in all directions. When heated at constant pressure, faster-moving gas particles spread out even further, causing a large increase in volume.

Thermal expansion has many important everyday applications. For example, metal bridge sections are built with small gaps, known as expansion joints, so that the materials can expand in hot weather without causing structural damage. Similarly, railway tracks are designed with small spaces between the rails to allow for expansion on warm days. Without these gaps, the expanding metal could bend or buckle, creating dangerous conditions. Thermal expansion is also the reason why a tight metal lid on a glass jar can be loosened by running it under hot water. The metal lid expands more quickly than the glass jar, making it easier to open.

Thermal expansion can be both helpful and harmful in real-life situations. It is helpful in devices such as thermostats, where materials expand and contract in response to temperature changes to control heating systems automatically. However, it can also cause problems if not properly managed. In buildings, pipelines, or roads, expansion during hot weather can lead to cracks, warping, or even structural failure if there is no room for the material to expand. By understanding how and why materials expand when heated, engineers and designers can create safer and more reliable structures and systems that account for changes in temperature.

2. Conduction

Learning outcomes

  • I can describe conduction as heat transfer through direct particle collisions.

  • I can explain why conduction is most effective in solids.
  • I can compare thermal conductors and thermal insulators.
  • I can identify materials with high and low thermal conductivity.
  • I can apply conduction concepts to real-world situations.

Thermal Conductors and Insulators

Materials that allow heat to pass through them quickly are called good thermal conductors. Metals such as copper, aluminium, silver, and steel are excellent examples — they are used for cooking pans, radiators, and the bases of kettles because they transfer heat efficiently.
Materials that slow the transfer of heat are called thermal insulators or poor conductors. Wood, plastic, rubber, glass, wool, and many foams are common insulators; they are used for saucepan handles, refrigerator doors, or building insulation to keep heat in or out.

How Conduction Works in Solids

In solids, the particles (atoms or molecules) are tightly packed in a regular arrangement called a lattice. When one part of a solid is heated, those particles vibrate faster. Their vibrations pass along the lattice, transferring energy from particle to particle — this is thermal conduction by lattice vibrations.
Metals conduct heat much more efficiently than non-metals because, in addition to lattice vibrations, they have delocalised (free) electrons that move easily through the metal. These electrons carry energy quickly from hot regions to cooler ones, making metals the best thermal conductors.

Thermal Conduction

A. Key Ideas

  1. Define thermal conduction in your own words.

  2. Give two examples of good thermal conductors and two poor conductors (insulators).

  3. Why are saucepan handles often made of plastic or wood instead of metal?

  4. Metals are better thermal conductors than non-metals. Explain why, using the idea of delocalised electrons.

B. Understanding the Lattice

  1. Draw a simple diagram of particles in a solid showing how lattice vibrations transfer energy. Label your diagram.

  2. Why is conduction usually slower in non-metals than in metals?

  3. If a 15 cm metal rod and a 15 cm wooden rod are heated at one end, which one will get hot at the other end first? Explain.

C. Real-World Links

  1. A company is designing a cooking pot. Which material would you suggest for the pot body and which for the handle? Give reasons.

  2. Name one situation where it’s useful to prevent conduction and one where it’s useful to encourage it.

  3. A student says, “Styrofoam keeps drinks warm because it generates heat.” Correct this misunderstanding.

Convection

A. Concepts

  1. Define convection.

  2. Convection happens only in liquids and gases, not in solids. Why?

  3. What property of a liquid or gas changes when it is heated, allowing convection currents to form?

B. Explaining the Process

  1. Use the words density, rise, sink, and current to describe what happens when water at the bottom of a beaker is heated.

  2. Draw and label a diagram showing convection currents in a pan of water being heated from below.

  3. Describe how convection helps a radiator heat a room.

C. Applications

  1. Sea breezes are an example of convection. Explain how they form during the day.

  2. Why does a refrigerator have a cooling unit at the top and not at the bottom?

  3. A hot-air balloon rises when the air inside is heated. Explain this using density.

  4. List three everyday examples where convection transfers heat.

3. Convection and Phase Change

Learning outcomes

  • I can describe convection as heat transfer through moving fluids.
  • I can explain how convection currents form because of density differences.
  • I can distinguish between boiling and evaporation.
  • I can explain why evaporation causes cooling.
  • I can identify examples of convection and evaporation in nature and everyday life.

Evaporation is the process where particles from the surface of a liquid escape into the air. Not all particles in a liquid have the same energy; some have more energy than others. The particles at the surface that have the most energy can break away from the liquid and become gas particles. This is why evaporation can happen at any temperature, not just at the boiling point. For example, puddles slowly dry up on a warm day even if the temperature is well below 100 °C.

When particles with higher energy leave the liquid, the average energy of the particles left behind becomes lower. Since temperature is a measure of the average energy of the particles, the liquid becomes cooler as evaporation takes place. This is why sweating cools the human body. The sweat evaporates, carrying away the more energetic particles, and the skin feels cooler because the liquid left behind has less energy.

Melting and boiling are different from evaporation because they involve a change of state caused by continuous energy input. When a solid melts at its melting point, or a liquid boils at its boiling point, the temperature does not rise even though energy is being added. Instead, the energy goes into breaking the forces between particles rather than increasing their speed. This means the phase change happens at a constant temperature until the entire substance has melted or boiled.

Evaporation and boiling are related but not the same. Evaporation happens only at the surface of a liquid and can occur at any temperature. Boiling, on the other hand, happens throughout the liquid, not just at the surface, and it only occurs at the boiling temperature. That is why you see bubbles forming throughout water when it boils, but you do not see this during evaporation.

Temperature plays an important role in evaporation. The higher the temperature, the more particles have enough energy to escape, so the rate of evaporation increases. This is why clothes dry faster on a hot day than on a cold day. It is also why wet floors dry more quickly in the summer.

Surface area also affects evaporation. A liquid spread out over a wide surface has more particles at the surface that can escape into the air. For instance, water spilled on the ground dries faster than the same amount of water kept in a tall, narrow glass. More surface area allows more particles to escape at the same time.

Air movement makes a big difference in evaporation as well. When air moves across the surface of a liquid, it carries away vapor particles that have just escaped. This prevents them from falling back into the liquid and allows more particles to escape. This is why clothes dry faster on a windy day than on a still day. It is also why we feel cooler if a breeze passes over damp skin—the moving air helps the sweat evaporate faster.

Altogether, evaporation and boiling show how particle energy and motion control everyday processes. From sweating and cooling, to drying clothes, to cooking food, the principles of particle energy and phase change explain many observations in daily life. By understanding the roles of temperature, surface area, and air movement, we can better explain why these processes happen at different rates under different conditions.

Practice Questions: Evaporation, Cooling, and Boiling

1. Short Answer
a) What type of particles escape during evaporation?
b) Why does evaporation happen only at the surface of a liquid?

2. Multiple Choice
Evaporation causes cooling because:
A. All particles lose energy at once
B. The most energetic particles leave, lowering the average energy of those left behind
C. The liquid absorbs heat from the air
D. Evaporation only happens at boiling point

3. Explanation
Why does the temperature of boiling water remain constant at 100 °C until all of the water has turned to steam?

4. True or False
a) Evaporation can only happen at the boiling point.
b) Boiling takes place throughout a liquid, not just at the surface.

5. Everyday Life
Give one example of how evaporation cools the human body.

6. Compare & Contrast
State two differences between evaporation and boiling.

7. Multiple Choice
Which factor will not increase the rate of evaporation?
A. Higher temperature
B. Larger surface area
C. Faster air movement
D. Decreasing the surface area

8. Problem Solving
Two identical wet cloths are left to dry. One is spread out flat, and the other is folded into a small square. Which will dry faster, and why?

9. Short Answer
How does air movement over the surface of a liquid affect evaporation?

10. Application
A student spills 50 mL of water on the floor. Another 50 mL is left in a cup. Which one will evaporate faster, and what is the scientific reason?

4. Radiation

Learning outcomes
  • I can describe radiation as heat transfer by electromagnetic waves.
  • I can explain why radiation does not require a medium.
  • I can compare conduction, convection, and radiation.
  • I can explain how surface color and texture affect heat absorption and emission.
  • I can identify examples of radiative heat transfer in everyday life.

Thermal Radiation and the Need for a Medium

Thermal radiation is a way of transferring energy as electromagnetic waves, mainly in the infrared region of the spectrum. Unlike conduction and convection, radiation does not need a medium; it can travel through empty space. This is how energy from the Sun reaches the Earth—by radiation across the vacuum of space. Any object above absolute zero emits infrared radiation, and the amount depends on its temperature and the nature of its surface.

Effect of Surface Colour and Texture

The colour and texture of a surface greatly affect how it emits, absorbs, and reflects thermal radiation. Dull, dark surfaces (especially black) are good absorbers and good emitters of heat. They absorb radiation quickly and also lose heat rapidly by emission. Shiny or light-coloured surfaces (like polished silver or white paint) are poor absorbers and emitters but excellent reflectors. This is why radiators are often painted matte black, while reflective foil is used to reduce heat loss or gain.

Earth’s Temperature and Radiation Balance

The temperature of Earth is controlled by the balance between the radiation it absorbs from the Sun and the radiation it emits back into space as infrared energy. If more energy is absorbed than emitted, the planet warms; if more is emitted than absorbed, it cools. This balance is influenced by the atmosphere, clouds, ice, and greenhouse gases, which can trap heat and slow the escape of infrared radiation, affecting global climate.

Experiments on Emitters and Absorbers

To compare emitters, you can take two identical metal cans: paint one black and the other shiny silver. Fill both with hot water and measure how fast they cool. The black can loses heat faster, showing it is a better emitter. For absorbers, place the same cans in sunlight or under a heat lamp, and measure how fast they warm up. The black can heats up more quickly, proving it is a better absorber of radiation.

Everyday Applications and Consequences

Knowledge of conduction, convection, and radiation helps explain many everyday situations. Cooking pans often have shiny outsides to reduce heat loss and dark interiors to absorb heat efficiently. Space blankets have shiny surfaces to reflect body heat back to a patient. Houses use insulation to limit conduction and convection, while radiators rely on both convection and radiation to warm a room. Wearing white clothes in summer keeps you cooler because they reflect sunlight, while black clothing in winter helps absorb warmth.

Cooling Curves

Goal: Identify better emitters using given data.
Materials: Handout, pencils, ruler, calculator (optional).
Student steps:
a) Plot both data sets on the same time–temperature graph.
b) Estimate the cooling rate (slope) for the first 6 minutes.
c) Write a 3-sentence CER (Claim–Evidence–Reasoning) naming the better emitter.

Data (identical cans, same start temp 80 °C):

  • Matte black can (°C): 80, 74, 69, 65, 62, 60 at t = 0,2,4,6,8,10 min

  • Shiny silver can (°C): 80, 76, 73, 71, 70, 69 at t = 0,2,4,6,8,10 min

Learning Outcomes
  • Understand the mechanisms of heat transfer.

Key Topics:
  • Conduction, convection, and radiation.
  • Real-world examples of heat transfer.

Mechanisms of Heat Transfer 

Heat transfer occurs in three main ways: conduction, convection, and radiation. Each mechanism transfers thermal energy from a hotter object to a colder one.

1. Conduction: Heat Transfer Through Direct Contact

Definition:

Conduction is the transfer of thermal energy through direct contact between molecules without the movement of the material itself.

How It Works:

  • Heat flows from hotter molecules to cooler molecules by collisions.
  • Metals conduct heat well due to free-moving electrons.
  • Insulators (wood, rubber) conduct poorly because they lack free electrons.

Real-World Examples:

Touching a metal spoon in hot soup → Heat moves from the spoon to your hand.
Cooking on a stovetop → Heat transfers from the burner to the pan.
Ice melting in your hand → Heat flows from your hand to the ice.

Equation for Heat Conduction:

\( Q = \frac{kA(T_1 - T_2)t}{d} \)

where:

  • Q = heat transfer (J),
  • k = thermal conductivity (W/m·K),
  • A = surface area (m²),
  • = temperatures of materials,
  • d = thickness of material,
  • t = time.

Higher k = Better conductor (e.g., metals).
Lower k  = Better insulator (e.g., wood, Styrofoam).

2. Convection: Heat Transfer by Fluid Movement

Definition:

Convection is the transfer of heat by the motion of fluids (liquids or gases). Hot fluid rises, cold fluid sinks, creating a convection current.

How It Works:

  • Hot fluids expand and become less dense, rising.
  • Cooler fluids contract and become denser, sinking.
  • This movement transfers heat efficiently in gases and liquids.

Real-World Examples:

Boiling Water → Hot water rises, cool water sinks, forming a circulation pattern.
Hot Air Balloons → Heated air inside the balloon rises, lifting it.
Ocean Currents & Weather Systems → Warm ocean water moves towards colder regions.

Types of Convection:

  1. Natural Convection → Caused by density differences (e.g., air rising above a fire).
  2. Forced Convection → Requires external forces (e.g., a fan cooling a CPU).

Convection is why warm air rises and cool air falls!

3. Radiation: Heat Transfer Through Electromagnetic Waves

Definition:

Radiation is the transfer of heat through electromagnetic waves (infrared radiation) without needing a medium.

How It Works:

  • Heat radiates from a hot object in all directions.
  • No direct contact is needed (heat can travel through a vacuum).

Real-World Examples:

The Sun heating Earth → Heat travels 93 million miles through space.
Feeling warmth from a campfire → Infrared waves transfer heat.
Microwaves heating food → Uses electromagnetic waves to heat molecules.

Equation for Radiative Heat Transfer:

Q = σAe(T4 - To4)

where:

  • σ = Stefan-Boltzmann constant (5.67•10-8 W/m²·K⁴),
  • A = surface area (m²),
  • e = emissivity (0 to 1),
  • T, To = temperatures of object and surroundings (K).

Objects with higher emissivity radiate more heat (e.g., black surfaces).

4. Summary Table: Heat Transfer Mechanisms

Heat Transfer Type Process Medium Required? Example
Conduction Heat moves through direct contact Yes (solids, especially metals) Spoon in hot soup
Convection Heat moves via fluid motion Yes (liquids, gases) Boiling water, ocean currents
Radiation Heat moves via electromagnetic waves No (can travel in vacuum) Sun heating Earth

Key Takeaways

  • Conduction transfers heat through solids by molecular collisions.
  • Convection moves heat through fluids by rising warm particles.
  • Radiation transfers heat without a medium using electromagnetic waves.
  • All three methods work together in real-life heat transfer!

These mechanisms explain how we cook, stay warm, and why the Earth has weather! 


Activities:

  • Experiment: Measure heat transfer through conduction and convection.
  • Class discussion on radiative heat transfer (e.g., sunlight).

Assessment: Lab report on heat transfer mechanisms.

5. Applications of Heat Transfer

Learning outcomes
  • I can identify situations where conduction, convection, and radiation occur together.
  • I can explain how insulation reduces heat transfer.
  • I can describe how heat transfer is used in buildings, cooking, and engineering.
  • I can explain how evaporation can cool objects and living organisms.
  • I can evaluate methods of improving thermal efficiency in practical applications.

Convection: Heat Transfer in Liquids and Gases

Liquids and gases can also transfer heat, but instead of conduction through particles locked in place, the fluid itself moves — this is called convection.
When a fluid is heated, the particles near the heat source gain energy, spread out, and become less dense. The warmer, less-dense region rises, while cooler, denser fluid sinks to take its place. This cycle sets up a convection current that continuously carries energy through the liquid or gas.

Convection explains why warm air rises above radiators, why sea breezes blow toward the shore on hot days, and why water circulates in a kettle as it boils. Any situation where a fluid is heated unevenly will create convection currents.

Summary

  • Good thermal conductors: metals such as copper, aluminium, silver (used for cooking pots, radiators).

  • Poor conductors / insulators: wood, plastic, rubber, wool, polystyrene.

  • Conduction in solids: energy passes through lattice vibrations; in metals, mobile electrons speed up the process.

  • Convection in liquids/gases: heating lowers density, causing warm regions to rise and cooler regions to sink, setting up currents that move thermal energy through the fluid.