🌡️ 2.3 Transfer of Thermal Energy

🔑 Key Idea

Thermal energy (heat) is transferred from hotter regions to colder regions by three main processes:

• Conduction
• Convection
• Radiation

🔥 2.3.1 Conduction

✅ Definition

Conduction is the transfer of thermal energy through a substance without bulk movement of the substance.

🧪 Experiments: Good vs Bad Conductors

🔬 Example Experiment 1: Metal Rod Test

• Attach drawing pins to a metal rod using wax.
• Heat one end of the rod.
• Pins fall off in order.

Observation:

• Heat travels along the rod → pins drop sequentially.

Conclusion:

• Metals are good thermal conductors.

🔬 Example Experiment 2: Comparing Materials

• Use rods of different materials (copper, glass, wood).
• Heat one end.

Observation:

• Copper heats fastest → best conductor
• Glass/wood heat slowly → insulators

⚛️ How Conduction Works (Particle Model)

🧱 In ALL solids:

• Particles vibrate more when heated.
• Vibrations pass energy to neighboring particles.

⚡ In Metals (Important!)

• Metals have free (delocalised) electrons.
• These electrons:
  • Move quickly through the metal
  • Carry thermal energy faster than vibrations

👉 This is why metals conduct heat better than non-metals.

🌬️ Why Conduction is Poor in Gases & Liquids

• Particles are far apart
• Fewer collisions → slower energy transfer

👉 Therefore:

• Gases = very poor conductors
• Most liquids = poor conductors

📝 Key Fact

• Some solids (e.g. glass) conduct heat better than gases but worse than metals.

📌 Exam Tips (Conduction)

• Always mention:
  • Particle vibrations
  • Free electrons (for metals)
• Don’t say particles “move along” → they vibrate in place

🌊 2.3.2 Convection

✅ Definition

Convection is the transfer of thermal energy by the movement of fluids (liquids or gases).

🌍 Where It Happens

• Only in liquids and gases
• Not in solids

⚙️ How Convection Works

Step-by-step:

1. Fluid is heated → particles gain energy
2. Fluid expands → density decreases
3. Warm fluid rises
4. Cooler fluid sinks
5. Cycle repeats → convection current

🧪 Experiment: Convection in Water

🔬 Setup:

• Add potassium permanganate crystals to water
• Heat gently from below

Observation:

• Colored currents show movement of water

👉 This visualizes convection currents.

🌬️ Example: Air Convection

• Hot air rises (less dense)
• Cool air sinks (more dense)

📝 Key Points

• Convection depends on density differences
• Causes circular flow (currents)

📌 Exam Tips (Convection)

• Must include:
  • Heating → expansion → lower density → rising
  • Cooling → higher density → sinking
• Use the term “convection current”

☀️ 2.3.3 Radiation

✅ Definition

Radiation is the transfer of thermal energy by infrared waves.

🌈 Key Facts

• All objects emit infrared radiation
• Does NOT require a medium → can travel through vacuum

👉 Example: Heat from the Sun reaches Earth through space

🎨 Effect of Surface Type

Surface	Absorption	Emission	Reflection
Black, dull	Good	Good	Poor
White, shiny	Poor	Poor	Good

🧪 Experiments

🔬 Emitters:

• Compare black vs shiny surfaces
• Use thermometer

Result:

• Black emits more radiation → cools faster

🔬 Absorbers:

• Shine infrared on surfaces

Result:

• Black absorbs more → heats faster

🌡️ Energy Balance

Constant Temperature:

• Energy in = Energy out

If:

• Energy in > Energy out → temperature rises
• Energy in < Energy out → temperature falls

🌍 Earth’s Temperature (Very Important)

• Sun emits radiation → Earth absorbs it
• Earth emits infrared radiation back

Affected by:

• Greenhouse gases (trap heat)
• Surface properties
• Clouds

📊 Factors Affecting Rate of Radiation

1. Temperature
   • Higher temperature → more radiation
2. Surface area
   • Larger area → more emission

📌 Exam Tips (Radiation)

• Always say infrared radiation
• Mention:
  • No medium required
  • Travels in vacuum
• Know surface effects clearly (black vs shiny)

⚙️ 2.3.4 Consequences of Thermal Energy Transfer

🍳 (a) Heating Objects (e.g. Pans)

Processes involved:

• Conduction: Heat spreads through metal pan
• Convection: Hot liquid inside moves
• Radiation: Heat emitted from surface

👉 Metal pans are used because they are good conductors

🏠 (b) Heating a Room

By Convection:

1. Heater warms air
2. Warm air rises
3. Cool air sinks
4. Circulation heats room

👉 Forms convection currents in the room

🔥 Complex Example 1: Fire (Wood/Coal)

All 3 processes:

• Radiation: Heat felt from fire
• Conduction: Heat through solid fuel
• Convection: Hot air rises

🚗 Complex Example 2: Car Radiator

How it works:

• Hot coolant flows into radiator
• Heat lost by:
  • Conduction (through metal)
  • Convection (air flow)
  • Radiation

👉 Fans increase convection → faster cooling

🧠 Summary Table

Method	Medium Needed?	Where Occurs	Mechanism
Conduction	Yes	Solids	Particle vibration + free electrons
Convection	Yes	Liquids & gases	Density changes
Radiation	No	Anywhere	Infrared waves

🎯 Final Exam Tips

• Always identify the type of heat transfer
• Use correct scientific terms:
  • “Infrared radiation”
  • “Convection current”
  • “Density decreases”
• For 3–4 mark questions:
  • Explain step-by-step (especially convection)
• Learn real-life examples (very common in exams!)