Ch 2 · Thermal Properties and Transfer of Thermal Energy · IGCSE Physics 0625
Topic 2 of 2 in Thermal Physics — IGCSE Physics 0625 — 49 lessons.
Introduction
Why do railway tracks have gaps? Why does a swimming pool take so long to warm up? Why does sweating cool you down, and why does a vacuum flask keep drinks hot? This lesson applies the particle model from the previous lesson to thermal expansion, specific heat capacity, changes of state, and the three ways thermal energy is transferred: conduction, convection and radiation.
Key ideas — Thermal expansion
1. What expands. When heated at constant pressure, solids, liquids and gases all expand. The particles move or vibrate more, so they take up more space; the particles themselves do not get bigger.
🧑🏫 Teacher's explanation
Imagine a crowded dance floor. When the music is slow, everyone stays close together, just swaying slightly in place. This is like a solid: the particles are locked in a lattice, and heating them just makes them vibrate a little more vigorously, pushing their neighbors slightly further away.
📖 Definitions
🔍 Worked example — Easy
A metal rod is 20.0 cm long at 20 ^circC. When heated to 100 ^circC, it expands to 20.1 cm. Calculate the change in length Δ L.
🔍 Worked example — Standard
A gas is heated at constant pressure. Its volume increases from 2.0 m^3 to 2.5 m^3. Calculate the fractional increase in volume.
🔍 Worked example — Exam-style
A bimetallic strip consists of brass and iron. Brass expands more than iron for the same temperature rise. If the strip is heated, describe the direction of the bend and explain why using particle theory.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Explain, in terms of particles, why the liquid level rises when the thermometer is placed in a hot liquid. [2] (b) State one everyday application of thermal expansion. [1] [3 marks]
Key ideas — Internal energy and specific heat capacity
2. Temperature rise. When an object's temperature rises, its internal energy increases.
🧑🏫 Teacher's explanation
Think of an object as a massive crowd of tiny, vibrating dancers. The 'internal energy' is the total amount of energy held by all those dancers combined. When you heat the object, you aren't just adding energy to the group; you are making each individual dancer move faster and shake more violently.
📖 Definitions
🔍 Worked example — Easy
Calculate the energy required to raise the temperature of 0.5 kg of water from 20 ^circC to 50 ^circC. Use c = 4200 J/(kg ^circC).
🔍 Worked example — Standard
An electric heater transfers 1.2 × 10^5 J of energy to a 2.0 kg block of metal. The temperature of the block rises by 15 ^circC. Calculate the specific heat capacity of the metal.
🔍 Worked example — Exam-style
A 0.25 kg piece of iron is heated from 30 ^circC to 110 ^circC. If the iron absorbs 14000 J of energy, calculate its specific heat capacity. Give your answer to 2 significant figures.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Calculate the specific heat capacity of the copper. [2] (b) State what happens to the average kinetic energy of the copper particles during this process. [1] [3 marks]
Key ideas — Melting, boiling and evaporation
3. Changes of state. During melting and boiling, energy is supplied but the temperature stays constant: the energy is used to overcome the forces between particles, not to speed them up. Condensation (gas → liquid) and solidification (liquid → solid) release energy as particles move closer and the attractive forces pull them together. At standard atmospheric pressure, pure water melts at 0\ ^circC and boils at 100\ ^circC.
🧑🏫 Teacher's explanation
Imagine a crowded dance floor. When people are dancing normally, they have a certain amount of energy (kinetic energy). If you want to change the 'state' of the dance floor—say, from a slow dance to a mosh pit—you have to add energy to break the social bonds between people. During this transition, even if you keep pumping music (adding energy), the 'vibe' (temperature) doesn't change because all that energy is being used to break the bonds, not to make individuals move faster.
📖 Definitions
🔍 Worked example — Easy
Calculate the energy required to raise the temperature of 0.5 kg of water from 20 ^circC to 50 ^circC. The specific heat capacity of water is 4200 J/(kg·^circC).
🔍 Worked example — Standard
A 2.0 kg block of metal absorbs 12,000 J of thermal energy, and its temperature rises by 15 ^circC. Calculate the specific heat capacity of the metal.
🔍 Worked example — Exam-style
A student wants to increase the temperature of 0.2 kg of a liquid by 10 ^circC. If the liquid has a specific heat capacity of 2500 J/(kg·^circC), how much energy is needed? (Give answer to 2 sig figs).
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Calculate the specific heat capacity of the liquid. [2] (b) Explain, in terms of particles, why the temperature of the liquid decreases during evaporation. [2] [4 marks]
Key ideas — Transfer of thermal energy
5. Conduction. Thermal energy passes through a material from hot to cold without the material moving. Metals are good conductors; plastics, wood, glass and air are poor conductors (insulators).
🧑🏫 Teacher's explanation
Think of thermal energy transfer like a crowd of people in a room. Conduction is like people standing in a line and passing a heavy ball from person to person; in metals, it's even faster because some people (electrons) can sprint through the crowd to deliver the ball quickly. Convection is like the crowd itself moving; if one group gets excited and starts jumping, they take up more space and move around, forcing others to move out of the way. Radiation is like someone throwing a ball through the air; it doesn't need anyone to be standing in a line or moving in a group; it just flies straight to the target.
📖 Definitions
🔍 Worked example — Easy
Calculate the energy required to raise the temperature of 2.0 kg of water from 20circC to 80circC. The specific heat capacity of water is 4200 J/(kgcdotK).
🔍 Worked example — Standard
A 0.50 kg block of metal absorbs 1500 J of energy. Its temperature rises by 15circC. Calculate the specific heat capacity of the metal.
🔍 Worked example — Exam-style
An object is placed in a room where it receives 50 W of radiation. It emits 30 W of radiation. What happens to its temperature? Explain using the concept of energy balance.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Practical notes
Worked example
Example 1 — heating water (Supplement). A kettle contains 0.80 kg of water at 20\ ^circC. The specific heat capacity of water is 4200\ J/(kg\ ^circC). Calculate the energy needed to bring it to 100\ ^circC and the time taken by a 2.4 kW heating element.
🔍 Worked example — Standard
A 1.0 kg aluminium block is heated by a 50 W heater for 300 s. Its temperature rises from 20.0 ^circC to 36.5 ^circC. Calculate the specific heat capacity of the aluminium.
🔍 Worked example — Exam-style
A 0.50 kg block of copper (c = 390 J/(kg ^circC)) is heated from 30 ^circC to 80 ^circC. How much energy is required? If the heater has an efficiency of 80 \%, what is the total energy supplied by the heater?
⚠️ Common errors
Common mistakes and examiner tips
Practice
Answers: (1) In hot weather the bridge sections expand; the gaps give room to expand without buckling or cracking. (2) Copper is a good conductor, so energy passes quickly to the food; plastic is a poor conductor (insulator), so the handle stays cool to hold. (3) The most energetic particles escape from the surface, so the average kinetic energy of the remaining particles decreases and the temperature falls. (4) Air next to the heater warms, expands and becomes less dense, so it rises; cooler, denser air sinks to replace it, forming a convection current that circulates warm air around the room. (5) Shiny silver surfaces are poor emitters and poor absorbers of infrared (good reflectors), so they reduce energy transfer by radiation. (6) Δ E = 2.5 × 2000 × 160 = 8.0 × 10^5\ J. (7) c = 5000/0.50 × 25 = 400\ J/(kg\ ^circC). (8) Evaporation happens at any temperature and only at the surface; boiling happens only at the boiling point and throughout the liquid (bubbles). Hang the washing somewhere warmer, spread it out (larger surface area), or in a breeze (more air movement). (9) Metals contain free (delocalised) electrons that move quickly through the metal and transfer energy in collisions, in addition to lattice vibrations; non-metals transfer energy only by lattice vibrations. (10) Energy passes through the thin metal walls by conduction (metal is a good conductor); the fins give a large surface area; air moving over the fins carries the energy away by convection.