Ch 6 · 2.3.1 Conduction · O Level Physics 5054
Topic 6 of 9 in Thermal Physics — O Level Physics 5054 — 49 lessons.
Introduction
Stir a hot cup of tea with a metal spoon and the handle soon feels hot. A wooden spoon stays cool. Thermal energy travels through the metal by conduction, and metal is far better at it than wood. (Syllabus section 2.3.1.)
Key ideas — What conduction is
Conduction is the transfer of thermal energy through a material without the material itself moving. Energy passes from the hotter part to the colder part.
🧑🏫 Teacher's explanation
Imagine a long line of people standing shoulder to shoulder. If the person at one end starts shaking vigorously, they will bump into the person next to them, who bumps into the next, and so on. Even though nobody actually moves down the line, the 'vibration' travels from one end to the other. This is exactly how heat moves through a solid.
📖 Definitions
🔍 Worked example — Easy
A metal rod is heated at one end. Explain, in terms of particles, why the temperature at the other end eventually increases.
🔍 Worked example — Standard
A student performs an experiment to compare the thermal conductivity of a copper rod and a glass rod. They mark the rods at 5 cm intervals and heat one end. Which rod will show a temperature rise at the 10 cm mark first?
🔍 Worked example — Exam-style
A wooden block is placed between two metal plates. One plate is heated to 100 ^circC and the other is kept at 20 ^circC. If the wood is a poor conductor, describe the temperature distribution within the block.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Conduction in all solids: lattice vibrations
In a solid the particles are held in a lattice and vibrate about fixed positions.
🧑🏫 Teacher's explanation
Imagine a crowded room where everyone is standing in a strict grid, holding hands. If one person starts shaking their arms vigorously, they will bump into the person next to them, who then bumps the next person, and so on. Even though no one actually moves from their spot, the 'energy' of that shaking travels across the room. This is exactly how heat moves through a non-metallic solid.
📖 Definitions
🔍 Worked example — Easy
A metal rod is heated at one end. Explain, in terms of particles, why the temperature at the other end eventually increases.
🔍 Worked example — Standard
A student performs an experiment to compare the thermal conductivity of copper and glass. They use a metal rod with a layer of wax at one end. If the copper rod conducts heat faster, what will be observed regarding the wax?
🔍 Worked example — Exam-style
In a metallic crystal, conduction occurs via two mechanisms. Name them and explain why metals are generally better conductors than non-metals.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) State the observation that would prove copper is a better thermal conductor than glass. [1] (b) Using the idea of particles, explain why the copper rod conducts heat more effectively than the glass rod. [3] [4 marks]
Key ideas — Conduction in metals: free electrons
Metals also have free (delocalised) electrons that can move through the whole metal.
🧑🏫 Teacher's explanation
Imagine a crowded hallway in a school. If everyone is standing in a fixed grid (like atoms in a non-metal), and you start pushing people, the 'shove' travels slowly as people bump into their immediate neighbors. This is lattice vibration. Now, imagine if there were also a few very fast runners (free electrons) weaving through the crowd. These runners can sprint from one end of the hallway to the other almost instantly, bumping into people along the way and spreading the 'energy' of the movement much faster than the slow, bumping crowd could.
📖 Definitions
🔍 Worked example — Easy
A metal rod is heated at one end. Explain why the temperature at the far end rises faster in a copper rod than in a glass rod.
🔍 Worked example — Standard
A student performs an experiment to compare the thermal conductivity of two rods: one made of iron and one made of plastic. The rods are of equal length and diameter. If the iron rod reaches a steady temperature T_1 at its midpoint after t seconds, and the plastic rod reaches T_2 at its midpoint after t seconds, where T_1 > T_2, what does this indicate about their conduction properties?
🔍 Worked example — Exam-style
In a metal, the kinetic energy of free electrons at the hot end is E_k. If the speed of these electrons is v, and the mass of an electron is m_e, write the expression for the kinetic energy of a single electron.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Describe, in terms of the movement of particles, how thermal energy is transferred from the hot end to the cold end. [2] (b) Explain why a metal rod is a better conductor of heat than a glass rod. [2] [4 marks]
Key ideas — Why liquids and gases are poor conductors
Their particles are further apart, so vibrations are not passed on easily. In gases the particles are so far apart that conduction is very poor.
🧑🏫 Teacher's explanation
Think of a solid as a tightly packed crowd of people standing shoulder-to-shoulder. If the person at one end starts shaking, they immediately bump into the person next to them, passing that energy through the crowd very quickly. This is how heat moves through a solid lattice.
📖 Definitions
🔍 Worked example — Easy
A metal rod has a length of 0.5 m. If heat travels at a constant rate, and the temperature at the hot end is 100 ^circC and the temperature at the cold end is 20 ^circC, calculate the temperature gradient Δ T/Δ x across the rod.
🔍 Worked example — Standard
A student performs an experiment to compare conduction in a copper rod and a glass rod. The copper rod has a thermal conductivity kappa_Cu = 400 W/(m· K) and the glass rod has kappa_Gl = 1.0 W/(m· K). Calculate the ratio of their conductivities frackappa_Cukappa_Gl.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Practical notes
Fair test: same length, thickness and starting temperature; same heat source; wax at the same distance.
Worked example
The metal base is a good conductor: its free electrons and lattice vibrations transfer thermal energy quickly from the flame to the food. The plastic handle is a poor conductor (insulator), so very little energy reaches your hand and it stays cool enough to hold.
🔍 Worked example — Easy
A metal rod is heated at one end. Explain, in terms of particles, why the temperature at the other end eventually increases.
🔍 Worked example — Standard
A student performs an experiment to compare the thermal conductivity of a copper rod and a glass rod. They wrap both in a layer of wet paper and heat one end of each. Which rod will cause the paper to dry faster, and why?
🔍 Worked example — Exam-style
A heat source provides 50 W of power to one end of a metal rod. If the rod is a good conductor, describe the energy transfer process and why the temperature gradient (difference in temperature between ends) might be low.
⚠️ Common errors
Common mistakes
Practice
Answers: (1) The transfer of thermal energy through a material without the material moving, from hotter to colder parts. (2) Particles at the hot end vibrate more strongly and pass the vibrations on to neighbouring particles through the lattice. (3) Copper has free (delocalised) electrons that move quickly through the metal, carrying energy and colliding with particles in colder parts; glass has no free electrons and conducts only by the slower lattice vibrations. (4) Fix wax at the same distance on copper and steel rods of the same length and thickness; heat one end of each equally; time how long the wax takes to melt: the faster it melts, the better the conductor.