Welcome to Heat Transfer!
Have you ever wondered why a metal spoon gets burning hot when left in a cup of tea, why hot air balloons float into the sky, or how the Sun warms our planet from millions of miles away across empty space? The answer to all of these questions is heat transfer.
Thermal energy (heat) is always on the move, and it always travels from a hotter place to a cooler place until everything reaches the same temperature. In this unit, we will explore the three methods of heat transfer—conduction, convection, and radiation—and learn how we use this knowledge to insulate our homes and save money on energy bills.
Don't worry if physics sometimes feels tricky! We will break down each idea step-by-step with clear examples, simple memory tricks, and examiner tips to help you get full marks.
1. Conduction: Passing the Energy Along
Conduction is the main way heat travels through solids (especially metals).
How Conduction Works Step-by-Step:
Step 1: When you heat one end of a solid, the particles at that end gain thermal energy and vibrate more vigorously.
Step 2: These vibrating particles bump into their neighbouring particles.
Step 3: The collisions transfer kinetic energy along the solid, passing heat from the hot end to the cold end.
Why are Metals the Best Conductors?
Non-metal solids conduct heat quite slowly because energy can only be passed from particle to particle through vibrations. However, metals are exceptional thermal conductors because they contain free (delocalised) electrons.
When heated, these free electrons gain kinetic energy and can move quickly throughout the metal lattice, colliding with distant particles and transferring thermal energy much faster.
What is a Thermal Insulator?
Materials that are poor conductors of heat are called thermal insulators. Non-metals (such as wood, plastic, glass) and fluids (liquids and gases) are poor conductors.
Trapped air is one of the best everyday insulators! Because air is a gas, its particles are spread very far apart, making conduction extremely slow. Materials like wool, fleece, and foam work by trapping pockets of air so heat cannot easily conduct through them.
Everyday Analogy: Imagine passing a bucket of water down a long queue of people without moving your feet (particle vibrations). Now imagine someone on a bicycle grabbing the bucket and racing it straight to the front (free delocalised electrons in metals)!
Key Takeaway for Conduction: Occurs mainly in solids. Vibrating particles pass energy to neighbours. Metals conduct fastest because of free electrons. Trapped air is a great insulator because gas particles are far apart.
2. Convection: Heat on the Move in Fluids
Convection is the primary method of heat transfer in fluids (which means liquids and gases). Unlike solids, particles in fluids are free to move around.
How a Convection Current Works:
To get full marks in an exam question, you must describe the steps of a convection current in the correct scientific order:
1. Heating: When a region of fluid is heated, its particles gain kinetic energy and move faster and further apart.
2. Expansion: Because the particles are spread further apart, the heated fluid expands.
3. Density Change: As it expands, the warm fluid becomes less dense than the surrounding cooler fluid.
4. Rising: The warm, less dense fluid rises upwards.
5. Replacement: Cooler, denser fluid moves in to take its place at the bottom, where it is heated in turn.
6. Convection Current: This continuous rising and sinking creates a circular flow known as a convection current.
Examiner Warning — The "Heat Rises" Trap:
Never write "heat rises" in your exam! Heat is a form of energy, not a substance. Examiners will penalise this shortcut. Always write: "The warm fluid expands, becomes less dense, and the less dense fluid rises."
Key Takeaway for Convection: Occurs only in liquids and gases. Fluid particles spread out when heated \(\rightarrow\) fluid expands \(\rightarrow\) becomes less dense \(\rightarrow\) rises, setting up a continuous convection current.
3. Radiation: Travelling as Waves
Thermal radiation (also known as infrared radiation) is very different from conduction and convection because it does not need particles to travel! It moves as electromagnetic infrared waves.
Because it requires no medium, thermal radiation is the only method of heat transfer that can travel through a vacuum (empty space). This is how heat from the Sun reaches Earth across space.
Surface Colour and Texture Matter:
All objects emit and absorb infrared radiation, but the surface colour and finish determine how quickly this happens:
• Matt Black / Dark Surfaces: These are the best absorbers and the best emitters of thermal radiation. They take in heat quickly and radiate it away quickly.
• Shiny White / Silver Surfaces: These are the poorest absorbers and poorest emitters. Instead, they are excellent reflectors of thermal radiation.
Did you know? In hot, sunny countries, houses are often painted white because white surfaces reflect the Sun's infrared radiation, keeping the interior cool!
Key Takeaway for Radiation: Travels as infrared waves through air and empty space (vacuums). Matt black surfaces absorb and emit heat best; shiny silver surfaces reflect heat and emit heat poorly.
4. Domestic Insulation: Keeping Our Homes Warm
Heating our homes costs money and uses energy resources. By understanding conduction, convection, and radiation, we can install domestic insulation to reduce heat loss through roofs, walls, windows, and doors.
Common Insulation Methods in the Home:
1. Loft Insulation:
Thick layers of fibreglass or mineral wool are laid across the attic floor. These materials trap tiny pockets of air. Trapped air prevents convection currents from forming and acts as a poor conductor, greatly reducing heat loss by conduction and convection through the roof.
2. Cavity Wall Insulation:
Most modern houses have an inner and outer brick wall with a gap (cavity) between them. Pumping insulating foam or mineral wool into this gap traps the air. This stops convection currents circulating within the cavity and reduces heat loss by conduction.
3. Double Glazing:
Windows made of two glass panes separated by a narrow gap of trapped dry air or a vacuum. The trapped air is a poor conductor, reducing conduction. If the gap contains a vacuum, it eliminates both conduction and convection entirely (as there are no particles).
4. Draught Excluders / Draught Proofing:
Strips fitted around doors and window frames physically block cold air from blowing in and warm air from escaping, preventing heat loss by convection.
5. Reflective Foil Behind Radiators:
Placing shiny silver foil on the wall behind a radiator reflects infrared radiation back into the living room rather than allowing it to be absorbed by cold exterior walls.
Key Takeaway for Home Insulation: Trapping air stops convection and reduces conduction. Shiny silver surfaces reflect radiation.
5. Formulae & Quantitative Calculations
In your exam, you may need to calculate the financial and energy performance of insulation and heating systems.
A. Payback Time
Payback time tells a homeowner how many years it will take for energy bill savings to cover the initial cost of installing insulation.
\(\text{Payback Time (years)} = \frac{\text{Installation Cost (\pounds)}}{\text{Annual Saving on Bills (\pounds/year)}}\)
Worked Example:
A family installs cavity wall insulation costing \(\pounds 600\). The insulation saves them \(\pounds 150\) each year on their heating bills. Calculate the payback time.
\(\text{Payback Time} = \frac{600}{150} = 4\text{ years}\)
B. Efficiency
Efficiency measures what percentage of the total energy supplied to a device is converted into useful output energy.
\(\text{Efficiency (\%)} = \left( \frac{\text{Useful Energy Output}}{\text{Total Energy Input}} \right) \times 100\)
Worked Example:
An electric heater takes in \(2000\text{ J}\) of electrical energy and supplies \(1800\text{ J}\) of useful thermal energy. What is its efficiency?
\(\text{Efficiency} = \left( \frac{1800}{2000} \right) \times 100 = 0.9 \times 100 = 90\%\)
C. Specific Heat Capacity Formula
When calculating the energy required to heat a substance, use the thermal energy equation:
\(\Delta E = m \times c \times \Delta\theta\)
Where:
• \(\Delta E\) = change in thermal energy in joules (\(\text{J}\))
• \(m\) = mass in kilograms (\(\text{kg}\))
• \(c\) = specific heat capacity in joules per kilogram degree Celsius (\(\text{J}/\text{kg}^\circ\text{C}\))
• \(\Delta\theta\) = temperature change in degrees Celsius (\(^\circ\text{C}\))
Unit Checklist: Make sure mass is in \(\text{kg}\) (if given in \(\text{g}\), divide by \(1000\)) and energy is in \(\text{J}\) (if given in \(\text{kJ}\), multiply by \(1000\)).
6. Summary: Avoiding Common Exam Pitfalls
• Mistake 1: Writing "heat rises".
Correct answer: The warm fluid expands, becomes less dense, and rises.
• Mistake 2: Thinking particles travel along a solid bar during conduction.
Correct answer: Particles remain in fixed positions and only vibrate, colliding with neighbouring particles.
• Mistake 3: Confusing silver surfaces.
Correct answer: Shiny silver surfaces do not "attract cold". They are poor emitters and poor absorbers because they are excellent reflectors of thermal radiation.
• Mistake 4: Forgetting why trapped air is an insulator.
Correct answer: Air is a poor conductor because its particles are far apart; trapping air in small spaces prevents it from circulating in convection currents.