Some materials pass thermal energy on quickly, others hardly at all
Definition
Thermal conduction
The transfer of thermal energy through a substance without the substance itself moving from place to place.
- A good thermal conductor passes thermal energy along quickly, and every one of them is a metal: copper, aluminium, steel, brass.
- A thermal insulator, also called a bad thermal conductor, passes energy along very slowly; wood, plastic, glass, wool and trapped air all behave this way.
- This is why a metal bench feels colder than a wooden one at the same temperature: the metal conducts energy away from your hand faster, and it is that flow, not the temperature, that your skin reports.
- Kitchen design uses both at once: the base of a pan is metal so energy reaches the food quickly, while the handle is plastic or wood so the energy does not reach your hand.
- Most household insulation works by trapping air, because still air is one of the worst conductors there is, and that is what loft insulation, a jumper and a duvet all have in common.
Practical
Investigation: comparing thermal conduction in different metals
- Aim: to compare how well different metals conduct thermal energy.
- Apparatus: eye protection, rods of different metals of equal length and thickness, wax, a Bunsen burner and tripod, a heat-proof mat and a stopwatch.
- Method:
- Fix an equal blob of wax to the far end of each rod, the same distance from the end that will be heated.
- Arrange the rods so that their other ends are gathered together over the same point of the flame, so every rod is heated equally and at the same moment.
- Light the Bunsen and start the stopwatch together, then time when the wax first melts and drops off each rod.
- Measure and record: tabulate the metal against the time for the wax to melt, with headings such as metal and time / s.
- Result: the wax melts soonest on the best conductor, which is usually copper, and last on the poorest of the metals tested.
- Watch out:
- The rods must be the same length and thickness, or a thicker rod carries more energy and the comparison is worthless.
- The wax blobs must be equal in size and equally far from the flame, since a larger blob needs more energy to melt.
- Judging the exact instant the wax melts is difficult, so repeat the test and average the times.
- Safety: wear eye protection, and treat the flame, the hot rods and the molten wax as burn hazards; leave the rods to cool on the mat rather than picking them up.
Water is a bad conductor, as a test tube of ice can show
- Push a small lump of ice to the bottom of a test tube of water and hold it down with a piece of wire gauze, so it cannot float back up.
- Heat the water near the top of the tube, well away from the ice, holding the tube in a clamp and angling it away from anyone nearby.
- The water at the top can be brought to the boil while the ice at the bottom stays frozen for a surprisingly long time.
- The water in between conducts so badly that almost no energy reaches the ice, which is the point of heating at the top: heating the bottom would set up a convection current and carry energy down by a different route entirely.
- The same idea, run with a rod half wrapped in paper, shows insulation directly: paper wrapped round a metal rod chars slowly, while paper wrapped round a wooden rod scorches quickly, because the metal keeps carrying the energy away.
Between the two extremes lies a whole middle range
- Materials do not split neatly into "metal" and "insulator", because many solids sit in between.
- Concrete, brick, glass, stone and ice all conduct thermal energy better than wool or trapped air but far worse than copper.
- The useful mental picture is a ranking rather than two boxes, running from metals at the top, through dense non-metallic solids, down to gases at the very bottom.
- That middle range is exactly why a solid brick wall loses heat steadily even though brick is not a metal, and why cavity wall insulation is worth fitting.
Example
- A tiled bathroom floor and the bath mat next to it have been in the same room all night, so they are at the same temperature.
- The tile is a much better conductor, so it draws energy out of your foot quickly and feels cold.
- The mat traps air in its fibres, so it barely conducts and your foot stays warm.
- The difference you feel is a difference in the rate of energy transfer, not in temperature.
Common Mistake
- Do not say metal is "colder" than wood at room temperature, because both are at the same temperature and the metal simply conducts energy away from you faster.
- Do not claim an insulator "stops" thermal energy, because it only slows the transfer down.
- Do not forget the control variables in the rod experiment, because rods of different thickness make the result meaningless.
- Do not treat "not a metal" as the same as "a good insulator", because glass, brick and ice conduct noticeably better than wool.
Exam technique
- Describing an experiment is point-marked, so name the variable you change, the variables you keep the same, and what you measure.
- Say what the result would look like as well as the method, because "the wax melts first on copper" is a separate mark from the description.
- Where a question mentions how something feels, answer in terms of the rate of energy transfer to or from your skin.
- For an insulation question, name the trapped air rather than the material, since that is what is really doing the work.
Self review
- Name two good thermal conductors and two thermal insulators.
- Describe an experiment to compare how well different metals conduct thermal energy.
- In the rod experiment, state two variables that must be kept the same.
- Why is the top of the test tube heated in the ice-and-water demonstration?
- Give one solid that conducts better than wool but far worse than copper.
