Heat Transfer
ICSE Class 8 · Physics · Chapter 6
What this chapter covers
- Heat and moving particles
- Evaporation and boiling
- Boiling and evaporation compared
- Thermal expansion
- Linear, superficial and volume expansion
- Comparing expansion in solids, liquids and gases
- Applications of expansion
Key points
- Heating makes the particles of a substance move faster; this can raise its temperature, make it expand, or change its state.
- Evaporation is slow, happens only at the surface, takes place at all temperatures and cools the remaining liquid.
- Boiling is rapid, happens throughout the liquid, and takes place only at the boiling point, where the temperature stays constant.
- The boiling point of pure water is 100 °C at sea level; it is lower where air pressure is lower, such as in the hills.
- Thermal expansion is the increase in size on heating; contraction is the decrease in size on cooling.
- Linear expansion is an increase in length, superficial expansion an increase in area, and volume expansion an increase in volume.
- For the same rise in temperature, gases expand most, liquids less and solids least.
- Water is an exception: it contracts when heated from 0 °C to 4 °C.
- Gaps in rails, expansion joints in bridges and loose overhead wires allow for expansion.
- Fitting cartwheel rims, loosening jar lids, bimetallic strips in irons and fire alarms, thermometers and hot-air balloons make use of expansion.
Questions answered
State two differences between evaporation and boiling.
Evaporation happens only at the surface of a liquid, while boiling happens throughout the liquid. Evaporation takes place at all temperatures, while boiling takes place only at the boiling point.
Why does water kept in a clay pot stay cooler than water kept in a steel bottle?
Some water seeps through the pores of the clay pot and evaporates from its outer surface. Evaporation takes heat from the pot and the water inside, so the water cools.
While water is boiling, the flame under it is kept on, yet the temperature does not rise above 100 °C. Where does the heat go?
The heat is used to change the liquid water into vapour. It gives particles throughout the water the energy to escape, instead of raising the temperature.
Name the three types of thermal expansion and give one object in which each type is important.
Linear expansion, important in railway tracks; superficial expansion, important in metal sheets or a lid on a jar; volume expansion, important in the liquid of a thermometer or the air in a tyre.
A steel rod expands by 0.5 mm for every 10 °C rise in temperature. By how much will it expand if its temperature rises by 40 °C?
40 °C is four times 10 °C, so the rod expands by 0.5 mm × 4 = 2 mm.
Arrange solids, liquids and gases in increasing order of expansivity, and explain the order using particles.
Solids, then liquids, then gases. The forces holding particles together are strongest in solids and weakest in gases, so the same heating moves gas particles apart the most.
A hole is drilled in a copper plate. What happens to the size of the hole when the plate is heated?
The hole becomes bigger. The whole plate, including the hole, expands outwards in every direction.
Why are gaps left between the slabs of a concrete road?
In hot weather the slabs expand. The gaps give them room to expand without pushing against each other and cracking.