Overhead telephone lines are hung loosely between utility poles so that they sag. Why is this slack left in the lines?
to prevent the lines from snapping when they contract in cold weather
to reduce the electrical resistance of the lines
to allow wind to pass through them more easily
to prevent electrical short-circuits
528 exam-style questions on CIE IGCSE Physics 2.2 Thermal properties and temperature, covering 2.2.1 Thermal expansion of solids, liquids and gases, 2.2.2a Internal energy and temperature, 2.2.2b Specific heat capacity, 2.2.3a Melting, boiling, condensation and solidification, 2.2.3b Evaporation, and 2.2.3c Cooling by evaporation. Each one has a worked solution and a mark scheme showing where the marks go.