Practical: Energy exchange
When bodies of different temperatures touch, the warmer body gives heat and the cooler one receives it until their temperatures become equal – thermal equilibrium. For example, ice dropped into hot water takes heat from the water and melts while the water cools. If equal masses of hot and cold water are mixed and no heat is lost, the result is the average temperature: (t1 + t2) ÷ 2. Energy takes many forms: mechanical, heat, light, electrical, chemical. It changes from one form to another: in a lamp electrical energy becomes light and heat, in a plant light energy becomes chemical energy, in a footballer’s leg chemical energy becomes motion, in a kettle electrical energy becomes heat. The law of conservation of energy: energy is neither created from nothing nor destroyed; it only changes from one form to another or passes from one body to another. How “hot or cold” something feels to the hand is not a measure of temperature: a metal cup feels colder than plastic at the same temperature because it takes heat from the hand faster; for accurate measurement a thermometer is needed. Safety: do not use boiling water; an adult handles the electric kettle.
Experiment: place identical metal, glass and plastic cups on a table and judge how warm each feels to your palms, then pour the same amount of warm (40–50 °C) water into each and judge again; also measure the water temperature with a thermometer. In part two drop one piece of ice into 200 ml of warm water and, when the ice has melted, record the water’s temperature change. Safety: use warm, not boiling, water; an adult pours it; do not put ice in the mouth; wipe up spills.