Celsius, kelvin and Fahrenheit, and the three ways heat travels
Heat is a main factor of climate: it affects pressure, wind, humidity, condensation and rain. Sunlight warms the land and water, they warm the air above, so the lowest air is the warmest. Three scales measure temperature: Celsius (used in Egypt), Fahrenheit (used in the United States) and kelvin (used in science). To convert: T(K) = t(°C) + 273 and t(°F) = 1.8 × t(°C) + 32. Water freezes at 0 °C = 32 °F = 273 K and boils at 100 °C = 212 °F = 373 K. Write kelvin without a degree sign. A change of 1 K equals a change of 1 °C. Heat moves from hot to cold; cold does not move. Conduction: in solids or touching bodies, the particles pass energy on without travelling; metals conduct well, cork and wood poorly. A metal spoon feels colder than a wooden one at the same temperature because it takes heat from your hand faster. Convection: in fluids the warmer, less dense part rises and the cooler, denser part sinks, forming currents; birds soar on rising warm air. It cannot happen in a solid. Radiation: heat travels as electromagnetic waves in all directions and needs no medium, so it crosses a vacuum; this is how the Sun warms Earth.
| Event | °C | K | °F |
|---|---|---|---|
| water freezes | 0 | 273 | 32 |
| a mild day | 25 | 298 | 77 |
| water boils | 100 | 373 | 212 |
Three spoons, with a grown-up. Stand a metal, a plastic and a wooden spoon in a cup of warm tap water (about 40 °C at most, no hot water). After a minute touch the top ends and rank them from warmest to coolest. Then put a drop of food colouring in a glass of warm tap water and one in a glass of cold, and describe how the colour moves (convection). Finally hold your hand near a sunny window and then in the shade: which kind of heat transfer is this?