Thermal Properties of MatterTemperature and Heat
On a May afternoon Sina pours herself a cup of hot chai and, for Riya, a glass of nimbu pani with ice. Then the doorbell rings, and both drinks sit forgotten on the table for half an hour. The room is at 34 °C.
Temperature is a measure of how hot or cold something is. A hot utensil has a high temperature and an ice cube a low one; hot and cold are relative words, like tall and short. Touch can compare a kettle with an ice tray, but it is easily fooled and works only over a small range, so physics needs something better. Predict what happens to each drink, then start the clock.
The orange curve falls, the blue curve rises, and both creep towards the dashed room line. Each drink exchanges energy with the room around it until the two are at the same temperature; then the exchange stops. Energy flowed out of the chai into the room, and from the room into the cold glass.
That is what physics means by heat: energy transferred between a system and its surroundings, or between two systems, because of a difference in temperature, always from the hotter one to the colder one. Heat is energy, so its SI unit is the joule (J). The SI unit of temperature is the kelvin (K); the degree Celsius (°C) is the everyday unit.
Heat or Temperature?
The two words are easy to mix up. Temperature is something a body has at a given moment; heat is energy on the move from one body to another. When a body is heated, its temperature may rise, it may expand, or it may melt or boil. Sort each statement.
A reading in °C, °F or K describes the state of a body. A number of joules passing from one thing to another describes heat.
Note
Heat always involves a temperature difference. Energy that passes from one body to another without any temperature difference, such as the energy you give a cart by pushing it, is not heat: it is work.