Heat and Work

Temperature tells you how fast particles in a substance are jiggling on average. Heat is energy in transit, flowing from a hotter object to a cooler one until they match. These are not the same quantity: a bathtub of warm water holds far more total energy than a lit match, even though the match is hotter, because the tub has so much more mass absorbing and storing that motion. That is why the heat equation uses mass: Q = m times specific heat times the temperature change. Specific heat is just how much energy a gram of a material needs to change its temperature by one degree, and it differs by substance, which is why metal and wood sitting in the same room feel different even at the same reading.

Heat moves by three distinct routes. Conduction passes energy through direct contact, particle to particle, which is fast in metals and slow in wood or air, so metal yanks heat out of your warm hand quickly and feels cold. Convection carries heat through the bulk movement of a fluid, like warm air rising off a radiator or water circulating in a pot. Radiation needs no contact or medium at all; it travels as electromagnetic waves, which is how the sun heats the earth across empty space.

During a phase change, added heat stops raising temperature and instead goes into breaking or forming the bonds between particles. That is why melting ice holds steady at 0 degrees Celsius while it absorbs energy, and why a heating curve plotted over time shows flat plateaus exactly at the melting and boiling points. This links heat and work to energy more broadly: work is energy transferred by a force acting through distance, while heat is energy transferred by a temperature difference, and both change a system's internal energy.