Heat and Thermal Energy

Touch a struck match and touch a radiator that has been running all afternoon. The match is far hotter, but it burns out in a second and warms nothing beyond your fingertip. The radiator is cooler but keeps warming the room for hours. What decides how much a room actually heats up isn't which object is hotter. It's how much energy is moving, and for how long.

Heat is thermal energy in transit: energy flowing from a higher-temperature object to a lower-temperature one. It is not a substance and not a property you can measure in a single object. You can measure temperature (a property). You can calculate thermal energy (depends on mass, temperature, and specific heat). But "heat" only exists as a flow between objects at different temperatures.

The amount of thermal energy stored in an object depends on three factors: mass, temperature, and specific heat capacity (how much energy it takes to raise 1 kg by 1°C). Water has an unusually high specific heat (4,186 J/kg·°C), meaning it takes a lot of energy to heat and releases a lot when it cools. This is why water is the working fluid in most power plants and heating systems: it stores and transports thermal energy efficiently.

Worked Example

A natural gas water heater raises 150 liters (150 kg) of water from 15°C to 60°C.

• Find the temperature change. ΔT = 60 - 15 = 45°C.
• Apply Q = mcΔT. Q = 150 x 4,186 x 45 = 28,255,500 J = 28.3 MJ.

The gas burner is rated at 10 kW. How long does this take at 100% efficiency?

Time = Energy / Power = 28,255,500 J / 10,000 W = 2,826 seconds = about 47 minutes. Real water heaters are about 80-95% efficient, so actual time would be somewhat longer.

Heat flow is the mechanism behind every thermal power plant, every building heating system, and every refrigeration cycle.

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