Heat Transmission

In certain situations, even if there is no physical contact between the bodies, it is possible to feel that something is warmer. Like when you get close to the fire in a fireplace. Thus, we conclude that somehow heat emanates from these "warmer" bodies and can spread in various ways.

As we have seen before, the heat flow happens from the highest to the lowest temperature.

This thermal energy transit can happen in the following ways:

  • driving;
  • convection;
  • irradiation.

Heat flow

In order for a body to be heated, a constant power thermal source is used, that is, a source capable of providing a quantity of heat per unit of time.

We define heat flow (Φ) that the source constantly provides as the quotient between the amount of heat (Q) and the exposure time interval (Δt):

Being the unit adopted for heat flow in the international system, the Watt (W), which corresponds to Joule per second, although the unit is also widely used calorie / second (cal / s) and their multiples: calorie / minute (cal / min) and kilocalorie / second (kcal / s).


A constant power source equal to 100W is used to increase the temperature 100g of mercury 30 ° C. Since the specific heat of mercury is 0.033cal / g. ° C and 1cal = 4.186J, how long does the source take to heat up?

Applying the heat flow equation: