Physics

Calorimetry


Heat

When we put two bodies with different temperatures in contact, we can observe that the temperature of the "warmer" body decreases, and that of the "colder" body increases, until the moment both bodies have equal temperature. This reaction is caused by the thermal energy passing from the "warmer" body to the "colder" body, the energy transfer is what we call heat.

Heat It is the transfer of thermal energy between bodies with different temperatures.

The most commonly used unit for heat is calorie (cal), although its SI unit is the joule (J) A calorie equals the amount of heat required to increase the temperature of one gram of pure water under normal pressure from 14.5 ° C to 15.5 ° C.

The relationship between calorie and joule is given by:

1 cal = 4.186J

From this, conversions between units can be made using a simple three rule.

Since 1 calorie is a small unit, we use its multiple, kilocalorie.

1 kcal = 10³cal

Sensitive heat

It is called sensitive heat, the amount of heat that only has the effect of changing a body's temperature.

This phenomenon is governed by the physical law known as Fundamental Equation of Calorimetry, which says that the amount of sensible heat (Q) is equal to the product of its mass, temperature variation and a proportionality constant depending on the nature of each body called specific heat.

Like this:

Where:

Q = amount of sensitive heat (lime or J).

c = specific heat of the constituent body (cal / g ° C or J / kg ° C).

m = body mass (g or kg).

Δθ = temperature change (° C).

It is interesting to know some specific heat values:

Substance c (cal / g ° C)
Aluminum 0,219
Water 1,000
Alcohol 0,590
Copper 0,093
Lead 0,031
Tin 0,055
Iron 0,119
Ice 0,550
Mercury 0,033
Gold 0,031
Silver 0,056
Water vapor 0,480
Zinc 0,093

When:

Q> 0: the body gets hot.

Q <0: the body loses heat.

Example:

How much sensitive heat does it take to heat a 2kg iron bar from 20 ° C to 200 ° C? Data: specific iron heat = 0,119cal / g ° C.

2 kg = 2000 g