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In addition to the revision notes for Absorption of Heat on this page, you can also access the following Thermodynamics learning resources for Absorption of Heat
Tutorial ID | Title | Tutorial | Video Tutorial | Revision Notes | Revision Questions | |
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13.3 | Absorption of Heat |
In these revision notes for Absorption of Heat, we cover the following key points:
"Heat energy always flows from the hottest object to the coldest one."
The absorption of heat energy is not equal for all materials. Some materials can absorb the heat more easily (good absorbers of heat) while some other materials show resistance to heat absorption (bad absorbers of heat).
The factors affecting the amount of heat absorbed by a substance are:
Putting all the above factors together, we obtain the formula for the specific heat capacity, c:
The SI unit of specific heat capacity is [J/(kg × K)].
Rearranging the above formula, we obtain for the heat energy absorbed by an object of mass m and specific heat capacity c when its temperature increases by ΔT:
In general, dense materials such as metals, are better absorbers of heat than less dense materials.
Initially, another unit known as calorie, cal was used as a unit of energy instead of Joule. It represents the amount of heat supplied to 1 g of water to increase its temperature by 1°C. Nowadays, calorie is still used as a unit of energy in food industry. Since calorie is very small, a multiple of it, known as kilocalorie, kcal (or Cal with uppercase C) is used as a unit of food energy. Thus,
The conversion factor between calorie and Joule is
Matter exists in three stable states: solid, liquid and gas.
When material changes its phase, we say there is a change of state in it. There are six possible phase changes during a change in state.
By definition, latent heat of fusion, Qf, is the amount of heat absorbed by a substance at the melting temperature in order to melt it completely.
Similarly, latent heat of vaporization, Qv, is the amount of heat absorbed by a substance at the boiling temperature in order to evaporate it completely.
Not all substances require the same amount of heat energy to change their state. Therefore, similarly as in the case of specific heat capacity, we introduce a new quantity known as the specific latent heat, L, which represents the amount of heat per kilogram mass needed to change the phase of a material. By definition,
Specific latent heat of fusion Lf is the amount of heat supplied to 1 kg of a substance in its melting temperature in order to make it melt completely.
Similarly, specific latent heat of vaporization Lv is the amount of heat supplied to 1 kg of a substance in its boiling temperature in order to make it evaporate completely.
Both specific latent heats are measured in Joules per kilogram, [J/kg]. The equation of latent heat for both cases therefore is
and
We can find the heat released by a hot object when it cools down by using the same procedure as when it is heated up. The only difference is that the heat value will result a negative number because t2 < t1 and therefore, Δt = t2 < t1 < 0.
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