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Absorption of Heat Revision Notes

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13.3Absorption of Heat


In these revision notes for Absorption of Heat, we cover the following key points:

  • What are good and bad absorbers of heat?
  • How does the heat travel between objects?
  • What are the factors affecting the amount of heat absorbed by a substance?
  • What is the specific heat capacity?
  • What is the conversion factor between units of heat energy?
  • What are the main features of the stable states of matter?
  • What are the phases of state change?
  • What is latent heat and why it is called so?
  • What is the specific latent heat of fusion and vaporization?
  • How to calculate the amount of heat absorbed or released by a substance?

Absorption of Heat Revision Notes

"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:

  1. The amount of substance. Mathematically, this factor is represented through the mass of substance, m.
  2. The increase in temperature of the substance. This factor is represented in formulae by the symbol ΔT = T - T0, where T is the final temperature of the substance and T0 is its initial temperature.
  3. The type of substance. This factor is represented mathematically through a new quantity, called specific heat capacity, c, which is the amount of heat absorbed by a 1 kg of a substance to increase its temperature by 1°C (or Kelvin).

Putting all the above factors together, we obtain the formula for the specific heat capacity, c:

c = Q/m × ∆T

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:

Q = m × c × ∆T

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,

1 Cal = 1 kcal = 1000 cal

The conversion factor between calorie and Joule is

1 cal = 4.186 J

Matter exists in three stable states: solid, liquid and gas.

  1. In solid state, particles are very close to each other. They are strongly bound to each other because the binding potential energy prevails over the kinetic energy of atoms. As a result, atoms can only vibrate around their equilibrium position but they cannot leave their predefined place. This means a certain atom in solid state has always the same "neighbours" around.
  2. In liquid state, the particles slide over each other, as they are not so tightly bound. The atoms potential energy is still greater than their kinetic energy, so atoms still stay together. It is like opening a sack full of tennis-table balls and spread them throughout the room. The balls will slide over each other forming horizontal layers. A liquid takes the shape of the container it is poured.
  3. In gaseous state, atoms are far away from each other; they move freely in space, as the atoms kinetic energy is greater than their potential energy. A gas fills the whole space of the container it is poured

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.

  1. Melting. It occurs when a solid turns into liquid. During this process, temperature remains constant as the heat supplied to the object goes for breaking the strong molecular bonds, not for the increase in their kinetic energy.
  2. Freezing. It is the reverse process of melting that occurs when a liquid turns into solid. It occurs when the object gives off heat to the surroundings. After the termination of this process, atoms are more structured than before. Again, the freeing process occurs without any change in temperature.
  3. Evaporation. It occurs when a liquid turns into gas. Again, the heat supplied during evaporation does not contribute in the increase in temperature. It only allows atoms leave the liquid and move freely in space.
  4. Condensation. It is the reverse process of evaporation, i.e. it occurs when a gas turns into liquid giving off heat, without any change in temperature.
  5. Sublimation. Sometimes, a solid turns directly into gas due to the very large amount of heat it absorbs. During this process, the liquid phase is skipped.
  6. Deposition. It is the reverse process of sublimation, i.e. it occurs when a gas turns directly into solid without passing through the liquid state. This process occurs when the object gives off large amounts of heat at a very short time.

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

Qf=m × Lf

and

Qv = m × Lv

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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