Heat and Work

Heat and Work:

What is Heat?

Heat is the form of energy that flows from a body at higher temperature to a body at lower temperature. It is expressed in calories or joules. 1 calorie = 4.186 joule.

Characteristics of Heat:

  • Heat is form of energy. A system does not possess any heat. It contains only energy. Heat comes into picture when there is temperature difference.
  • Heat is a path dependent quantity.
  • When heat flows from surroundings into the system, the energy of the system increases but when heat flows from system to the surroundings, the energy of the system decreases.

Heat Sign Conventions:

The heat exchanged between the system and its surroundings is denoted by the symbol ‘q‘. In thermodynamics, q is taken as positive when heat flows into the system i.e. heat is absorbed by the system. But q is taken as negative when heat flows out of the system i.e. heat is liberated by the system.

What is Work?

Work is said to be done when a body moves through a certain distance by the application of a certain force. It is denoted by symbol ‘W‘.

Mathematically, work done, W = force X displacement = F X  Δx

If F = 0, then W = 0, thus when there is no force, there will be no work. Similarly if Δx = 0 then W = 0. Thus, when there is no displacement there will be no work. The units of work are erg or joules. 1 joule = 107 erg.

Characteristics of Work:

  • Work is manifestation of energy. Work comes into picture only when there is displacement. If there is no displacement, work is zero.
  • Work is a path function. Thus, work is not a state function.
  • When work is done by the system, its energy decreases and when work is done on the system, its energy increases.

Work Sign Conventions:

In thermodynamics, ‘w’ is taken as positive, when work is done on the system and ‘w’ is taken as negative, when work is done by the system.

There are two main types of work. These are mechanical work and non-mechanical work. Mechanical work is important in systems that contains gases. It is generally considered as the work done in expanding (or compressing) a gas at constant external pressure. It is known as pressure volume work or PV work or expansion work.

Wmechanical = Wexpansion = -P ΔV

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