First Law of Thermodynamics JPJC Notes

1 JURONG PIONEER JUNIOR COLLEGE 9749 H2 PHYSICS FIRST LAW OF THERMODYNAMICS Content • Specific heat capacity and specific latent heat • Internal energy • First law of thermodynamics Learning Outcomes Candidates should be able to: (a) define and use the concepts of specific heat capacity and specific latent heat. (b) show an understanding that internal energy is determined by the state of the system and that it can be expressed as the sum of a random distribution of kinetic and potential energies associated with the molecules of a system. (c) relate a rise in temperature of a body to an increase in its internal energy. (d) recall and use the first law of thermodynamics expressed in terms of the increase in internal energy, the heat supplied to the system and the work done on the system.

2 Introduction Heat capacity and specific heat capacity allows for the calculation of temperature changes in the interaction between two substances in thermal contact. Latent heat is used to calculate the energy required to change the phase of a substance. Thermodynamics is the study of relationships involving heat, mechanical work, and other aspects of energy and energy transfer. The first law of thermodynamics is central to understanding thermodynamic processes which involve heat and mechanical work. This law extends the principle of conservation of energy in Newtonian mechanics, by introducing the concept of internal energy. 1 Specific Heat Capacity and Specific Latent Heat (a) Candidates should be able to define and use the concepts of specific heat capacity and specific latent heat. 1.1 Heat capacity Heat capacity C of a substance is the amount of thermal energy required to cause a unit rise in temperature of the substance. SI units of heat capacity: J K−1 In symbols, we have QC T=  Hence, Q = C T where Q is the heat supplied (or removed), C is the heat capacity, T is the change in temperature. Note: Heat capacity is dependent on both the mass of the body and the material of the body. Example 1 How much heat must be supplied to an object with a heat capacity of 150 J K−1 in order to increase its temperature from 350 K to 400 K? Solution: We assume that the object does not change phase in this temperature range. Heat supplied = C T = (150)(400 – 350) = 7500 J

3 1.2 Specific heat capacity The specific heat capacity c of a substance is defined as the thermal energy per unit mass required to raise the temperature of the substance by one unit of temperature. SI units of specific heat capacity: J kg−1 K−1 In symbols, we have Qc mT=  Hence, Q = m c T where Q is the heat supplied (or removed), m is the mass of substance, c is the specific heat capacity, T is the change in temperature. Note: The specific heat capacity is the heat capacity per unit mass of a substance and it is characteristic of the material of the s