ASRJC First Law of Thermodynamics

ANDERSON SERANGOON JUNIOR COLLEGE PHYSICS 9749 1 Additional Notes Section III Thermal Physics Topic 9: 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 the 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.

ANDERSON SERANGOON JUNIOR COLLEGE PHYSICS 9749 2 Additional Notes A.1 What is Specific Heat Capacity • Heating is a process whereby thermal energy is being transferred. • Thermal energy is transferred by conduction, convection or radiation from one body to another due to a temperature difference. • When a body is heated, its temperature rise depends on: 1. its mass - the smaller the mass, the larger the temperature rise for the same amount of heat supplied; 2. the material of which the body is made - objects of the same mass but different materials experience different temperature rise when supplied with the same amount of heat. Th is is determined by a property called the specific heat capacity c of the body. The specific heat capacity c of a substance is the thermal energy per unit mass per unit change in temperature of the substance. where Q = heat supplied m = mass of the substance ∆T = rise in temperature of the substance • SI unit of c is J kg−1 K−1. The heat capacity C of a substance is the thermal energy required per unit change in temperature of the substance. • SI unit of C is J K−1. Qc mT=  Q mc T= A Specific Heat Capacity Δ Δ QC= T Q = C T memorize

ANDERSON SERANGOON JUNIOR COLLEGE PHYSICS 9749 3 Additional Notes A.2 Practical Uses of Specific Heat Capacity • For a given heat input, if it causes only a small temperature rise in the substance, this means the substance should have a high specific heat capacity. • An example is the use of water as an efficient coolant in a car radiator. The specific heat capacity of water is 4.2  103 J kg−1 K−1 (i.e. 4.2  103 J of energy is required to increase the temperature of 1 kg of water by 1 K). • The table below shows the mean specific heat capacity for some other materials at ordinary temperatures: Mean Specific Heat Capacities / J kg−1 K−1 Aluminium 9.1  102 Ice 2.1  103 Copper 3.9  102 Rubber 1.7  103 Glass (ordinary) 6.7  102 Wood 1.7