Thermodynamics An Introduction

What is Thermodynamics? Thermodynamics is the science that is concerned with energy, particularly ‘energy-in-transit’ in the forms of heat and work, and those properties of systems that are related to energy.

Three Kinds of Energy Energy – the ability to do work. All energy is relative! Energy-in-transit is not relative. Three kinds of energy: (1) potential – Stored Energy (2) kinetic - Energy at work (or in motion) (3) internal - the sum of all potential and kinetic energies of the constituent parts [atoms, molecules, etc.] of a system. TME, TIE – Total Internal Energy

Energy in Transit Two kinds of ‘energy-in-transit’: (1) heat – energy transferred between the system and surroundings because of a temperature difference, or gradient. (2) work - energy transferred between the system and surroundings because of a pressure difference, or gradient. (Another way of thinking of unbalanced forces.)

What is Thermal Energy? Thermal energy is energy possessed by an object or system due to the movement of particles within that object or system. Measured as temperature, thermal energy has to do with the speed at which molecules move within an object or system. We can’t see individual atoms vibrating, but we can feel their kinetic energies as temperature. When there’s a difference between the temperature of the environment and a system within it, thermal energy is transferred between them as heat.

What is Temperature? Temperature is a measure of the average heat (or thermal energy) of the particles in a substance. Since it is an average measurement, it does not depend on the number of particles in an object, meaning that it does not depend on the size of the object. SI Unit of Measurement: °K (0°K = -273.15°C)

Kelvin Comparison

Absolute Zero Absolute Zero is the point at which matter has zero vibrational/motion energy. While atoms still have their mass-related quantum energy, which is the total energy of the mass itself if it were to be converted into pure energy (E=mc2), there is no thermal energy remaining in the matter for molecules to vibrate or move, and the matter is, for all intents and purposes, absolutely frozen.

What is Heat? Heat is the quantity of the transfer of thermal energy. This transfer moves from hotter to colder materials and temperatures, in order to reach a state of thermal equilibrium, which is a state in which all matter has the same average temperature. Measured in Joules (as it is a measure of energy) Mathematically, it is expressed with the variable Q.

Types of Heat There are three means of thermal energy transfer (heat): Conduction: The transfer of energy through the collision of molecules; transfer through direct contact. Convection: The transfer of energy through a liquid or gas due to a temperature difference; change in density due to heating and cooling. Radiation: The transfer of energy by electromagnetic waves.

Specific Heat Specific Heat is the amount of energy that must be added to a unit of mass of a material to raise its temperature by one temperature unit. In SI, the unit for specific heat is J/(kg·°K) Mathematically, it is expressed with the variable C.

Specific Heat

Heat Formula The formula for heat is: Q = mCΔT = mC(Tf-Ti) Q = Heat m = mass of object C = specific heat of object ΔT = Difference between initial and final temperatures. *T can be expressed in Kelvin or Celsius, as they both have the same temperature interval. ** Heat can be a negative value if ΔT is negative (meaning heat is removed from the object)

What is a Calorie? 1 Calorie (small calorie) is the amount of energy it takes to heat 1g of water by 1°C. This amount is 4.19 J. 1 Large Calorie is the amount of energy it takes to heat 1kg of water by 1°C. This amount is 4.19 kJ. (This is the ‘calorie’ used to determine the energy content in food.) Joules are the SI standard for heat and energy.

Heat Problem A cast-iron skillet (5.10 kg) is heated on the stove from 295K to 373K. How much thermal energy (heat) was transferred to the iron to raise its temperature that amount? (Specific heat of iron: 450 J/(kg·K))

Heat Problem When you turn on the hot water to wash your dishes, some of the heat from the water transfers to the pipes, heating them up. How much heat is absorbed by a 2.3kg section of copper pipe (C=385 J/(kg·K))) when its temperature is raised from 20C to 80C?

Heat Problem 50,000 J of heat is added to a 2kg pot of water (C=4180 J/(kg·K)) whose temperature was at 2C. What is the water’s new temperature?

Assignment Heat Practice Problems Sheet