Thermal Force Unit 1.4

Objectives ~ TEKS 6.D Define thermal energy Name the property of a body that determines its temperature. Given Celsius or Fahrenheit temperature and the formula for conversion, find the equivalent temperatures. Explain the difference between heat and thermal energy Explain the relationship between heat transferred to an object and the change in the object’s temperature Use specific heat, heat of fusion, and heat of vaporization to solve problems involving heat transfer

Terms (We will study energy in the next unit) Energy – the ability to do work. Potential energy – the stored or build up of energy; the capacity/ “potential” to do work. Kinetic energy – energy of motion. Thermal motion – the random motion of vibration of an object’s atoms and molecules. Thermal energy – the total energy of the thermal motion of all particles that make up an object

Temperature Temperature is the “hotness” property of a body; it is the average kinetic energy of the random motion of the atoms and molecules in a body. Heat flows from regions of higher temperature to regions of lower temperature. Measured in degrees Fahrenheit or degrees Celsius. Thermal Energy is measured in Joules, Calories, and BTU’s (British Thermal Unit)

Measuring Temperature Thermometer – uses expansion and contraction of a liquid usually colored alcohol or mercury. Celsius scale – developed by Anders Celsius and based on the properties of water. (0° = freezing; 100° = boiling) Fahrenheit scale - used in the US (32° = freezing and 212° = boiling)

Absolute Zero Absolute Zero is the temperature at which all molecular motion stops 0° Kelvin = C° 0° Rankine = F°

Temperature scales T C = 5 9 (T F – 32) T F = 9 5 T C + 32 T K = T C – 273 T R = T F – 460

Heat Conduction – thermal energy flows from the hotter object to the cooler object as kinetic energy is transferred when particles collide. Heat is the energy that flows from one object to another because of temperature difference. Heat flow depends on temperature difference. As the temperature difference increases so does the heat transfer rate. (no temp. diff = no heat transfer) Temperature difference is the prime mover in thermal systems.

Analogies to Prime Movers Mechanical – Force Fluid – Pressure Electrical – Voltage Thermal – Temperature Difference

Specific Heat The specific heat (C) of a substance is the amount of energy that must be added to raise the temperature of a unit mass of the substance one unit of temperature. Specific heat of water =1 cal g C° energy mass Unit of temp.

Change of State States of matter – Solid, Liquid, Gas, and Plasma.

Changes of state

Changes in State Freezing Point – Temperature at which a substance changes from a liquid to a solid. Boiling Point – Temperature at which a liquid changes to a gas. Heat of Fusion (H f )– the amount of energy required to change and melt a solid. Heat of Vaporization (H v ) – the amount of energy required to vaporize a liquid.

Changes of State The amount of heat (Q) needed to melt a solid of mass (m) is Q = m H f The amount of heat (Q) needed to vaporize a liquid of mass (m) is Q = m H v

Measurements One calorie is the amount of thermal energy that must be added to water to change the temperature of 1 gram of water by 1 degree Celsius. One BTU is the amount of thermal energy that must be added to water to change the temperature of 1 pound of water by 1 degree Fahrenheit.

Heat Transfer Q = m C  T Heat transferred Object’s mass Specific heat Temperature Change

Summary The thermal energy of a body is the total kinetic energy of motion of all the particles that make up the body. The temperature of a body is determined by the average kinetic energy of the particles that make up the body. A thermometer measures temperature in degrees Celsius, degrees Fahrenheit, degrees Kelvin, or degrees Rankine. Heat is the energy that flows from one body to another because of a temperature difference. Whenever two bodies are brought together, heat flows from the body with higher temperature to the body with the lower temperature. The amount of heat transferred to an object varies linearly with the object’s temperature change, as long as there is no change of state: Q = mc  T If heat is transferred to a substance and it changes state, its temperature does not change.