THERMAL ENERGY
Thermal Energy can best be understood by gaining an understanding of exactly what we mean by thermal energy. The Kinetic Theory of Matter states that all matter is made of tiny particles in constant motion. The range of the motion determines the physical states of matter.Solids: limited movement, rigid long range structural order. It has a fixed volume and shape
Liquids: loosely bonded together, much greater range of motion. Liquids have a fixed volume, but no fixed shape, it conforms to its container.
Gases: no bonds between particles. Each moves independently, no fixed shape or volume. Gases will expand to fit whatever space and shape is available.
TEMPERATURE VS. HEAT
Temperature is the average kinetic energy of an objects particles. Heat, which can never be totally measured, is the total kinetic energy of an object's particle. Heat is said to measure the flow of energy. In the strictest sense this is true. A change in the total kinetic energy, or a change in the heat, causes the temperature of nearby objects to change. According to the Law of Conservation of Energy what one loses the other must gain. To get a better understanding of these definitions, think of a coffee cup of boiling water and a swimming pool of cold water. The coffee cup's particle on average, have much greater energy. The pool has more heat since its particles vastly outnumber the molecules of a cup of boiling water.This example clearly illustrates the difference between heat and temperature.
Heat is transferred in one of three ways:
Conduction is actual physical contact
Convection is through the use of a moving fluid, a liquid or a gas.
Radiation uses electromagnetic waves, like in a microwave
A phase diagram represents the change in kinetic energy of a substances' molecules as heat (energy) is added.
Notice that the temperature increases except during a phase change. During a phase change the energy is going to break or loosen the bonds. The energy that doesn't contribute to a temperature rise is "latent" or hidden heat.
When we calculate the heat that flows from an object we need to know three things.
1) How much mass is present?
2) What type of material is this? Every substance has a unique ability to absorb heat and thus the amount of energy required to raise its temperature by 1o C is different. This unique value is the specific heat. By definition specific heat is the energy required to raise the temperature of a substance by one degree Celsius.
Water has a very high specific heat, 4.184 J/g *Co. This means water requires a lot of energy to heat up and gives up a lot of energy when it cools down. Metals usually have a very low specific heat.
3) Finally we need to know the difference in temperature. If two things are very close to the same temperature, not much energy will flow between them until equilibrium is established.
Q represents heat flow, so Q = mc/\T
This equation allows us to calculate the flow of heat into or from a material within a specific phase. How do we calculate the heat during a phase change when the temperature doesn't change? We must know the values or the energy required to melt of fuse a material or to vaporize or condense a material.
The heat required to convert liquids to solids, or solids to liquids, = Hf, or heat of fusion. The energy required to convert a liquid to a gas or a gas to a liquid is Hv, the heat of vaporization. Q =mHf or
Q = mHv during a phase change. This Q is the "latent" or hidden heat.
As with all energy we measure in Joules. Formerly, another unit, the calorie was used. By definition a calorie is the energy required to raise one gram of water by one degree Celsius. A calorie equals 4.184 joules. The calorie we normally use, a food calorie, is actually a kilocalorie or 1000 regular calories
When we measure temperature with a thermometer, we are using the concept of Thermal Equilibrium. We've placed an object in a different heat content and the energy will transfer from hot to cold due to contact.
Virtually all matter expands when heated and contracts when cold. When a thermometer is placed in a substance it contracts or expands to move down or up the column. The property of increased volume for increased temperature is Thermal Expansion.
Temperature is measured using three (3) different scales: Fahrenheit, Celsius, and Kelvin.
The conversions are made with these three equations:
F = 9/5C + 32
K = C+273.15
C = 5/9(F-32)
