Energy and Heat Transfer Mechanisms

Energy Energy – the ability to do work or transfer heat. – Energy used to cause an object that has mass to move is called work. – Energy used to cause the temperature of an object to rise is called heat. Thermodynamics – study of energy and its transformations – Thermochemistry – branch of thermodynamics involving chemical reactions and changes in heat 2

There are 6 Major Types of Energy – Mechanical Thermal Chemical Electrical Electromagnetic Nuclear

Kinetic Energy Kinetic energy is energy an object possesses by virtue of its motion 1 2 KE =  mv 2 m = mass v = velocity Atoms and molecules can have mass and motion and therefore possess kinetic energy! 4

Potential Energy Potential energy is energy an object possesses by virtue of its position or chemical composition. Potential energy arises when a force operates on an object – Force – any kind of push or pull exerted on an object Ex: Gravity is the force exerted on the cyclist at the top of the hill. The cyclist possesses potential energy. PE = mgh m = mass g = gravitational constant, 9.8 m/s 2 h = height of an object relative to some reference height 5

Potential Energy Potential energy is energy an object possesses by virtue of its position or chemical composition. Many substances (ie, fuels) release energy when the react. The chemical energy of these substances is due to the potential energy stored in their chemical bonds. 6

Units of Energy The SI unit of energy is the joule (J) (“jewel”). An older, non-SI unit is still in widespread use: the calorie (cal). 1 cal = J – In nutrition, the Calorie (Cal) is used 1 Cal = 1000 cal = 1 kcal 1 J = 1  kg m 2 s2s2 7

Definitions: System and Surroundings The system includes the molecules we want to study (here, the hydrogen and oxygen molecules). The surroundings are everything else (here, the cylinder and piston). 8

Classifications of Systems 1.Open: matter and energy can be exchanged with the surroundings – Example: Boiling pot of water on stove with no lid – heat enters the system from the stove and water vapor escapes from the system into the surroundings. 2.Closed: energy, but not matter, can be exchanged with the surroundings – Example: Cylinder with movable piston containing H 2 (g) and O 2 (g) – these form water but mass is not lost. But energy is exchanged with surroundings in the form of heat and work. H 2 (g) + O 2 (g)  H 2 O(g) + energy 3.Isolated: neither energy nor matter are exchanged with the surroundings – Example: An insulated thermos containing hot coffee is an approximation. 9

Transferring Energy: Work Work – the energy used to move an object over some distance w = F  d w = work F = force d = distance over which the force is exerted The object being moved is the system Whatever is moving the object is part of the surroundings and is performing work on the system 10

Transferring Energy: Heat Energy can also be transferred as heat. Heat flows from warmer objects to cooler objects spontaneously. Heat is the transfer of energy due to a difference in temperature. Temperature and heat are not the same thing! Temperature is a measure of the average kinetic energy of the molecules that make up a substance. In this combustion reaction, the chemical energy stored in the fuel molecules is released as heat (energy). The heat is transferred from the hotter system to the cooler surroundings. – Substances involved in combustion reaction = system – Everything else = surroundings 11

Methods of Heat Energy Transfer Conduction is the transfer of heat energy by – Between particles of objects in direct contact Convection is the transfer of heat energy by – the movement of fluids(gas or liquid) – convection currents due to hot fluid rising and cold fluid sinking Radiation is the transfer of heat energy by – electromagnetic waves – does not involve the movement of matter

A. Conduction transfers energy as heat along the wire and into the hand B. Embers swirl upward in the convection currents that are created by the warmed air above the fire which rises C. Electromagnetic waves emitted by the hot campfire transfer energy by radiation

Heat Transfer by Conduction Conduction is the transfer of thermal energy without any net movement of the material itself. When a metal poker is put in a hot fire, the exposed end of the poker soon becomes hot as well, even though it is not directly in contact with the source of heat. We say that heat has been conducted from the hot end to the cold end.

Heat conduction in many materials can be visualized as the result of molecular collisions. As one end of the object is heated, the molecules there move faster and faster. As they collide with their slower-moving neighbors, they transfer some of their energy to these molecules whose speeds thus increase. These in turn transfer some of their energy by collision with molecules farther along the object. Thus the energy of thermal motion is transferred by molecular collision along the object. Good thermal conductors such as silver, copper, aluminum, and gold are also good electrical conductors

Heat Transfer by Convection Convection is the process of heat transfer through the mass motion or flow of some fluid, such as air or water. When a pot of water is heated, convection currents are set up as the heated water at the bottom of the pot rises because of its reduced density and is replaced by cooler water from above.

Heat Transfer by Convection Although liquids and gases (fluids) are generally not very good conductors of heat, they can transfer heat quite rapidly by convection. Convection is the process whereby heat is transferred by the mass movement of molecules from one place to another. Whereas conduction involves molecules (and/or electrons) moving only over small distances and colliding, convection involves the movement of molecules over large distances.

Perhaps the first thing that most people say is "heat rises". What you should say is "hot air rises" or "hot water rises". Anything fluid - that is gases or liquids - will tend to change density with changes in temperature. For example, if heated, air decreases in density. The surrounding air is cooler and denser. This makes it heavier, so it falls beneath the hot air, forcing it upwards.

Heat Transfer by Radiation Radiation is a more rapid transfer of thermal energy in the form of electromagnetic radiation accomplished by a process that requires neither contact nor mass flow. A hot object also loses heat energy by radiation. This radiation is similar to light and can pass through empty space. The warmth you fell when you warm yourself by a fire is due to this radiation. If the object is hot enough, some of the radiation is visible and can indeed be seen.

What type of heat transfer is shown in the following pictures? A. B. C. D. Convection Radiation Conduction Radiation