Food and Energy The Basics of Heat Transfer

The Flow of Energy Thermochemistry - concerned with heat changes that occur during chemical reactions

Cranking up the Heat: Basic Thermodynamics Energy - capacity of an object to do work or produce heat

Kinetic Energy q The energy an object has due to its motion Potential Energy Stored energy q Energy of position q

External Energy q The energy applied to an object by another source Internal Energy q Energy within an object

Energy in the universe is constant, though it changes forms. Law of Conservation of energy – energy can neither be created nor destroyed

Forms of Energy n n n Mechanical Chemical Electrical Nuclear Radiant

Mechanical Energy n n The total kinetic and potential energy of a system Examples- lifting weights, chewing food, opening the refrigerator

Chemical Energy n n n The energy resulting from the forming or breaking of bonds in a chemical reaction Endothermic reaction- A reaction whose products have less total heat than the reactants Exothermic reaction- where energy is released during the reaction

Electrical Energy n n Energy that is produced by the movement of electrons Example- electricity

Radiant Energy n n n Energy transmitted in the form of waves through space or some medium. Also known as the electromagnetic spectrum Examples- light, radio waves, UV waves, microwaves A microwave oven has a magnetron, which is an electron tube that converts electrical energy into microwaves The more water in food, the faster a microwave oven will cook it since the energy agitates water molecules

Nuclear Energy n n Energy from the splitting or combining of atoms of certain elements that then gives off radiation Examples- X-rays, nuclear power plants, irradiation

Heat vs. Temperature and heat are not the same thing. n n Temperature- the measure of the average kinetic energy of a group of individual molecules Heat- describes amount of energy transferred from one object to another caused by a temperature difference between the two bodies

Describing Energy Changes Heat flow is from the warmer object to the cooler object Heat Flow

Describing Energy Changes Hot day 50°C = 122°F Energy transferred from fire to beans. Change in energy for beans would be positive. Beans lost energy, it was transferred to its surroundings. This change of energy would be negative change = ∆

Describing Energy Changes Endothermic reactions – absorbs heat, ∆H is positive, the surroundings cool down Surroundings Heat Feels cold to the touch Exothermic reactions – releases heat, ∆H is negative, the surroundings heat up Surroundings Heat Feels warm to the touch

Heat Transfer n n Three methods of heat transfer Conduction- the transfer of heat through matter from particle to particle collisions Convection- the transfer of heat by the motion of fluids, such as water and air Radiation- the transfer of heat by electromagnetic waves

Units used to describe Energy: n calorie – called “little calorie” – (cal) n kilocalorie – called “Big calorie”- (CAL) q n 1000 calories – same as a food calorie Joule – the SI unit of heat - (J)

CONVERSION FACTOR n 1 cal = 4. 184 joules 1 cal 4. 184 J 1 cal

Energy Flow and Phase Changes n n n Fusion- The movement from a solid to a liquid phase, aka melting Crystallization- The movement from a liquid to a solid phase, aka freezing Latent heat of Fusion- The energy needed to melt or freeze a substance

Energy Flow and Phase Changes n n n Evaporation or Vaporization- The phase change from the liquid to the gaseous phase Condensation or Liquefaction- The change of a gas to a liquid Latent heat of vaporization- The amount of heat needed to either evaporate or condense a substance

Energy Flow and Phase Changes n n n Latent heat- The energy required to complete a phase change without a change in temperature Deposition- The changing of a substance directly from a gas to a solid, ex. Frost forming on a window Sublimation- The changing of a substance directly from a solid to a gas

Phase Change Diagram