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

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

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

Forms of Energy Mechanical Chemical Electrical Nuclear Radiant

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

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

Radiant Energy 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 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 Temperature and heat are not the same thing. 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 Beans lost energy, it was transferred to its surroundings. This change of energy would be negativechange = ∆ Energy transferred from fire to beans. Change in energy for beans would be positive.

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

Heat Transfer 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: calorie – called “little calorie” – (cal) kilocalorie – called “Big calorie”- (CAL)  1000 calories – same as a food calorie Joule – the SI unit of heat - (J)

CONVERSION FACTOR 1 cal = joules 1 cal4.184 J J 1 cal

Energy Flow and Phase Changes 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 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 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