Heat and Heat Transfer

Scales of Measurement Celsius scale – based on where water freezes (0  C) and where water boils (100  C) Kelvin scale – based on the movement of particles

Absolute Zero At 0 K, all particle movement has ceased It is impossible to have a temperature lower than 0 K 0 K = -273  C

T C =5/9 (T F -32º) T F = T C 9/5+32º T K = T C ºC 273 K 10ºC 283 K 20ºC 293 K 30ºC40ºC50ºC 303 K313 K323 K

Kinetic-Molecular Theory The faster particles move, the greater the kinetic energy or thermal energy.

Definitions Temperature –average kinetic energy of the particles in a substance –Thermometers measure temperature Heat – amount of energy transferred

Thermal equilibrium Energy will always travel from an area of higher energy to an area of lower energy.

Thermal Equilibrium, cont. Two substances with different energies transfer energy (higher  lower) until their energies are equal. This point is “thermal equilibrium”.

Heat transfer Conduction: molecular agitation; no motion as a whole Convection: mass motion of a fluid Radiation: emission of EM waves, no medium needed

Conduction As materials are heated, electrons gain thermal energy which means they move faster. As the electrons in a substance collide, the energy is transferred to surrounding electrons. The actual molecules do not change places.

Convection Heating occurs due to the motion of a fluid. When a fluid is heated, it becomes less dense and rises. The cooler air is more dense and circulates to the bottom where it is heated and begins the process again.

Radiation Radiation does not require a medium to transmit energy. This type of energy is called radiant energy and it travels in electromagnetic waves. High temperatures emit short wavelengths whereas low temperatures emit long wavelengths.

Specific Heat Amount of energy that must be added to the material to raise the temperature of a unit mass one temperature unit. The units of specific heat are J/kg·K or J/kg·°C

Specific Heat Formula Q = mC  T Q = mC (T final – T initial ) Q = Heat (J) m = mass (kg) C = Specific heat (J/kg·K or J/kg·°C)  T = change in temperature (K or °C)

Example #1: A kg block of iron is heated from 295 K to 325 K. How much heat had to be transferred to the iron if the specific heat of iron is 450 J/ kg·K?

Example #1: Q = mC  T Q = (0.400 kg)(450 J/ kg·K)( K) Q = (0.400 kg)(450 J/ kg·K)(30 K) Q = 5400 J

Law of Conservation of Energy Energy lost by one object must be equal to the amount gained by another object. Energy lost = - Energy gained m A C A  T A = -m B C B  T B

Example #2: A container has 0.50 kg of water at 15  C. A kg block of zinc at 115  C is placed in the water. What is the final temperature of the system? (C zinc = 388 J/kg·  C and C water = 4180 J/kg·  C)

Example #2: m A C A  T A = -m B C B  T B (0.5)(4180)(T f -15)= - (.04)(388)(T f -115) 2090(T f -15) = (T f -115) 2090 T f = T f T f = T f = ºC