Thermal energy Heat and heat effects

KINETIC MOLECULAR THEORY: (KMT) 1) All matter is composed of very small particles. 2) The particles of matter are in constant random motion and possess kinetic energy. 3) Particles collide with each other and the container walls with perfectly elastic collisions. 4) Empty space exists between particles which is large compared to the size of the particles. 5) The particles move faster with an increase of temperature absorbing the heat energy transforming it into kinetic energy of the particles.

KINETIC MOLECULAR THEORY: (KMT) At a specific state change, heat is no longer used to increase kinetic energy of atoms or molecules but rather used to break bonds. There is no increase in temperature during a change of state. Thermal energy is defined as the total kinetic and potential energy of the atoms or molecules of a substance. It depends on mass, temperature, nature and state.

Definitions Heat: A measurement of energy transfer from a hot body to a cold one. Temperature: A measure of the average kinetic energy of the atoms or molecules of a substance. T increases as the motion of the particles increases. Thermometers are used to measure temperature. They contain mercury or alcohol. Mercury freezes at -39oC and boils at 357oC. Alcohol freezes at -117oC and boils at 79oC

Thermometer scales Anders Celsius (1701-1744) based his temperature scale on water 0oC to 100oC and used 100 divisions on a scale; 1 division/degree. It is possible to use the known boiling and freezing temperatures of any liquid to make your own thermometer!! William Thompson Kelvin (1824-1907) invented a scale with a zero point of -273.14oC, the coldest temperature possible; absolute zero when matter collapses. One Kelvin degree = one Celsius degree.

Methods of Heat Transfer Temperature decides the direction of spontaneous energy transfer by; Conduction Convection Radiation

An insulator does not conduct heat well (wood, wool, paper, cork). Conduction Takes place within materials and from one to another while in contact; metals make the best conductors. An insulator does not conduct heat well (wood, wool, paper, cork). When heated, electrons move rapidly in conductors (as electrons are held loosely) which collide with other electrons transferring kinetic energy and therefore temperature.

Blankets slow the transfer of heat. Conduction Ex: Touch wood and metal; the metal feels cool as it transfers your heat. Tiles and carpet are similar. Blankets slow the transfer of heat. Snow slows down the escape of heat from Earth’s surface. Liquids and gases are generally insulators. Cold is the absence of heat

Convection Air in contact with a heat source rises to warm the region above occurs in all fluids via currents. A fluid heated from below, expands and becomes less dense, and then rises.

Ex: You can place your hand around a candle flame but not above it: Convection Ex: You can place your hand around a candle flame but not above it: Air is an insulator but convection occurs above the candle flame. Wind is the result of convection currents stirring the atmosphere (hot air rises and cool air rushes in underneath).

Convection Earth heats unevenly as it absorbs heat differently. (causes convection currents) At the beach: the sand warms faster than the water so the air rises up causing a greater air pressure above the shore. At a ~2 km height, air blows out to the water, on the beach, a wind blows onshore (cyclic). At night, the process reverses as sand gives up heat faster than water.

Radiant energy is transmitted by radiation in the form of waves Ex: Radio, micro, infrared, visible, ultraviolet, x- rays, gamma rays( in order from long to short wavelengths.) All objects continually give off radiant energy of different wavelengths.

Low temperature objects emit longer wavelengths. radiation Low temperature objects emit longer wavelengths. When an object becomes hot enough, the wavelength decreases to visible light (like light bulbs). (Red is ~500oC and white at 1200oC ..... white hot!!) Objects absorb and reflect Infra Red (IR) and the visible light of the sun. Absorption increases the internal energy of the object.

HEAT CAPACITY The heat capacity of a material determines the amount of heat that can be added to a sample of matter. It is the amount of heat, Q, required to raise an object’s temperature 1oC measured in J/oC

Specific Heat Capacity The amount of heat required to raise the temperature of a mass of 1 kg by 1oC measured in J/kgoC. Different materials can absorb heat differently. Water has a very high specific heat capacity making it useful for radiators: cw = 4184 J/kgoC

Specific Heat Capacity Q = mc∆T where Q is in Joules m is in kg c is in J/kgoC ∆T is in oC 1 cal = 4.184 J

Factors affecting Heat Capacity 1) Mass: an increase in mass will increase heat capacity 2) Temperature change: a large ∆T requires more heat 3) Material: different materials require different Q to raise the T by oC, so a different c value. (specific heat capacity). *You should be able to explain these factors using KMT.

example Find the amount of heat transferred to 200 g of water heated from 20 oC to 60oC. Q = mc∆T = (.200 kg) (4184 J/kgoC) (60-20)oC = 3.3 x 104 J

Heat of mixtures The cold component gains heat (heat always flows from hot to cold). We will assume no energy is lost to the surroundings to simplify these problems. Qlost= Qgained - (mc∆T)hot = (mc∆T)cold

Example 1 You mix 100 g of water at 80oC with 100 mL of water at 20oC .What is the final temperature of this mixture? Qlost = Qgained -(mc∆T)hot= (mc∆T)cold -(.100 kg)(4184J/kgoC)(Tf - 80oC) = (0.100kg)(4184J/kgoC)(Tf - 20oC) -(Tf - 80oC) = (Tf - 20oC) -2Tf = -20oC - 80oC Tf = 50oC

Example 2 Jello™ Party!! Jello™ has a specific heat capacity of 480 J/kgoC and the human body has a specific heat capacity of 3470 J/kgoC. You dive into 100 kg of Jello™ and your mass is 55 kg. Your body temperature is 37oC and the Jello™ is 10oC, Find the final temperature of this mixture. Qlost= Qgained -(mc∆T)body = (mc∆T)Jello = - (55kg)(3470J/kgoC)(Tf - 37oC) = (100kg)(480J/kgoC)(Tf - 10oC) Tf= 32oC

Standard units are: (J/kg) Q = mLf Lf (H2O) = 80. cal/g Heat of fusion The quantity of heat required to melt a unit of mass of a solid at constant temperature. The heat released by a unit mass of a liquid crystallizing at constant temperature. Standard units are: (J/kg) Q = mLf Lf (H2O) = 80. cal/g

Standard units are: (J/kg)oC Q = mLv Lv (H2O) = 540. cal/g Heat of vaporization The quantity of heat required to vaporize a unit of mass of a liquid at constant temperature The heat released by a unit of mass of a gas condensing at a constant temperature Standard units are: (J/kg)oC Q = mLv Lv (H2O) = 540. cal/g

Example How much heat is required to melt 100 g of ice and then raise its temperature to 70.0oC? As we have to change state and raise temperature; Q = mLf + mc∆T = (100g)(80 cal/g)(4.184 J/cal) + (0.100kg)(4184J/kgoC)(70.0oC) = 6.28 x 104 J Unit analysis in this unit will be very helpful!