Presentation on theme: "Thermal Energy and Heat. Work and Heat Heat- the transfer of thermal energy from one object to another because of a temperature difference Heat flows."— Presentation transcript:
Thermal Energy and Heat
Work and Heat Heat- the transfer of thermal energy from one object to another because of a temperature difference Heat flows from higher temperatures to lower temperatures. Temperature is related to the average kinetic energy of the particles in an object due to their random motion through space. Collisions between particles transfer thermal energy from hot objects to cold objects.
Thermal Energy- Total PE and KE of all particles in an object. Depends on: mass, temperature, and phase. Thermal energy depends on mass and temperature. A The tea is at a higher temperature than the lemonade because its particles have a higher average kinetic energy. B The lemonade is at a lower temperature, but it has more thermal energy because it has many more particles.
Thermal Expansion- an increase in the volume of a material due to a temperature increase. Remember Charless Law? As temperature increases, volume increases. particles speed up, they have more collisions, and produce more collisions, and produce more force. thermal expansion/contraction are used in thermometers.
Specific Heat- the amount of heat needed to raise the temperature of 1 gram of a material by 1ºC. The lower the specific heat, the more its temperature rises when a given amount of energy is absorbed by a given mass. Specific heat is measured in joules per gram per degree Celsius Specific heat is the heat needed to raise the temperature of 1 gram of material by 1°C.
Q = mc Δ T Q = heat absorbed by the material m = mass c = specific heat Δ T = change in temperature Ex: An iron skillet has a mass of 500 g. The specific heat = J/g ·ºC. How much heat must be absorbed to raise the temperature by 95 ºC?
1.How much heat is needed to raise the temperature of g of water by 85.0°C? 2.How much heat is absorbed by a 750-g iron skillet when its temperature rises from 25°C to 125°C? 3.In setting up an aquarium, the heater transfers 1200 kJ of heat to 75,000 g of water. What is the increase in the water's temperature? (Hint: Rearrange the specific heat formula to solve for Δ T.) 4.To release a diamond from its setting, a jeweler heats a 10.0-g silver ring by adding 23.5 J of heat. How much does the temperature of the silver increase? 5.What mass of water will change its temperature by 3.0°C when 525 J of heat is added to it?
Calorimeter- an instrument used to measure changes in thermal energy A calorimeter uses the principle that heat flows from a hotter object to a colder object until both reach the same temperature A sample to be tested is heated and placed in the calorimeter. The lid is put on and the temperature change is observed.
There are three ways heat can be transferred. 1. Conduction- The transfer of thermal energy with no overall transfer of matter. Conduction can occur within a material or between materials that are touching. Ex: Newtons Cradle
Conduction in gases is slower than in liquids and solids because the particles in a gas collide less often. In metals, conduction is faster due to the sea of electrons Thermal Conductor- A material that conducts thermal energy well. Ex: wire racks, copper, aluminum. The arrows show how thermal energy is conducted away from the heat source in a metal frying pan.
A thermal conductor does not have to be hot. Tile floors feel colder than wood floors even though they are at the same temperature. Tile floors feel colder because it is a better conductor. (It rapidly pulls the heat away from your foot.) Thermal Insulator- A material that conducts energy poorly. Ex: Wood and air. (double paned windows have a pocket of air.)
2. Convection- The transfer of thermal energy when particles of a fluid move from one place to another. Convection Current- Occurs when a fluid circulates in a loop as it alternately heats up and cools down. As air heats up, it expands and rises. As it rises, it begins to cool. As it cools, it condenses and sinks. A Passing sandbags along a line is like transferring thermal energy by convection. B The arrows show convection of air in an oven.
3. Radiation-The transfer of energy by waves moving through space. Ex: Heat lamps used in restaurants. All objects radiate energy. As an objects temperature increases, the rate at which it radiates energy increases. When you go to the beach, heat from the sun warms you by radiation. A heating coil on a stove radiates thermal energy. The changing color of the red arrows indicates that the farther you are from the coil, the less radiation you receive
Thermodynamics- The study of conversions between thermal energy and other forms of energy. There are three laws. 1 st Law of Thermodynamics The first law of thermodynamics states that energy is conserved. (Remember the law of conservation of energy?)
2 nd Law of Thermodynamics The second law of thermodynamics states that thermal energy can flow from colder objects to hotter objects only if work is done on the system. This is how refrigerators work. Remember, heat is supposed to flow from hot to cold. A refrigerator has to work hard to oppose this. Heat Engine- any device that converts heat into work. Waste Heat- thermal energy that is not converted into work. (lost to friction)
3 rd Law of Thermodynamics The third law of thermodynamics states that absolute zero cannot be reached. This physicist is adjusting a laser used to cool rubidium atoms to 3 billionths of a kelvin above absolute zero. This record low temperature was produced by a team of scientists at the National Institute of Standards and Technology