Thermal energy!

• matter is made up of atoms or molecules which are in continual random motion and so have kinetic and potential energy (KMT theory)

• thermal energy of an object is the sum of the kinetic energy and potential energy associated with the random motion of the particles in the object

Temperature

• temperature is a measure of how hot or cold an object is

  • is the property that determines whether or not an object is in thermal equilibrium with other objects

• subjective to touch, objective with scientific measurement

• a hot object has a high temperature, with faster moving particles and have a higher thermal energy compared to a cold object

• thermometers are used to measure the temperature of an object

  • infrared, alcohol, thermacouple

• SI unit of temperature is kelvin (K)

• thermal equilibrium is when two objects at different temperature are placed in thermal contact and thermal energy passes from the object at the higher temperature to the object at the lower temperature and the two objects reach the same temperature

Temperature scales

• temperature can be measured on the Celsius, Fahrenheit, or Kelvin scales

• two standard points on each scale are the freezing point and the boiling point of water

  • freezing point: 0°C, 32°F, 273.15K

  • boiling point: 100°C, 212°F, 373.15K

• the size of a degree on the Celsius scale is larger than that on the Fahrenheit scale by a factor of 9/5

• the size of a degree on the Celsius scale is equal to that on the Kelvin scale

• the lowest temperature attainable is absolute zero (0 K) which is the basis for the Kelvin scale

• at absolute zero, the kinetic energy of molecules is minimal - molecules have stopped moving and have no kinetic energy

  • 0 K = -273.15°C = -459.67°F

Heat

• heat is the transfer of energy from one object to another due to a temperature difference between them

• Si unit for heat is Joule (J)

• heat flows from an object at a higher temperature to an object at a lower temperature

• when it flows within an isolated system, the thermal energy of the hot temperature decreases and the thermal energy of the cold object increases

• heat can be specified in calories or kilocalories

• one calorie is the amount of heat needed to raise the temperature of 1 g of water by 1°C

  • 1cal = 4.186 J

  • 1kcal = 1 Cal = 4186 J = heat needed to raise 1 kg of water by 1°C

Specific heat

• when energy is added to an object, the temperature of the object usually rises

• the amount of energy required to raise the temperature of a given mass of an object by some degrees varies from one object to another

• the specific heat of an object is the amount of energy required to change the temperature of unit mass of the object by 1°C

• as the specific heat of an object is greater, more energy is required to a given mass of the object to cause a particular temperature change

Conservation of energy: calorimetry

• when objects at different temperatures are placed in contact, energy is exchanged between them and the transfer of energy continues until thermal equilibrium is reached

• calorimetry is the quantitative measurement of energy exchange

• calorimeter is a device that is used to make such measurements

• when heat flows within an isolated system, he heat gained by one part of the system is equal to the heat lost by the other part of the system by the conservation of energy

• hot objects will lose thermal energy and have negative heat flow - cold will gain and have positive

• start @ the same temp = no net heat flow

Phase change of matter

• when energy is added or removed, matter can change from one phase to another without a change in temperature - the energy goes to breaking bonds between atoms and molecules so they can slide past each other without change in temp.

• every phase change involves a change in thermal energy

• when heat is added, a solid can melt into a liquid or sublime into a gas or a liquid can evaporate into a gas

• when heat is removed, a liquid can freeze into a solid, and a gas can condense into a liquid or deposit into a solid

Latent heat

• latent heat is the heat per kilogram that must be required or given off when an object changes from one phase to another without a change in temperature

• latent heat depends on the nature of the phase change and on the object

• the latent heat of fusion is when an object goes from solid to liquid or heat given off in liquid to solid

• the latent heat of vaporization is when a liquid goes to vapour and is equal to the heat given off during the change of vapour to liquid

Heat transfer

• heat is transferred from one object to another in three different processes of conduction, convection, and radiation

• any two or all three processes may operate at the same time in practical situations

Conduction

• conduction is direct heat transfer through physical contact and involves energy exchange through molecular collisions

• occurs only if there is a difference in temperature

• the rate of heat transfer through a substance is proportional to the cross-sectional area (A) and to the temperature difference

• substances with large thermal conductivities such as most metals conduct heat well and are good conductors

• substances with small thermal conductivities such as wood and glass conduct heat poorly and are good insulators

Convection

• convection is heat transfer by the mass movement of molecules of a fluid over considerable distances

• natural convection occurs when the warmer, less dense part of a fluid is pushed upward by the buoyant force provided by the surrounding cooler and denser part

• forced convection occurs when an external device such as a fan or a pump causes the fluid to move

Radiation

• radiation is heat transfer by electromagnetic waves that can travel through empty space

• all objects radiate energy continuously in the form of electromagnetic waves

• the rate at which an object radiates temperature is proportional to the fourth power of its absolute temperature

• an object simultaneously emits energy by radiation and absorbs energy radiated by other objects

• the net rate of energy produced by an object as a result of radiation depends on the temperature difference with its surroundings

• stefan-boltzmann constant

• emissivity is largely determined by the colour of the object - black objects have higher rate of heat transfer by radiation than white or reflective objects