Lecture 33 Temperature and Thermal Energy Ozgur Unal NIS – PHYSICAL SCIENCE Lecture 33 Temperature and Thermal Energy Ozgur Unal

Thermal Energy Remember a form of energy, thermal energy, we briefly mentioned few chapters ago. Do you think the gases in the air are moving? Or, are they suspended in the air? How about the particles (atoms or molecules) in a liquid and solid?

Kinetic Theory of Matter Particles (atoms and molecules) in solids, liquids and gases are randomly moving around. The motion of these particles are described by the kinetic theory. According to the kinetic theory, the particles also collide with each other. Since these particles are moving, they have kinetic energy. As the particles collide, the kinetic energy can be transferred from one particle to another particle. http://www.chm.davidson.edu/vce/kineticmoleculartheory/basicconcepts.html http://preparatorychemistry.com/KMT.html

Temperature How does the temperature of an object change if you give energy to it (heating, leaving under the Sun etc.)? How do you think the energies of the particles change if you increase the temperature of an object? The temperature of a substance is a measure of the average kinetic energy of its particles. As the average speed of the particles in random motion increases, the temperature of the substance increases.

Temperature The SI unit of temperature is Kelvin (K). Celsius scale (C) and Fahrenheit (F) scale are commonly used. Conversion equations: Fahrenheit to Celsius  oC = 5*( oF – 32)/9 Celsius to Fahrenheit  oF = 9*oC/5 + 32 Celsius to Kelvin  oC = K – 273 Example: What is a temperature of -40 oF in degrees Celsius? Example: What is a temperature of 300 K in degrees Fahrenheit?

Thermal Energy Particles in an object do not only have kinetic energy, but also potential energy. This potential energy is due to the interaction of particles. The potential energy of particles increase as they get farther apart  Figure 2 on page 256. The sum of kinetic and potential energies of all the particles in an object is the thermal energy of the object.

Thermal Energy Which ball has more thermal energy: A ball at rest at 300 K or a rolling ball at 300 K? Thermal energy is the sum of the potential and kinetic energies of the particles in the ball. Since the balls have the same temperature, their thermal energies are the same. The rolling ball’s kinetic energy is NOT included in thermal energy.

Heat What is heat? Heat is the thermal energy that flows from something at a higher temperature to something at a lower temperature. Heat is a form of energy  Unit of heat is Joules. Heat flows from hot objects to cold objects. State in which direction heat flows for the following examples: 1- An object at 10 K in contact with another object at 20 K. 2- Ice in a cup of liquid water.

Lecture 34 Specific Heat Ozgur Unal NIS – PHYSICAL SCIENCE Lecture 34 Specific Heat Ozgur Unal

Specific Heat Have you ever noticed that some materials heat up faster than other materials? Can you provide some examples? In a hot summer day, although the sea and the sand gets equal amount of sunlight, sand gets hotter than sea.

Specific Heat As a substance is heated, its temperature change depends on the amount of thermal energy added and the nature of the substance. Example: For an equal amount of water and iron, it takes about 6 times more thermal energy to increase the temperature equal amounts. The amount of thermal energy needed to raise the temperature of 1 kg of some material by 1 K is called the specific heat. The unit of specific heat is J/(kg*K) Check out Table 1!

change in thermal energy = mass*change in temperature* specific heat For a given amount of substance, if you increase its thermal energy, how does the temperature change? For a given amount of thermal energy, if you increase the mass, how does the temperature change? change in thermal energy = mass*change in temperature* specific heat Q = m*(Tf - Ti)*C Q = Change in thermal energy m = mass Tf = Final temperature Tf = Initial temperature C = Specific heat

Specific Heat Example: Find the change in thermal energy of a 20-kg wooden chair that warms from 15 K to 25 K, if the specific heat of wood is 1,700 J/(kg*K). Example: The air in a living room has a mass of 72 kg and a specific heat of 1,010 J/(kg*K). What is the change in thermal energy of the air when it warms from 20 oC to 25 oC?

Measuring Specific Heat In order to find the specific heat of an object, C, we must know Q, m and ΔT. Specific heat is measured using a device called calorimeter. In a calorimeter, a heated sample of substance transfers thermal energy to a known mass of water. The energy absorbed by water can be calculated by measuring the water’s temperature. The thermal energy released by the sample is equal to the thermal energy absorbed by the water.

Activity Measuring Specific Heat of Water Develop a procedure to find the specific heat of water. Use microwave to heat up the water.