Chapter 16 Thermal Energy and Heat
Section 1 Thermal Energy and Matter
Key Concepts In what direction does heat flow spontaneously? What is the temperature of an object related to? What two variables is thermal energy related to? What causes thermal expansion? How is a change in temperature related to specific heat? On what principle does a calorimeter operate?
In the 1700s, most scientists thought heat was a fluid called caloric that flowed between objects. In 1798, the American-born scientist Benjamin Thompson (1753–1814), also known as Count Rumford, challenged this concept of heat. Rumford managed a factory that made cannons.
Work and Heat Heat is the transfer of thermal energy from one object to another because of a temperature difference. Heat flows spontaneously from hot objects to cold objects.
Temperature Temperature is a measure of how hot or cold an object is compared to a reference point. Temperature is related to the average kinetic energy of the particles in an object due to their random motions through space. As an object heats up, its particles move faster, on average. As a result, the average kinetic energy of the particles, and the temperature, must increase. Thermal energy depends on mass and temperature. The tea is at a higher temperature than the lemonade because its particles have a higher average kinetic energy. The lemonade is at a lower temperature, but it has more thermal energy because it has many more particles.
Thermal Energy Thermal energy depends on the mass, temperature, and phase (solid, liquid, or gas) of an object. Thermal energy, unlike temperature, depends on mass. Suppose you compare a cup of tea and a teapot full of tea. Both are at the same temperature, so the average kinetic energy of the particles is the same in both containers. However, there is more thermal energy in the teapot because it contains more particles.Suppose you compare a cup of tea and a teapot full of tea.
Thermal Contraction and Expansion Thermal expansion is an increase in the volume of a material due to a temperature increase. Thermal expansion occurs when particles of matter move farther apart as temperature increases. Gases expand more than liquids and liquids usually expand more than solids.
Specific Heat Specific heat is the amount of heat needed to raise the temperature of one gram of a material by one degree Celsius. The lower a material’s specific heat, the more its temperature rises when a given amount of energy is absorbed by a given mass. Specific heat is often measured in joules per gram per degree Celsius, or J/gC
Specific Heat Specific Heats of Selected Materials Material (at 100 kPa)Specific Heat (J/gC) Water Plastic (polypropylene) Air Iron Silver –
Specific Heat The heat (Q) absorbed by a material equals the product of the mass (m), the specific heat (c), and the change in temperature (ΔT). Formula: Q = m x c x ΔT
How much heat is needed to raise the temperature of g of water by 85.0°C?
How much heat is absorbed by a 750-g iron skillet when its temperature rises from 25°C to 125°C?
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 ΔT.)
Measuring Heat Changes A calorimeter is 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 calorimeter is used to measure specific heat. A sample to be tested is heated and placed in the calorimeter. The lid is put on and the temperature change is observed. Stirrer Thermometer Lid Water Aluminum sample
Reviewing Concepts 1. In what direction does heat flow on its own spontaneously? 2. How is the temperature of an object related to the average kinetic energy of its particles? 3. Name two variables that affect the thermal energy of an object. 4. What causes thermal expansion of an object when it is heated? 5. How do the temperature increases of different materials depend on their specific heats? 6. What principle explains how a calorimeter is used to measure the specific heat of a sample material?