Thermal Properties of Materials Specific Heat Capacity Specific Latent Heat Internal Energy First Law of Thermodynamics

Quantifying Heat Heat and Temperature Equation Specific Heat Capacity Change of Phase Equation Latent Heat

Kinetic-Molecular Theory Postulates Matter is made of particles containing mass. Particles of substances are in constant, rapid, and random motion. Collisions are either elastic. Temperature measures average kinetic energy. Particles exert intermolecular forces. These forces are “non-existent” in gases. These forces are strong in liquids and solids.

Specific Heat Capacity

Latent Heat

Questions Which is greater, an increase in temperature of 1C or an increase of 1F? If you drop a hot rock into a pail of water, the temperature of the rock and water will change until both are equal. The rock will cool and the water will warm. Does this hold true if the rock is dropped in the Pacific Ocean? Explain.

Questions Adding the same amount of heat to two different objects does not necessarily produce the same increase in temperature. Why not? Why will watermelon stay cool for a longer time than sandwiches when both are removed from a cooler on a hot day? The desert sand is very hot in the day and very cool at night. What does this tell you about its specific heat?

Examples 135 g of aluminum (initially at 400oC) are mixed with an unknown mass of water (initially at 25oC). When thermal equilibrium is reached, the system has a temperature of 80oC. Find the mass of the water.

Examples In the lab, an experimenter mixes 75.0 g of water (initially at 30oC) with 83.8 g of a solid metal (initially at 600oC). At thermal equilibrium, he measures a final temperature of 50.0oC. What metal did the experimenter probably use?

Examples A 38 g sample of ice at –11oC is placed into 214 g of water at 56oC. Find the system’s final temperature.

Examples 25 g of 116oC steam are bubbled into 0.2384 kg of water at 8oC. Find the final temperature of the system.

Measuring and Determining

Kinetic Theory of Ideal Gases Point masses The gas consists of very small particles with non-zero mass The average distance separating the gas particles is large compared to their size. No intermolecular forces They exert no forces on one another except during collisions. Random motion The rapidly moving particles constantly collide with the walls of the container.

Kinetic Theory of Ideal Gases Elastic collisions There is no change in potential energy since intermolecular forces are negligible (recall gravitation) The average kinetic energy of the gas particles depends only on the temperature of the system.

Energy Mechanical Energy: KE, PE, SE Work is done by energy transfer. Heat is another form of energy. Need to expand the conservation of energy principle to accommodate thermal systems.

First Law of Thermodynamics Energy cannot be created or destroyed but can only be transformed from one form to another Conservation of energy The heat energy supplied to the system increases the internal energy Or enables it to do work Or both Equation

Internal Energy Sum of the kinetic and potential energies of a system or substance.

1st Law of Thermodynamics Consider an example system of a piston and cylinder with an enclosed dilute gas.

1st Law of Thermodynamics What happens to the gas if the piston is moved inwards?

1st Law of Thermodynamics If the container is insulated the temperature will rise, the atoms move faster and the pressure rises. Is there more internal energy in the gas?

1st Law of Thermodynamics External agent did work in pushing the piston inward. W = Fd =(PA)Δx W =PΔV Dx

1st Law of Thermodynamics Work done on the gas equals the change in the gases internal energy, W = ΔU Dx

1st Law of TD Let’s change the situation: Keep the piston fixed at its original location. Place the cylinder on a hot plate. What happens to gas?

Heat flows into the gas. Atoms move faster, internal energy increases. Q = ΔU

1st Law of TD F What if we added heat and pushed the piston in at the same time?

Examples