Lab7physics

LAB 7 Ideal Gases and Thermodynamics 7.1 Laboratory 7.1.1 Forces Exerted by a Gas In this laboratory you will make qualitative observations of a gas in a sealed system and explain your observations in terms of the individual molecules making up the gas using the Kinetic Theory of Gas. The simulation titled “Molecules in Motion” may help you to formulate your explanations. Kinetic Theory allows us to predict macroscopic, observable phenomena of gases, i.e. change in pressure and in temperature, by understanding the behaviors of many individual molecules of gas. The simplest model of a gas assumes each molecule is a particle and that the collisions between individual particles and between particles and the walls of the container are perfectly elastic. Gases modeled this way are called ideal gases and their behavior is described by the Ideal Gas Law: PV = NkT . Where P is the pressure of the gas, V is the volume of the gas, N is the number of molecules, k is Boltzmann’s constant, and T is the temperature of the gas. Temperature is a measure of the average kinetic energy of the particles in a gas and pressure is a measure of the force exerted on the walls of the container by the particles that collide with it. (You may be more familiar with the Ideal Gas Law in terms of the number of moles of gas, n, written as PV = nRT .) • The equipment for this laboratory includes a piston/syringe apparatus, rubber tubing, rubber stopper and a metal can. Pull the piston approximately half way out of the syringe then seal the system by holding your finger firmly over the opening in the rubber stopper (you will not be using the metal can in this experiment). Rubber hose 1. Consider the air in the entire system (syringe, rubber hose and rubber stopper). When you let go of the piston, while keeping the system sealed, which of the following properties change? Wherever possible, explain your answers in terms of what changes with the individual molecules of air that produces the macroscopic changes. Volume: Pressure: 137 stays the same increases because molecules become more compacted

7. I DEAL G ASES AND T HERMODYNAMICS Temperature: Number of air molecules: 2. Draw a free body diagram of the piston, after you have released it, but before you remove your finger from the opening of the rubber stopper. For each force, indicate 1) the type of force, 2) the object on which the force is exerted, and 3) the object exerting the force. Neglect friction between the piston and the syringe. 3. Write an equation that relates the magnitudes of all the forces in your diagram. Solve for the magnitude of the force exerted on the piston by the air inside the syringe. 4. Imagine that you hold the syringe upside down by the base while keeping the opening in the rubber stopper sealed with your finger without resting the syringe on the table. Do not carry out the experiment yet . Predict whether the piston will drop out of the syringe or remain inside. Explain your reasoning. 5. Check your prediction and describe your observations. 6. Draw a free-body diagram for the piston in the base-up position. For each force, indicate 1) the type of force, 2) the object on which the force is exerted, and 3) the object exerting the force. (Hint: gases can only push on an object; they cannot pull.) Neglect friction between the piston and the syringe. 7. Write an equation that relates the magnitudes of all the forces in your diagram. Solve for the magnitude of the force exerted on the piston by the air inside the syringe. 8. Compare the free-body diagrams for the piston in both the base-up and base-down positions. Is the magnitude of the force exerted by the air inside the syringe in the base-up position greater than, less than or equal to the magnitude of the force exerted by the air inside the syringe in the base-down position? 9. What does this suggest about the air pressure inside the syringe in the base-up position as compared to the base-down position? Explain your reasoning. 10. Explain the difference in the magnitude of the force exerted by the air in the syringe on the piston in the base-up and base-down positions in terms of the air molecules in the syringe in each case. 138 increases stays the same Fontside is the exerted Fatm= Atmosphenc force ↑ Finside air on tneplaton. Fg = force of gravity A late Fair= inside air Fatm-t= InM ma The syringe remains inside because there I no change in pressure, since the syringe is sealed. Nothing happens, our predictions are correct. it Fairin Fairont Ig = 0 Emitirout + Ey Pressure in base up is less than the base down. Pressure in base up is less than the base down. This suggests that force & pressure are directly related. Intrebase down position, gravity acts downward and pushes the molecules away from the piston so the molecules also exert the same amount of pressure to keep the platon from going down.

7. I DEAL G ASES AND T HERMODYNAMICS 2. Are the following quantities positive or negative? Be sure to explain your answers. Work done by the gas: Heat added to the gas: 3. Place a few small weights on top of the cylinder to slightly compress the gas inside. Consider the air in the entire system. Assuming that the system remains perfectly sealed, describe how each of the following changes (if at all). Wherever possible, explain your answers in terms of changes to the individual molecules of air that produces the macroscopic changes. Volume: Pressure: Temperature: 4. Are the following quantities positive or negative? Be sure to explain your answers. Work done by the gas: Heat added to the gas: 140 negative, piston moves down negative, neat transferred decreases, the syringe has less space in it increases because very tightly sealed staystmesame negative o, no change