Systems and Surroundings

Transcript

0:00Introduction

Hello everyone! Picture a pressure cooker on the flame, lid sealed, heating up. Then the whistle blows, and steam rushes out. Same cooker, same flame. So, is it the same kind of system in both moments? Hmm. Keep that question in mind; by the end of this video you will answer it in two seconds. And look, every thermodynamics problem starts with one question: what is my system? And JEE does ask questions on exactly this. So, let's begin.

0:31System, surroundings and boundary

So, first things first. We cannot study the whole universe at once, right? So we choose one part of it to study, and that part is called the system. Just the part we are interested in. Okay? Now, everything else in the universe, everything outside the system, is called the surroundings. So the system and its surroundings together make up the whole universe. Clear? Between them is the boundary, also called the wall. It can be real, like the side of a container, or imaginary, a surface we draw in our mind. Either way, it separates the two. Now, a quick test. Suppose a reaction is carried out in a beaker standing on the bench. What is the system here? Yes, exactly! The reaction mixture. That is the part we are actually studying, right? And careful! Many students put the beaker inside the system. No! The beaker belongs to the surroundings, with the air and the rest of the laboratory. The boundary is the surface of the liquid and the inner wall of the beaker. Got it?

1:30Open, closed and isolated systems

Now, two things can cross this boundary: matter and energy. Look at the figure. The beaker on the left has no lid: the arrow going up is matter leaving, and the two-headed arrow at the side is energy, going in or out. Both cross, so it is an open system: it exchanges matter and energy. Okay? In the middle, the same beaker with a lid. Matter cannot get out now, but see the energy arrow? Energy still passes through the walls. So, energy only, no matter: a closed system. Oh no, careful! Closed does not mean nothing crosses. It blocks matter, not energy. Right? And on the right, the beaker sits inside a thick grey box, and there are no arrows at all. Neither matter nor energy crosses. That is an isolated system. Clear? So here is a simple trick: two questions, in this order. First, does matter cross? If yes, open. If not, does energy cross? If yes, closed; if no, isolated. Two questions, three answers. Really simple! Now, back to our pressure cooker. First sealed and heating up, then whistling, with steam rushing out. Use the two questions. What kind of system is its content in each case? Sealed and heating: no matter leaves, but heat from the flame comes in. So, closed. Whistling: steam is matter, and it is leaving. So, open! See? Same cooker, two answers. It depends on what crosses the boundary, not on the object. Got it?

3:01Kinds of wall

Now, the wall itself decides what can cross. Diathermic wall: it lets heat pass, like the walls of our closed beaker. Adiabatic wall: the opposite, no heat passes at all. So remember, adiabatic means no heat. Okay? Then, the rigid wall: a wall that cannot move. The P dash V on screen means pressure volume work, the work done when the volume changes. The wall cannot move, so the volume cannot change, and there is no P V work. Right? So, tell me. Which kind of wall makes a process adiabatic? And which makes it isochoric, that is, at constant volume? Correct! Adiabatic: an insulating wall, so no heat crosses. Isochoric: a rigid wall, since a wall that cannot move keeps the volume fixed. Very good!

3:55State, equilibrium and the isolated system

Next, the state of a system is fixed by its state variables: capital P for pressure, capital V for volume, capital T for temperature, and small n for the amount, in moles. Quick one. For one mole of an ideal gas, how many of P, V and T must I give you to fix its state? Two! Very good. Because P V equals n R T, where R is the gas constant. Give me any two, and this equation gives the third. That holds for a fixed amount of an ideal gas. Got it? Now, thermodynamic equilibrium needs three kinds of equilibrium at once. Thermal: the temperature T is uniform. Mechanical: the pressure P is uniform. Chemical: the composition is fixed; it does not change with time. Miss even one, and it is not in equilibrium. Clear? And the isolated system again. Nothing crosses, so small q, the heat, equals zero, and small w, the work, equals zero. So delta U, the change in internal energy, equals zero, always. That small triangle is the Greek letter delta, meaning change, okay? Its internal energy stays constant. Now, a beautiful question. Think of the whole universe as one system. Is it open, closed or isolated? Isolated! Yes, exactly. There is nothing outside the universe, so it has no surroundings to exchange matter or energy with. Nice, isn't it?

5:24Example 1: classify a coffee cup, can, thermos and body

Alright, Example one. Classify each as open, closed or isolated. Part a, hot coffee in an open cup. Part b, an unopened can of a soft drink. Part c, coffee in an ideal stoppered thermos. Part d, the human body. Keep the three pictures in front of you, pause the video, and try it. Done? Okay, let's check. Hot coffee in an open cup: steam rises into the air, and that is matter crossing the boundary. First question, first yes. So, open, right? The unopened can: does matter cross? No, it is sealed. Does energy cross? Yes, it cools in a fridge and warms on a hot day, so heat passes through the metal. Energy only: closed. With me? Now part c, the stoppered thermos. It is sealed with a stopper, nothing gets out, so it is closed, right? No, no, careful! Read the word ideal. An ideal thermos also stops heat; that is its whole point. Neither matter nor energy crosses: isolated. One word changed the answer, okay? And part d, the human body. We eat, we drink, we breathe in and out: matter crosses all the time. First question, first yes: open! So the answers are open, closed, isolated and open. Perfect!

6:48Example 2: bomb calorimeter, closed or isolated

Now a harder one, JEE style. Example two. Here, a sample burns in a sealed steel bomb, immersed in water inside an insulated jacket. Classify, part a, the contents of the bomb, and part b, the bomb together with the water and jacket. Pause and try. Hint: draw the boundary first. Done? Let's check. Part a first. The bomb is sealed steel. Nothing can get in or out. So it is isolated, right? No, no, wait! The sample is burning and giving out heat, and that heat passes through the steel into the water. Matter stays in, but energy goes out. So the contents of the bomb form a closed system. Same trap as the thermos, just the other way round. Clear? Now part b. Move the boundary outwards: bomb plus water plus jacket. The jacket is insulated, so heat cannot escape, and no matter crosses either. So this bigger system is isolated, to a good approximation, since no real insulation is perfect. See? Same experiment, two answers. It all depends on where you draw the boundary. Got it?

7:57Quick revision: open, closed, isolated and kinds of wall

So, quick revision. Open: matter and energy. Closed: energy only. Isolated: neither. Adiabatic wall: no heat. Rigid wall: no P V work. Isolated system: q equals zero, w equals zero, so delta U equals zero. And always fix the system and its boundary first. Now go and try the four practice problems on this page. You will find them easy. All the best!