Princeton Ch 7 - Thermodynamics Flashcards
Define work in terms of energy transfter.
Work is a transfer of mechanical energy into or out of a system, from or to the environment.
System.
The objects or objects under examination.
Environment.
The environment is just the other objects and external forces outside of the system. The environment may or may not interact with the system.
Closed system.
The environment cannot contribute matter to it.
Isolated system.
The environment cannot contribute matter or energy to it.
Open system.
An open system is free to interact with the environment.
Temperature.
Temperature(T) is the macroscopic measure of thermal energy per molecule. Absolute temperature is measured in Kelvins.
Heat (Q).
The transfer of thermal energy between a system and its environment. Measured in joules.
Differences between temperature and heat.
Units (K versus Joules). Temperature is an intensive property and thermal energy is an extensive one. Temperature (like density) does not depend on the amount of material present but thermal energy does. A block of stone cut in half with temp of 300K is still 300K. But if it had 20,000 J of thermal E, it is now cut in half.
Zeroth law of thermodynamics.
If one object is in thermal eq with a second, and the second object is in thermal eq with a third object, then the first and third objects are in thermal eq with each other. The two bodies are in such a way that heat is free to pass between them but the same amount of heat passes each way.
If we measured the temperature of two objects, a rod and wooden stick to be the same, then if we put the two in contact, how much heat would be transferred between the two?
No NET heat would be transferred between them.
Heat transfer is the movement of thermal energy from one point to another. There are three mechs by which this is acheived. Name them.
Conduction, convection, radiation.
You touch a hot stove and burn your hand. Que pasa? What kind of heat transfer is this?
Conduction is the transfer of heat by microscopic collisions of particles and movement of electrons within a body. The highly agitated atoms of the hot skillet bump into the atoms of your hand, making them move more rapidly, thus heating up your hand.
Convection.
The movement caused within a fluid (liquid or gas) by the tendency of hotter and therefore less dense material to rise, and colder, denser material to sink under the influence of gravity, which consequently results in the transfer of heat. A convection current is formed, and the fluid is heated completely.
In convection, what happens to fluids near the heat source.
They gain energy, become less dense, and rise.
Radiation.
Absorption of energy carried by electromagnetic waves (like radiant energy from the sun’s fusion reactions). Remember sunlight on your face warms your skin.
True or false. Most materials expand as their temperature increases.
True. Moving particles = less dense.
First law of thermodynamics.
The total energy of the universe is constant. Energy may be transformed from one form to another, but it cannot be created or destroyed.
The equation for the first law of thermodynamics and why we should pay attention to signs.
ΔE = Q - W. Q is positive when heat is moving into the sys and neg when it’s coming out. W is positive when the work is being done by the sys on the environment. W is negative when work is being done by the environment on the system. Sometimes you’ll see ΔE = Q + W. Just note when its positive or negative.
When we talk about an ideal gas in a sealed container with a piston on top, typically, KE = PE = __, and the only form of energy present is ___.
When we talk about an ideal gas in a sealed container with a piston on top, typically, KE = PE = 0, and the only form of energy present is THERMAL E. We use E(internal) = E(thermal) to indicate the total E of the system. In those systems, we have ΔE(system) = Q(net into the sys) - W (net by the system).
In a piston, ΔE(internal) is (greater/less/proportional) to the object’s absolute temperature.
Proportional.
Assume we have a metal container with a pin that prevents the piston from moving. We heat the container. What happens to ΔE(internal)? Q?
Since the piston doesn’t move, no work is done, but energy is transferred as heat. ΔE = Q, and Q is positive. W = 0.
Assume we have an metal container with a pin that prevents the piston from moving. We heat the container and then let it cool at room temp. Describe ΔE(internal) for this process of cooling down.
The hot cylinder and gas will cool down as they lose heat to the room, until they’re at room temp again. ΔE = Q, and Q is negative. W=0. Energy is lost form the system in the form of heat.
Assume we have an metal container with a movable piston. We heat the container and then add insulation so no heat is lost. Describe what happens in terms of work.
If we heat the container, the piston will move up. When the weight on our piston moves up, it gains PE (mgh, h gets bigger) from work being done by the gas (the system). If piston is insulated, such that no heat (Q) can go in or out, ΔE(internal) = -W; W >0. Energy is lost from our sys in the form of doing work.
