First Law of Thermodynamics

Please hand in your Labs at the front of room

Take out a piece of paper

No need for notes!

Thermodynamic Process

A thermodynamic process is one in which heat is added to or taken away from a system. ΔQ = change in heat

 Any system has some internal energy (ΔU) state. The energy state is defined by the systems temperature (T), volume (V), and pressure (P)

Laws of Thermodynamics…. kind of….

Zeroth Law of Thermodynamics

Zeroth: Two systems are at thermal equilibrium if they have the same temperature.

The significant consequence of the Zeroth Law is that, when a hotter object and a colder object are placed in contact with one another, heat will flow from the hotter object to the colder object until they are in thermal equilibrium.

First Law of Thermodynamics

First: also known as Law of Conservation of Energy, states that energy can not be created or destroyed; it can only be redistributed or changed from one form to another.

ΔU = Q + W

ΔU is the change in internal energy of the system, Q is the heat added to the system, and W is the work added to the system

Q is positive when it is added to the system and negative if it is taken out of the system.

Output Work (done on other systems) is negative

Input Work (done on the system) is positive

Sign Convention

ΔU = Q + W

+Q when heat is added to the system. - Q when heat is lost from the system. +W when work is done on the system. - W when work is done by the system.

Second Law of Thermodynamics

Second: The entropy of a closed system never decreases as time goes by. Reversible processes do not change the entropy of a system, while irreversible processes increase a system’s entropy.

S ≥ 0

S = Entropy of a System

When we discuss these Laws think of….

Zeroth: Temperature Exists and therefore you must play the game.

Heat will flow from hot to cold always and forever.

First: You can't win.

You can't get more energy out of a heat engine than you put in.

Second: You can't break even.

No heat engine can use all the heat produced by a fuel to do work.

First Law Question

In a thermodynamic process, a system absorbs 450 kJ of heat and does 87 kJ of work on its surroundings. By what amount did the system’s internal energy change?

ΔU = Q + W

First Law Question

In a thermodynamic process, a system absorbs 450 kJ of heat and does 87 kJ of work on its surroundings. By what amount did the system’s internal energy change?

ΔU = Q + W

Note that the work done by the system ON the surroundings is negative.

First Law in a “Nutshell”

Here is a diagram of a generic thermodynamic process.

A system has an initial internal energy.

Heat (Q): can be added to the system, or taken away from the system or there may be no heat transfer.

Work (W): may be done on the system or the system may do work or there may be no work involved at all.

In tern a l E n ergyU 1

T 1 P 1 V 1

In tern a l E n ergyU 2

T 2 P 2 V 2

T h erm odyn am icP rocess

Q

W

For everything we are about to do….Always keep in mind the Ideal Gas Law…

PV = nRT

Isobaric Process

Greek for…

“Iso” – refers to something remaining constant

“Baric” refers to pressure

Collect and read the handout from the front and read the

The 4 “`Processes” ~ cases of the First Law

Isothermal Process – thermodynamic process in which the temperature of the system remains constant. • Occurs often in heat reservoirs (surrounds can supply or soak up excess heat)

Adiabatic Process – thermodynamic process in which no heat is exchanged between the system and its environment. • Occurs rapidly or when well insulated (think a thermos!)

Isobaric Process – thermodynamic process in which the pressure of the system remains constant.

Isochoric Process – thermodynamic process in which the volume of the system remains constant.

Please collect a Whiteboard and Pens and draw the following!

PV = nRT PV = nRT

For the following P vs. V diagram… determine which of the Special Cases applies

A. Isobaric: pressure is constant

Is the 1st Law Equation affected by an Isobaric process?

No!

W = PΔV

Work can be done if the volume of the system undergoes a change.

P

V

A.

Video

For the following P vs. V diagram… determine which of the Special Cases applies

B. Isochoric: Volume is constant (taking up the same space)

How does the 1st Law Equation change during an Isochoric process?

In this type of process, the volume stays constant. This means that ΔV is zero. If ΔV is zero, then the work must also be zero.

P

V

B.

For the following P vs. V diagram is an Adiabatic Process

C. Adiabatic Process: no heat is exchanged between the system an its environment

How does the 1st Law Equation change during an Adiabatic process?

ΔQ = 0

P

V

C.

For the following P vs. V diagram is an Isothermal Process

D. Isothermal Process: Temperature is constant

How does the 1st Law Equation change during an Isothermal process?

The isothermal process happens at a constant temperature. The pressure and volume change, so work is done, but ΔU is zero.

D.

P

V

PV = nRT = constant!

Summary of Processes

What now?

1. Collect Worksheet for 1st Law of Thermodynamics – Key Posted

2. Watch the notes for the four species cases of the 1st Law!

3. Next Class: Heat Engines