The internal energy of a substance can be changed in different ways. Work can transfer energy to a substance and increase its internal energy.

Heat can be lost by the substance, which results in a decrease of internal energy.

Energy can also be transferred to the substance as heat and from the substance as work.

Energy is added to substances or groups of substances. It is also removed from these substances. Such a substance or combination of substances is called a system.

The surroundings with which the system interacts is called the environment.

Work done on or by a gas is the pressure multiplied by the change in volume.

W = P∆V

An engine cylinder has a cross-sectional area of 0.010 m2. How much work can be done if a gas exerts a constant pressure of 7.5 x 105 Pa and moves the piston 0.040 m?

We have been discussing three quantities and how they relate to each other: internal energy (U), heat (Q), and work (W). The study of how these relate is called thermodynamics.

We can simplify our discussion by considering situations where one of these properties (U, Q, or W) does not change.

No work is done in a constant volume process.

W = P∆VSuch processes are isovolumetric.

From PV/T, a change in P without a change in V requires a change in T.

When the temp of a gas changes without a change in volume, no work is done on or by the system.

(Energy must be added to or taken from the system by some other means.)

Internal energy is constant in a constant temperature process (isothermal). This remains constant even as energy is transferred to or from the system as heat or work.

When heat energy is not transferred in a process it is called adiabatic. Any change in internal energy is due to work done on or by the system.

The first law of thermodynamics states that any change in internal energy is equal to the energy transferred to or from the system as heat and the energy transferred to or from the system as work.

∆U = Q + W

This is another statement of the law of conservation of energy. In this case, it is not just mechanical energy that is conserved, but all the energy of a system.

∆U = Q + W

Q is positive if heat is added to a system.Q is negative is heat is removed from a system.

∆U = Q + WW is positive if work is done on a system (gas compression).W is negative if work is done by a system (gas expansion).

Important Note: The signs for “work done on the system (+)” and “work done by the system (-)” are the opposite listed in your book to reflect changes in the AP test.

A gas in a cylinder with a movable piston is submerged in ice water. The initial temperature of the gas is 0°C. 1200 J of work is done by a force that slowly pushes the piston inward. A) Is this process isothermal, adiabatic, or isovolumetric?B) How much energy is transferred as heat?

A total of 135 J of work is done on a gas through compression. If the internal energy of the gas increases by 114 J, what is the total amount of energy transferred as heat? Has energy been added to or removed from the gas as heat?

A refrigerator does work to create a difference in temperature between its closed interior and its environment. This is a cyclic process and the change in internal energy of a system is zero in a cyclic process.

In a cyclic process:∆Unet = 0 and Qnet = Wnet

The difference in the transfer of heat from the system ,Qh, and the transfer of heat to the system ,Qc, is equal to Wnet: Qh - Qc = Wnet.

A refrigerator uses work to remove heat from the system. A heat engine does the opposite, it uses heat to do mechanical work.

It is still a cyclic process, and the formulas and relationships are still the same: ∆Unet = 0 and Qnet = Wnet

Qh - Qc = Wnet

The second law of thermodynamics states that no cyclic process that converts heat entirely into work is possible.

W can never be equal to Qh, there must always be a value Qc > 0 indicating a loss of heat to the environment.

The efficiency of a thermodynamic system is calculated using these equations:

eff = Wnet/Qh eff = (Qh - Qc)/Qh

eff = 1 - Qc/Qh

A heat engine can only be 100% efficient if no energy is removed as heat Qc = 0.Engines are most efficient if Qh is high and/or Qc is low.

Find the efficiency of a gasoline engine that, during one cycle, receives 204 J of energy from combustion and loses 153 J as heat to the exhaust.

In thermodynamics, a system tends to change from a very ordered set of energies to one where there is less order. The measure of a system’s disorder is called entropy.

Systems with maximum disorder are favored.Greater disorder means there is less energy available to do work.

The second law of thermodynamics, (No cyclic process that converts heat entirely into work is possible.), can be stated in terms of entropy: The entropy of the universe increases in all natural processes.

Entropy can increase or decrease within a system. Entropy can decrease for parts of a system as long as this decrease is offset by a greater increase in entropy elsewhere in the universe.