Thermodynamics: First Law

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Questions and Answers

What is the sum of the changes in internal, kinetic, and potential energies equal to?

The net work done

The change in thermal energy

The net heat transfer

The total energy of the system (correct)

What is the unit of energy in the first law of thermodynamics written on a unit-mass basis?

kJ/m

kJ/kg (correct)

kJ/s

kJ/L

What is the difference between heat transfer and work from the first law point of view?

There is no difference (correct)

Heat transfer is energy out, work is energy in

Heat transfer is energy in, work is energy out

Heat transfer is kinetic energy, work is potential energy

What is the energy stored in chemical bonds called?

Chemical energy Signup and view all the answers

What is the energy associated with the motion of an object?

Kinetic energy Signup and view all the answers

What is the law of energy conservation also known as?

The first law of thermodynamics Signup and view all the answers

What is the first law of thermodynamics based on?

Experimental observations Signup and view all the answers

What is the sign convention for heat transfer in a system?

Heat transfer to a system is positive, and heat transfer from a system is negative Signup and view all the answers

What is the unit of heat transfer rate?

kW Signup and view all the answers

What is heat defined as, based on kinetic theory?

Energy associated with random motions of atoms and molecules Signup and view all the answers

What do both work and heat transfer have in common?

Both are recognized at the boundaries of the system Signup and view all the answers

What is the first law of thermodynamics also known as?

The conservation of energy principle Signup and view all the answers

What is the fundamental principle of physics stated by the Law of Conservation of Energy?

Energy can be converted from one form to another Signup and view all the answers

What was the main purpose of Joule's experiment?

To show that mechanical work can be converted into heat Signup and view all the answers

What is the relationship between heat and work?

Heat and work are two different forms of energy Signup and view all the answers

What is internal energy (U) in a thermodynamic system?

The sum of kinetic energy, potential energy, and thermal energy Signup and view all the answers

What is the equation for the work done on a system?

W = P dV Signup and view all the answers

What is the significance of the equation dU = dQ - P dV?

It proves that energy is a property of the system Signup and view all the answers

What is the primary factor that internal energy is a function of in ideal gases?

Temperature Signup and view all the answers

What is the definition of enthalpy?

The total energy of a system plus the energy associated with the pressure and volume of a system Signup and view all the answers

What is the relationship between the specific heat at constant volume and the specific heat at constant pressure?

Cv is always lower than Cp Signup and view all the answers

What is the change in enthalpy equal to in a constant pressure process?

The heat added to the system only Signup and view all the answers

What is the internal energy of a system a function of in non-ideal gases and other systems?

Temperature and other factors Signup and view all the answers

What is Cv related to in a thermodynamic system?

Changes in internal energy Signup and view all the answers

What is the relationship between specific heat at constant volume and specific heat at constant pressure for an ideal gas?

R = Cp - Cv Signup and view all the answers

What is the characteristic of a reversible adiabatic process?

Q = 0 and W ≠ 0 Signup and view all the answers

What is the condition for an ideal gas to follow the equation h = h(T)?

The internal energy is a function of temperature only Signup and view all the answers

What is the definition of enthalpy for an ideal gas?

h = u + Pv Signup and view all the answers

What is the characteristic of a constant volume process?

The volume is constant and the pressure is not constant Signup and view all the answers

What is the equation for the change in internal energy of an ideal gas in a constant volume process?

?U = mCv(T2-T1) Signup and view all the answers

What is the method of heat transfer involving the movement of fluids?

Convection Signup and view all the answers

What is the change in internal energy of a system represented by?

?U Signup and view all the answers

What is the work done on a system during compression of a gas equal to?

-PdV Signup and view all the answers

What is the energy transferred through a force applied over a distance?

Work Signup and view all the answers

What is the process called when a gas is compressed or expanded in a cylinder while maintaining a constant temperature?

Isothermal Process Signup and view all the answers

What is the change in internal energy of a system due to heat transfer and work done?

U = Q - W Signup and view all the answers

What is the work done by a gas during expansion equal to?

PdV Signup and view all the answers

What is the force applied to lift a weight against gravity?

F = mg Signup and view all the answers

What is the unit of internal energy typically measured in?

Joules Signup and view all the answers

What is the process called when a gas is compressed or expanded in a cylinder without any heat transfer?

Adiabatic Process Signup and view all the answers

Study Notes

First Law of Thermodynamics

The first law of thermodynamics states that energy cannot be created or destroyed, only converted from one form to another.
The law can be mathematically expressed as: Q - W = ΔE (energy transferred as heat minus energy transferred as work equals the change in internal energy)
The law is based on experimental observations and cannot be proved mathematically.

Energy Transfer Mechanisms

Heat (Q): energy transferred due to a temperature difference between the system and its surroundings.
Work (W): energy transferred when a force is applied over a distance or when a system changes its volume under pressure.

Energy Conservation in a Closed System

For a closed system, the total energy (kinetic, potential, and internal energies) remains constant, although it can change forms.
The energy balance needs to account for the energy carried by the mass entering and leaving the system.

Law of Conservation of Energy

The law of conservation of energy states that the total energy in an isolated system remains constant over time, although energy can transform from one form to another.
This fundamental principle of physics implies that energy cannot be created or destroyed, only converted or transferred.

Joule's Experiment

James Prescott Joule's experiment demonstrated the mechanical equivalent of heat.
The experiment showed that mechanical work could be converted into heat, providing strong evidence for the conservation of energy.

Energy as a Property of a System

Energy is a property of a thermodynamic system, meaning it is a state function that describes the energy content of the system.
Energy is a scalar quantity that encompasses various forms, including kinetic, potential, internal, thermal, chemical, electrical, nuclear, and radiant energy.

Internal Energy

Internal energy (U) is a fundamental concept in thermodynamics that represents the total energy contained within a system.
Internal energy is a state function that depends on the temperature of the system.
For an ideal gas, internal energy (U) is a function of temperature (T) only.

Enthalpy

Enthalpy (H) is a thermodynamic property that represents the total energy of a system.
Enthalpy is defined as: H = U + PV, where U is the internal energy, P is the pressure, and V is the volume.
Enthalpy is a useful concept for analyzing thermodynamic systems, especially in chemical engineering and chemistry.

Specific Heats

The specific heat is defined as the energy required to raise the temperature of a unit mass of a substance by one degree.
There are two kinds of specific heats: specific heat at constant volume (Cv) and specific heat at constant pressure (Cp).
Cp is always higher than Cv because at constant pressure the system is allowed to expand and energy for this expansion must also be supplied to the system.

First Law of Thermodynamics for Non-Flow Processes

The First Law of Thermodynamics can be applied to non-flow processes, also known as closed systems or batch processes.
The First Law for a closed system is: ΔE = Q - W, where ΔE is the change in internal energy, Q is the heat added to the system, and W is the work done on the system.

Examples of Non-Flow Processes

Heating a solid: heat is added, and the internal energy increases.
Compression of a gas: work is done on the system, and the internal energy increases.
Phase change: heat is added, and the internal energy increases.
Expansion of a gas: work is done by the system, and the internal energy decreases.
Cooling a solid: heat is removed, and the internal energy decreases.

Isothermal and Adiabatic Processes

Isothermal process: a process in which the temperature remains constant, and no heat is added or removed.
Adiabatic process: a process in which there is no heat transfer between the system and the surroundings, and the internal energy changes due to work done on or by the system.### Work Done in Different Scenarios

Work Done in Compressing a Gas

Work done on the system: W = -PdV (negative sign indicates work done on the system)
Force applied by piston: F = PA (where P is pressure and A is area)
Displacement of piston: dx = dV/A (where dV is change in volume)
Work done: W = Fdx = PAdx = PdV

Work Done in Expanding a Gas

Work done by the system: W = PdV (positive sign indicates work done by the system)
Force applied by gas: F = PA
Displacement of piston: dx = dV/A
Work done: W = Fdx = PAdx = PdV

Work Done in Lifting a Weight

Work done by force applied to lift weight against gravity: W = mgh (where m is mass, g is acceleration due to gravity, and h is height lifted)
Force lifting weight: F = mg
Displacement of weight: h (height lifted)
Work done: W = Fdh = mgh

Work Done in Stirring or Agitation

Work done by stirrer or agitator: W = τθ (where τ is torque and θ is angular displacement)
Torque applied: τ
Angular displacement: θ
Work done: W = τdθ = τθ

Work Done in Isothermal and Adiabatic Processes

Isothermal compression or expansion: W = 0 (no work is done since process occurs at constant temperature and pressure)
Adiabatic compression: W = -PdV (compression)
Adiabatic expansion: W = PdV (expansion)
Sign convention for work done:
- Negative sign (-) indicates work done on the system
- Positive sign (+) indicates work done by the system

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Description

This quiz covers the basics of the First Law of Thermodynamics, including energy exchange between systems and surroundings through heat and work transfer.