Thermodynamics Unit-II: First Law

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Questions and Answers

What is the primary statement of the First Law of Thermodynamics?

Energy can be transferred, but it cannot be conserved.

Energy can neither be created nor destroyed, but it can change forms. (correct)

Energy can be created or destroyed, but not changed from one form to another.

Energy can be conserved, but it cannot be transferred.

What is the unit of heat transfer?

kW/kg

kJ/kg

kJ (correct)

What is the sign convention for heat transfer?

Heat transfer to a system is zero, and heat transfer from a system is zero.

Heat transfer to a system is positive, and heat transfer from a system is negative. (correct)

Heat transfer to a system is negative, and heat transfer from a system is positive.

Heat transfer to a system is infinite, and heat transfer from a system is infinite.

What do both heat and work have in common?

They are boundary phenomena. Signup and view all the answers

What is the basis of the First Law of Thermodynamics?

Experimental observations Signup and view all the answers

What is heat, according to kinetic theory?

The energy associated with the random motions of atoms and molecules. Signup and view all the answers

What is the primary component of internal energy in an ideal gas?

Both kinetic and potential energy of the molecules Signup and view all the answers

What is the mathematical relationship between enthalpy and internal energy?

H = U + PV Signup and view all the answers

What is the specific heat at constant pressure Cp related to?

Changes in enthalpy Signup and view all the answers

Why is the specific heat at constant pressure Cp higher than Cv?

Because the system is allowed to expand Signup and view all the answers

What is internal energy a function of, in an ideal gas?

Temperature only Signup and view all the answers

What is the change in enthalpy (?H) equal to, in a constant pressure process?

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

What is the relationship between the internal energy and temperature for an ideal gas?

u is a function of temperature only Signup and view all the answers

What is the specific heat ratio for monatomic gases?

1.67 Signup and view all the answers

What is the assumption made for incompressible substances?

The volume remains constant during a process Signup and view all the answers

What is the equation for the enthalpy change of an incompressible substance?

h = u + Pv Signup and view all the answers

What is the characteristic of a constant temperature process for an ideal gas?

pV = constant Signup and view all the answers

What is the equation for the First Law of Thermodynamics for a closed system?

ΔU = Q - W Signup and view all the answers

What is the fundamental principle stated by the first law of thermodynamics?

Energy cannot be created or destroyed, only converted from one form to another. Signup and view all the answers

What is the correct expression for the change in total energy of a system during a process?

E = U + KE + PE Signup and view all the answers

What is the term for the energy stored in an object due to its position in a force field?

Potential Energy Signup and view all the answers

What is the unit of energy typically used in the first law of thermodynamics?

Kilojoules per kilogram (kJ/kg) Signup and view all the answers

What is the term for the energy carried by electromagnetic waves?

Radiant Energy Signup and view all the answers

What is the characteristic of a cyclic process in thermodynamics?

The initial and final states of the system are identical. Signup and view all the answers

What is the fundamental principle of physics implied by the law of conservation of energy?

Energy cannot be created or destroyed, only converted or transferred. Signup and view all the answers

What was the key observation made by James Joule in his experiment?

Mechanical work done by the falling weights resulted in a measurable increase in the water's temperature. Signup and view all the answers

What is internal energy (U) in thermodynamics?

The energy of motion of particles, vibrational energy, rotational energy, and bond energy. Signup and view all the answers

What is the mathematical expression for the work done on a system in terms of pressure and volume?

W = P dV Signup and view all the answers

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

The change in internal energy is a state function, meaning it depends only on the current state of the system. Signup and view all the answers

What is the condition for a process to be quasi-static?

The process must be slow and reversible. Signup and view all the answers

What is the primary method of energy transfer in a conduction process?

Heat Signup and view all the answers

What is the change in internal energy of a system during an isothermal process?

Zero Signup and view all the answers

What is the direction of work done during the expansion of a gas in a cylinder?

Work is done by the system Signup and view all the answers

What is the equation for work done during the compression of a gas?

W = -PdV Signup and view all the answers

What is the change in internal energy of a system during a process where 100 J of heat is added and 50 J of work is done on the system?

150 J Signup and view all the answers

What is the equation for work done in lifting a weight against gravity?

W = mgh Signup and view all the answers

What is the condition for an adiabatic process?

No heat transfer occurs Signup and view all the answers

What is the unit of internal energy?

Joules Signup and view all the answers

What is the effect of heat transfer on the internal energy of a system?

It increases the internal energy Signup and view all the answers

What is the direction of work done during the compression of a gas in a cylinder?

Work is done on the system 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

Understand the First Law of Thermodynamics, which states that energy gained by a system is equal to energy lost by its surroundings. Learn about energy transfer through heat and work in closed systems.