Chapter 2 - Energy & The First Law of Thermodynamics

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Questions and Answers

Which forms of energy are included in the change in energy of a closed system?

Mechanical energy, electrical energy, and kinetic energy

Kinetic energy, thermal energy, and work

Potential energy, heat, and internal energy

Kinetic energy, gravitational potential energy, and internal energy (correct)

What does the first law of thermodynamics state about energy?

Energy is a non-conservative property.

Energy can be created and destroyed.

Energy cannot be created or destroyed, only transformed. (correct)

Energy is always conserved in all conditions.

In a closed system, energy is transferred across the system boundary primarily through which means?

Pressure and volume change

Conduction and radiation

Work and heat (correct)

Friction and inertia

Which equation represents the change in kinetic energy for a system of mass m?

$DKE = KE2 - KE1$ Signup and view all the answers

What is the primary contributing factor to changes in gravitational potential energy in a system?

The system's position within a gravitational field Signup and view all the answers

Which of the following is NOT considered a mode of heat transfer?

Potential energy change Signup and view all the answers

In energy analyses of thermodynamic cycles, what is thermal efficiency?

The ratio of useful work output to heat input Signup and view all the answers

What role does work play in the energy transfer of a closed system?

Work transfers energy into or out of the system. Signup and view all the answers

What does Qout represent in the heat pump cycle?

Heat transfer to the hot body Signup and view all the answers

Which equation represents the energy balance in a heat pump cycle?

Wcycle = Qout - Qin Signup and view all the answers

How is the coefficient of performance (g) for a heat pump cycle calculated?

g = Qout / Wcycle Signup and view all the answers

In the example given, what is the net work input (Wcycle) for the heat pump cycle?

300 kJ Signup and view all the answers

What is the value of the coefficient of performance (g) calculated in the example?

3.0 Signup and view all the answers

What does the equation $\Delta PE = mg(z_2 - z_1)$ represent?

The change in gravitational potential energy of a mass Signup and view all the answers

What is represented by 'U' in the context of internal energy?

The total internal energy of the system Signup and view all the answers

Which statement is true regarding the change in energy of a system from state 1 to state 2?

The change in energy is calculated using the equation $\Delta E = \Delta U + \Delta KE + \Delta PE$ Signup and view all the answers

How is internal energy typically evaluated for a wide range of applications?

Consulting tables found in appendices of textbooks Signup and view all the answers

Which of the following statements correctly describes work in thermodynamics?

Work depends on the interactions between the system and surroundings Signup and view all the answers

What does the equation $E_2 - E_1 = Q - W$ represent in closed system energy balance?

The energy content change between two states Signup and view all the answers

What does the term 'extensive property' refer to in the context of internal energy?

It is proportional to the mass of the system Signup and view all the answers

In the equation $DKE + DPE + DU = Q - W$, what does DU represent?

Change in internal energy Signup and view all the answers

What are the two means by which energy can be transferred to and from closed systems?

Work and heat Signup and view all the answers

What is crucial to distinguish when applying the energy balance equations?

Positive and negative signs of energy Signup and view all the answers

What is the significance of changes in the energy of a system between states?

Only changes in energy have significance, not absolute values Signup and view all the answers

Why is the location of the system boundary important in energy balance?

It determines how heat and work are defined Signup and view all the answers

What does the time rate form of the closed system energy balance describe?

Instantaneous energy change within the system Signup and view all the answers

Which component of the energy balance equation is treated as positive when energy is transferred from the system?

Work done Signup and view all the answers

What does the negative sign before W in the energy balance equations signify?

Energy is leaving the system Signup and view all the answers

Which element is NOT part of the closed system energy balance considerations?

Thermal conductivity of the container Signup and view all the answers

What does Fourier's law primarily describe in heat transfer?

The relationship between the rate of heat transfer and temperature gradient. Signup and view all the answers

What is the significance of the minus sign in Fourier's law?

It signifies the direction of energy transfer. Signup and view all the answers

Which statement regarding thermal radiation is true?

It can occur in a vacuum. Signup and view all the answers

In the context of convection, what does the convection heat transfer coefficient represent?

A constant indicating the rate of heat transfer per unit area. Signup and view all the answers

What is the primary factor that differentiates convection from conduction?

Convection involves bulk fluid motion. Signup and view all the answers

What best describes a thermodynamic cycle?

A series of processes that returns to the original state. Signup and view all the answers

Which law is applied to quantify energy transfer by convection?

Newton's law of cooling. Signup and view all the answers

What role does emissivity play in thermal radiation exchange?

It characterizes the efficiency of a surface to emit energy. Signup and view all the answers

What is the net work developed for a system that receives 1000 kJ of heat and discharges 600 kJ?

400 kJ Signup and view all the answers

What is the thermal efficiency of a cycle that generates 400 kJ of work from 1000 kJ of heat?

40% Signup and view all the answers

How does the refrigeration cycle define the relationship of work to heat transfers?

Wcycle = Qout - Qin Signup and view all the answers

Which equation expresses the efficiency of a refrigeration cycle?

Efficiency = Qin / Wcycle Signup and view all the answers

What is the coefficient of performance for a refrigeration cycle based on its definition?

COP = Qin / Wcycle Signup and view all the answers

In a heat pump cycle analysis, how does it compare with the refrigeration cycle?

It operates on the same principle as a refrigeration cycle. Signup and view all the answers

During a refrigeration cycle, which statement is true regarding energy balance?

Energy change per cycle is zero. Signup and view all the answers

Which of the following is true about the heat exchanged during a refrigeration cycle?

Qin is absorbed from the cold body while Qout is released to the hot body. Signup and view all the answers

Study Notes

Chapter 2: Energy and the First Law of Thermodynamics

This chapter focuses on energy and the first law of thermodynamics.
Learning outcomes include explaining key concepts like energy, internal energy, kinetic energy, potential energy, work, power, heat transfer, heat transfer modes, heat transfer rate, power cycles, refrigeration cycles, and heat pump cycles.
Analyzing closed systems, including applying energy balances, modeling the specific case, and observing proper sign conventions for work and heat transfer.
Conducting energy analyses of systems undergoing thermodynamic cycles and evaluating thermal efficiencies, as well as coefficients of performance for refrigeration and heat pump cycles.

Closed System Energy Balance

Energy is an extensive property, encompassing kinetic and gravitational potential energy.
For closed systems, energy transfer occurs across the boundary through heat and work.
Energy is conserved (this is the first law of thermodynamics).
The closed system energy balance states that the change in the amount of energy within a closed system during a time interval equals the net amount of energy transferred in and out across the system boundary by heat and work during that time interval.
ΔE = Q-W
Q = transfer of heat into the system, W=work done by the system

Change in Energy of a System

In engineering thermodynamics, the change in energy of a system is composed of kinetic energy, gravitational potential energy, and internal energy.
Kinetic energy change (ΔKE) is associated with the system's motion relative to an external frame (e.g., the Earth). The formula is ΔKE = ½m(V₂² - V₁²), where m is mass and V₁ and V₂ are initial and final velocities.
Gravitational potential energy change (ΔPE) is related to the system's position in a gravitational field, calculated as ΔPE = mg(z₂ - z₁), where m is mass, g is acceleration due to gravity, and z₁ and z₂ are initial and final elevations.
Internal energy change (ΔU) is associated with the system's internal composition and is represented by U. There's no simple expression, so data from tables are usually used.

Change in Kinetic Energy

The change in kinetic energy is related to the motion of the system relative to an external reference frame (e.g., the Earth).
The formula for this change is given by ΔKE = KE₂ – KE₁ = ½m(V₂² – V₁²), where m is the mass, and V₁ and V₂ are the initial and final velocity magnitudes, respectively.

Change in Gravitational Potential Energy

The change in gravitational potential energy depends on the position of the system within Earth's gravitational field.
The change in potential energy from state 1 to state 2 is given by ΔPE = PE₂ – PE₁ = mg(z₂ – z₁), where m is the mass, g is the acceleration due to gravity, and z₁ and z₂ are the initial and final elevations relative to a reference plane.

Change in Internal Energy

Internal energy is associated with the makeup of a system, including its chemical composition.
Internal energy is represented by U.
The specific internal energy on a mass basis is u.
The specific internal energy on a molar basis is u.
There's no single expression for ΔU; tables often provide data for calculations.

Energy Transfer by Work

Energy can be transferred to or from a closed system through work or heat.
Work is done by a system on its surroundings if the sole effect on everything external to the system could have been the raising of a weight.
Work is considered positive when done by the system and negative when done on the system.
The integral of force-displacement is often used to determine work.
Work is not a property of the system itself.

Energy Transfer by Heat

Heat transfer is due to a temperature difference between the system and its surroundings.
Heat transfer occurs in the direction of decreasing temperature (hot to cold).
Q is used as an indicator of heat transfer to or from the system.

Modes of Heat Transfer

Energy can be transferred by conduction, radiation, and convection.

Conduction

Energy transfer through a substance due to particle interactions and temperature differences.
Fourier's law quantifies the rate of heat conduction.

Radiation

Transfer of energy through electromagnetic waves (no medium required).
Quantified by the Stefan-Boltzmann law.

Convection

Energy transfer through the motion of fluids (gases or liquids).
Heat transfer through fluid motion and conduction.
Newton's law of cooling quantifies convection's rate.

Thermodynamic Cycles

A thermodynamic cycle is a sequence of processes that begins and ends at the same state.
Examples include power cycles (with work output), refrigeration cycles (for cooling), and heat pump cycles (for heating).

Power Cycle

It produces a net work output.
The net work is equal to the difference between heat input and heat output for a complete cycle.

Refrigeration Cycle

It removes heat from a cold body.
Work input is needed to transfer heat from a cold spot to a hot one.
The coefficient of performance (COP) measures a refrigeration cycle's efficiency.

Heat Pump Cycle

It supplies heat to a warm body.
Work input is needed to pump heat from a cold source to a hot one.
The coefficient of performance (COP) measures its efficiency.

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Description

Test your understanding of thermodynamics concepts, including the first law, forms of energy, and modes of heat transfer. This quiz covers crucial principles such as energy changes in closed systems, kinetic and potential energy, and thermal efficiency in thermodynamic cycles.