INTRODUCTION
• Energy is a scalar quantity.
• It was first hypothesized by Newton to express kinetic and potential energies.
• We cannot observe energy directly, but we can measure it using indirect methods and analyze its value.
• Energy may be in different forms, such as potential, kinetic, magnetic or electrical.
Potential energy of a system is by virtue of its location with respect to gravitational field. If an object has a mass m, located at elevation h, and acceleration due to gravity is g, then the potential energy is, EPE = mgh (1.1)
Kinetic energy of an object is due to its velocity. If an object is moving with a velocity u, and it has mass m, then its kinetic energy is, EKE = 1/2mu2 (1.2) • Both kinetic and potential energies are macroscopic, that is, they represent energy of a system due to its entire being. • This is an contrast to internal energy, which is due to the microscopic nature of a system. • At the molecular scale, the atoms of a substance are continuously in motion. • They move in random direction, collide with each other, vibrate, and rotate. • Energies related to all these movements, including energy of attractions between the atoms is combined into one lump sum and is called the internal energy. Internal energy is an extensive property, and it is independent of the path of a process, and it is independent of value of the path of a process. We cannot measure an absolute value of internal energy; we can relate changes in internal energy to other properties such as temperature and pressure. In many engineering systems, one or two forms of energy may dominate while others can be neglected. For example, when a sugar beet is dropped from a conveyor into a bin, the potential and such as a chemical, magnetic and electrical, do not change and may be neglected in the analysis. Similarly, when tomato juice is heated in a hot-break heater, the potential or kinetic energy of the juice does not change, but the internal energy will change as temperature increases. The total energy of a system can be written in the form of an equation, ETOTAL = EKE + EPE + EELECTRICAL + EMAGNETIC + ECHEMICAL + ……..+ Ei (1.3) Where Ei is the internal energy, kJ. …show more content…
If the magnitude of all other energy forms are small in comparison with the kinetic, potential and internal energies, then ETOTAL = EKE + EPE + Ei (1.4)
PROBLEM 1.
A small object of mass m=234g slides along a track with elevated ends and a central flat part. The flat part has a length L=2.16m. The curved portions the tracks are frictionless; but it travelling the flat part, the object loses 688 mJ of mechanical energy, due to friction. The object is released at point A, which is height h= 1.05 m above the flat part of the track. Where does the object finally come to rest?
Concept:
The potential energy U is defined as:
U=mgh
Here, m is the mass of the body, g is the free fall acceleration and h is the fall of height.
Solution:
To find the potential energy of the object at the point A, substitute 234g for mass of the object m, 9.81 m/s2 for free fall acceleration g and 1.05 m for height h in the equation U=mgh = (234g) (9.81m/s2) (1.05m) = (234g* 10-3kg/1g) (9.81m/s2) (1.05m) = 2.41 kg.m2/s2 = (2.41 kg.m2/s2) (1J/1kg.m2/s2) = 2.41 J The curved portion of
• Energy is a scalar quantity.
• It was first hypothesized by Newton to express kinetic and potential energies.
• We cannot observe energy directly, but we can measure it using indirect methods and analyze its value.
• Energy may be in different forms, such as potential, kinetic, magnetic or electrical.
Potential energy of a system is by virtue of its location with respect to gravitational field. If an object has a mass m, located at elevation h, and acceleration due to gravity is g, then the potential energy is, EPE = mgh (1.1)
Kinetic energy of an object is due to its velocity. If an object is moving with a velocity u, and it has mass m, then its kinetic energy is, EKE = 1/2mu2 (1.2) • Both kinetic and potential energies are macroscopic, that is, they represent energy of a system due to its entire being. • This is an contrast to internal energy, which is due to the microscopic nature of a system. • At the molecular scale, the atoms of a substance are continuously in motion. • They move in random direction, collide with each other, vibrate, and rotate. • Energies related to all these movements, including energy of attractions between the atoms is combined into one lump sum and is called the internal energy. Internal energy is an extensive property, and it is independent of the path of a process, and it is independent of value of the path of a process. We cannot measure an absolute value of internal energy; we can relate changes in internal energy to other properties such as temperature and pressure. In many engineering systems, one or two forms of energy may dominate while others can be neglected. For example, when a sugar beet is dropped from a conveyor into a bin, the potential and such as a chemical, magnetic and electrical, do not change and may be neglected in the analysis. Similarly, when tomato juice is heated in a hot-break heater, the potential or kinetic energy of the juice does not change, but the internal energy will change as temperature increases. The total energy of a system can be written in the form of an equation, ETOTAL = EKE + EPE + EELECTRICAL + EMAGNETIC + ECHEMICAL + ……..+ Ei (1.3) Where Ei is the internal energy, kJ. …show more content…
If the magnitude of all other energy forms are small in comparison with the kinetic, potential and internal energies, then ETOTAL = EKE + EPE + Ei (1.4)
PROBLEM 1.
A small object of mass m=234g slides along a track with elevated ends and a central flat part. The flat part has a length L=2.16m. The curved portions the tracks are frictionless; but it travelling the flat part, the object loses 688 mJ of mechanical energy, due to friction. The object is released at point A, which is height h= 1.05 m above the flat part of the track. Where does the object finally come to rest?
Concept:
The potential energy U is defined as:
U=mgh
Here, m is the mass of the body, g is the free fall acceleration and h is the fall of height.
Solution:
To find the potential energy of the object at the point A, substitute 234g for mass of the object m, 9.81 m/s2 for free fall acceleration g and 1.05 m for height h in the equation U=mgh = (234g) (9.81m/s2) (1.05m) = (234g* 10-3kg/1g) (9.81m/s2) (1.05m) = 2.41 kg.m2/s2 = (2.41 kg.m2/s2) (1J/1kg.m2/s2) = 2.41 J The curved portion of